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thesis entitled

ULTRAFAST DYNAMICS OF POLYATOMIC MOLECULES
UNDER INTENSE LASER RADIATION

presented by

Vahan Artsruni Senekerimyan

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ULTRAFAST DYNAMICS OF POLYATOMIC MOLECULES UNDER
INTENSE LASER RADIATION

BY

Vahan Artsruni Senekerimyan

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
Department of Chemistry

2004

ABSTRACT

ULTRAFAST DYNAMICS OF POLYATOMIC MOLECULES UNDER INTENSE
LASER RADIATION

BY

Vahan Artsruni Senekerimyan

The goal of the research was to develop methods to study and control the behavior of
molecules on an ultrashort time-scale. First, the behavior of molecules under high
intensity femtosecond laser irradiation is discussed. In this part, we discuss the electric
field produced by the lasers, and how this field interacts with molecules causing
multiphoton or field ionization and Coulomb explosion. Results obtained with the help of
femtosecond CPM dye laser and molecular beam apparatus coupled with a time-of-flight
mass spectrometer are shown for triatomics such as CS; and N02. Analysis of angular
distributions of molecular and fragment ions of these molecules is presented and
structural information of Coulomb exploding molecular ions of C82 is retrieved. Second,
preliminary experimental and theoretical results for ozone are presented. It is shown that
ozone does not behave like N02 or CS; under the same experimental conditions. Possible

explanations of this result are discussed.

ACKNOWLEDGMENTS

I would like to thank my adviser Professor Marcos Dantus and all Dantus research group
members and express thanks to my Guidance Committee members -

Professors John McCracken, Katharine Hunt, and Jack Watson for their help and
comments. I am also gratefiJl to Professor Kaoru Yamanouchi from University of Tokyo
for giving me permission to use a figure from his article in Science (Science, 295, 1659-

1660, 2002) in my thesis.

iii

TABLE OF CONTENTS

LIST OF TABLES .................................................................................... vi
LIST OF FIGURES ................................................................................. vii
INTRODUCTION .................................................................................... 1
EXPERIMENTAL .................................................................................... 7
EXPERIMENTAL RESULTS ...................................................................... 9

Experimental results of Coulomb explosion in CS; and NO; .......................... 9

Preliminary Experimental and Theoretical results for O3 ............................. 11
DISCUSSION ............................................. . ......................................... 1 3
CONCLUSIONS ................................................................................... 1 8
APPENDICES ......................... . ............................................................. 20
Appendix A .......................................................................................... 21
Figure 1 ........................................... . ................................................... 22
Appendix B .......................................................................................... 23
Figure 2 ............................................................................................... 24
Appendix C .......................................................................................... 25
Figure 3 ............................................................................................... 26
Appendix D .......................................................................................... 27
Figure 4 ............................................................................................... 28
Appendix E .......................................................................................... 29
Figure 5 ............................................................................................... 30
Appendix F ........................................................................................... 31
Figure 6 ............................................................................................... 32
Appendix G .......................................................................................... 33
Figure 7 ............................................................................................... 34
Appendix H .......................................................................................... 35
Figure 8 ............................................................................................... 36
Appendix I ........................................................................................... 37
Figure 9 ............................................................................................... 38
Appendix J ........................................................................................... 39

iv

Figure 10 .............................................................................................. 40

Appendix K .......................................................................................... 41
Figure 11 ............................................................................................. 42
BIBLIOGRAPHY ................................................................................... 43

LIST OF TABLES

Table 1. First and second vertical ionization potentials (VIP) of ozone and geometries of
03+ and 032+ obtained in the CCSD(T) calculations ............................................ 12

Table 2. Comparison of experimental values of first and second ionization potentials of
03, C82, C02, 802, and N02 ....................................................................... 16

vi

LIST OF FIGURES

Figure 1. Laser field induced processes in various intensity regimes (from K
Yamanouchi: Science 295 (2002) 1659-60) ..................................................... 22

Figure 2. Schematic of 3-step multiphoton ionization in two cases: resonant (on the left)
and non-resonant (on the right). Typical laser electric field intensities required for these
two processes are 1 — 104 W/cm2 and >10'2 W/cm2 for resonant and non-resonant
processes respectively. Non-resonant absorption of the photon proceeds through the
virtual state with lifetimes on the order of 1 fs .................................................. 24

Figure 3. Schematic representation of field (tunneling) ionization for an atomic system.
Keldysh adiabaticity model is reasonable for field ionization when y = Funnel/Tim, << 1.
In the opposite limit multiphoton ionization mechanism is appropriate. . . . . . . . . . .. ....... '. .26

Figure 4. a) Schematic for the experimental setup. b) Schematic representation of the
Coulomb explosion process leading to the splitting in the mass spectrum (from Comstock
M, Senekerimyan V, Dantus M: J Phys Chem A 107 (40): 8271-8281 2003) .............. 28

Figure 5. Glass container with the stored ozone on silica gel in the isopropyl/dry ice
slurry bath at around - 85° C (from V Senekerimyan, M McGuire, M Comstock, M
Dantus: Quantum Control of Light and Matter, Gordon Research Conference, Mount
Holyokee, MA 2003) ............................................................................... 30

Figure 6. a) Mass spectrum of CS2, where the zoomed part shows the splitting of the
peaks. b) The isotropic angular distribution of the C8; parent ion (from Comstock M,
Senekerimyan V, Dantus M: J Phys Chem A 107 (40): 8271-8281 2003) ................... 32

Figure 7. Angle dependent detection of singly and doubly charged fragment ions
following Coulomb explosion. pronounced. . Sz+ shows a maximum ion yield at 0° and
180°, while C2+ shows a maximum at 90° and 270° (from Comstock M, Senekerimyan V,
Dantus M: J Phys Chem A 107 (40): 8271-8281 2003) .................................................... 34

Figure 8. Mass spectrum of NO; taken at laser peak power 4.6x 10l4 W/cmz. Notice the
splitting in the peaks of N and 0+ (from V Senekerimyan, M McGuire, MComstock, M
Dantus: Quantum Control of Light and Matter, Gordon Research Conference, Mount
Holyokee, MA 2003) ............................................................................... 36

Figure 9. a) Mass spectrum of O3 (top) and air (bottom) taken under the same
experimental conditions as that of NO;. The dashed arrow points where 032+ should

vii

appear. (b) Polar plot of angular distribution for 02+ (solid line) and 0+ (dashed line,
twice reduced in scale) (from V Senekerimyan, M McGuire, M Comstock, M Dantus:
Quantum Control of Light and Matter,. Gordon Research Conference, Mount Holyokee,
MA 2003) ............................................................................................. 38

Figure 10. Conceptual drawings of the Coulomb Explosion of CS; when the electric field
is parallel or perpendicular to the time-of-flight axis for the a) linear molecules and b)
bent molecules ....................................................................................... 40

Figure 11. A typical potential energy curve for the diatomic dication ...................... 42

viii

1. Introduction

Electric forces underlie almost all of chemistry and molecular physics. Precise
manipulation of external electric fields, therefore, may be a natural choice to explore the
study and control of atomic and molecular dynamics. Ultrashort lasers allow probing of
molecular processes in real time on the femtosecond time scale (1). Another unique
feature of these lasers is the ability to get extremely high peak powers. The advent of
modern amplification methods has greatly increased the output energies of femtosecond
pulses. Today even in small university labs, laser fields with peak powers as high as 10'8
W/cm2 can be routinely generated; this intensity is comparable in magnitude to the
intensity generated by the atomic nucleus. For instance, the electric field strength of a
1018 W/cm2 pulse is approximately 5"‘109 V/cm, which is the same as the hydrogen
intratomic field.

Such fields will help in studying atomic and molecular processes, and will cause a
number of physical phenomena in atoms and molecules such as multiphoton or field
ionization, structural deformations, and Coulomb explosion (CE). The behavior of
molecules in such fields has been studied intensively in the last decade to clarify complex
dynamics caused by the interplay among electrons, nuclei, and a photon field.

When electric field intensities approach 10lo W/cmz, the alignment of molecules along

the laser field direction takes place (Fig. 1). At these intensities, the probability of

ionization is low, thus, we can investigate the alignment mechanism of neutral species
subjected to the laser field.

The next electric field intensity regime, where one may observe structural
deformations of molecules, multiple ionization, and Coulomb explosion, is reached when
the field intensity is increased to between 1013 — 10'6 W/cmz. In this power region, which
is called Coulombic regime (Fig.1), the removal of two or more electrons via the
muliphoton or field ionization leads to CE because of the strong Coulombic repulsion in
the resulting positively charged molecular ion island (2). Thus, it is the multiple
ionization that leads to CE. A simple schematic for resonant and non-resonant
multiphoton ionization is given in Figure 2. In case of non-resonant ionization, the
absorption of photons proceeds through the virtual states having lifetimes on the order of
1 fs. Therefore, non-resonant multiphoton absorption requires much higher laser field
power compared to the case of resonant absorption. Resonant absorption of photons
enormously facilitates the transitions; therefore, the multiphoton absorption probability is
dependent on the laser wavelength resonant or non-resonant with the molecular
transitions. A simple schematic for field ionization (also called tunneling ionization) for
an atomic system is shown on Figure 3. As an approximation, we may treat the effective
electric field due to the laser as a static field. That is, we will assume we can ignore the
fact that the field actually oscillates sinusoidally in time. If we combine the Coulomb
potential of the nucleus, q/r, with the static field potential, eEr, the electrons then move in
a distorted potential similar to that shown in Figure 3. The central features of this
potential are (1) the electrons are only quasi-bound in one side of the potential since they

can tunnel through the barrier to freedom and (2) the higher the intensity, the more the

ground state energy level (potential barrier) is pushed upward. Clearly at some intensity
the ground level may reach the top of the barrier. This occurs at about 10'4 W/cm2 for the
ground state of the hydrogen atom. It turns out that this is also true for many electron
systems; they too will not survive as neutral species at intensity of about 10'4 W/cmz.
This field ionization picture depends on the validity of treating the laser field as a static
electric field. This Keldysh adiabaticity model is reasonable if there is enough time for
the electron to escape before the direction of field changes sign. Often the Keldysh
parameter (3) is used as a measure of when the static field approach is valid. When the

Keldysh parameter, given by

7 = mums... = Janna/77...... = (Um/1.87*10 "“1919” . (I)

is <<1, the field approach is a reasonable assumption. In Eq. (1) U”: is the ionization
potential of the atom, I the intensity in W/cm2 and 2 the wavelength in microns. In the
opposite limit it is appropriate to view ionization as a multiphoton process and employ
perturbation theory to determine the ionization rate. For molecular systems, by
convention when 7 < 0.5 the system is in the tunnel ionization regime, otherwise it is in
multiphoton ionization regime.

When the laser field intensity is increased to 1014 - 1015 W/cmz, strong mixing of
electronic states also occurs. This implies that the potential energy surface (PES) of the
molecule, whose shape is determined by the electronic configuration, becomes deformed
(4). Since the angular distributions of fragment ions following CE will be sensitively

dependent upon the geometry of exploding molecular ions, the molecular and ionic

deformations resulting from the formation of these light-dressed states may be
investigated by studying the angular distributions of the fragment ions ejected upon CE
and analyzing them either by mass-resolved momentum imaging (MRMI) or coincidence
momentum imaging (2,5).

In fact, the dynamics in Coulombic regime is rather complex. This kind of
research helps us understand the fundamentals and nature of high intensity laser and
matter interactions and serves as a good tool in investigating the behavior of molecules
subject to laser fields. The ability to apply strong electric forces with high precision and
little energy has led to various applications in science and technology. For example,
intense femtosecond lasers have found their way as novel ionization sources in mass
spectrometry. There is also a great commercial interest to femtosecond laser machining
of materials due to the high precision and reduced local heat deposition of this process.
Studies of atmospherically relevant molecules such as 03, CS;, CO;, NO;, 0;, H20, etc in
high - intensity laser fields is also important since it will allow us to understand how the
molecules and the laser field aremodified during the propagation of the pulses in the
atmosphere.

As will be shown in this work, not all the molecules subject to high - intensity
fields in the Coulombic regime behave similarly, which may be caused by the multiple
ionization dependence on laser wavelength. A number of dissociation channels may be
accessed, each having a partial cross section depending upon the focused laser peak
intensity, laser wavelength, and pulse duration. For instance, for CS; we observe CS;2+,
CS;3+, CS;4+, etc in the mass spectrum, which Coulomb explode producing different

fragment ions via various channels. For example, CS;3+ -—> CS+ + S+ + C“, CS;3+ —> Sz+ +

CS+, or CS;3+ -—> C” +2S+, CS;4+ ——> S;2+ + CH, etc. The identification of dissociation
channels is also a quite complicated aspect since some of the exploding molecular ions
may not be present in the TOF mass spectrum. This happens when the lifetime of the
multiply charged ion is shorter than the time of flight in the TOF spectrometer.

Experimentally, one can vary the wavelength, risetime, focused intensity and state
of polarization of the laser and measure the kinetic energy and angular distributions of the
photoions or photoelectrons. One usually uses time-of-flight (TOF) spectroscopy for such
measurements.

At laser field intensities about 1017 W/cm2 and higher, we can observe X-Ray
emission and nuclear fusion (See Fig.1). At 1020 W/cmz, we approach the electric field
strengths comparable to intranuclear forces, which can cause nuclear fusion. This is
another hot scientific topic nowadays, and scientists are trying to take advantage of
ultrafast lasers to generate highly energetic proton and electron beams, electron-positron
pairs, and use them for plasma and astrophysical research.

Here I present experimental studies of triatomics such as CS;, NO;, and 03 under
high intensity laser irradiation in the Coulombic regime. TOF mass spectra of these
molecules following Coulomb explosion are collected along with the angular
distributions of molecular and fragment ions of CS; and 03. Data analyses are made to
retrieve structural information about exploding molecular ions of CS;. Preliminary
experimental and theoretical data for ozone are also presented. The difference in behavior
of ozone compared to NO; and CS; under Coulombic regime laser fields is discussed.

Thus, we can summarize the task and the scope of this thesis in the following way:

The dynamics of polyatomic molecules subject to high intensity laser
irradiation is studied and discussed.

At laser intensities between 1013 — 1016 W/cm2 molecules undergo
structural deformations, multiple ionization (via field or multiphoton
ionization mechanism), and Coulomb explosion. Depending on the
estimated value of Keldysh parameter, ionization mechanism will be either
field or multiphoton. Mutiple ionization of molecules leads to Coulomb
explosion resulting in fi'agment ions flying in all directions. Experimental
studies of angular distributions of these fragments and analysis of the data
will reveal information about the geometry of exploding molecular ions,
thus, providing dynamical information about the molecules and molecular
ions under high - intensity laser fields.

Target molecules in our studies are triatomics important in the atmosphere,
such as CS;, 03, and NO;. In our experiments we observed Coulomb
explosion for CS; and NO; and studied the angular distributions of
fragment ions following Coulomb explosion. This helped us retrieve
structural information about the exploding particles. We do not observe
signs of Coulomb explosion for ozone. Possible explanations of this result

are discussed in the text.

2.Experimental

The experiments presented here were carried out with a molecular beam (MB)
apparatus (See Fig.4) combined with a time-of-flight (TOF) spectrometer and an intense
femtosecond laser source. As shown on Figure 4a, there are two electrodes in the vacuum
chamber of the MB apparatus, the extractor and the repeller, to extract the resulting
fragment ions after the CE, initially moving towards the detector as well as in the
backward direction. Figure 4b shows in more detail how the fragment ions fly afier CE,
how they are extracted, and the splitting in the mass spectrum caused by the CE. The first
peak, observed at earlier arrival time in the mass resolved spectrum, is caused by
fragments initially moving towards the detector, whereas the second peak is caused by
fragment ions originally moving towards the repeller. Rotating the polarization of the
laser pulses we can study the angular distributions of the fragment ions and the structural
deformations the molecular ions undergo prior to the CE (5,6).

In the case of CS; and NO;, the samples were introduced using He as a carrier gas,
which entrained some of the vapor pressure of the sample (7). The sample and carrier gas
(He, 1700 Torr) expanded through the l-mm diameter aperture in the pulsed valve. A 2-
mm diameter skimmer produced the molecular beam which proceeded to the ionization
chamber at ~ 8x10'8 Torr with valve closed and ~ 7x10'7 Torr when the valve was

operated at 30 Hz. He flowed through a metallic tank containing the samples. The

voltages on the extractor and repeller were kept at 1800 and 2500 V, with a distance
between them of 1.5 cm.

Ozone was generated by passing ultrapure 0; through a commercial ozonizer and
trapped on silica gel (180 g) at isopropyl alcohol/dry ice slurry temperature (— 84° C). At
this temperature, above the boiling point of liquid oxygen, only 03 is trapped and no
further steps are needed to separate it from O; (8). Figure 5 shows the container with the
ozone trapped on silica gel in a dry ice/isopropyl bath (9). The He carrier gas flowed
through the silica gel ozone trap and expanded through the pulsed valve. The skimmer
produced the molecular beam, which proceeded to the ionization chamber.

The experiments were performed using a colliding pulse mode-locked dye laser
(CPM), pumped by a CW NszLF laser. The output from the CPM was amplified by a
four-stage dye amplifier pumped by a 30-Hz Nd-YAG laser, resulting in 60 fs and 200 rd
pulses centered at 620 nm (2 eV/photon). For the CE experiments, the forth dye stage
was double-passed to obtain up to 0.5 mJ pulses with a longer duration of ~80 fs. The
laser beam was focused on the molecular beam by an f=150 mm lens. The focal spot size
of the beam was estimated (Gaussian beam optics) at 20 pm, the measured size (from a
burn mark) was 26 um. Laser pulse intensity was varied from 0.25 to 0.49 In], giving a
peak power at the focus from 4.6 to 9x10‘4 W/cmz. The ions formed in the interaction
region traveled in a drift free zone (50 cm) before being detected by a micro-channel
plate. Twenty time-of-flight mass spectra (TOF-MS) were averaged before analysis. The
polarization dependence of the ion intensities was obtained by a zero-order half—wave
plate, which was rotated manually from O to 180° in 15° increments with respect to the

TOF axis. A calcite polarizer was used to ensure high polarization purity of the beam.

3. Experimental Results

A. Experimental results of Coulomb explosion in CS; and NO;. In Figure 6a, we show
the TOF-MS resulting from excitation with pulses with 4.6"‘10l4 W/cm2 peak intensity. In
addition to the singly and doubly charged molecular ions (CS;+, CS;2+) multiple
fragments are observed. We distinguish the singly ionized fragments (C+ and 8+) from
the doubly ionized fragments (C2+ and 82+). In Figure 6b, we show the ionization yield of
the CS;+ molecular ion as a function of the polarization vector of the excitation beam.
Notice that the yield is isotropic, as expected, because of the isotropic distribution of CS;
molecules in the molecular beam. The data in Figure 6b serve also to confirm that there is
no systematic bias in the probability of ion detection as a function of the polarization
vector of the excitation field with respect to the time-of-flight axis. A similar isotropic
distribution was observed for the CS;2+ ion. The angle - dependent ionization yield for 8+
and for CI was measured, and the results are presented in Figure 7a in Cartesian and
polar coordinates. The experimental data obtained for three separate runs are depicted as
circles, squares, and triangles. The lines are best fitted by cosine or sine functions to the
fourth power. Notice that the yield for S“ follows a cosinusoidal distribution, with
maxima at zero and 180° (see Figure 7a). The yield for C+ ions follows a sinusoidal
distribution with maxima at 90 and 270° (see Figure 7a). The angle dependent ionization
yield for S2+ and for C2+ was measured, and the results are presented in Figure 6b in

Cartesian and polar coordinates. The experimental data obtained for three separate runs

are depicted as circles, squares, and triangles. The lines are best fitted by cosine or sine
functions to the fourth power. Notice that the yield for S2+ follows a cosinusoidal
distribution, with maxima at zero and 180°degrees (see Figure 7b). The yield for C” ions
follows a sinusoidal distribution with maxima at 90 and 270° (see Figure 7b). Notice that
the results for the doubly ionized fragments show greater sensitivity toward the angle of
the polarization vector of the excitation pulse compared to the case of the singly ionized
fragments in Figure 7. This indicates the doubly charged particles are ejected from
multiply charged molecular ions with more significant bending than that of the singly
charged ions. The ionization data shown in Figure 6 are consistent with the angular
distribution of the fragment ions obtained with a titanium sapphire laser centered at 800
nm (10). Similarly, we are able to interpret our data in terms of a molecular structure
deformation that is consistent with bending.

In Figure 8, we show the mass spectrum of NO; undergoing CE. The mass spectrum
contains molecular ions, such as NO;, as well as CE fragments, such as NO”, N+, 0+,
etc. As in our earlier work on CS;, performed under similar experimental conditions,
some of the peaks are split, which is the signature of CE.

Anisotropic distributions are observed for (a) C+ and 8+ and (b) C2+ and S2+ ions in
polar and Cartesian plots. Data for three independent experiments (squares, circles,
triangles) are accompanied by a guide to the eye (solid line) in the form of a 0054(0)

function. S+ shows a maximum ion yield at 0° and 180°, while C+ shows a maximum at

90° and 270°.

10

B. Preliminary Experimental and Theoretical results for 03. An interesting and
important molecule to study at high intensity laser fields is ozone (03). The vital role of
atmospheric ozone for the biosphere has motivated a large number of experimental and
theoretical studies regarding its spectroscopy and electronic structure. However, the
behavior of 03 under the intense laser irradiation has not yet been investigated.
Knowledge from such a study would be valuable for predicting the effects high intensity
laser pulses might have on the upper atmosphere.

We designed an experiment to study the behavior of ozone under high - intensity laser
excitation (10l4 — 10l5 W/cmz). As mentioned above, at these intensities molecules
undergo multiphoton or field-induced ionization, and the removal of two or more
electrons leads to CE. Figure 9a (top) presents the mass spectrum for 0;; taken at a peak
power of 4.6x10l4 W/cmz, where ion signals for 03+, 02+, and 0+ are observed. The data
show no evidence of splitting in the fragment ion peaks, which indicates that CE did not
occur under these experimental conditions. It should be noted that 0; and 0+ signals are
partly due to the molecular oxygen present in the molecular beam as a result of ozone
decomposition upon the desorption from the silica gel.

Since the absence of CE for ozone was surprising, we increased the laser’s peak
power up to 9x1014 W/cm2 (the maximum intensity that could be achieved by the laser
system in our experiments), but this increase did not change the spectrum. Our ability to
resolve peak splitting depends on the voltage difference between the extractor and
repeller. We reduced this difference from 700 V to 200 V. This change did not reveal

evidence for fragment ion peak splitting.

ll

The collaboration with Piecuch Group helped us get first and second ionization

potentials for 03 as well as structural data about ozone and its mono- and dications (see

Table] ).

Table 1. First and second vertical ionization potentials (VIP) of ozone and geometries of
03+ and 032+ obtained in the CCSD(T) calculations.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

of

Ionized Electrona Energy (lst VIP)/eVb R(O - 0), Ac 40 — O — O/degree°
6a1 12.67 (12.85) 1.2371 132.44

4b; 12.82 (12.99) 1.2733 104.23

1a; 13.39 (13.56) 1.2912 112.81

o32+

Ionized Electronsa Energy (2nd VIP)/eVb R(O - 0), Ac 40 — O — O/degreec
6a1 35.37 (35.62) 1.1749 180.00

4b; 35.25 (35.51) d d

la; 38.54 (38.75) 1.3437 107.34

 

 

 

 

' The three highest occupied orbitals of 03 are 6a., 4b;, and la;.

 

b Calculated with the cc-pVTZ and aug-cc-pVTZ basis sets (the latter results in parantheses). The
experimental first and second VIPs are 12.43 eV (S Katsumata, H Shiromaru, T Kimura: Bull. Chem. Soc.
Jpn. 57 (1984) 1784) and 34.3 eV, respectively.
‘ Calculated with the cc-pVTZ basis set. For comparison, the experimental geometry of ozone is R(O - O) =
1.272 A, and 4 (O - O — O) = 116.8 ° (S Katsumata, H Shiromaru, T Kimura: Bull.ChemSoc.Jpn. 57
$1984) 1784, K Kimura, S Katsumata, Y Achiba, T Yamazaki, A Iwata: Jap. Sci. Soc. Press. (1981)).

The optimum geometry could not be determined.

12

4. Discussion

Let us examine the behavior of triatomics (as an example we take CS;) in the
molecular beam subject to high intensity laser irradiation. If the molecules remained
linear, a case depicted in Figure 9a, the signal from the S" ions would show a marked
anisotropy. The anisotropy would result from two situations. When the electric field
vector is aligned with the TOF axis, the S+ ions are ejected toward and away from the
detector. Because of the repeller plate, the fragments that are ejected away from the
detector are deflected back toward the detector. This leads to a well-known splitting in
the ion signal, evident in the zoomed inset of the mass spectrum in Figure 5 for S2+ ions,
and used in photofiagment spectroscopy analysis (1 1). Note that molecules perpendicular
to the field are not affected by the field and yield no ions. When the field is perpendicular
to the TOF axis, the S+ ions are ejected at a high velocity in a direction that is
perpendicular to the TOF axis. Despite the acceleration voltage, the Coulomb repulsion is
significant, and in the time it takes those ions to reach the detector plane (estimated to be
~4 us), they are already several centimeters apart and are not detected. For this reason,
the S" ions show a distribution that can be simulated by a cosinusoidal function. For
linear molecules, the detection of C+ ions is not affected by the angle of the polarization
vector of the electric field. The predicted detection is therefore isotropic, a fact that is not
supported by the data shown in the experimental data in Figure 7. In Figure 10b, we

consider the angle dependent detection of fragment ions resulting from bent CS;

13

molecules. Unless the bending angle is very large, there is almost no change in the
anisotropic dependence of the S+ ions discussed above for linear molecules; see Figure
9b, top. However, for bent molecules the C+ signal becomes anisotropic. The anisotropy
results from the following observations, based on the assumption that the bending angle
is small and that the polarizability of the molecule remains along the two S atoms. When
the electric field vector is aligned parallel to the TOF axis, the bent CS; molecules can
have the two S atoms oriented parallel with the field or perpendicular to the field. If the
molecules are oriented along the field, then the C+ ions are ejected in a direction that is
perpendicular to the detector and, thus, will miss it. When the electric field vector is
aligned perpendicular to the TOF axis, the bent CS; molecules oriented along the field
eject C+ ions toward the detector and register a maximum signal. When the molecules are
oriented perpendicular to the field, the signal is minimal because the perpendicular
component of the polarizability is very small for small bend angles. From this analysis so
far, it is clear that C+ ions are predicted to show anisotropic detection that is
perpendicular to that observed for S+ ions if the CS; molecules are bent. The predicted
anisotropic ion signal from the Coulomb explosion from bent CS; molecules is shown in
Figure 10. A simple simulation of the parallel and perpendicular components of the
polarizability as a firnction of the bend angle leads us to estimate the bend angles from
the singly and doubly ionized data sets to be 170° and 155°, respectively. The agreement
between the findings of Iwasaki et al. using mass - resolved momentum imaging (MRMI)
and the 800-nm excitation wavelength and our angle determination is good (12).

Similar analysis may be performed for other linear and bent molecules, such as

NO;, SO;, CO;, etc. The molecules have isotropic distributions in the molecular beam;

14

however, there is a clear directionality in the angular distributions (Fig.10), where the
maxima of the signals for the fragment ions are reached in the case of parallel or
perpendicular polarizations. This result is due to the phenomenon called enhanced
ionization (13,14); the ionization is enhanced for the molecules that are aligned in the
direction of laser field polarization.

As was mentioned in our Experimental Section, ozone does not undergo CE under
the same experimental conditions as NO; or CS;, a behavior that differs from other
similar molecules (such as NO; or SO;). Multiply charged ozone ions are not observed,
nor is splitting of the fragment ion signals (See Fig.9). It should be pointed out again that
the existence of 0+ and 0; ions in the spectrum may be due to the dissociation of ozone
ions as well as to the existence of molecular oxygen in the molecular beam due to the
decomposition of 03 upon the desorption from the silica gel surface. This aspect cannot
be addressed quantitatively because of the absence of specific experimental data.

In Figure 8, we showed the mass spectrum of NO; undergoing CE. It is interesting
that NO; and 03, which have very similar molecular structures, behave differently under
the same experimental conditions. What can be concluded from this result is that the
response to the high intensity laser field, in the Coulombic regime, is different even for
structurally similar molecules. This might be due to the difference in ionization potentials
of NO; and 03. However, this is not supported by the experimental and theoretical data
available for these molecules (See Table 1 and Table 2). As one can see from Table 2, the
experimental values for first and second vertical ionization potentials of NO; and O; are
close (within 1 eV). Therefore, the absence of CE for ozone cannot be due to the

ionization potentials.

15

Table 2. Comparison of experimental values of first and second ionization potentials of
03, CS;, CO;, SO;, and NO;.3

 

 

 

 

 

 

 

Molecule 1“ ionization potential, eV 2“3 ionization potential, eV
03 12.43 34.3
SO; 12.50 34.5
NO; 1 1.23 35.5
C0; 13.78 37.7
CS; 10.06 27.0

 

 

 

 

a Ionization potentials taken from F Harris: J. Mass Spec. Ion Proc. 120 (1992) 1, P Millie, I Nenner, P
Archirel, P Lablanquie, P Foumier, J Bland: J. Chem. Phys. 84 (1986) 1259, S Katsumata, H Shiromaru, T
Kimura: Bull. Chem. Soc. Jpn. 57 (1984) 1784, S Katsumata, H Shiromaru, K Mitani, S Iwata, K Kimura:
Chem Phys. 69 (1982) 423, K Kimura, S Katsumata, Y Achiba, T Yamazaki, A Iwata: Jap. Sci. Soc. Press.
(1981), I Griffiths, D Parry, F Harris: Chem Phys. 238 (1998) 21, P Fournier, J Eland, P Millie,S.Svensson,
S Price, J Fournier, G Comtet, B Wannberg, L Karlsson, P Baltzer, A Kaddouri, U Gelius: J. Chem Phys.
89 (1988) 3553.

One might explain the result by analyzing the stability of the ozone ions, particularly the
ozone dication 032+, and their dissociation pathways. Generally, but not exclusively,
molecular dications are energy-rich species with few long-lived electronic states (15).
Usually dication electronic states lie above the thermodynamic asymptote for charge
separation so any states possessing a potential barrier to fragmentation are metastable.
Figure 10 presents a general schematic illustrating typical potential energy curve for a
diatomic dication. One may think of the dication energy curve as being a result of the
repulsive Coulombic potential superimposed on the potential curve of the diatomic.
Depending on the depth of the minimum in the potential energy curve of the diatomic
species, the minimum for the dication curve will be either deep or shallow. Thus, some of

the molecules may have metastable dications whereas others may have unstable ones (16).

16

This implies that stability properties of the dication are determined by the competition
between the chemical bond energy and the Coulomb repulsion strength. We assume that
the absence of 032+ in the spectrum and CE may be due to the lifetime of 032+. It needs to
be less than the acceleration time within the repeller and extractor plates (in us time
scale), but more than the rotational dephasing time (estimated at ~ 5 ps, assuming Trot =
10 K). However, this needs further experimental and theoretical studies of ozone
subjected to high intensity laser fields in the Coulombic regime.

Another reason for the observed difference in behavior of structurally similar
molecules may be due to the laser wavelength dependence of multiphoton ionization.
Multiphoton ionization is facilitated for photons with wavelengths that are resonant with
excited states of the molecule (See Fig.2). Thus, the wavelength of the laser used might
also affect the probability of multiple ionization and Coulomb explosion. For instance,
He is doubly ionized at lower peak powers of the laser centered at 780 nm compared to
laser centered at around 614 nm (17, 18). Therefore, careful experiments performed at
wavelengths in infrared (around 780 nm) would be necessary to address this aspect. In
addition, the availability of a laser system such as Ti:Sapphire centered at 780 or 800 nm
that would produce intensities beyond 1015 W/cm2 would allow studying CE for ozone
because at these intensities production of highly charged ozone ions would be possible.
This would lead to CE because of much stronger Coulombic repulsion and thus would

allow studying the dynamics of the changes in geometry of ozone at high laser fields.

17

5. Conclusions

The behavior of triatomics such as CS;, 03, and NO; subject to high intensity
femtosecond laser fields in Coulombic regime was studied. The experiments were
performed in the molecular beam coupled with TOF mass spectrometer. TOF mass
spectra of CS;, 03, and NO; following Coulomb explosion were collected along with the
angular distributions of melecular and fragment ions. Anisotropic distributions of
fragment ions from Coulomb explosion of CS; confirmed the prediction about bending of
linear triatomics under high intensity laser fields in Coulombic regime. Analysis of
anisotropic distributions of singly and doubly charged carbon and sulfur fragment ions
allowed retrieving structural information about the molecular ions of CS; undergoing
Coulomb explosion. This, in fact, provides information about the dynamic behavior of
CS; irradiated by high intensity femtosecond laser fields.

Two triatomics with similar structures and close first and second ionization
potentials, ozone and nitrogen dioxide, were also studied under similar experimental
conditions as CS;. Whereas we observed Coulomb explosion for NO;, we did not see any
signs of it for 03. The absence of doubly charged ozone ion in the mass spectrum and the
absence of splitting of fragment ions imply that there is no Coulomb explosion for ozone
at intensities between 1014 W/cm2 and 1015 W/cmz. This may be due to the inability of
our laser pulses to doubly ionize 03. However, this does not seem likely since we are able

to multiply ionize NO;, which has first and second vertical ionization potentials very

18

close to those of 03, as discussed in the text. It should be noted that the wavelength
dependence of multiphoton ionization also might play a role. As discussed in the text, for
higher excitation wavelengths (for example, 780 nm) multiphoton ionization is facilitated.
Carefirl experimental studies of ozone at these wavelengths would answer the question
whether or not ozone would undergo Coulomb explosion at intensities in Coulombic
regime.

Another scenario is that doubly charged molecular ions of ozone have a lifetime
shorter than the acceleration time within the repeller and extractor plates (in us time
scale), but longer than the rotational dephasing time (~ 5ps). This assumption needs to be

supported by further experimental measurements.

19

APPENDICES

20

APPENDIX A

Figure 1

21

3
He

 

 

 

 

 

 

 

 

 

 

 

 

1}: o” a
6 T. r
\r‘r‘! 0| . v
3.1;“ I
g 14-0 X:
2.“:— \ \L o
i 0‘ o Neutron
Alignment Structural Coulomb X-ray Nuclear
deformation explosion emission fusion

Coulombic regime

1012

I
1015

Relativistic regime

I
1018

Laser Field intensity (Wicmzi

I' r

Figure 1. Laser field induced processes in various intensity regimes (from K

Yamanouchi: Science 295 (2002) 1

659-60).

22

APPENDIX B

Figure 2

23

 

 

 

 

 

Ground State Ground State

Figure 2. Schematic of 3-step multiphoton ionization in two cases: resonant (on the lefi)
and non-resonant (on the right). Typical laser electric field intensities required for these
two processes are l — 104 W/cm2 and >1012 W/cm2 for resonant and non-resonant
processes respectively. Non-resonant absorption of the photon proceeds through the

virtual state with lifetimes on the order of 1 fs.

24

APPENDIX C

Figure 3

25

 

Tunneling Time T

 

Figure 3. Schematic representation of field (tunneling) ionization for an atomic system.
Dashed and solid curves represent the potential energy surfaces in the absence and
presence of external electric field respectively.

Keldysh adiabaticity model is reasonable for field ionization when y = Funnel/Tim, << 1.

In the opposite limit multiphoton ionization mechanism is appropriate.

26

APPENDIX D

Figure 4

27

(a)

 

            
  

MCP Detector
m
M2 plate ‘ o
.1, Time of Flight
119 °
.11 i \ @4— ground
immr '-..1|1
“-ir ,/ \~;s'.','.".'.';; 4—extractor
molecular """"""
beam
Molecular
Beam setup

 

 

 

(b)
------------------ Extractor
+ 2000 i
O .
Splitted mass spectrum
0
+ 2500

------------------- Repeller

Figure 4. a) Schematic for the experimental setup. b) Schematic representation of the

Coulomb explosion process leading to the splitting in the mass spectrum (pointed by the

arrow), (from Comstock M, Senekerimyan V, Dantus M: J Phys Chem A 107 (40): 8271

28

APPENDIX E

Figure 5

29

Adsorbed Ozone
Ozone/He He
(— +—
to MB j

   

 

 

 

Silica gel crystals with
adsorbed ozone

Figure 5. Glass container with the stored ozone on silica gel in the isopropyl/dry ice
slurry bath at around - 85° C (from V Senekerimyan, M McGuire, M Comstock, M
Dantus: Quantum Control of Light and Matter,. Gordon Research Conference, Mount

Holyokee, MA 2003 ).

30

APPENDIX F

Figure 6

31

 

 

   
 

2.5

(a) 5+ 082*
1""?! 2+
3 4 Zoomed 5‘ ,N 3,,
52: 33+ C+ “1 + Z :2)
‘2‘ .flfrkxnlwr...) 'x 9V (:3
n— 3 I +
g is
‘3:
.9 . N
C NN (Z)
_ + 8 + r
c: N . g
o (O O
_ +6 0 ‘6
+ + a as
I ‘2') U) L.

 

5

 

 

 

Figure 6. a) Mass spectrum of CS;, where the zoomed part shows the splitting of the
peaks. b) The isotropic angular distribution of the CS;+ parent ion (from Comstock M,

Senekerimyan V, Dantus M: J Phys Chem A 107 (40): 8271-8281 2003).

32

APPENDIX G

Figure 7

33

a +
( I 120 90 60 S
150 30
180 0

   

 

3’ __
3' =3
3 31.0 3 ‘3, ‘v
8 .é‘ 0.8 ‘2 i
0 g r ; 1
g 9 0'6 1 1" z -'
fi 5 0.4 r y‘ ,5
x ‘1 I" ‘1'. f
& E 02 ‘1'..." kn}...
g c 0.0 .......-....... a”...
T o 90 130 270 360 2 0 9A”? 270 36"
n e
Angle 9
0+
120 9° 60
150 30
100 o
210 330
300
240 270 _..
-.~ . 3:
3 1-03 (*2 7‘1 32.
E: i. 1 1’ '1 3;
g, 0.9; ' 1. ,1 1 g
0 i 1'. 2i z ‘, g
E 0.8i ' “l, , ' ‘x -
a i y] k '1 K? S
31’ 0.7% ° ' ' CL
3 0 90 180 270 360 5 0 90 180 270 360
Angle Angle

Figure 7. Angle dependent detection of singly and doubly charged fragment ions
following Coulomb explosion. pronounced. . S2+ shows a maximum ion yield at
0° and 180°, while C2+ shows a maximum at 90° and 270° (from Comstock M,

Senekerimyan V, Dantus M: J Phys Chem A 107 (40): 8271-8281 2003 ).

34

APPENDIX H

Figure 8

35

 

NO+

Ion Intensity [a.u.]

 

 

 

Time of Flight, us

Figure 8. Mass spectrum of N02 taken at laser peak power 4.6x10l4 W/cmz. Notice the
splitting in the peaks of N+ and 0+ (from V Senekerimyan, M McGuire, MComstock, M
Dantus: Quantum Control of Light and Matter,. Gordon Research Conference, Mount

Holyokee, MA 2003).

36

APPENDIX 1

Figure 9

37

A
(D
V

 

A
‘2’

‘3’

 

lon Intensity [a.u.]

 

 

 

. + N2+ +
All’ H20 \ \102
0 3 6
Time of Flight. us

TOF Axis

(b)

120 90 ’60
1.073 150 30

Figure 9. a) Mass spectrum of 03 (top) and air (bottom) taken under the same

experimental conditions as that of N02. The dashed arrow points where 03,2+ should

appear. (b) Polar plot of angular distribution for 02+ (solid line) and 0+ (dashed line,

twice reduced in scale) (from V Senekerimyan, M McGuire, M Comstock, M Dantus:

Quantum Control of Light and Matter,. Gordon Research Conference, Mount Holyokee,

38

APPENDIX I

Figure 10

39

(a

 

Electric Field
Polarization
<——>

00
9001
TOFAms

eé 0"

Predicted Ion Detection

Coulomb Explosion

  

 

 

 

W)

 

 

 

 

Electric Field Coulomb Explosion
Polarization .-
( , C

()0
900$ %
TOFAms

o .5 u
00 V
900 i C+g§o -)-
900

 

 

 

e): co

Predicted Ion Detection

 

Figure 10. Conceptual drawings of the Coulomb Explosion of CS; when the electric

field is parallel or perpendicular to the time-of—flight axis for the a) linear molecules

and b) bent molecules.

40

 

APPENDIX K

Figure 1 1

41

 

Energy

 

Dication Potential Energy Curve
L -...._....-_.._-...- -....-A-._._-_,_... ’.

A-A distance

 

 

Figure 11. A typical potential energy curve for the diatomic dication.

42

(1)
(2)
(3)
(4)
(5)
(6)

(7)

(8)

(9)

(10)

(11)
(12)
(13)

(14)

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44

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