Competing mechanisms for intramolecular vibrational redistribution in the ν14 asymmetric methyl stretch band of trans-ethanol

Abstract

The extensively perturbed spectrum of the asymmetric methyl stretching vibration of trans-ethanol near 2990 cm−1 has been reinvestigated via direct absorption infrared spectroscopy at a resolution of 30 MHz. A ground state combination difference analysis of the vibrational state mixing is presented for the upper state levels Ka′ = 0–2 and J′=0–4. The analysis indicates that the rotationless 000 level is anharmonically coupled to the dark bath states. The effective number of perturbing states in each rovibrational transition increases with both J and Ka providing evidence for rotational involvement in intramolecular vibrational redistribution (IVR). The decrease of the average dilution factor from φd=0.41 at Ka′ = 0 to φd=0.09 at Ka′ = 2 and the increase of the average interaction width from Δε=0.04 cm−1 at Ka′ = 0 to Δε=0.19 cm−1 at Ka′ = 2 indicate an a-type Coriolis component to the bright-bath coupling. In the Ka′ = 0 series the dilution factor decreases rapidly from φd=0.92 at J′=0 to φd=0.14 at J′=3 indicating that b,c-type Coriolis coupling also plays a significant role in the IVR process. The effective level density ρeffc for all of the observed transitions lie above the total vibrational state density ρvib=9 levels per cm−1 and most are closer to the total rovibrational state density ρrovib=(2J+1)ρvib. This suggests that following a coherent preparation of the asymmetric methyl stretching vibration, the ensuing dynamics explores all of the energetically accessible vibrational phase space of both the gauche and trans forms and much of the accessible rovibrational phase space, i.e., that the Ka quantum number is at least partially destroyed. The C–H stretch is deduced to decay with a 59 ps IVR lifetime to the asymptotic probability of 0.24.