High-resolution, slit jet infrared spectroscopy of hydrocarbons: Quantum state specific mode mixing in CH stretch-excited propyne

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Abstract
A direct absorption, difference frequency, infrared laser spectrometer with 10−4 cm−1 resolution combined with slit supersonic jet optical pathlengths is presented as a tool for the study of mode–mode vibrational coupling in laser-excited hydrocarbons. These weak mode–mode couplings are evidenced in our frequency domain studies by virtue of transitions to isolated upper J states that are split into multiplets under sub-Doppler resolution. Instrument performance is demonstrated by investigating vibrational coupling in the 3000–3300 cm−1 C–H stretch fundamental region of 12C3 propyne, as well as the 12C213C propynes observed in natural isotopic abundance. No appreciable state mixing is observed in ν1=1←0 and ν6=1←0 spectra at T=4 K. However, near-resonant two-state mixing of ν2 and ν5+ν8+3ν10 in the ν2=1←0 transition of 12C3 propyne is detected and deperturbed to provide an anharmonic coupling matrix element of 0.096 41(38) cm−1. This matrix element is independent of J′ and thus arises from purely anharmonic, non-Coriolis-mediated couplings. The implications of anharmonic coupling matrix elements of this magnitude in overtone vibrational dynamics are discussed.