Relaxation of vibrationally highly excited diatomics in van der Waals clusters

Abstract

The dynamics of vibrational relaxation in polyatomic van der Waals clusters is investigated theoretically, using as an example a one-dimensional model of I₂(Ne) _N with excited I₂ vibration. The method employed is classical dynamics simplified by the time-dependent self-consistent-field (TDSCF) approximation. The dependence of the relaxation on cluster size is examined, with calculations for N = 4, 8, 16 and with extrapolation to N → ∞ representing I₂ in a solid Ne matrix. Also studied is the variation of the relaxation mechanism with initial vibrational state v, in the range v = 25–60. The main results are: (i) Relaxation can be interpreted best as affected by collisions between I₂ and neighbouring Ne atoms. Collective modes do not, in most cases, play a significant role in the relaxation. (ii) The I₂ centre-of-mass vibration plays a less significant role than the Ne atoms in the redistribution of the released vibrational energy in the cluster. (iii) The initial relaxation rate decreases with cluster size, the lifetime of v = 28 being τ = 3·6 ps for N = 4 and τ = 51 ps for N = 8. This is due to decreasing frequency of impulsive I₂-Ne ‘collisions’ in the larger clusters. (iv) Initial relaxation behaviour and rates for the N = 16 cluster appear converged to those of the corresponding solid matrix. (v) relaxation dynamics switches from weak to strong coupling-type behaviour as v increases from 35 to 60.