We consider light trapping in an amplifying medium consisting of cold alkali-metal atoms; the atomic gas plays a dual role as a scattering and as a gain medium. We perform MonteCarlo simulations for the combined processes. In some configurations of the inhomogeneous distribution this leads to a point of instability behavior and a signature of random lasing in a cold atomic gas.
In this paper we calculated various correlated photoionization processes in helium by direct solution of the time-dependent Schrödinger equation. In Tables II through V we compared our theoretical results with various recent experiments. In all the tables we took the ratio of the specific correlated photoionization process to the single photoionization of helium leaving the atom in the nϭ1 ground state. However, this ratio is not the one measured in the double photoionization experiments. Thus, we have corrected Tables IV and V by calculating the ratio of the correlated double photoionization process to the single photoionization of helium, leaving the atom in any of the nϭ1, 2, and 3 bound states. The agreement between theory and experiment has noticeably improved.
Experimental, polarization-dependent excitation spectra for excited-state-Mg-rare-gas-atom optical collisions are reported. In these first studies of the process, polarized Mg atoms in the 3p 'Pl level are produced by absorption of linearly polarized light tuned to the 3s 'S0~3p 'Pl resonance transition at 285.2 nm. Detuning-dependent, collision-induced polarization spectra are measured in a +200-cm range around the Mg 3p 'Pl~5s 'So transition at 571.2 nm. The spectra correspond to probing transient Mg-Ne and Mg-Ar molecules on 3p 'n. &~5s 'Xo+ and 3p 'Xo~5s 'Xo electronic transitions. Measurements of these excited-state polarization spectra for Mg-Ne optical collisions reveal that for detunings to the red of the atomic Mg 3p 'P&-+5s 'So transition, electronic linear polarization greater than 50%%uo survives far into the molecular regime. This represents a direct measure of the polarization important to alignment-dependent inelastic processes in alkaline-earth-metal-rare-gas-atom collisions. The polarization spectra are discussed in terms of existing information on the interatomic potentials and through an axial recoil limit for the polarization degree for parallel and perpendicular molecular transitions. Rate coeicients k for disalignment of Mg 3p 'P, atoms by collisions with Ar [k =9.4(5) X 10 cm'/s) and with Ne [k =6.5(7) X 10 ' cm3/s] are also extracted from the data.
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