Sub-Doppler, infrared laser spectroscopy of the propyne 2ν1 band: Evidence of z-axis Coriolis dominated intramolecular state mixing in the acetylenic CH stretch overtone

Abstract

The eigenstate‐resolved 2ν₁ (acetylenic CH stretch) absorption spectrum of propane has been observed for J’=0–11 and K=0–3 in a skimmed supersonic molecular beam using optothermal detection. Radiation near 1.5 μm was generated by a color center laser allowing spectra to be obtained with a full‐width at half‐maximum resolution of 6×10⁻⁴ cm⁻¹ (18 MHz). Three distinct characteristics are observed for the perturbations suffered by the optically active (bright) acetylenic CH stretch vibrational state due to vibrational coupling to the nonoptically active (dark) vibrational bath states. (1) The K=0 states are observed to be unperturbed. (2) Approximately 2/3 of the observed K=1–3 transitions are split into 0.02–0.25 cm⁻¹ wide multiplets of two to five lines. These splittings are due to intramolecular coupling of 2ν₁ to the near resonant bath states with an average matrix element of 〈V²〉^1/2=0.002 cm⁻¹ that appears to grow approximately linearly with K. (3) The K subband origins are observed to be displaced from the positions predicted for a parallel band, symmetric top spectrum. The first two features suggest that the coupling of the bright state to the bath states is dominated by parallel (z‐axis) Coriolis coupling. The third suggests a nonresonant coupling (Coriolis or anharmonic) to a perturber, not directly observed in the spectrum, that itself tunes rapidly with K; the latter being the signature of diagonal z‐axis Coriolis interactions affecting the perturber. A natural interpretation of these facts is that the coupling between the bright state and the dark states is mediated by a doorway state that is anharmonically coupled to the bright state and z‐axis Coriolis coupled to the dark states. Z‐axis Coriolis coupling of the doorway state to the bright state can be ruled out since the ν₁ normal mode cannot couple to any of the other normal modes by a parallel Coriolis interaction. Based on the range of measured matrix elements and the distribution of the number of perturbations observed we find that the bath levels that couple to 2ν₁ do not exhibit Gaussian orthogonal ensemble type statistics but instead show statistics consistent with a Poisson spectrum, suggesting regular, not chaotic, classical dynamics.