Doorway state enhanced intramolecular vibrational energy redistribution in the asymmetric =CH2 hydride stretch of methyl vinyl ether

Abstract

The microwave-infrared double-resonance capabilities of an electric-resonance optothermal spectrometer have been used to assign the high resolution (5 MHz) infrared spectrum of the asymmetric =CH2 stretch of the cis conformer of methyl vinyl ether near 3130 cm−1. This vibrational state is anharmonically coupled to a near-resonant bath state by a 0.69 cm−1 matrix element resulting in two vibrational bands separated by about 1.44 cm−1. The two mixed states resulting from this interaction are further coupled to other near-resonant bath states with an average matrix element of about 0.01 cm−1. The coupled state density increases weakly with the total angular momentum, J, however, the intramolecular vibrational energy distribution (IVR) rate is approximately independent of the total angular momentum quantum number. Therefore, the rotationally mediated coupling mechanisms are weaker than the anharmonic terms in the redistribution process. A two-state analysis of the strong coupling, which includes a phenomenological IVR rate constant, suggests that the IVR rate in the two mixed states is dominated by the contribution from the coupled dark state. From the deconvolution of the IVR rates to remove the contribution from the dark state, the IVR lifetime of the asymmetric =CH2 stretch is determined to be 660 ps.