Lineshapes of infrared zero frequency double resonance spectra in CH₃F : Infrared-microwave sideband laser spectroscopy of the v₃ and 2v₃ - v₃ bands of ¹³CH₃F and ¹²CH₃F. Infrared-infrared double resonance of ¹³CH₃F by means of a wave guide CO₂ laser a...