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Laser-assisted reactive collisions

The two-photon laser-assisted reaction (LAR) in rare gas and halogen gas mixtures has been investigated. Product state distributions from the Xe + Cl₂ LAR were measured via monitoring the (bound-free) excimer emission from the XeCl* reaction product. The LAR is described as a direct two-photon transition to a Xe⁺Cl₂^- charge transfer state, and the subsequent half collision on the reactive potential energy surface (PES) for this triatomic system leads to efficient formation of the XeCl* exciplex (excited complex). The LAR may thus provide a unique spectroscopic probe of the Xe⁺Cl₂^- transition state ...

A major portion of this research effort involved the numerical simulation of the bound-free continuum spectra from the electronically excited XeCl molecule in order to determine the quantum state distribution of the reaction products. The XeCl* exciplex is also vibrationally excited, and may involve vibrational quanta up to v=150 or higher, depending on excitation wavelength ...

See our recent publication for more details.

Maximum entropy analysis

An information theoretic technique known as surprisal analysis, developed by Levine and Bernstein, can be used in the analysis of energy disposal or product specificity in a reactive process. The technique consists of applying maximum entropy (MaxEnt) considerations (subject to the experimentally determined constraints) to the description of the quantum state outcomes of a reaction. The surprisal, then, is a measure of the deviation of the measured final quantum state distribution from the predicted outcome of the unconstrained reaction (referred to as the prior expectation or prior distribution). This approach to determining the surprisal assumes that the dynamical constraints associated with the reaction can be inferred; in these cases the surprisal distribution will form the most compact possible representation of the final state distribution.

See also Charlie Strauss’ page at Los Alamos for a general discussion of this technique.

Reaction dynamics

A classical direct interaction model has been developed to describe the two-photon LAR of Xe and Cl₂. This model is based on the DIPR (direct interaction with product repulsion) model, as formulated by Polanyi, which emphasizes the repulsive interaction in harpoon-type reactions. The current model also includes an attractive interaction as the Xe-Cl bond is “switched on” between the extended nuclei, and employs Monte Carlo techniques to sample initial conditions on the reactive potential energy surface (PES).

Collisional energy transfer

In rare gas excimer and atomic rare gas lasers, the upper lasing level is populated primarily through dissociative recombination of ionic states and collisional deactivation in the lower excited-state manifolds. In order to improve models of these laser systems, we have undertaken an extensive study of energy disposal in rare gas mixtures. Lifetimes and bimolecular quenching rates were determined for the Xe^* and Kr^* two-photon excited states in krypton and xenon buffer gases. State-to-state rate constants for collisional deactivation and excitation transfer were also measured.

The Xe^* 5p⁵(6p,7p,6p') and Kr^* 4p⁵5p excited states were selectively excited using two-photon laser excitation at wavelengths between 210-255 nm, produced using a frequency-doubled tunable pulsed dye laser. Fluorescence scans were taken to identify the collisional product channels following each laser excitation, and a time-correlated single photon counting technique was used measure the decay profile of the fluorescence from each state. The instrumental response (PMT and electronics) was deconvolved from the measured time profile using a nonlinear least squares fit algorithm, and the deconvolved kinetic profiles were fit to an exponential to determine the decay rates for the excited states. These rates were determined for varying buffer gas pressures, so as to obtain a Stern-Volmer plot of the deactivation of the laser-excited state. The zero-pressure intercepts of these plots give the radiative lifetimes of the laser excited states. A two-component decay was seen for the Kr^* 5p[1/2]₂ state, indicating that this laser-excited state is strongly coupled to the nearby 5p[1/2]₃ level. The kinetic profile for this laser-excited state was fit to a double exponential function to determine the slow and fast decay rates, and the collisional mixing was analyzed using a coupled two-state model. State-to-state rates were determined from the pressure dependence of the time-integrated fluorescence combined with the total loss rates from each state determined from the time dependent measurements.

Related publications

“Energy disposal in the two-photon laser-assisted reactive collisions in xenon and chlorine gas mixtures,” J. Chem. Phys. 113 (23), 10551 (2000).

“A direct interaction model for the laser-assisted reaction of xenon and chlorine,” (in preparation).

“Deactivation of two-photon excited Xe(5p⁵6p,6p',7p) and Kr(4p⁵5p) in xenon and krypton,” J. Chem. Phys. 102 (5), 1965 (1995).

“Two-photon laser-assisted reactive collisions in Xe/Cl₂ gas mixtures,” The Meeting of the Texas Section of the American Physical Society, Lubbock, TX, October 1995. [Abstract available]

“Vibrational energy disposal in two-photon laser-assisted reactions of Xe + Cl₂ collision pairs,” The 26st Annual Meeting of the Division of Atomic, Molecular and Optical Physics of the American Physical Society, Toronto, Canada, May 1995; published in Bull. Am. Phys. Soc. 40 (4), 1346 (1995).

“State-to-state quenching reactions of two-photon excited xenon and krypton,” The 1991 Spring Meeting of the American Physical Society, Washington, DC, April 1991; published in Bull. Am. Phys. Soc. 36 (4), 1378 (1991).

“Energy disposal studies of two-photon excited xenon and krypton,” The 21st Annual Meeting of the Division of Atomic, Molecular and Optical Physics of the American Physical Society, Monterey, California, May 1990; published in Bull. Am. Phys. Soc. 35 (5), 1195 (1990).