We present a time resolved study of excitation transfer processes between the low lying excited states of barium (6s6p
3PJ, 6s5d
1D2, 6s5d
3DJ) induced by collisions with noble gases. The transient excited state populations have been measured by means of fluorescence and absorption spectroscopy. Cross sections for spin-changing collisions from
3P1 to
1D2 and from
1D2 to
3D3 as well as for fine-structure mixing within the
3D multiplet were obtained by fitting the numerical solution of a seven-state system of rate equations including diffusion to the experimental data. We compare our results to experimental findings for other alkaline earth metals and report an improved value of the lifetime of the 6s6p
3P1 state of barium.
We describe a set of experiments that investigate diffusion of barium 6s5d
3DJ metastable level and 6
s21S0 ground-state atoms through the noble gases: He, Ne, Ar, Kr, and Xe. The barium metastable levels were populated through optical pumping of the 6
s21S0\ensuremath{\rightarrow}6s6p
3P1o intercombination transition with a pulsed laser, followed by spontaneous or stimulated emission into the 6s5d
3D1,2 levels. Collisional mixing then distributed population throughout the 6s5d
3DJ levels. A weak narrow-band cw laser was tuned to appropriate transitions to probe the spatial distribution of metastable level atoms, as well as the hole created in the ground-state spatial distribution, by the pulsed laser. Time-dependent absorption coefficients were obtained by measuring the transmission of the probe laser at several different pump-probe spatial separations. From these absorption data, spatial number density profiles were mapped out for various times following the firing of the pump laser. These density profiles were used to determine the diffusion coefficient for each state at a particular buffer-gas pressure and temperature. From these pressure- and temperature-dependent diffusion coefficients D, we determined the thermally averaged diffusion cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$(6s5d
3DJ) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$(6
s21S0) for the metastable and ground-state atoms diffusing through the noble gases, respectively. We also report values for the scaled standard diffusion coefficients
D0. Our experimental values of the thermally averaged diffusion cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$(6s5d
3DJ) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$(6
s21S0) for the barium-helium system are compared with values calculated from published, theoretical barium-helium potentials and the agreement is excellent. \textcopyright{} 1996 The American Physical Society.
We describe a set of experiments that investigate excitation transfer among the barium 6s5d
3DJ metastable levels due to collisions with argon perturber atoms, and quenching of atoms in these levels by nitrogen perturber molecules. The metastable levels were populated through optical pumping of the 6
s2 1S0\ensuremath{\rightarrow}6s6p
3P1o intercombination transition with a pulsed laser, followed by stimulated emission or stimulated Raman scattering into the 6s5d
3D1,2 levels. Collisional mixing then distributed population throughout the
3DJ levels. Time-dependent absorption coefficients were obtained by measuring the transmission of a weak cw probe laser beam (after the pulsed laser fired) at several perturber number densities. Argon buffer gas was used to investigate fine-structure mixing, and nitrogen was used to study quenching. The time-dependent absorption coefficients were fitted to appropriate fitting functions, obtained by solving the set of coupled rate equations used to model the time evolution of the metastable level populations. The buildup rates of the initially unpopulated
3D3 and underpopulated
3D1 levels were used to determine the collisional mixing rates, while the measured decay rates of the
3DJ level populations yielded the quenching rates. From plots of buildup rates versus argon number density for the
3D1,3 states, we were able to determine four of the six fine-structure mixing rate coefficients
kij.The results are
k12=(1.71\ifmmode\pm\else\textpm\fi{}0.18),
k21=(1.39\ifmmode\pm\else\textpm\fi{}0.13),
k23=(1.42\ifmmode\pm\else\textpm\fi{}0.13), and
k32=(1.92\ifmmode\pm\else\textpm\fi{}0.16)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$
cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. The experiment is not sensitive to the values of
k13 and
k31. Estimated rate coefficients for excitation transfer among the
3DJ levels due to collisions with barium atoms are
k21Ba=1.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$
cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and
k23Ba=5.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$
cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. A plot of the population decay rates versus
N2 number density for each of the three metastable levels yielded the average nitrogen quenching rate coefficient
kq=(3.14\ifmmode\pm\else\textpm\fi{}0.19)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$
cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.
Collisional deactivation of the 5d7p (3)D1 state of Ba by noble gases is studied by time- and wavelength-resolved fluorescence techniques. A pulsed, frequency-doubled dye laser at 273.9 nm excites the 5d7p (3)D1 level from the ground state, and fluorescence at 364.1 and 366.6 nm from the 5d7p (3)D1 --> 6s5d (3)D1 and 5d7p (3)D1 --> 6s5d (3)D2 transitions, respectively, is monitored in real time to obtain the deactivation rate constants. At 835 K these rate constants are as follows: He, (1.69 +/- 0.08) x 10(-9) cm(3) s(-1); Ne, (3.93 +/- 0.14) x 10(-10) cm(3) s(-1); Ar, (4.53 +/- 0.15) x 10(-10) cm(3) s(-1); Kr, (4.64 +/- 0.13) x 10(-10) cm(3) s(-1); Xe, (5.59 +/- 0.22) x 10(-10) cm(3) s(-1). From time-resolved 5d7p (3)D1 emission in the absence of noble gas and from the intercepts of the quenching plots, the lifetime of this state is determined to be 100 +/- 1 ns. Using time- and wavelength-resolved Ba emission with a low background pressure of noble gas, radiative lifetimes of several near-resonant states are determined from the exponential rise of the fluorescence signals. These results are as follows: 5d6d (3)D3, 28 +/- 3 ns; 5d7p (3)P1, 46 +/- 2 ns; 5d6d (3)G3, 21.5 +/- 0.8 ns; 5d7p (3)F3, 48 +/- 1 ns. Integrated fluorescence signals are used to infer the relative rate constants for population transfer from the 5d7p (3)D1 state to eleven near-resonant fine structure states.