IR and FTMW-IR Spectroscopy and Vibrational Relaxation Pathways in the CH Stretch Region of CH₃OH and CH₃OD

Abstract

Infrared spectra of jet-cooled CH(3)OD and CH(3)OH in the CH stretch region are observed by coherence-converted population transfer Fourier transform microwave-infrared (CCPT-FTMW-IR) spectroscopy (E torsional species only) and by slit-jet single resonance spectroscopy (both A and E torsional species, CH(3)OH only). Twagirayezu et al. reported the analysis of ν(3) symmetric CH stretch region (2750-2900 cm(-1); Twagirayezu et al. J. Phys. Chem. A 2010, 114, 6818), and the present work addresses the more complicated higher frequency region (2900-3020 cm(-1)) containing the two asymmetric CH stretches (ν(2) and ν(9)). The additional complications include a higher density of coupled states, more extensive mixing, and evidence for Coriolis as well as anharmonic coupling. The overall observed spectra contain 17 interacting vibrational bands for CH(3)OD and 28 for CH(3)OH. The sign and magnitude of the torsional tunneling splittings are deduced for three CH stretch fundamentals (ν(3), ν(2), ν(9)) of both molecules and are compared to a model calculation and to ab initio theory. The number and distribution of observed vibrational bands indicate that the CH stretch bright states couple first to doorway states that are binary combinations of bending modes. In the parts of the spectrum where doorway states are present, the observed density of coupled states is comparable to the total density of vibrational states in the molecule, but where there are no doorway states, only the CH stretch fundamentals are observed. Above 2900 cm(-1), the available doorway states are CH bending states, but below, the doorway states also involve OH bending. A time-dependent interpretation of the present FTMW-IR spectra indicates a fast (∼200 fs) initial decay of the bright state followed by a second, slower redistribution (about 1-3 ps). The qualitative agreement of the present data with the time-dependent experiments of Iwaki and Dlott provides further support for the similarity of the fastest vibrational relaxation processes in the liquid and gas phases.

Single and double resonance spectroscopy of methanol embedded in superfluid helium nanodroplets

“…17 At the lower wavenumbers relevant to this study, vibrational coupling is evidenced by splittings and shifts of gas phase lines, in addition to intensity borrowing, especially in the highly congested CH stretching, bending-overtone, and bendingcombination band region. 13,14 In this region, the contribution of the dominant normal mode to the vibrational wavefunction for a particular band is rather low (29%-67% for the fundamental bands; see Table I), thus illustrating the high level of vibrational mode mixing that leads to intensity borrowing by, in particular, the bending-overtone and bending-combination bands. 18 In addition to the gas phase, 19 methanol has been studied by IR spectroscopy in several different matrices, including solid para-hydrogen (p-H 2 ), 20 neon, 21 nitrogen, 22 and argon.…”

“…paul.raston@adelaide.edu.au resonance (DR) spectroscopy, 13,14 further work is necessary to analyse, in particular, the highly congested region around 3.4 μm.…”

Millimeter-wave optical double resonance schemes for rapid assignment of perturbed spectra, with applications to the C̃1B2 state of SO2

“…The technique enables rapid spectral acquisition and assignment of congested regions. Our millimeter-wave implementation of the background-free CCPT scheme, originally developed by Twagirayezu and coworkers, [20][21][22] significantly increases the mmODR sensitivity and allows us to observe weak transitions to levels that perturb theC(B a 1 ) level of SO 2 . Due to the relatively short Dopplerdephasing coherence time, we had difficulty implementing a multiplexed CCPT scheme.…”

“…We have also made a millimeter-wave implementation of the background free coherence-converted population transfer (CCPT) technique. [20][21][22] We find that CCPT provides an excellent improvement in signal-to-noise over traditional population labeling schemes, and is a promising alternative to polarization-based background-free double-resonance methods. 23 However, there are challenges to implementing the technique in our current experimental configuration, where the coherent experiment must be completed in a time window that is short compared to the millimeter-wave Doppler lifetime of <2 µs.…”

“…[20][21][22] The technique is made possible by taking advantage of phase-coherent excitation and detection of the microwave FID. CCPT is a three-pulse sequence.…”