Vibrational relaxation in infrared excited SF6⋅Arn+ cluster ions

Abstract

An attempt is made to characterize the increase in internal temperature that should accompany the partitioning of a single quantum of vibrational energy within a small SF6⋅Arn+ cluster ion. For each value of n, the kinetic energy release associated with unimolecular (metastable) decay is used to establish an initial temperature for the cluster ion; ∼950 cm−1 of vibrational energy is then deposited into the ν3 vibrational mode of the SF6 moiety (using a CO2 laser). This step promotes additional dissociation which is accompanied by an increase in kinetic energy. From a model due to Klots [J. Chem. Phys. 58, 5364 (1973)] photofragment kinetic energies are predicted on the assumption that energy from the photon is partitioned statistically and leads to an overall increase in the temperature of each ion. Comparisons of experimental and calculated results clearly show that the infrared photoexcitation of SF6 in the ν3 mode leads to incomplete energy randomization. An improved description of the energy relaxation process is provided on the assumption that SF6 undergoes partial vibrational relaxation to either the ν2 or ν4 mode. The energy difference (∼300 cm−1) is then randomized throughout each cluster ion, and is reflected in the magnitude of the measured kinetic energy release accompanying the loss of a single argon atom. The estimated time scale for this process is an order of magnitude faster than the experimentally measured time for the total relaxation of SF6 (ν3=1) in an argon matrix.

Temperature Measurement from the Translational Kinetic Energy Release Distribution in Cluster Dissociation: A Theoretical Investigation

“…At lower excitations, the distribution of kinetic energy released after dissociation carries a lot of information about the energetics and kinetics of the reactions . In general, the binding energies or temperatures are deduced in experiments − through a comparison with some established statistical rate theories such as those pioneered by Weisskopf or by Rice, Ramsperger, and Kassel (RRK) . Significant progress was made by Klots − who developed the quasi-equilibrium theory (QET) of unimolecular decomposition and later introduced the concept of the “evaporative ensemble”.…”

Product Energy and Angular Momentum Partitioning in the Unimolecular Dissociation Of Aluminum Clusters

“…The other statistical family of models used to extract structural information from the experimental translational energy for metastable cluster dissociation is based on the Klots theory of cluster evaporation. − This model was applied with some success to a number of clusters, including (H 2 O) n H + clusters 10 and {(NH 3 ) n CH 3 CN}H + and (NH 3 ) n H + clusters. , The advantage of the Klots evaporative ensemble approach over the Engelking model is that both cluster heat capacities and binding energies can be determined with no adjustable parameter. Klots theory has been used for a wide variety of applications, such as the (statistical) modeling of the competitive coordination of different solvent molecules in mixed ligand−metal complexes, and to estimate cluster temperatures or internal energies …”

Infrared photodissociation of (NO)n+·X cluster ions (n ≤ 5)

“…A further important aspect that is relevant to the outcome of these experiments is the fate of the photon's energy. In a recent series of experiments, the kinetic energy release that accompanies photofragmentation was measured for the cluster ions SF 6 Á (NO) n 1 , 14 SF 6 Á (Ar) n 1 , 36 and SF 6 Á (C 6 H 6 ) n 1 , 37 where each size-selected species was photoexcited at ca. 950 cm À1 .…”