engleski

Reverse energy pooling in lithium upon two-photon excitation of 3d state

Endothermic energy-pooling (EP) collisions in lithium, Li(2P)+Li(2P) -> Li(3D)+Li(2S) ((E=1475 cm-1) have recently been observed, but the cross section was not determined due to the large uncertainty regarding the Li(2P) atom density1. Very recently, high efficiency of exothermic reverse heteronuclear EP collisions in NaK system has been demonstrated2. This additionally proved empirical behavior of endothermic and exothermic EP rate coefficients in alkali metal vapors, in which later are about an order of magnitude more efficient3. Reverse EP collisions offers an advantage over forward EP collisions since one does not have to determine the excited state populations and their spatial distributions which is a main source of error4, but rather the ground state atom density.It is not only a missing number for the EP cross section opposite to all other measured in alkali metal vapors, but the aspect of REP as a source of large 2P population essential in various ionization processes in dense lithium vapor5 and the perspective to achieve a laser control over that collisional process6, which motivated us to study the reverse EP process in lithium.In the present work we study conditions necessary to obtain relevant REP and EP cross sections.The main difference to the REP study in heteronuclear NaK case is that here the lower 2P state can (mainly) be populated by radiative decay (3D->2P). In addition various nonlinear effects could occur7. We try to overcome this disadvantages by performing temporal analysis of the fluorescence at different atom densities (radiation trapping conditions). Similar experiment in homonuclear Cs case using a stepwise cw laser excitation of 5D state has been performed by Yabuzaki et al 8. In present experiment the lithium atoms are excited by two-photon absorption of 639.1 nm pulsed nanosecond dye laser beam. The lithium vapor is generated by use of a modified heat-pipe oven9, with vertical pipe filled with sodium. This allows accurate determination of lithium vapor temperature and the ground state atom density. The fluorescence is dispersed by a 0.6 m monochormator, detected by a photomultiplier and signals are processed by a box-car averager and stored in a PC-computer. At the preliminary stage we have measured the excitation spectra and temporal behavior of the emission at 610.3 nm (3D->2P) and 670.8 nm (2P->2S) for different vapor temperatures (densities) and laser fluences. At temperatures above 750 K we see the evidence of simultaneous molecular excitations and also the radiative recombination process as a consequence of ionization processes. Above 1 mJ laser pulse energy the saturation of the 610.3 nm emission is observed. Additionally lithium vapor was excited resonantly with 670.8 nm photons under the same vapor conditions. The effective lifetimes of the 3D->2P and 2P->2S emission are found to be strongly dependent on laser pulse energy in both cases of excitation. Taking into account simultaneous effects of saturation and the radiation trapping, a proper set of time-dependent rate equations for the 2S, 2P, 3P, 3D lithium levels is solved numerically, in the attempt to extract relevant REP rate coefficients from the measured quantities. Parallel to the experiment, the calculations are carried out using a multicrossing Landau-Zener model and Li dimer potential energy curves given by Spies10 to investigate various symmetry paths of the collisional process. References 1. Chun He and R. A. Bernheim, Chem. Phys. Lett. 190, 494 (1992). 2. S. Guldberg Kjear, G. De Filippo, S. Milošević, S. Magnier, M. Allegrini and J. O. P. Pedersen, Phys. Rev. A, 55 April (1997). 3. C. Gabbanini, S. Gozzini, G. Squardito, M. Allegrini and L. Moi, Phys. Rev. A 39, 6148 (1989). 4. Z. J. Jabbour, R. K. Namiotka, J. Huennekens, M. Allegrini, S. Milošević, F. de Tomasi, Phys. Rev. A 54, 1372 (1996). 5. D. Veža and C. J. Sansonetti, Z. Phys. D 22, 463 (1992). 6. M. Shapiro and P. Brumer, Phys. Rev. Lett. 77, 2574 (1996). 7. B. Nikolaus, D. Z. Zhang and P. E. Toschek, Phys. Rev. Lett. 47, 171 (1981). 8. T. Yabuzaki, A. C. Tam, M. Hou, W. Happer and S. M. Curry, Opt. Commun. 24, 305 (1978). 9. H. Scheingraber and C. R. Vidal, Sci. Rev. Instrum. 52, 1010 (1981). 10. N. Spies, Phd Thesis, Univ. Kasierslautern 1990.

energy-pooling collisions

nije evidentirano