Structures in the long-range potential curves of Na₂. II. Application to the semiclassical study of the energy pooling process between two excited sodium atoms

Abstract

Recent accurate calculations by Magnier et al. (1994) have demonstrated that for some symmetries the long-range excited adiabatic potential curves of the Na2 molecule display many avoided crossings, some of them being due to competition between ionic and covalent dissociation. We have used those molecular data to estimate cross sections for the energy pooling reaction: Na(3p)+Na(3p) to Na(3s)+Na(5s, 4d, 4f) in the framework of a semiclassical multicrossing Landau-Zener model. All the pseudocrossings of the adiabatic potential curves from the Na(3s)+Na(4p) up to the Na(3s)+Na(4f) dissociation limit have been considered. At thermal collision energy (T approximately=500 K) cross sections are in good agreement with available experimental data. At lower collision energy (T approximately=200 K), we show that a 1 Sigma g+ and a 3 Sigma u+ channel correlated to Na(3s)+Na(5s) dissociation limit become dominant. This illustrates the crucial role of long range crossings in the dynamics of collisions between excited sodium atoms at very low energies.

The temperature dependence of the cross section for the energy pooling process Na(3P)+Na(3P)to Na(4D)+Na(3S)

“…The obtained value is in fair agreement with the values obtained by fitting the theoretical results to the same dependence. Namely, the σ 4D (T ) curves of Yurova et al (1994), Geltman (1989) and Kowalczyk (1984) yield E values of 487, 475 and 632 cm −1 , respectively. As for the absolute values of σ 4D , our experimental data are higher than all theoretical calculations.…”

“…Experimental data are labelled with symbols: -Allegrini et al (1983), -Huennekens and Gallagher (1983), -present work. Theoretical calculations are represented with curves (data are digitized from the figures in the original papers, except for Kowalczyk (1984) who tabulated the values): dot- Yurova et al (1994), dash-Geltman (1989, dash-dot- Kowalczyk (1984). The full curve is the Arrhenius-type fit σ 4D ∼ σ ∞ exp(− E/kT ), through the present experimental data.…”

“…The results of the theoretical calculations (Yurova et al 1994, Geltman 1989, Kowalczyk 1984 are also displayed.…”

“…The theoretical value of the k 4F /k 4D is available from the data of Yurova et al (1994) which yield k 4F /k 4D = 0.13-0.14 in the temperature range 600-700 K. We have not used this theoretical value either, since the calculations disregard the role of the spin-orbit coupling.…”

“…Among them the calculations performed by Yurova et al (1994) should be considered most reliable since they are based on the accurate ab initio potential curves for the Na 2 molecule (Magnier et al 1993, while the other authors used a very approximate estimation of the molecular potential curves. Regarding the temperature dependence all theoretical results predict a mild rise in the cross section value with increasing temperature.…”