[Reactive and nonreactive energy transfer in Cs(6D(J))+ (H2, He) collisions].

Cs vapor, mixed with a gas was irradiated in a glass fluorescence cell with pulses of 886nm radiation from a YAG-laser-pumped OPO laser, populating 6D3/2 state by two-photon absorption. Cross sections for 6D3/2 --> 6D5/2 transition induced by collisions with various H(e) atoms and H2 molecules were determined using methods of atomic fluorescence. The resulting fluorescence included a direct component emitted in the decay of the optically excited state and a sensitized component arising from the collisionally populated state. At the different densities, we have measured the relative time-integrated intensities of the components and fitted a three-state rate equation model to obtain the cross sections for 6D3/2 --> 6D5/2 transfer: sigma = (55 +/- 13) x 10(-16) and (16 +/- 4) x 10(-16) cm2 for H2 and H(e), respectively. The cross sections for the effective quenching of the 6D5/2 state were also determined. The total transfer rate coefficients from the 6D5/2 state for H(e) is small [1.2 x 10(-10) cm3 x s(-1)]. The total quenching rate coefficient of the 6D5/2 state is larger for H2 [6.7 x 10(-10) cm3 z s(-1)]. For H2 case, the quenching rate coefficient corresponds to reaction and nonreactive energy transfer. Evidence suggests that the nonreactive energy transfer rate coefficient is [6.3 x 10(-10) cm3 x s(-1)]. Hence the authors estimated the cross section (2.0 +/- 0.8) x 10(-16) cm2 for reactive process Cs(6D5/2) + H2 --> CsH + H. Using the dependence on the pressure of H2 (or H(e)) of the integrated fluorescence monitored at the 6D5/2 --> 6P3/2 transition the cross section (4.0 +/- 1.6) x 10(-16) cm2 for Cs (6D3/2) + H2 --> CsH + H was obtained. Thus, the relative reactivity with H2 follows an order of Cs (6D3/2) > Cs (6D5/2).

[Experimental evaluation of cross section for Rb(5Dj)+H2-->rbH+H reaction].

The Rb(5Dj )+H2-->RbH[X 1sigma+(v"==0)]+H photochemical reaction was studied in a cell experiment applying a laser pump-absorption technique. Using two-photon excitation of the Rb5 (2)D atomic level in a Rb-H2 vapor mixture, the resulting fluorescence includes a direct component arising from the optically excited state and a sensitized component due to the collisionally populated fine-structure state. The RbH molecules are formed in three-body reactive collisions between excited Rb5 (2)D atoms and ground state H2 molecules. Near-infrared absorption band RbH X (1)sigma+ (v"==0-->v'==17) near 852 nm by using a diode laser was measured. The absorbed intensity of laser beam through a length L of the RbH vapor is defined as deltaI' and deltaI" where deltaI' and deltaI" are the absorbed intensity of pumping 5D(3/2) and 5D(5/2) levels, respectively. The ratio of deltaI' to deltaI" contains information on reactivity. w5D(3/2) and W5D(3/2) are the production rates of Rb in the 5D(5/2) and 5D(3/2) levels by direct laser excitation from the 5S(1/2) level. Using a second experiment in which pump laser is used to pump the 5D(3/2) and 5D(5/2) states in a pure Rb vapor (T = 290 K), and the i'/i" where i' and i" are measured intensities of the 5D(3/2)-->5P(1/2) and 5D(5/2)-->5P(3/2) transition, respectively, is determined. At low density of Rb atoms, the 5D mixing rate is neglect. The rate of 5D(3/2) and 5D(5/2) fluorescence yields the ratio of 5D(3/2) to 5D(5/2) pump production rate. The rate equations were solved, and the authors estimate the value of the cross section at T=385 K and P(H2) =400 Pa for collisional energy transfer from Rb5D(3/2) to 5D(5/2), from Rb(5D)to Rb states other than Rb(5D)to be 9.8 x 10(-16) cm2 and 2.0 x 10(-16) cm2, respectively. The reaction cross sections [i.e., Rb(5Dj)+H2-->RbH+H] for j being 3/2 and 5/2 are 5.4 x 10(-7) and 2.3 x 10(-17) cm2, respectively. The relative reactivity with H2 for two studied atoms is in an order of Rb(5D(3/2)>Rb(5D(5/2)), and this is consistent with the result obtained from a laser pump-probe technique.

[Collisional transfer of the Cs(8S) state and excitation of high lying atomic states].

Using selective stepwise excitation of the cesium 8S atomic level in Cs vapor, the collisional transfer and the population of the high-lying atomic states were studied in detail. At cesium densities of 10(16)-10(17) cm(-3), the rate coefficient of excitation collision [i. e., 8S+6S --> 6D+6S] was measured. Measurement of fluorescence intensities as a function of Cs density yields k6D = (2.4 +/- 0.5) x 10(-10) cm3 x s(-1). The 5D state was populated by the 8S --> 7P --> 5D transitions. The energy pooling collisions 5D+5D --> nL+6S (nL = 9D, 11S, 7F) were also studied. The rate coefficients were measured relative to the known rate coefficient of the collision [i. e. 6P+5D --> 6S+7D]. The densities of 6P state were combined with measured fluorescence ratios to determine rate coefficients for EP process. The average values (in unites of 10(-10) cm3 x s(-1)) for nL = 9D, 11S and 7F are 6.4 +/- 3.2, 1.0 +/- 0.5 and 8.4 +/- 4.2, respectively.

[Energy-pooling collisions of rubidium atoms: Rb (5P(J)) + Rb (5P(J))--> Rb (5S) + Rb (nl = 5D,7S)].

An experimental study of rubidium energy pooling collisions, Rb(5P(J)) + Rb(5P(J))-->Rb(nlJ') + Rb(5S), at thermal energies, was carried out in a cell. Atoms were excited to either the 5P 1/2 or 5P 3/2 state using a single-mode diode laser. The excited atom density and spatial distribution were mapped by monitoring the absorption of a counter-propagating single-mode diode laser beam, tuned to either 5P 1/2-->5D 3/2 or 5P 3/2-->7S 1/2 transition, which could be translated parallel to the pump beam. The excited atom densities were combined with the measured fluorescence ratios to determine cross sections for the rubidium energy pooling process. For 5P 3/2 excitation the cross sections for nlJ' being 5D 5/2, 5D 3/2, and 7S 1/2 are (1.32+/-0.59) x 10(-14), (1.18+/-0.53) x 10(-14) and (3. 21+/-1. 44) x 10(-15) cm2, respectively. For 5P 1/2 excitation the cross sections for nlJ' being 5D 5/2 and 5D 3/2 are (6.57+/-2.96) x 10(-15), and (5.90+/-2.66) x 10(-15) cm2, respectively. The results were compared with those of other experiments.

[Study of resonance exchange collisions in cesium vapour by optical-optical double resonance spectroscopy].

An experimental study of cesium resonance exchange collision, Cs(6P(3/2), v) + Cs(6S(1/2), v')-->Cs(6S(1/2), v) + Cs (6P(3/2), v'), was carried out. Populations of excited atoms that all have the same z component of velocity were produced by pumping a vapor with a narrow-band laser. A counterpropagating single-mode diode laser was used to probe the excited atom velocity distribution in the 6P(3/2)-->8S(1/2) transition. Fluorescence was monitored in the 8S(1/2)-->6P(3/2) transition. The magnitude of the thermalizinng effect of resonance exchange collisions was estimated by measuring the ratio of the intensity in the narrow features to that associated with the Doppler pedestals. The rate coefficient of 9.62 x 10(-7) cm3 x s(-1) for the resonance exchange collisions was yielded. This work demonstrates that, in a pure metal vapor, the thermalization of velocity-selected excited-atom distribution by the mechanism of resonance exchange can be three orders of magnitude greater than that from velocity-changing collisions.