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Journal of Physics B Atomic Molecular and Optical Physics

Collisional excitation transfer between Rb(5P) states in 50–3000 Torr of4He

J. F. Sell, M. A. Gearba, Brian M. Patterson, D Byrne, Gregory Jemo, Taylor Lilly, Ryan Meeter, R. J. Knize

Published Feb 21, 2012• 14 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/45/5/055202

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Shinjang dashösi ilmiy jurnili•Jan 1, 2010•0 citations
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.
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.
Employing ultrafast laser excitation and time-correlated single-photon counting, we have measured the fine-structure transfer between Rb 5P states induced by collisions with 4He buffer gas at temperatures up to 150 °C. The temperature dependence of the binary cross section agrees with earlier measurements. Our data show that the temperature dependence of the three-body rate is about the same as that of the binary rate. The three-body rate can be described as arising from the reduction of the rubidium fine-structure splitting due to nearby helium atoms.
Using ultrafast laser excitation and time-correlated single-photon counting techniques, we have measured the collisional mixing rates between the rubidium 5(2)P fine-structure levels in the presence of (4)He gas. A nonlinear dependence of the mixing rate with (4)He density is observed. We find Rb fine-structure transfer is primarily due to binary collisions at (4)He densities of < or = 10(19) cm(-3), while at greater densities, three-body collisions become significant. We determine a three-body collisional transfer rate coefficient (5(2)P(3/2) --> 5(2)P(1/2)) of 1.25(9)x10(-32) cm(6)/s at 22 degrees C.
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.
Cross sections for inelastic transfer between the 82D3/2 and 82D5/2 fine-structure states in rubidium, induced in resonant collisions with ground-state Rb atoms, have been determined using an experimental method involving two-photon excitation of atomic fluorescence. Rubidium vapor in a fluorescence cell was irradiated with pulses of 641 nm radiation from a N2 laser-pumped dye-laser tuned to excite one of the 82D states. The resulting fluorescence included the direct component originating from the optically excited state and a sensitized component arising from the other fine-structure state populated by collisions. Relative intensities of the fluorescent components, determined by photon-counting techniques, yielded the cross sections for excitation transfer: Q(2D3/2 → 2D5/2) = 8.1 × 10−13 cm2; Q(2D3/2 ← 2D5/2) = 5.5 × 1013 cm2; as well as [Formula: see text], the effective quenching cross section. The excitation transfer cross sections which are considered accurate to within ±20% are in the ratio predicted by the principle of detailed balancing.
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.