The inelastic collisional cross sections of the
nD states of Cs are measured by the technique of selective laser excitation and observation of collision-induced fluorescence. The cross sections (in units of ${10}^{\ensuremath{-}14}$
cm2) for the fine-structure-changing collisions [i.e., $\mathrm{Cs}(n^{2}D_{\frac{3}{2}})+\mathrm{Cs}(6^{2}S_{\frac{1}{2}})\ensuremath{\rightarrow}\mathrm{Cs}(n^{2}D_{\frac{5}{2}})+\mathrm{Cs}(6^{2}S_{\frac{1}{2}})$] for
n being 6-10 are 2.7\ifmmode\pm\else\textpm\fi{}0.5, 3.7\ifmmode\pm\else\textpm\fi{}0.7, 8.6\ifmmode\pm\else\textpm\fi{}1.7, 18.4\ifmmode\pm\else\textpm\fi{}3.7, and 29.8\ifmmode\pm\else\textpm\fi{}6.0, respectively. The cross sections (in ${10}^{\ensuremath{-}14}$
cm2) for excitation transfer out of the
nD doublet [i.e., $\mathrm{Cs}(n^{2}D_{\frac{5}{2}})+\mathrm{Cs}(6^{2}S_{\frac{1}{2}})\ensuremath{\rightarrow}\mathrm{states}\mathrm{other}\mathrm{than} \mathrm{Cs}(\mathrm{nD})$] for
n being 6-10 are 1.1\ifmmode\pm\else\textpm\fi{}0.6, 5.3\ifmmode\pm\else\textpm\fi{}1.5, 3.6\ifmmode\pm\else\textpm\fi{}1.5, 6.3\ifmmode\pm\else\textpm\fi{}2.3, and 15.5\ifmmode\pm\else\textpm\fi{}4.0, respectively. The fine-structure-changing cross sections are found to be nearly equal to the geometrical cross section $\ensuremath{\pi}{n}^{*4}{a}_{0}^{2}$ for all the
nD states studied, except
6D, whose fine-structure-changing cross section is unusually large.
Cross sections for the transfer of excitation between fine-structure sublevels of the 9D state of caesium due to collisions with noble-gas atoms are measured. The experimental method employs stepwise excitation with a pulsed dye laser in the second step (6 2 P 3/2 to 9 2 D 5/2 ) and time-resolved detection of sensitised fluorescence 9 2 D 3/2 to 6 2 P 1/2 . The cross sections for the process Cs(9 2 D 3/2 )+A to Cs(9 2 D 5/2 )+A are (in units of 10 -14 cm 2 ) 7.6(1.5), 2.5(5), 2.7(5), 3.0(6) and 3.2(6) for A=He, Ne, Ar, Kr and Xe, respectively. For all the noble gases except helium the experimental cross sections are a few times smaller than the ones obtained in a recent semiclassical calculation.
Cross sections for
72D and
82D fine-structure mixing in Rb, induced by collisions with various ground-state atoms and
N2 molecules, have been determined using methods of atomic fluorescence. Rb vapor, pure or mixed with a buffer gas, was irradiated in a glass fluorescence cell with pulses of radiation from a
N2 laser-pumped dye laser, populating a
2D fine-structure state by two-photon absorption. The resulting fluorescence included a direct component arising from the optically excited state and a sensitized component due to the collisionally populated fine-structure state. Measurements of relative intensities of the two components in relation to buffer-gas pressure yielded the following cross sections (in units of ${10}^{\ensuremath{-}14}$
cm2): $Q({7}^{2}{D}_{\frac{3}{2}}\ensuremath{\rightarrow}{7}^{2}{D}_{\frac{5}{2}})=14.1,15.5,\mathrm{and}10.0$; $Q({7}^{2}{D}_{\frac{3}{2}}\ensuremath{\leftarrow}{7}^{2}{D}_{\frac{5}{2}})=9.0,10.5,\mathrm{and}6.9$, for Kr, Xe, and
N2, respectively; $Q({8}^{2}{D}_{\frac{3}{2}}\ensuremath{\rightarrow}{8}^{2}{D}_{\frac{5}{2}})=8.9,4.9,12.4,24.9,25.8,12.1,\mathrm{and}43.1$; $Q({8}^{2}{D}_{\frac{3}{2}}\ensuremath{\leftarrow}{8}^{2}{D}_{\frac{5}{2}})=5.8,3.2,9.4,15.1,17.6,8.3,\mathrm{and}28.5$, for He, Ne, Ar, Kr, Xe,
N2, and Rb, respectively. Cross sections for the effective depopulation of the
2D states were also determined.
We report an application of laser spectroscopy techniques to a study of collision-induced interactions in atomic excited states. Due to pulsed dye laser excitation a considerable selective population of highly excited states of ccsium is obtained. Collision-induced transfer of excitation energy between the excited states results in modifications in time and spectral characteristics of observed atomic fluorescence. Quantitative information on the efficiency of collisional processes can be obtained from the measurements of time constants of the time-resolved fluorescence signals and/or from those of the integrated intensities of the fluorescence lines. Both possibilities are used in the present work. Perturbation of nD (n=8-14) and nS (n=1O-15) states of cesium in collisions with noble-gas atoms is investigated. The cross sections for the transfer of excitation between fine-structure substates of the nD states (J mixing) and for the nS-(n-4)F intermultiplet transfer are obtained.
Cross sections for inelastic transfer between the 82D3/2 and 82D5/2 fine-structure states in rubidium, induced in resonant collisions with ground-state Rb atoms, have been determined using an experimental method involving two-photon excitation of atomic fluorescence. Rubidium vapor in a fluorescence cell was irradiated with pulses of 641 nm radiation from a N2 laser-pumped dye-laser tuned to excite one of the 82D states. The resulting fluorescence included the direct component originating from the optically excited state and a sensitized component arising from the other fine-structure state populated by collisions. Relative intensities of the fluorescent components, determined by photon-counting techniques, yielded the cross sections for excitation transfer: Q(2D3/2 → 2D5/2) = 8.1 × 10−13 cm2; Q(2D3/2 ← 2D5/2) = 5.5 × 1013 cm2; as well as [Formula: see text], the effective quenching cross section. The excitation transfer cross sections which are considered accurate to within ±20% are in the ratio predicted by the principle of detailed balancing.
The method of sensitized fluorescence has been employed to investigate the [Formula: see text] excitation transfer processes in cesium induced in collisions with ground state cesium atoms. The cesium vapor density was kept sufficiently low to enable the cross sections for the mixing process to be determined under single collision conditions and to ensure that radiation trapping and quenching were negligible. A nitrogen laser pumped dye laser was used to excite the cesium atoms to each of the 72P levels in turn and measurements of the relative intensities of fluorescence yielded the following cross sections: [Formula: see text] These results are consistent with the empirical relationship between the magnitude of the cross sections and the fine structure splitting that has previously been determined for the alkalis.