Rotational energy transfer in highly vibrationally excited HCN

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Abstract
The state-resolved collisional self-relaxation of HCN at a vibrational energy content of 10 000 cm−1 is probed directly by combining direct overtone vibration excitation, to prepare energized molecules in the (0,00,3) level, with a laser induced fluorescence monitor of the population evolution from different rotational states. Pure rotational energy transfer dominates the collision dynamics while vibrational relaxation results from only a small fraction of the inelastic events. The depopulation of single j levels proceeds with high efficiency. It is characterized by rates up to 14 times faster than the Lennard-Jones gas kinetic rate conforming to a j dependent distribution which peaks near the Boltzman population maximum and decreases to higher and lower angular momentum values. Approximately 70% of the collisional population removal from the j=4 level proceeds via the ΔJ=±1 channel and 28% proceeds via the ΔJ=±2 direct population transfer step. The results support a long range dipole–dipole mechanism for the energy transfer. This work also investigated various empirical scaling relations and determined that a two parameter fitting law based on the momentum gap or a three parameter modified scaling expression based on the energy gap successfully models the rotational relaxation.

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