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Acta Physica Sinica

NEAR RESONANT SCATTERING OF COLLISION PERTURBED ATOMS WITH DOUBLET EXCITED STATES

Yixiang Zhang, HAN QUAN-SHENG, LUO ZHENG-JI, CHENG WEN-QIN, 中国科学院物理研究所

Published Jan 1, 1981• 0 citations
Publisher

Abstract

We investigated the near resonant scattering and collision induced fluorescence of sodium atoms with doublet excited states in a weak laser field. By changing the frequency of the incident light and measuring the integral intensities of the components of the scattered light, the near resonant excitation cross-section of each sublevel was obtained as functions of the frequency detuning. Their variations are qualitatively explained by the interaction potential between the sodium atom and various collisional perturbing species. We have measured the variation of the induced D1 and D2 fluorescence with laser power, density of sodium atoms and pressure of buffer gas. It is shown that the relation is linear. The experimental results are in agreement with the theory.

DOI
10.7498/aps.30.962

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We investigated the near resonant scattering and collision induced fluorescence of sodium atoms with doublet excited states in a weak laser field. By changing the frequency of the incident light and measuring the integral intensities of the components of the scattered light, the near resonant excitation cross-section of each sublevel was obtained as functions of the frequency detuning. Their variations are qualitatively explained by the interaction potential between the sodium atom and various collisional perturbing species. We have measured the variation of the induced D1 and D2 fluorescence with laser power, density of sodium atoms and pressure of buffer gas. It is shown that the relation is linear. The experimental results are in agreement with the theory.
We report observations of inelastic collisions of atoms excited by a weak monochromatic laser field whose frequency is close to the resonance frequency of the transition from the ground state to an excited state. Our experiments use sodium atoms in a gas cell with the laser frequency detuning from resonance ($|\ensuremath{\Delta}|<6$ GHz) much smaller than the inverse of a collision duration. In this frequency region, our results may be interpreted in terms of inelastic collisions involving virtually excited atoms. The inelastic collision rate is determined by measuring the ratio of fluorescence from the collisionally populated level to emission from the laser populated level. By studying this emission ratio as a function of perturber pressure we show that the inelastic collision rate constant for virtually excited atoms is the same as the thermally averaged rate constant for level-to-level population transfer. Measurements of the emission ratio as a function of the laser detuning from resonance explicitly show how the correlation of collision rates with excitation frequency due to the Doppler effect is lost as the laser is tuned more than several Doppler widths from line center. The collisions of virtually excited atoms fundamentally limit experiments utilizing the Doppler effect to obtain velocity-dependent excited-state collision rates.
The intensity ratio D2/D1 for the sodium doublet has been measured for radiation from excited sodium atoms which result from the optical dissociation of sodium iodide vapor by ultraviolet light in the 2082→2500 A region. With shorter wavelength ultraviolet light as exciting source, the D2/D1 ratio is 0.98 and increases to above 1.5 with longer exciting wavelengths. Addition of foreign gases cause the observed D2/D1 ratio to increase to 1.89 for large foreign gas pressure. Cross sections for the transfer of excited sodium atoms from the 32P32 to the 32P½ state and vice versa due to collisions with foreign molecules have been measured as they depend on the relative velocity between excited sodium atom and foreign molecule. The Doppler broadening of the separate D lines emitted in the atomic fluorescence of sodium iodide has been estimated from absorption measurements. Both lines show equal broadening within the experimental error of the observations. Measurements were made with a photomultiplier tube. The sodium doublet was resolved with a reflection grating monochromator.
P^2P3/2 m ^-x^n8 *n sodium vapour, induced by inelastic collisions with the diatomic molecules HD, and N^, was studied using techniques of sensitized fluorescence.The mixtures of sodium vapour with the various molecular gases were irradiated with one component of the appropriate resonance doublet.The fluorescence which contained both components of the doublet, was monitored at right angles to the direction of excitation.The measurement of the relative intensities of the fluorescent components 2 yielded the total inelastic cross sections for P mixing, Ql2(2Pi/2---> 2p3/2^ and ^21^2pi/2^---' 2p3/2^* The sodium vaPour *n 5. Cross Sections for 3 P. ---3 P. Mixing Induced by Sodium Molecule Collisions.38 6. Cross Sections for Excitation Transfer in Na-Molecule Collisions, together with the Corresponding Relative Velocity of the Colliding Partners 42 2 2 7. Comparison of the Cross Sections for P. ---^-s/o Mixing Induced by Alkali Atom-Molecule Collisions 43 vi
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.
Redistribution and depolarisation of light scattered by atoms undergoing collisions outside the impact region of the line profile has been studied experimentally and theoretically. In a cell experiment intensity and linear degree of polarisation of the fluorescence components 32S1/2 to or from 32P1/2,3/2 of Na atoms induced by collisions with Ar buffer gas atoms were measured as functions of the laser frequency omega L. The decrease of polarisation of the D2 component observed when omega L is shifted away from the D2 toward D1 resonance disagrees with calculations on the basis of the factorisation approximation of the theory of redistribution. Using Baylis (1975) potentials for the Na/Ar system, the effect is interpreted in terms of adiabatic coupling of the 32P3/2 electronic angular momentum to the collision axis during 'strong' collisions, giving rise to a greater than average depolarisation.
We observe the production of Na2+ and Na+ arising from single collisions between crossed beams of sodium atoms when a laser field is tuned near the Na(3p 2P3/2) and Na(3p 2P1/2) transitions. Measurements of ion intensity vs laser intensity show that at moderately high power true laser-induced processes dominate over purely collisional effects. Relative intensity of mass-selected ions produced at either member of the Na resonance doublet shows conclusively that Na+ does not arise simply from photodissociation of Na2+ but must result from a direct, laser-induced collisional ionization.