Saturation and power broadening of the calcium singlet resonance line (422.7 nm) was found to show strong pressure dependence for helium, argon and xenon. This was ascribed to transfer of excitation from the (4s 4p) 1 P 1 level through the sequence of levels 1 D 2 , 3 D J , 3 P J by predominantly collisional processes. The first step of this cascade, 1 P 1 - 1 D 2 , was identified as the slowest process and cross sections are presented. The temperature independence of the process for xenon indicates that the large cross section for this gas may be due to a favourable potential curve crossing. Quenching of the 3 P J levels is also detected and cross sections are given for argon and xenon.
Saturation and power broadening of the calcium singlet resonance line (422.7 nm) was found to show strong pressure dependence for helium, argon and xenon. This was ascribed to transfer of excitation from the (4s 4p) 1 P 1 level through the sequence of levels 1 D 2 , 3 D J , 3 P J by predominantly collisional processes. The first step of this cascade, 1 P 1 - 1 D 2 , was identified as the slowest process and cross sections are presented. The temperature independence of the process for xenon indicates that the large cross section for this gas may be due to a favourable potential curve crossing. Quenching of the 3 P J levels is also detected and cross sections are given for argon and xenon.
Collisions of the second kind between caesium and caesium, caesium and helium, and caesium and xenon atoms, leading to P p the transfer of excitation between the 6 6 P^yg states in caesium, have been investigated by studying the sensitized fluorescence in caesium vapor at pressures in the range 2x10*^ • 7x10*5 mm Hg and in vapor-inert gas mixtures.These low caesium densities, at which trapping of resonance radiation is virtually absent, were previously Inaccesible to such measurements.The use of a high intensity light source and an improved fluorescence cell in conjunction with a very sensitive detection system, permitted accurate measurements of extremely low fluorescent intensities.The total collision cross sections for the processes ô^P^yg -> and 6 %]^yg<r-G^P^yg equal 0.64x10*^^ cm® and 3.1x10*15 ^^2 for caesium-caesium collisions, 0.41x10*®® cm® and 2.7x10*®® cm® for caesium-helium collisions, and 7 .6x10 *®! cm^ and 2.9x10*1^ cm® for caesium-xenon collisions respectively.The ratios of these cross sections agree well with the values predicted by the principle of detailed balancing except for the caesiumxenon collisions, where a shielding effect has been postulated to account for the discrepancy.ill
The destruction rates of the xenon $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0}$ metastable and $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{1}$ resonance atoms by collisions with ground-state xenon atoms have been measured by studying the temporal dependence of the number densities of the $6{s}^{\ensuremath{'}}$ and 6p atoms. The studies have been made both during the earlier afterglow of a xenon pulsed discharge and after the selective production of the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ atoms by a laser excitation in the late afterglow. An original optical pumping technique, using a tunable pulsed dye laser, was employed to populate the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ levels by excitation of the 6s[23]2 long-lived metastable atoms to $6{p}^{\ensuremath{'}}$ levels. The pressure dependence of the decay frequencies of these $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ atoms was analyzed in the xenon pressure range of 0.04 to 1 torr and a kinetic model describing the evolution of the densities of these atoms after their production was established. The rate coefficients for the relaxation of the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0}$ metastable and $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{1}$ resonance atoms by two-body collisions involving a ground-state xenon atom are (8.45\ifmmode\pm\else\textpm\fi{}0.85) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}12}$ cm3 ${\mathrm{sec}}^{\ensuremath{-}1}$ and (6.65 \ifmmode\pm\else\textpm\fi{} 1.00) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}11}$ cm3 ${\mathrm{sec}}^{\ensuremath{-}1}$, respectively. Results show that, in fact, at least 75% of this relaxation is a transfer of population to the 6p[21]1 level following ${\mathrm{Xe}}^{*}(6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1})+\mathrm{Xe}\ensuremath{\rightleftarrows}{\mathrm{Xe}}^{*}(6p{[\frac{1}{2}]}_{1})+\mathrm{Xe}\ensuremath{-}\ensuremath{\Delta}E$. These collisional transfer processes have been explained by means of Mulliken's xenon molecular potential curves.
This paper reports on our studies of near-resonant scattering of laser light in a collisional environment. A pulsed dye laser with a peak power of 55 MW/cm2 was tuned near the 460.73-nm resonance line of strontium and the side emission was observed from an oven containing both strontium vapor and argon buffer gas. The emission was composed of three spectral components: Rayleigh scattering at the frequency of the laser ${\ensuremath{\omega}}_{L}$, fluorescence at the resonance frequency ${\ensuremath{\omega}}_{0}$ of strontium, and a third component at $2{\ensuremath{\omega}}_{L}\ensuremath{-}{\ensuremath{\omega}}_{0}$. These three components have been studied as a function of the frequency and intensity of the laser and also as a function of the argon buffer gas pressure. While the Rayleigh emission was found to vary as ${\ensuremath{\Delta}}^{\ensuremath{-}2}$ ($\ensuremath{\Delta}={\ensuremath{\omega}}_{0}\ensuremath{-}{\ensuremath{\omega}}_{L}$), the fluorescence component, which was produced by Sr-Ar collisions, was found to be asymmetric with the sign of $\ensuremath{\Delta}$ as predicted by line-broadening theory. By measuring the ratio of the intensities of the fluorescence and Rayleigh components, we were able to measure directly the collisional redistribution function, important in the study of radiative transfer in stellar and planetary atmospheres. At high laser intensities all three components were found to saturate. The results were compared with the theoretical predictions of Mollow's steady-state theory. Theoretical fits for the high-intensity results were obtained when the collisional cross sections were taken to be considerably smaller than in our low-intensity measurements. We believe the discrepancy lies in the use of a steady-state theory for a transient experiment. Effects of radiative trapping and spatial averaging are also discussed.
We present a kinetic study of the low-lying electronically excited state of atomic calcium, Ca[4s4p(3PJ)], 1.885 eV above the 4s2(1S0) ground state. Ca(4 3P1) was generated by dye-laser pulsed excitation at λ= 657.3 nm of calcium vapour [Ca(4 3P1)â†� Ca(4 1S0)] at elevated temperatures (1000 K), in equilibrium with solid calcium, in a slow-flow system, kinetically equivalent to a static system. Following rapid Boltzmann equilibration within Ca(4 3PJ) through collisions, the forbidden time-resolved atomic resonance fluorescence at the same wavelength was monitored using boxcar integration. The decay of Ca(4 3PJ) in the presence of all the noble gases, He, Ne, Ar, Kr and Xe, was studied, yielding measurements of relative diffusion coefficients. Extrapolation of the diffusional component of the decay in each noble gas to infinite pressure yields an independent value of the mean radiative lifetime for Ca(4 3P1)→ Ca(4 1S0)+hν(λ= 657.3 nm), the average of which from the measurements in all the noble gases yields τe= 0.34 ± 0.02 ms (error 2σ). This result is compared with values reported from previous measurements, particularly atomic beams and time-resolved measurements. The kinetic analysis also gives rise to data for the collisional quenching of Ca(4 3PJ), which is found to be insignificant for most of the noble gases [kHe,Ne,Ar,Kr⩽ 4 × 10–15 cm3 molecule–1 s–1(1000 K)], in contrast to measurements from earlier work using phase-shift techniques. Xenon, however, exhibited measurable rates of collisional quenching of Ca(4 3PJ) characterised by kXe= 2.4 ± 0.3 × 10–14 cm3 molecule–1 s–1(1000 K). The time-resolved emission technique is further applied to the study of the removal of Ca(4 3PJ) by added gases, for which we report (errors 1σ)kN2=(8.9 ± 0.9)× 10–13 cm3 molecule–1 s–1(1000 K), kH2=(6.0 ± 0.6)× 10–14 cm3 molecule–1 s–1(1000 K), kD2=(2.7 ± 0.3)× 10–14 cm3 molecule–1 s–1(1000 K).
Experiments on collisionally assisted two-photon absorption (TPA) in calcium vapour buffered by argon gas have been performed using two tunable dye lasers. Both direct TPA and stepwise transitions through the collisionally populated resonance level. 4s4p 1 P 1 , have been observed. Detailed study of the lineshapes of the stepwise transitions to the 4p 2 1 S 0 , 4p 2 1 D 2 and 4s6s 1 S 0 levels reveals large differences in the strengths of the Ca-Ar interatomic potentials for these levels. These differences probably arise from configuration mixing.
We describe collision studies on the two-photon excitation and fluorescence of the xenon 6p[1/2,0] excited state. Molecular nitrogen is the major collision partner. The 250-nm second harmonic of a line-narrowed Hansch-type dye laser excites the 6p state of xenon by two-photon absorption. Sample mixtures contain xenon at a partial pressure of 1 Torr and nitrogen at partial pressures of up to several hundred Torr. Time-resolved and time-integrated measurements of the intensity of the 828-nm 6p[1/2,0]—6s[3/2,1] allowed transition are obtained with photomultipliers, a transient digitizer, and a computerized data acquisition system. We extract pressure-dependent and pressure-independent components of the fluorescence lifetime and fluorescence yield from measurements over a range of nitrogen pressures. When the excitation power density is of the order of 104 W/cm2, the excited xenon atoms decay spontaneously. For spontaneous fluorescence, we obtain an inelastic (quenching) collision cross section of ~10–14 cm2 for xenon-nitrogen collisions, and a pressure-broadening coefficient of ~3 × 107 Hz/(Torr N2) for the two-photon transition rate. For excitation intensities above 2.6 × 106 W/cm2, the xenon atoms radiate cooperatively in a subnanosecond pulse. The fluorescence yield of this cooperative emission increases with nitrogen pressure rather than decreasing as in the spontaneous case. Mechanisms are discussed.