Two‐Color Charge Transfer Transitions as a Probe of Electronic Relaxation and Photodissociation Dynamics at High Densities: Molecular Halogens in Xenon

Abstract

Two‐color charge transfer transitions are developed as a probe of intermolecular potentials, electronic relaxation, and photodissociation dynamics of halogens in condensed rare gases. Two illustrative examples are given: electronic relaxation of Br2 in liquid Xe and recombination dynamics of Cl atoms in high pressures of Xe. The Br2 A(3π1+u) and A'(3π2+u) states, which are in fast equilibrium, are not quenched by the solvent in liquid Xe. In dilute solutions, they relax radiatively via the A' state with a lifetime of ∼ 100 μs. In concentrated solutions, diffusion‐controlled self‐quenching of Br2(A/A') proceeds with a probability of 0.07 per encounter. An encounter is found to be equivalent to four gas kinetic collisions. The photodissociation of Cl2 and recombination of Cl atoms in gaseous Xe, 1–55 atm, are studied. The recombination kinetics is well reproduced by incorporation of diffusion‐controlled encounter frequencies in the rate constants. An equilibrium constant for formation of XeCl(X) of 2.3 × 10−22 cm3 is derived from the kinetic analysis.

On the relevance of OClO photodissociation to the destruction of stratospheric ozone

“…The technique relies on charge transfer excitation of C1-Xe collision pairs, with subsequent detection of emission from the xenon halide exciplexes. The methodology and its sensitivity have previously been demonstrated in this laboratory for the detection of C1 and other halogen atoms in the gas [Fajardo and Apkarian, 1989], solid [Lawrence and Apkarian, 1990], and liquid phases [Okada et al, 1989]. The principle is as follows.…”

Photodynamics of charge transfer and ion-pair states of Cl2:Xe complexes in liquid Ar

“…Laser induced harpoon reactions, the most common of which are those between rare gas atoms ͑Rg͒ and molecular halogens ͑X 2 ͒ or halogen containing polyatomic molecules ͑RX͒, have been extensively studied in the gas phase, [1][2][3][4][5][6][7][8] in clusters produced in molecular beams, [9][10][11] in the solid state, [12][13][14][15][16] and in the liquid phase. 13,14,[17][18][19] This class of photochemical reactions is useful in studies of fundamental chemical dynamics, and in particular, bimolecular chemistry, in condensed media. Specializing to the case of molecular halogens, the process can be generically summarized as…”

“…͑6͒, if X 2 * is bound or caged. 13,14,17 Obviously, the source of the wealth of laser induced harpoon reactions is also the source of complication. More often than not, the variety of listed entrance channels coexist in the same system, necessitating indirect interpretations.…”

“…Ϫ intermediate in its various electronic states; ͑b͒ a direct demonstration of the contrast between reactive dynamics on covalent versus ionic surfaces in condensed media; ͑c͒ relative energetics of solvated molecular ion-pair states and intermolecular charge transfer states, and their associated dynamics; ͑d͒ observation of the predicted dramatic lowering of intermolecular charge transfer resonances from excited Cl 2 ͑A/AЈ͒ versus Cl 2 (X) states controlled by vertical electron affinities; 17 ͑e͒ characterization of the neutral Xe:Cl 2 complex in its various electronic states. The ground state of this complex has been studied in molecular beams, 29 and considered theoretically, 30 as a van der Waals complex which shows incipient bonding.…”

“…The experimental paradigm and design has been completely discussed previously, 17 we repeat it here succinctly. In solutions of Cl 2 /LAr, we first excite Cl 2 to its dissociative 1 ⌸ u surface.…”