Collisions of excited Na atoms with H2 molecules. I. A b i n i t i o potential energy surfaces and qualitative discussion of the quenching process

Abstract

Potential energy surfaces have been calculated for the four lowest electronic states of Na (3 ²S, 3 ²P)+H₂(¹Σ⁺_g) by means of the RHF–SCF and PNO–CEPA methods. For the so‐called quenching process of Na (3 ²P) by H₂ at low initial translational energies (E–VRT energy transfer) the energetically most favorable path occurs in C_2v symmetry, since—at intermediate Na–H₂ separation—the ? ²B₂ potential energy surface is attractive. From the CEPA calculations, the crossing point of minimal energy between the ? ²A₁ and ? ²B₂ surfaces is obtained at R_c = 3.57 a.u. and r_c = 2.17 a.u. with an energy difference to the asymptotic limit (R = ∞, r = r_e) of −0.06 eV. It is thus classically accessible without any initial translational energy, but at low initial translational energies (∼0.1 eV) quenching will be efficient only for arrangements of collision partners close to C_2v symmetry. There is little indication of an avoiding crossing with an ionic intermediate correlating asymptotically with Na⁺ and H₂⁻ as was assumed in previous discussions of the quenching process. The dependence of the total quenching cross sections on the initial translational energy is discussed by means of the ’’absorbing sphere’’ model, taking the initial zero‐point vibrational energy of the hydrogen molecule into account. New experimental data of the product channel distribution in H₂ for center‐of‐mass forward scattering are presented. The final vibrational states v′ = 3, 2, 1, and 0 of H₂ are populated to about 26%, 61%, 13%, and 0%, respectively. The observed distributions in H₂ (and D₂) may be rationalized by simple dynamic considerations on the basis of the calculated surfaces.