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Journal of the Optical Society of America A

Measurements of velocity-changing collisions using two-photon spectroscopy with a resonant intermediate state (A)

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

Published Oct 1, 1978• 0 citations
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A theory of the influence of binary collisions on the line shapes associated with Doppler-free spectroscopy is presented. The specific calculation is for two-photon absorption through a real or virtual intermediate state, but the extension of the results to alternative level schemes is included. Collisions are treated quite generally at first, but, ultimately, a simple but reasonable collision model is adopted to properly account for both the "phase-interrupting" and "velocity-changing" aspects of collisions. It is shown that collisional processes can be used to distinguish between the "two-quantum" and "stepwise" contributions to the two-photon excitation rate. Moreover, it is demonstrated that systematic experimental line-shape investigations, in addition to providing tests of current collision theories, can lead to values for collision broadening and shift parameters, excitation transfer rates, magnetic substate relaxation rates, velocity-changing collision rates, and collision kernels. Consequently, Doppler-free spectroscopy can provide a new and important probe of collision effects in atomic and molecular systems.

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Distortion of on-resonance two-photon spectroscopic line shapes caused by velocity-selective optical pumping

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We have observed that spectral lines obtained in sodium vapor can be strongly distroted when using two-photon spectroscopy with a resonant intermediate state. In some cases, the spectral lines appear to be split into two components. We demonstrate that these effects are due to velocity-selective optical pumping of the atomic ground state; the line shapes reflect the velocity distribution of atoms in the excited intermediate state. We have analyzed the effects using the time-dependent solutions of the rate equations which describe the time evolution of optical pumping for the various atomic velocity groups, and we obtain good agreement with experiment. Optical-pumping problems are often analyzed with the use of steady-state rate equations which incorporate phenomenological relaxation; such approaches are not capable of describing our observations. Velocity-changing collisions are shown to suppress the effects and they explain why these effects were not previously observed; the previous experiments were carried out with slight amounts of a buffer gas in the vapor cell. The effects reported here must be accounted for when carrying out laser-induced line-narrowing experiments in atoms or molecules that can be optically pumped; they can also be used as a new and more sensitive probe of optical pumping in vapors.

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Interaction of Radiation with Matter: Atomic Collision Processes Occurring in the Presence of Radiation Fields

P. R. Berman

Abstract : Work is summarized in areas of: Laser Assisted Collisions; Velocity Changing collisions in Atomic Vapors; Coherent Transients; Collision Induced Extra Resonances; Breakdown of the Optical Bloch Equations; Saturation Spectroscopy; Dynamics of Two-and Three-level Atoms Interacting with Radiation Fields; Pulse Propagation. Keywords: Laser spectroscopy, Optical collisions, Radiative collisions, Two-level atom, Atomic coherence, Degenerate four-wave mixing, Saturation spectroscopy, Laser propagation,, Dressed atom, Photon echo, Bloch equations, Collision kernal, Collisions, Optical noise.

Annual Report•Sep 15, 1988•0 citations
A theory of the influence of binary collisions on the line shapes associated with Doppler-free spectroscopy is presented. The specific calculation is for two-photon absorption through a real or virtual intermediate state, but the extension of the results to alternative level schemes is included. Collisions are treated quite generally at first, but, ultimately, a simple but reasonable collision model is adopted to properly account for both the "phase-interrupting" and "velocity-changing" aspects of collisions. It is shown that collisional processes can be used to distinguish between the "two-quantum" and "stepwise" contributions to the two-photon excitation rate. Moreover, it is demonstrated that systematic experimental line-shape investigations, in addition to providing tests of current collision theories, can lead to values for collision broadening and shift parameters, excitation transfer rates, magnetic substate relaxation rates, velocity-changing collision rates, and collision kernels. Consequently, Doppler-free spectroscopy can provide a new and important probe of collision effects in atomic and molecular systems.
We have observed that spectral lines obtained in sodium vapor can be strongly distroted when using two-photon spectroscopy with a resonant intermediate state. In some cases, the spectral lines appear to be split into two components. We demonstrate that these effects are due to velocity-selective optical pumping of the atomic ground state; the line shapes reflect the velocity distribution of atoms in the excited intermediate state. We have analyzed the effects using the time-dependent solutions of the rate equations which describe the time evolution of optical pumping for the various atomic velocity groups, and we obtain good agreement with experiment. Optical-pumping problems are often analyzed with the use of steady-state rate equations which incorporate phenomenological relaxation; such approaches are not capable of describing our observations. Velocity-changing collisions are shown to suppress the effects and they explain why these effects were not previously observed; the previous experiments were carried out with slight amounts of a buffer gas in the vapor cell. The effects reported here must be accounted for when carrying out laser-induced line-narrowing experiments in atoms or molecules that can be optically pumped; they can also be used as a new and more sensitive probe of optical pumping in vapors.
Abstract : Work is summarized in areas of: Laser Assisted Collisions; Velocity Changing collisions in Atomic Vapors; Coherent Transients; Collision Induced Extra Resonances; Breakdown of the Optical Bloch Equations; Saturation Spectroscopy; Dynamics of Two-and Three-level Atoms Interacting with Radiation Fields; Pulse Propagation. Keywords: Laser spectroscopy, Optical collisions, Radiative collisions, Two-level atom, Atomic coherence, Degenerate four-wave mixing, Saturation spectroscopy, Laser propagation,, Dressed atom, Photon echo, Bloch equations, Collision kernal, Collisions, Optical noise.