A pressure-broadening study of three-photon and one-photon resonances was performed using counterpropagating, linearly polarized laser beams. Self-broadening, as well as the collision-induced effects due to the presence of a second noble gas, was investigated for the 6s, 6s′, 5d and 7s of xenon and the 5s′ of krypton. A multiphoton ionization scheme in a calibrated proportional counter is the basis of a new technique by which collisional processes can be investigated for high pressure regimes which have previously been inaccessible to investigation, particularly for self-broadening where the mean free path of the photons is of the order of 10−3 cm for pressures as low as 1 Torr. Using this technique, we have recorded the collision-induced widths and shifts of the resonance for pressures up to 1000 Torr. In the hundreds of torr range, asymmetric, self-broadening line shapes were observed with full widths and shifts which were greater than five times the laser bandwidth (0.002-0.005 nm).
A two-step excitation experiment has been employed to measure the collisional rate coefficients and to study the velocity distribution of
Cs(6P1∕2) atoms that have undergone a single hyperfine state-changing collision with Ar. In addition, argon pressure broadening rates and shifts of the cesium $6{P}_{1∕2}({F}^{\ensuremath{'}})\ensuremath{\rightarrow}8{S}_{1∕2}({F}^{\ensuremath{''}})$ transitions have been determined. In the experiment, a single mode, cw Ti:sapphire laser is tuned to line center of the $6{S}_{1∕2}(F=4)\ensuremath{\rightarrow}6{P}_{1∕2}$(${F}^{\ensuremath{'}}=3$ or 4) transition. Then, the frequency of a single mode cw dye laser is scanned over the $6{P}_{1∕2}\ensuremath{\rightarrow}8{S}_{1∕2}$ manifold to probe the populations of the
6P1∕2 hyperfine levels. Absorption of probe laser photons is monitored by detecting $8{S}_{1∕2}\ensuremath{\rightarrow}6{P}_{3∕2}$ fluorescence. The experiment is conducted at room temperature, where the Cs density is low $(n\ensuremath{\sim}3.4\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}\mathrm{atoms}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$, and thus the probability of a $\mathrm{Cs}\text{\ensuremath{-}}\mathrm{Cs}$ collision is negligible during the
Cs(6P1∕2) radiative lifetime. The Ar pressure is varied from 0 to
1.52Torr, and $\mathrm{Cs}\text{\ensuremath{-}}\mathrm{Ar}$ collisions cause population to be transferred from the directly excited $6{P}_{1∕2}({F}^{\ensuremath{'}})$ level to the other
6P1∕2 hyperfine level. The data are analyzed using a density matrix formalism to yield the rate coefficients for $\mathrm{Cs}(6{P}_{1∕2})\text{\ensuremath{-}}\mathrm{Ar}$ hyperfine state-changing collisions. In addition, the one-dimensional velocity changing collision kernel for
Cs(6P1∕2) atoms prepared with
vz=0 that undergo ${F}^{\ensuremath{'}}=3\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=4$ hyperfine state-changing collisions with argon is reported.
Measurements of collisional depolarization cross sections play an important role in gaining valuable information about the interaction between inert and rare-gas atoms.A key element in understanding collisional dynamics is the detailed analysis of the polarization of the emitted photons.In order to investigate the collisional cross section of the excited J=1/2 cesium atoms, by collisions with the ground level argon atoms, experimental measurements were made studying circular polarization spectra.Orientation in the J=1/2 level was optically induced by circularly polarized light with a positive helicity.A two-photon double-resonance (6s 2 S 1/2 →6p 2 P 1/2 → 10s 2 S 1/2 ) condition was achieved using nanosecond pulsed dye lasers, and the intensity of the cascade fluorescence was monitored in the presence of argon atoms ranging from 10 -4 torr to 100 torr.
A semiclassical treatment of collision-induced three-photon ionization of atoms with a near two-photon resonance is presented. The system is described by an effective two-level system. The wavelength dependence of the ionization yield does not obey a power law. When the first excited state is far from resonance, and with a
R6C6 interatomic potential, a first-order correction to the expected $\ensuremath{-}\frac{3}{2}$ power law obeys a - \textonehalf{} power law. This three-photon process promises to be an extremely sensitive method for the study of collisional effects in the far wing of two-photon $S\ensuremath{-}S$ and $S\ensuremath{-}D$ transitions, and without the requirement of high densities.
We apply two-photon resonant nondegenerate four-wave mixing with a resonant intermediate state for observing the broadening and the shifting of the barium Rydberg 6snd 1D2 series by collision with argon. The collision broadenings and the collision shifting cross sections are measured for n=1633. This technique is a purely optical means, and can achieve Doppler-free resolution of narrow spectral structures of Rydberg levels when the incident lasers have narrow bandwidths. Different from other experimental methods of studying the pressure dependence of the longitudinal relaxation rate of Rydberg states, our method is to investigate the pressure dependence of the transverse relaxation rate of the transition.
An experimental investigation of the atomic-Na 3s
2${\mathit{S}}_{1/2--}$5s
2S1/2 two-color two-photon polarization spectrum has been made. Measurements of the linear polarization degree of the two-photon transition were made for one laser tuned within a \ifmmode\pm\else\textpm\fi{}100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range of the atomic-Na 3p
2Pj levels, with the second adjusted to satisfy the two-photon resonance condition. The relatively short-pulse (\ensuremath{\sim}0.5 ns) lasers used eliminate effects on the spectrum due to atomic-Na hyperfine interactions and radiation trapping, while greatly reducing depolarization due to collisions with Ar gas. In the absence of collisions, the measured polarization spectrum is in excellent agreement with calculations. Measurements of the Ar pressure dependence of the spectrum revealed strong depolarization in the vicinity of resonant excitation through the 3p
2P3/2 level. A collisional depolarization cross section of 200(30) \AA{} for this process was deduced from the data.