A time-resolved study of the Sr(5 $^{3}\ifmmode^\circ\else\textdegree\fi{}_{J}$) intramultiplet collisional mixing, following optical excitation of the $^{3}\mathrm{\ifmmode^\circ\else\textdegree\fi{}}_{1}$ state, is presented. The degeneracy-averaged rate coefficients of Sr(5 $^{3}\ifmmode^\circ\else\textdegree\fi{}_{J}$\ensuremath{\rightarrow}5 $^{3}\ifmmode^\circ\else\textdegree\fi{}_{J\mathcal{'}}$) due to collisions with each of the noble gases in a quasiequilibrium cell are derived from least-squares fittings of the triexponential time behavior of 5 $^{3}\ifmmode^\circ\else\textdegree\fi{}_{1}$ fluorescence and
53P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{2}$, 0\ensuremath{\rightarrow}6
3S1 absorption data. A 5 $^{3}\ifmmode^\circ\else\textdegree\fi{}_{1}$ radiative rate of 22\ifmmode\pm\else\textpm\fi{}0.5 \ensuremath{\mu}s is also obtained. The dependence of the rates upon buffer pressure demonstrated the presence of both linear and quadratic terms, due to two- and three-body reactions, respectively. The behavior of the binary rate coefficients with varying noble-gas species and temperature is qualitatively consistent with St\"uckelberg's model for noncrossing levels.
Measurements of the mixing rates and cross sections for collisional excitation transfer between the 5P 1/2 and 5P 3/2 states of rubidium (Rb) in the presence of 4 He buffer gas are presented. Selected pulses from a high repetition rate, mode-locked femtosecond laser are used to excite either Rb state with the fluorescence due to collisional excitation transfer observed by time-correlated single-photon counting. The time dependence of this fluorescence is fitted to the solution of rate equations which include the mixing rate, atomic lifetimes and any quenching processes. The variation in the mixing rate over a large range of buffer gas densities allows the determination of both the binary collisional transfer cross section and a three-body collisional transfer rate. We do not observe any collisional quenching effects at 4 He pressures up to 6 atm and discuss in detail other systematic effects considered in the experiment.
Inert-gas-induced collisional transfer between the first four excited states of Sr has been measured by determining excited-state densities following pulsed laser excitation of the 5
1P state. We observe 5
1P collisional transfer exclusively to the 4
3D state, with (\ensuremath{\sim}800 K) rate coefficients ranging from 0.24 to 2.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$
cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the various gases. We also measure 4
1D2 transfer primarily to the 4
3D state, with rate coefficients of 0.65 to 6.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$
cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. The 4
3D state radiative lifetime is observed to be 2.9\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}s, collisional mixing between the 4
3Dj fine-structure levels is essentially gas kinetic (k\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$
cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$), and k${10}^{\mathrm{\ensuremath{-}}13}$
cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for 4
3D\ensuremath{\rightarrow}5
3P collisional transfer.
Using ultrafast laser excitation and time-correlated single-photon counting techniques, we have measured the collisional mixing rates between the rubidium 5(2)P fine-structure levels in the presence of (4)He gas. A nonlinear dependence of the mixing rate with (4)He density is observed. We find Rb fine-structure transfer is primarily due to binary collisions at (4)He densities of < or = 10(19) cm(-3), while at greater densities, three-body collisions become significant. We determine a three-body collisional transfer rate coefficient (5(2)P(3/2) --> 5(2)P(1/2)) of 1.25(9)x10(-32) cm(6)/s at 22 degrees C.
The collisional angular-momentum mixing of $n=5\ensuremath{-}15$ Na
d states with the
l>2 states of the same
n has been studied using He, Ne, and Ar as collision partners. At rare-gas pressures of \ensuremath{\sim}1 Torr, the $l\ensuremath{\ge}2$ states are thoroughly mixed and the mixture decays at the average radiative lifetime of all the $l\ensuremath{\ge}2$ states of the same
n. Using \ensuremath{\sim}1 Torr of neon to collisionally mix the $l\ensuremath{\ge}2$ states, the average lifetime for $l\ensuremath{\ge}2$ was measured for each
n and compared to the theoretical prediction for hydrogen, with which it is in excellent agreement. The cross section for
l mixing by each of the gases is found to increase sharply from
n=5to10 and decrease above
n=10. The magnitudes of the
l-mixing cross sections are found to correlate with the low-energy scattering lengths of the collision partners.
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.