Inelastic collisional processes in fine-structure-state mixing have been studied by selective stepwise excitation and observation of the resulting fluorescence during collisions of excited rubidium
62D25 state atoms with various noble-gas atoms. It is shown that for intermediately excited rubidium atoms the fine-structure-state mixing cross sections are generally increasing from helium to xexon. The cross sections for the process $\mathrm{Rb}(6^{2}D_{\frac{5}{2}})+A(n^{1}S_{0})\ensuremath{\rightarrow}\mathrm{Rb}(6^{2}D_{\frac{3}{2}})+A(n^{1}S_{0})+\ensuremath{\Delta}E$ have the values ${\ensuremath{\sigma}}_{\mathrm{fs}}^{*}=2.4\ifmmode\pm\else\textpm\fi{}0.9, 3.1\ifmmode\pm\else\textpm\fi{}1.2, 2.6\ifmmode\pm\else\textpm\fi{}1.0, 5.8\ifmmode\pm\else\textpm\fi{}2.2, \mathrm{and} 9.3\ifmmode\pm\else\textpm\fi{}5.6$ in units of ${10}^{\ensuremath{-}14}$
cm2, where
A denotes in sequence any one of the noble gas atoms He, Ne, Ar, Kr, and Xe, respectively. Transfer cross sections for the transfer of excited Rb
62D-state atoms out of the doublet were also determined and are about an order of magnitude smaller than the fine-structure-mixing cross sections.
Using an isoclinic-point technique, we measured the
D1 collision shift by Xe, $\ensuremath{\partial}[\ensuremath{\delta}\ensuremath{\nu}]/\ensuremath{\partial}P$, and the exponent $\ensuremath{\kappa}$ of the shift's temperature dependence (i.e., $\ensuremath{\delta}\ensuremath{\nu}\ensuremath{\sim}{T}^{\ensuremath{\kappa}}$). As demonstrated in our examination of the Rb-Kr system [N. P. Wells et al., Phys. Rev. A 89, 052516 (2014)], the isoclinic point provides (arguably) the only means of assessing $\ensuremath{\kappa}$ unambiguously: ${\ensuremath{\kappa}}_{\mathrm{Kr}}\phantom{\rule{4pt}{0ex}}=\phantom{\rule{0.16em}{0ex}}0.36\phantom{\rule{0.16em}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0.16em}{0ex}}0.06$ and in the present work ${\ensuremath{\kappa}}_{\mathrm{Xe}}\phantom{\rule{4pt}{0ex}}=0.32\ifmmode\pm\else\textpm\fi{}0.05$. With our estimate of $\ensuremath{\kappa}$ for the Rb-Kr and Rb-Xe systems, we were able to combine our Kr and Xe collision shift measurements with those of Rotondaro and Perram [M. D. Rotondaro and G. P. Perram, J. Quant. Spectrosc. Radiat. Transfer 57, 497 (1997)] (another set of high quality $\ensuremath{\partial}[\ensuremath{\delta}\ensuremath{\nu}]/\ensuremath{\partial}P$ measurements) to obtain a highly accurate experimental estimate for the
D1 collision shift resulting from Rb's interaction with the heavy noble gases: For the Rb-Kr interaction $\ensuremath{\partial}[\ensuremath{\delta}\ensuremath{\nu}]/\ensuremath{\partial}P{|}_{T=323\phantom{\rule{0.16em}{0ex}}\mathrm{K}}=\ensuremath{-}5.02\ifmmode\pm\else\textpm\fi{}0.07\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}/\mathrm{torr}$ and for the Rb-Xe interaction $\ensuremath{\partial}[\ensuremath{\delta}\ensuremath{\nu}]/\ensuremath{\partial}P{|}_{T=323\phantom{\rule{0.16em}{0ex}}\mathrm{K}}=\ensuremath{-}5.46\ifmmode\pm\else\textpm\fi{}0.09\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}/\mathrm{torr}$. These measured values for the collision-shift coefficient are approximately
20 smaller (in magnitude) than the best theoretical estimates, suggesting that there is room for theoretical improvement regarding our present understanding of how noble-gas collisions perturb the alkali-metal
P1/2 state.
The collision cross sections for alkali-metal--rare-gas spin orbit mixing between the ${n}^{2}{P}_{3/2}\ensuremath{\rightarrow}{n}^{2}{P}_{1/2}$ levels trend strongly with the Massey parameter, or adiabaticity of the collisions. The strength of the interaction, as characterized by the
C6 dispersion coefficient, is a secondary influence on the rates. An analytic expression for the probability of energy transfer in alkali-metal--rare-gas collisions is derived using time-dependent perturbation theory. The model agrees well with a broad literature survey of the observed temperature-dependent cross sections. A simple interaction potential successfully organizes the alkali-metal--rare-gas database. The rates become very large for high-lying states, as the collisions are quite sudden and the radius of the valence electron is large. In contrast, the highly adiabatic cesium
62P mixing rates are six to eight orders of magnitude smaller. The mixing rate for the Rb-He diode pumped alkali laser system varies from $0.20--1.53\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\mathrm{cm}{}^{3}/\mathrm{at}.\phantom{\rule{0.16em}{0ex}}\mathrm{s}$ for
T=279−−893K.
Measuring the energy dependence of optical-transition collision shifts has proven extremely difficult, in part because of Doppler broadening and the manifold of overlapping hyperfine components that must be disentangled in the spectra. Here, we demonstrate an approach to these measurements based on spectroscopic isoclinic points. To illustrate the approach's efficacy, we investigated the Kr collision shift of the Rb
D1 transition at 795 nm. For the expected Rb-Kr van der Waals interaction, the collision shift should scale like ${(T/{T}_{o})}^{\ensuremath{\kappa}}$, where
To is a reference temperature and \ensuremath{\kappa}
theo = 0.31. Exemplifying the difficulty of \ensuremath{\kappa} determinations, previous alkali-metal--noble-gas experimental measurements of \ensuremath{\kappa} have varied widely, sometimes in striking disagreement with theory (i.e., factor of 2 larger). In the present work, we not only demonstrate a measurement precision better than 10${}^{\ensuremath{-}10}$/\ifmmode^\circ\else\textdegree\fi{}C, but with our technique we validate the theoretical scaling constant, finding ${\ensuremath{\kappa}}_{\mathrm{expt}}$ = 0.36 \ifmmode\pm\else\textpm\fi{} 0.06.
The cross sections for excitation transfer between the 62P fine-structure substates in rubidium, induced in collisions with noble gas atoms, have been determined in a series of sensitized fluorescence experiments. Mixtures of rubidium vapor and noble gases at pressures varying in the range 0–5 Torr were irradiated with each component of the second 2P rubidium doublet in turn and the following cross sections for 2P mixing were obtained from measurements of sensitised-to-resonance fluorescent intensity ratios. Rb–He: Q12(2P1/2 → 2P3/2) = 29.3 Å2; Q21(2P1/2 ← 2P3/2) = 19.0 Å2. Rb–Ne: Q12 = 10.3 Å2; Q21 = 6.4 Å2. Rb–Ar: Q12 = 24.0 Å2; Q21 = 14.9 Å2. Rb–Kr: Q12 = 23.2 Å2; Q21 = 14.6 Å2. Rb–Xe: Q12 = 43.9 Å2; Q21 = 27.7 Å2 In their dependence on the magnitude of the fine-structure splitting, the values are consistent with previously determined cross sections for mixing in the first and third 2P doublets of alkali atoms.
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.