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УДК 544.17, 544.18, 544.174.2, 544.174.3 Вестник СПбГУ. Сер. 4. 2013. Вып. 4
A. K. Belyaev, A. Z. Devdariani, V. S. Rybak, I. A. Zlatkin
ELECTRONIC RADIATIVE TRANSITIONS IN He(1s221'3^)-Ne WEAKLY BOUND MOLECULES. TEMPERATURE DEPENDENCES*
The metastable states He(1s221,35) are the champions in terms of radiative lifetimes of neutral atomic states, т0 ~ 20-10-2, т3 ~ 7-103 s for the singlet and triplet states respectively [1, 2]. However, in numerous applications, e. g., in ionospheric and discharge plasmas, one deals with a metastable atom surrounded by buffer gas atoms. Thus, one needs to account for the influence of the interaction between atoms on radiative properties including lifetimes and spectral profiles. The issue is of topical in the case of low temperatures when processes of recombination lead to the formation of clusters and dimers containing excited atoms. This work accounts for reaction of radiative transitions in the weak bound He(21,35)-Ne dimmers and focuses on the calculations of the averaged lifetimes and spectral profiles produced by optical transitions from four vibrational states in case He(21^) and eleven vibrational state in He(23^) case. This influence is shown on the Ne atoms case as a particular example. For large interatomic distances about 10ao it has been shown that the interaction with a Ne atom destroys the spherical symmetry of the He(23^) state via admixture of p-atomic states of the He atom that increases the probability of radiation in ~ 103 times [3].
The starting point of the present calculations is a quantum chemical input that includes potential energy curves and electronic dipole transition moment. For small distances about 5a0 the recent ab initio calculations [4, 5] have revealed the existence of shallow potential wells with a few vibrational states. Discrete Variable Representation (DVR) basis functions sinc(fl) for nuclear wave functions were used to calculate the positions of vibrational levels and the nuclear wave functions by numerical solution of the time-independent Schrodinger equation. As discussed in [4], the asymptotic value of the calculated potential should be lowered in order to match the experimental data for the separated atoms. The new fitting of the potential performed in the present study leads to the correction of 469 cm-1 for He(21^) and 947 cm-1 for He(23^). It must be said that ambiguity in the asymptotic value for the potential energy curve with two potential wells implies an ambiguity in the number of vibrational states.
Now we have calculated the lifetimes of weakly bound He(21,35)-Ne molecules as well as temperature dependences of lifetimes and spectral profiles averaged over the Boltzmann distribution. The results for the average lifetime under the Boltzmann distribution for He(23^)-Ne are shown in figure. Numbers from 1 to 11 referred to the corresponding number of allowed vibrational levels which were supposed to populate in calculation.
Audrey Konstantinovich Belyaev — Dr. Sci. in physics and mathematics, Herzen University, St. Petersburg; e-mail: belyaev@herzen.spb.ru
Alexander Zurabovich Devdariani — Dr. Sci. in physics and mathematics, Herzen University, St. Petersburg; Saint Petersburg State University; e-mail: snbrn2@yandex.ru
Veronika Sergeevna Rybak — PhD student, Herzen University, St. Petersburg; e-mail: nika-spb_4@mail.ru
Ilya Aleksandrovich Zlatkin — PhD student, Saint Petersburg State University; e-mail: zlatkin_ilya@mail.ru
* По материалам международного семинара «Collisional processes in plasmas and gas laser media», 22—24 апреля 2013 г., физический факультет СПбГУ.
Семинар был проведён при софинансировании фондом «Династия». © A. K. Belyaev, A. Z. Devdariani, V. S. Rybak, I. A. Zlatkin, 2013
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Temperature, K
Temperature dependence of averaged lifetimes for the He(235)-Ne molecules
Bound—bound transition contributes 16 % in singlet case and 2 % for triplet case to Franck—Condon factor, the rest is coming from the bound-free transitions. Radiative transitions mainly lead to the formation of the ground-state He and Ne atoms with the kinetic energy of relative motion in the range of 0-0.03 eV. The interaction at small distances results in substantial decreasing of lifetimes: Ti = 6.7 • 1O~7t0 for singlet and t3 = 6 • 1O~10t3 for triplet. Although He-Ne gas mixtures and, in particular, radiative emission of the excited He atoms have been intensively studied since the advent of the first gas laser it would be interesting to study such transitions in the He-Ne gas mixtures under temperatures below 100 K, when the process of the excitation transfer is negligible [6], but recombination processes should produce a large number of the molecules. It is expected that the increase of the number of surrounding buffer gas atoms will further decrease the lifetimes.
1. Hodgman S. S., DallR. G., ByronL. J. et al. Metastable helium: A new determination of the longest atomic excited-state lifetime // Phys. Rev. Lett. 2009. Vol. 103. 053002.
2. van DyckS., Johnson C. E., ShugartH. A. Radiative lifetime of the 21S'0 metastable state of helium // Phys. Rev. (A). 1971. Vol. 4. P. 1327.
3. Devdariani A. Z., Zagrebin A. L., Blagoev K. B. Excitation transfer and intermultiplet transitions in collisions of He and Ne atoms at thermal energies // Ann. Phys. Fr. 1992. Vol. 17. P. 365.
4. Buenker R. J., Liebermann H.-P., Devdariani A. Z. Ab initio spin—orbit configuration interaction calculations for high-lying states of the HeNe quasimolecule //J. Phys. Chem. (A). 2007. Vol. 111. P. 1307.
5. Devdariani A. Z., Belyaev A. K., Alekseyev A. Ab initio study of the lifetime of weakly bound He(215)-Ne molecules // Molecular Physics. 2010. P. 108.
6. Devdariani A. Z., Buenker R. Квазимолекула He(21,35)-Ne при низких температурах // Khim. Fyz. 2004. Vol. 23, N 2. P. 61-64.
References
Статья поступила в редакцию 22 апреля 2013 г.
