Example: dissociative energy transfer Ar*+H₂O -> Ar+H+OH(A,X)

Quantitative understanding of the processes of quenching of electronic excitation in collisions involving triatomic molecules is of importance for modeling practically important plasma chemical systems, such as gas lasers, gas-discharge lamps, and upper atmosphere. Quenching of Ar* in the resonant excited states 4s[3/2]1 and 4s’[1/2]1 in collisions with H2O in the ground electronic state leads to the dissociation of the water molecule with the formation of OH in the ground electronic state X and excited electronic state A. OH(A) formed in this process is an effective radiator because of the absence of radiation trapping, in contrast to Ar* in the resonant excited states. The reaction Ar* + H2O proceeds in two steps, see left figure. First, excitation transfer from Ar* to H2O in the excited electronic state B takes place in the collision due to dipole-dipole interaction. Second, dissociation H2O(B) -> OH(A) + H(S) or H2O(B) -> OH(X) + H(S) occurs. The first step determines the rate constant, and the second one is responsible for the OH(A)/OH(X) branching ratio. The Ar* + H2O(X) -> H2O(B) reaction rate was calculated using the general theory of resonant dipole-dipole collisional energy transfer developed in the USSR in the 1980s and the experimental data on H2O photodissociation available in the literature. The OH(A)/OH(X) branching ratio for the H2O(B) dissociation was found following the work [D.S. Mordaunt, M. N. R. Ashfold, and R. N. Dixon, J. Chem. Phys. 100, 7360 (1994)]. The results of calculations are presented in the right figure.

Thus the approach based on the Kintech Lab expertise allowed us to gain quantitative information on the rate of the practically important process for which both experimental and theoretical data were lacking.