The collisional energy transfer process Rb(5PJ)+M, where M= He, N2, under gas cell conditions has been investigated. The Rb(5P3/2) state was excited by a diode laser. The direct 5P3/2-->5S1/2 fluorescence and the sensitized 5P1/2-->5S1/2 fluorescence as a function of quenching gas pressure were measured. The 5P3/2 and 5P1/2 states are separated by 238 cm(-1). The closest other states are >6500 cm(-1) or >28 KT. Neglect of these other states should be an excellent approximation. In the experiment the Rb density was 4.5 x 10(11) cm(-3). Using radiation trapping theory the effective radiative rate=Gammae5P3/2-->5S1/2 2.47 x 10(7) s(-1) was obtained. For quenching by He only electronic to translational energy transfer is possible. However, in the N2 case, electronic to vibrational or rotational transfer is important. The Rb(5P3/2) state is 13 cm(-1) lower than the N2[X'Sigmag+ (V= 5, J= 11)]state; this energy gap is near resonant. The authors have not attempted to directly observe this possible E-R quenching channel Using a two-state rate equation model the transfer rate coefficients from Rb(5PJ) were obtained. The rate coefficient (k21He) for 5P3/2-->5P1/2 transfer in collision with He is 2.61 x 10(-12) cm3 x s(-1). By comparing the direct and sensitized fluorescence intensities for He and N2 case, and fitting the experimental results to the rate equation analysis, the authors estimate that the rate coefficient (k21N2) for 5P3/2-5P1/2 transfer in collision with N2 is 2.36 x 10(-11) cm3 x s(-1). The E-V quenching rate coefficient (kN2) of the 5PJ state is 1.44 x 10(-10) cm3 x s(-1). The authors found find that the rate coefficient kN2 is about 6 times larger than the k21N2. The assumption that the Cs-N2 energy transfer occurs primarily in collinear collision geometry is supported. The results are discussed in relation to those of other experiments.
The radiation of a laser photoexcited Rb atoms from the ground state to the 5P3/2 level in a mixture of Rb vapor and hydrogen. The energy-pooling collision 5P3/2 + 5P3/2 --> 5S1/2 + 5D producted 5D state. The Rb (5P3/2) density and spatial distribution were mapped by monitoring the absorption of a counter-propagating laser beam, tuned to the 5P3/2 --> 7S1/2 transition, which could be translated parallel to the pump beam. In the presence of radiation trapping, the spontaneous radiation rate is multiplied by the transmission factor T5P3/2 --> 5S1/2, which describes the average probability that photons emitted within the fluorescence detection region can pass through the optically thick vapor without being absorbed. The T5P3/2 --> 5S1/2 is related to the frequency dependent absorption cross section and the density and spatial distribution of atoms in the level of the transition. The effective radiative rates of the Rb D2 line as a function of the H2 pressure were obtained. These quantities were combined with the measured excited atom density and fluorescence ratio to yield absolute energy-pooling rate coefficient. The quenching collision Rb (5P3/2) + H2 (v = 0) --> Rb(5S) + H2 (v = 2) producted state H2 (v= 2). This process is at least 16 times faster than the Rb (5P3/2) radiative decay rate. The reverse process of this process is relatively unlikely due to their large translational energy defect. The cross section for the process H2 (v = 2) + H2 (v = 0) --> H2 (vn = 1) + H2 (v = 0) + 3 920.2 cm(-1) is 7.7 x 10(-19) cm2. Hence the relaxation rate of this vibrational level is relatively slow and the nuclear spin statistics is conserved. The H2 (v = 2) density was determined by using the cross section for Rb (5P3/2)-H2 quenching. RbH was fromed by the Rb(5D) + H2 and Rb (5P3/2) + H2 (v = 2) reactions and observed by laser absorption. The ratio of 5D --> 5P3/2 to 5P3/2 --> 5S1/2 fluorescence was measured as a function of the H2 density. The absorption of the laser beam tuned to the X1sigma+ --> A1sigma+ line of RbH was also measured as a function of the H2 density. From these measurements we obtained the cross section of 4.02 x 10(-7) cm2 for the process Rb (5D) + H2 --> RbH + H. The cross section for Rb (5P3/2) + H2 (upsilon = 2) --> RbH + H is 1.00 x 10(-8) cm2. This experiment showed that under our experimental conditions, RbH molecules are not created by a direct interaction of Rb(5P) with H2 but through a two-step reaction.
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.