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Physical Review A

Study of collisional redistribution using two-photon absorption with a nearly-resonant intermediate state

P. F. Liao, John E. Bjorkholm, P. R. Berman

Published Oct 1, 1979• 35 citations
Publisher

Abstract

The authors report studies of collision-induced features of the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{3}{2}}$ two-photon absorption in atomic-sodium vapor undergoing collisions with several rare-gas perturbers. The results yield the collisional redistribution function which describes the nonresonant excitation of the $3{P}_{\frac{1}{2}}$ intermediate state. Good agreement with theory is obtained only if effects of $3{P}_{\frac{1}{2}}\ensuremath{\leftrightarrow}3{P}_{\frac{3}{2}}$ state-changing collisions are included in the calculations. Results are used to obtain a value for a pressure-broadening coefficient in the Na-Kr system.

DOI
10.1103/physreva.20.1489

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Study of collisional redistribution using two-photon absorption with a nearly resonant intermediate state

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We report studies of collision-induced features of the 3S(1/2) yields 3P(1/2) yields 4D(3/2) two-photon absorption in atomic sodium vapor undergoing collisionss with several rare gas perturbers. The results yield the collisional redistribution function which describes the nonresonant excitation of the 3P(1/2) intermediate state. Good agreement with theory is obtained only if effects of 3P(1/2) in equilibrium with 3P(3/2) state-changing collisions are included in the calculations. Results are used to obtain a value for a pressure broadening coefficient in the Na-Kr system. (Author)

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We report the results of an experimental study of the effects of velocity-changing collisions on two-photon and stepwise-absorption line shapes. Excitation spectra for the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{1}{2}}$ transitions of sodium atoms undergoing collisions with foreign gas perturbers are obtained. These spectra are obtained with two cw dye lasers. One laser, the pump laser, is tuned 1.6 GHz below the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ transition frequency and excites a nonthermal longitudinal velocity distribution of excited $3{P}_{\frac{1}{2}}$ atoms in the vapor. Absorption of the second (probe) laser is used to monitor the steady-state excited-state distribution which is a result of collisions with rare gas atoms. The spectra are obtained for various pressures of He, Ne, and Kr gases and are fit to a theoretical model which utilizes either the phenomenological Keilson-St\"orer or the classical hardsphere collision kernel. The theoretical model includes the effects of collisionally aided excitation of the $3{P}_{\frac{1}{2}}$ state as well as effects due to fine-structure state-changing collisions. Although both kernels are found to predict line shapes which are in reasonable agreement with the experimental results, the hard-sphere kernel is found superior as it gives a better description of the effects of large-angle scattering for heavy perturbers. Neither kernel provides a fully adequate description over the entire line profile. The experimental data is used to extract effective hard-sphere collision cross sections for collisions between sodium $3{P}_{\frac{1}{2}}$ atoms and helium, neon, and krypton perturbers.

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Our recent treatment [Phys. Rev. A 18, 1066 (1978)] of the broadening and shift of alkali-metal-atom two-photon $S\ensuremath{-}S$ transitions is applied to the Na $3S\ensuremath{-}\mathrm{nS}$ transitions ($n=6\ensuremath{-}9$) perturbed by Ar, Kr, and Xe rare gases. Comparison with recent experimental data [A. Flusberg et al., Phys. Rev. A 19, 1607 (1979)] on the Na-Ar pair for $n\ensuremath{\le}7$ shows good agreement. However, for transitions to $n>8$ the calculated pressure effects no longer agree well with experimental data; presumably the pressure effects cannot be adequately described by a simple two-body long-range impact theory.

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Journal of the Physical Society of Japan•Jul 15, 1987•1 citation
The authors report studies of collision-induced features of the 3S12\ensuremath→3P12\ensuremath→4D323{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{3}{2}}3S21​​\ensuremath→3P21​​\ensuremath→4D23​​ two-photon absorption in atomic-sodium vapor undergoing collisions with several rare-gas perturbers. The results yield the collisional redistribution function which describes the nonresonant excitation of the 3P123{P}_{\frac{1}{2}}3P21​​ intermediate state. Good agreement with theory is obtained only if effects of 3P12\ensuremath↔3P323{P}_{\frac{1}{2}}\ensuremath{\leftrightarrow}3{P}_{\frac{3}{2}}3P21​​\ensuremath↔3P23​​ state-changing collisions are included in the calculations. Results are used to obtain a value for a pressure-broadening coefficient in the Na-Kr system.
We report studies of collision-induced features of the 3S(1/2) yields 3P(1/2) yields 4D(3/2) two-photon absorption in atomic sodium vapor undergoing collisionss with several rare gas perturbers. The results yield the collisional redistribution function which describes the nonresonant excitation of the 3P(1/2) intermediate state. Good agreement with theory is obtained only if effects of 3P(1/2) in equilibrium with 3P(3/2) state-changing collisions are included in the calculations. Results are used to obtain a value for a pressure broadening coefficient in the Na-Kr system. (Author)
We report the results of an experimental study of the effects of velocity-changing collisions on two-photon and stepwise-absorption line shapes. Excitation spectra for the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{1}{2}}$ transitions of sodium atoms undergoing collisions with foreign gas perturbers are obtained. These spectra are obtained with two cw dye lasers. One laser, the pump laser, is tuned 1.6 GHz below the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ transition frequency and excites a nonthermal longitudinal velocity distribution of excited 3P123{P}_{\frac{1}{2}}3P21​​ atoms in the vapor. Absorption of the second (probe) laser is used to monitor the steady-state excited-state distribution which is a result of collisions with rare gas atoms. The spectra are obtained for various pressures of He, Ne, and Kr gases and are fit to a theoretical model which utilizes either the phenomenological Keilson-St\"orer or the classical hardsphere collision kernel. The theoretical model includes the effects of collisionally aided excitation of the 3P123{P}_{\frac{1}{2}}3P21​​ state as well as effects due to fine-structure state-changing collisions. Although both kernels are found to predict line shapes which are in reasonable agreement with the experimental results, the hard-sphere kernel is found superior as it gives a better description of the effects of large-angle scattering for heavy perturbers. Neither kernel provides a fully adequate description over the entire line profile. The experimental data is used to extract effective hard-sphere collision cross sections for collisions between sodium 3P123{P}_{\frac{1}{2}}3P21​​ atoms and helium, neon, and krypton perturbers.
Our recent treatment [Phys. Rev. A 18, 1066 (1978)] of the broadening and shift of alkali-metal-atom two-photon $S\ensuremath{-}S$ transitions is applied to the Na $3S\ensuremath{-}\mathrm{nS}$ transitions ($n=6\ensuremath{-}9$) perturbed by Ar, Kr, and Xe rare gases. Comparison with recent experimental data [A. Flusberg et al., Phys. Rev. A 19, 1607 (1979)] on the Na-Ar pair for $n\ensuremath{\le}7$ shows good agreement. However, for transitions to n>8n>8n>8 the calculated pressure effects no longer agree well with experimental data; presumably the pressure effects cannot be adequately described by a simple two-body long-range impact theory.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
In recent years, the collisional redistribution of radiation and collision-induced broadening of Rydberg atomic spectral lines by buffer gas perturbation have aroused the renewed interest. Rydberg atoms having a large dipole moment and long lifetime can interact with each other coherently for relatively long time, which makes them a potential candidate for quantum information processing. Besides, collisional redistribution has an important potential application in laser cooling and trapping. Based on previous experimental data, in this paper, two-nondegenerate four-wave mixing (NFWM) for studying atom collision, composed of two-photon resonant NFWM and collisional redistribution NFWM, is reported. The spectrum variation of the two-NFWM affected by the pressure, temperature, detuning and collision-broadening rate coefficient is analyzed. The principle of two-NFWM involving three incident beams is explained as follows. Consider two-NFWM in a |0-|1-|2 cascade three-level system, where states between |0 and |1 and between |1 and |2 ightangle are coupled by resonant frequencies 1 and 2 , respectively. Beam 1 with frequency 1 propagates along the direction opposite to the direction of beam 2, beams 2 and 2' have the same frequency 2, and between their directions there exists a small angle. Assuming that 1 1 and 2 2 so that 1 drives the transition from |0 to |1 while 2 drives the transition from |1 to |2, the simultaneous interactions of atoms with beams 1 and 2 will induce atomic coherence between |0 and |2 through two-photon excitation. This coherence is probed by beam 2', and as a result a two-photon resonant NFWM signal of frequency 1 is generated in the direction almost opposite to the direction of beam 2'. To avoid strong absorption at the resonant frequency of transition from |0 to |1, here the wavelength of beam1 is detuned from the exact resonance. An atom population of level |1 caused by collisional redistribution can be induced when a certain buffer gas pressure is imposed. The collisional redistribution NFWM process also exists in this case. Beam 2 drives the transition from |1 to |2 to induce an atomic coherence which is probed by beam 2' for giving rise to an atomic population grating. A collisional redistribution NFWM signal propagating along the same direction as the two-photon resonant NFWM signal is generated when beam 1 is scattered by the grating. Much information about atomic collisions can be obtained by analyzing the two NFWM signals. In a cascade three-level system composed of ground state, intermediate state and Rydberg state, and the two-NFWM can be used to investigate not only the broadening and shifting of the Rydberg level but also the collisional redistribution of the intermediate state. Unlike other experiments studying the pressure dependence of the longitudinal relaxation rate of atom states, this technique is a purely optical coherent means, and can measure the transverse relaxation rate 20 between Rydberg state and ground state as well as the pressure dependence of the transverse relaxation rate 21 between Rydberg state and intermediate state.
The relation between the spectral character of a completely off-resonant two-photon absorption and the non-impact collision dynamics of a three-level atomic system is studied. A collisionally induced resonance character in the spectral dependence of the absorption probability is shown theoretically. Measured results about the dependence of fluorescence intensity on the incident light frequencies are ingood agreement with the theory. The interatomic potential between the sodium 4D state and argon is also estimated from experimental results.