Hierarchies of intramolecular vibration–rotation dynamical processes in acetylene up to 13,000 cm⁻¹

Abstract

The vibration–rotation dynamics of acetylene are computed from a spectroscopic Hamiltonian with 468 parameters fit to 19,582 vibration–rotation transitions up to 13,000 cm⁻¹ of vibrational energy. In this energy range, both the bending and the CH stretching vibrations can reach large amplitudes, but the maximum energy remains below the threshold for isomerization to vinylidene. In contrast to the behavior at energies below 5000 cm⁻¹ [Mol. Phys. 108, 1115 (2010)], excitation of single bright states leads, in almost all cases, to computed intramolecular vibrational redistribution (IVR) that is irreversible on the timescales investigated. Hierarchies of IVR processes on timescales ranging from 20 fs to 20 ps result when different bright states are excited. Different parts of the vibrational quantum number space are explored as a result of the four different classes of coupling terms: vibrational l-type resonance, anharmonic resonances, the rotational l-type resonance, and Coriolis couplings. The initial IVR rates are very different depending on whether the bright states are bending states or stretching states, normal modes or local modes, edge states or interior states. However, the rates of the rotationally mediated couplings do not depend substantially on these distinctions.

Keywords:

• intramolecular vibrational redistribution
• acetylene dynamics
• effective Hamiltonian
• Coriolis
• l-type resonance

Acknowledgements

The authors thank André Fayt of Université Catholique de Louvain for his work on the spectroscopic Hamiltonian. Development of the Hamiltonian was accomplished in Brussels with support from the “Action de Recherches Concertées de la Communauté française de Belgique”. The time-dependent calculations conducted in Akron were supported by the Division of Chemical Sciences, Offices of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy under grant No. DE-FG02-90ER14151. Support of this work does not constitute endorsement by the DOE of views expressed in this paper. B.A. thanks FRIA for a research grant.