High resolution study of the ν1 vibration of CH3 by coherent Raman photofragment spectroscopy

Abstract

Coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the vibrational–rotational Q-branch structure of the ν1 symmetric stretch of methyl radicals produced by 266 nm laser photolysis of methyl iodide. Spectra were recorded in both flow cells and free jet expansions at instrumental resolutions ranging from 0.25 to 0.005 cm−1. Due to the high recoil velocity of the CH3 fragment, Doppler and collisional broadening of the transitions is appreciable. Even at the highest resolution such broadening of the transitions leads to interference effects among the closely spaced Raman transitions that influence both the line positions and intensities in the observed CARS spectra. The molecular parameters (cm−1) obtained from the analysis are ν1=3004.42(4), αB1=0.0851(8), αC1=0.0475(7), DN1−DN0=−0.000 046(8), DNK1−DNK0=0.000 083(20), and, with assumptions, DK1−DK0=−0.000 039. These results and infrared data in the literature yield a CH bond length of 1.08378(5) Å for the (1000) state and, with some assumptions, an equilibrium bond length Re of 1.076 Å for this prototypic case of sp2 bonding.