The energy dependence and the absolute value of the cross section for the P 1/2 to P 3/2 fine-structure transition (FST) in Rb(5P) induced by H 2 or N 2 molecules and He atoms has been measured for the first time in a crossed-beam apparatus. The results for He, in excellent agreement with previous experimental data, are used for comparing atomic and molecular target behaviour with regard to FST processes in heavy alkalis. One observes much larger cross sections for H 2 and N 2 at low energies with a much weaker energy dependence. Theoretical work suggests two seemingly competing mechanisms for including large FST cross sections for molecular targets. One, involving an intermediate ionic complex, accounts for the general trend (absolute value and energy dependence) of the experimental data for N 2 while the other, recently suggested by Hickman seems to play the major role when H 2 is the target.
The cross sections for fine-structure transition (FST) of Rb(7 2 D ) atoms induced by collision with molecules of H 2 and CO have been experimentally determined by the method of selective stepwise excitation of Rb atoms and observation of the resulting fluorescence. The cross sections (in units of 10 -14 cm 2 ) for Rb(7 2 D )-H 2 are σ fs *(7 2 D 5/2 → 7 2 D 3/2 ) = 0.16 ± 0.03, σ fs *'(7 2 D 3/2 → 7 2 D 5/2 ) = 0.24 ± 0.05, for Rb(7 2 D )-CO: σ fs * = 0.10 ± 0.06 and σ fs * ' = 0.15 ± 0.09. The cross sections σ tr * for transfer out of the 7 2 D doublet were also determined.
Cross sections of excitation transfer between the fine-structure sublevels of Cs(7 2 P), Rb(6 2 P) and Rb(7 2 P) induced by collisions with rare-gas atoms have been determined from the time dependence of the sensitised fluorescence. The Cs and Rb atoms, contained in absorption cells filled with inert gases at different pressures, were excited using a pulsed dye laser pumped by a nitrogen laser. The fluorescence was observed time resolved employing a photon counting technique. The use of a two-level approximation allowed the Maxwellian (velocity) averaged transfer cross sections to be calculated. Comparison of the cross section values determined in this work with the values quoted by other authors shows good consistency in the case of Rb 6 2 P inert-gas collisions while in the case of Cs 7 2 P inert-gas collisions there are some discrepancies. Rb 7 2 P excitation transfer has not been dealt with before.
Applying the CW laser absorption and fluorescence method,the cross sections for the fine structure mixing and quenching of the Rb(5P)state,induced by collision with N2 molecules,have been measured.Cesium atoms were optically excited to the 5P3/2 state.The excited atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single mode laser beam,turned to the 5PJ→7S1/2 transitions.The effective radiative rates were calculated for the 5PJ→5S transitions.The fluorescence intensity I794 of the sensitized 5P1/2→5S1/2 emission was measured as a function of N2 density in the range 0.5×1016N4×1016cm-3 at constant temperature T=340 K.It was found that the quantity N/I794 exhibited a parabolic dependence on N,confirming that the quenching of the 5PJ states is due to collision with N2 molecules,not Rb ground state atoms.The coefficients of the second-order polynomial fitted through the measured data yielded the cross sections σ3/2→1/2=(10.43±3.54)×10-16cm2 and σD=(9.8±3.4)×10-16cm2 for the 5PJ fine-structure mixing and quenching,respectively,due to collision with N2 atoms.Our values for these cross sections are in agreement,within combined error bars,with values we have recently obtained under different experimental conditions.
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.