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Journal of Physics B Atomic Molecular and Optical Physics

A study of collisional quenching and radiation-trapping kinetics for Rb(5p) in the presence of methane and ethane using time-resolved fluorescence

Nathan D. Zameroski, Wolfgang G. Rudolph, Gordon D. Hager, David A. Hostutler

Published Dec 4, 2009• 37 citations
Publisher

Abstract

An experimental study using time-resolved fluorescence techniques together with theoretical simulations has been conducted and used to determine the quenching cross-sections of rubidium–methane and rubidium–ethane. Radiation trapping was significant under many of the experimental conditions (temperatures 40–130 °C and pressures 50–700 Torr) and a detailed analysis of the interplay between radiation trapping and quenching kinetics was carried out. Modifications of the Holstein equation for radiation trapping were implemented to account for the quasi-2 level behaviour of the Rb atom for high buffer gas pressures, the absolute frequency-dependent absorption cross-section for Rb with variable buffer gas pressures which accounts for the hyperfine splitting of 87 Rb and modification of the trapping factors so that radiation trapping and quenching by an additive quenching gas could be treated simultaneously. Experimental results supported by theoretical simulations bound the quenching cross-sections (σ) of methane and ethane at 40 °C to be σ ⩽ 0.019 Å 2 and σ ⩽ 0.033 Å 2 , respectively. These values are nearly two orders of magnitude smaller than previously reported.

DOI
10.1088/0953-4075/42/24/245401

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Collisional excitation transfer and quenching in Rb(5P) -methane mixtures

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Collisional Energy Transfer for Rb_2 in Excited State by Rb Atoms

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At different Rb densities,Rb vapor was irradiated in a glass fluorescence cell with pulses of radiation from a pumped-laser,populating Rb 2 (B 1 Π u)state by single-photon absorption.Energy transfer in Rb 2 (B 1 Π u) + Rb(5S) collision is studied using methods of atomic and molecular fluorescence.The decay signal of time-resolved fluorescence from B 1 Π u → X 1 Σ + g transition was monitored.The effective lifetimes of the B 1 Π u state can be resolved.The plot of reciprocal of effective lifetimes of the B 1 Π u state against Rb densities yielded the slope that indicated the total cross section for deactivation and the intercept that provided the radiative lifetime of the state.The radiative lifetime 23.5ns for the B 1 Π u is obtained.The cross section for deactivation of the Rb 2 (B 1 Π u) molecules by collisions with Rb atoms is σ 3=(4.33 ± 0.14) × 10-15 cm 2.At the different Rb densities,the time-integrated intensities of the resulting Rb emission lines.I(5P 3/2 → 5S 1/2) and I(5P 1/2 → 5S 1/2) were measured.The absolute values for Rb 2 (B 1 Π u)+Rb→ Rb 2 (1 1 Σ + g) + Rb(5P J)collisional transfer cross sections have been obtained.The cross sections are σ 31=(2.86 ± 0.13) × 10-16 cm 2 (for transferring to 5P 1/2) and σ 32=(8.30 ± 0.38) × 10-16 cm 2 (for transferring to 5P 3/2),respectively.

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Radiative transport and collisional transfer of excitation energy in Rb(5P)atoms mixed with N_2

Shen Yi

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.

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[Experimental investigation of mechanisms of forming RbH by irradiating a Rb+H2 mixture with laser light].

Xiaoyan Shen, Jing Liu, Kang Dai +1 more

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.

•Nov 1, 2008•0 citations
An experimental study using time-resolved fluorescence techniques together with theoretical simulations has been conducted and used to determine the quenching cross-sections of rubidium–methane and rubidium–ethane. Radiation trapping was significant under many of the experimental conditions (temperatures 40–130 °C and pressures 50–700 Torr) and a detailed analysis of the interplay between radiation trapping and quenching kinetics was carried out. Modifications of the Holstein equation for radiation trapping were implemented to account for the quasi-2 level behaviour of the Rb atom for high buffer gas pressures, the absolute frequency-dependent absorption cross-section for Rb with variable buffer gas pressures which accounts for the hyperfine splitting of 87 Rb and modification of the trapping factors so that radiation trapping and quenching by an additive quenching gas could be treated simultaneously. Experimental results supported by theoretical simulations bound the quenching cross-sections (σ) of methane and ethane at 40 °C to be σ ⩽ 0.019 Å 2 and σ ⩽ 0.033 Å 2 , respectively. These values are nearly two orders of magnitude smaller than previously reported.
Optically pumped alkali vapor lasers have been constructed using excitation of the D2 line (2P3/2-2S1/2) followed by lasing on the D1 line (2P1/2-2S1/2). Collisional relaxation is used to transfer population from 2P3/2 to the 2P1/2 level. The collision partner used for this step must have a large cross section for inducing transfer between the 2PJ levels, combined with a very small cross section for electronic quenching of the form M(2PJ)+Q→M(2S1/2)+Q (where M is an alkali metal and Q is the energy transfer agent). Ethane has proved to be an effective energy transfer agent for optically pumped Rb and Cs lasers. However, modeling of data for the Rb/C2H6 laser with the literature value for the quenching rate constant was unsuccessful. We have reexamined the quenching of Rb(2PJ) by C2H6 using time-resolved fluorescence techniques. Radiation trapping was significant under the conditions of our measurements, and an analysis of the interplay between the kinetics of trapping and quenching was carried out. It was found that quenching of Rb(2PJ) by C2H6 was very inefficient. The upper bound established for the quenching cross section was two orders of magnitude lower than that indicated by the previous determination.
We have examined fine-structure mixing between the rubidium 52P3/25\phantom{\rule{0.16em}{0ex}}^{2}P_{3/2}52P3/2​ and 52P1/25\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}52P1/2​ states along with quenching of these states due to collisions with methane gas. Measurements are carried out using ultrafast laser-pulse excitation to populate one of the Rb 52P5{\phantom{\rule{0.16em}{0ex}}}^{2}P52P states, with the fluorescence produced through collisional excitation transfer observed using time-correlated single-photon counting. Fine-structure mixing rates and quenching rates are determined by the time dependence of this fluorescence. As Rb(52P\mathrm{Rb}(5\phantom{\rule{0.16em}{0ex}}^{2}PRb(52P) collisional excitation transfer is relatively fast in methane gas, measurements were performed at methane pressures of 2.5−−252.5--252.5−−25 Torr, resulting in a collisional transfer cross section ($5\phantom{\rule{0.16em}{0ex}}^{2}P_{3/2}\ensuremath{\rightarrow}5\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}$) of $(4.23\ifmmode\pm\else\textpm\fi{}0.13)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$. Quenching rates were found to be much slower and were performed over methane pressures of 50−−400050--400050−−4000 Torr, resulting in a quenching cross section of $(7.52\ifmmode\pm\else\textpm\fi{}0.10)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$. These results represent a significant increase in precision compared to previous work, and also resolve a discrepancy in previous quenching measurements.
Measurements of the mixing rates and cross sections for collisional excitation transfer between the 5P 1/2 and 5P 3/2 states of rubidium (Rb) in the presence of 4 He buffer gas are presented. Selected pulses from a high repetition rate, mode-locked femtosecond laser are used to excite either Rb state with the fluorescence due to collisional excitation transfer observed by time-correlated single-photon counting. The time dependence of this fluorescence is fitted to the solution of rate equations which include the mixing rate, atomic lifetimes and any quenching processes. The variation in the mixing rate over a large range of buffer gas densities allows the determination of both the binary collisional transfer cross section and a three-body collisional transfer rate. We do not observe any collisional quenching effects at 4 He pressures up to 6 atm and discuss in detail other systematic effects considered in the experiment.
At different Rb densities,Rb vapor was irradiated in a glass fluorescence cell with pulses of radiation from a pumped-laser,populating Rb 2 (B 1 Π u)state by single-photon absorption.Energy transfer in Rb 2 (B 1 Π u) + Rb(5S) collision is studied using methods of atomic and molecular fluorescence.The decay signal of time-resolved fluorescence from B 1 Π u → X 1 Σ + g transition was monitored.The effective lifetimes of the B 1 Π u state can be resolved.The plot of reciprocal of effective lifetimes of the B 1 Π u state against Rb densities yielded the slope that indicated the total cross section for deactivation and the intercept that provided the radiative lifetime of the state.The radiative lifetime 23.5ns for the B 1 Π u is obtained.The cross section for deactivation of the Rb 2 (B 1 Π u) molecules by collisions with Rb atoms is σ 3=(4.33 ± 0.14) × 10-15 cm 2.At the different Rb densities,the time-integrated intensities of the resulting Rb emission lines.I(5P 3/2 → 5S 1/2) and I(5P 1/2 → 5S 1/2) were measured.The absolute values for Rb 2 (B 1 Π u)+Rb→ Rb 2 (1 1 Σ + g) + Rb(5P J)collisional transfer cross sections have been obtained.The cross sections are σ 31=(2.86 ± 0.13) × 10-16 cm 2 (for transferring to 5P 1/2) and σ 32=(8.30 ± 0.38) × 10-16 cm 2 (for transferring to 5P 3/2),respectively.
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.
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.