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Zeitschrift für Naturforschung A

Depolarization of NaD2 Fluorescence under Pulsed Laser Excitation

V. Kroop, Wolfgang Behmenburg

Published Jul 1, 1976• 1 citation
Publisher

Abstract

Abstract The intensity and polarization state of the NaD2-fluorescence (3 2P3/2→3 2S1/2 , λ 5890 Å) from a cell containing pure Na vapor, selectively excited by a flash lamp pumped dye laser was investigated. Measurements were performed both for excitation with π-and σ-light of spectral density ranging between 10-23 and 10-17 Ws2 cm-3 and with Na number densities between 2.8 X109 and 4.0 X 1010 cm-3. The results are interpreted in terms of population of the 2P3/2 state and of hyperfine and Zeeman pumping of the 2S1/2 substates. The values for the degree of linear polarization measured at the smallest excitation densities and Na pressures agree well with theoretical values.

DOI
10.1515/zna-1976-0705

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Canadian Journal of Physics•Oct 1, 1968•23 citations
Abstract The intensity and polarization state of the NaD2-fluorescence (3 2P3/2→3 2S1/2 , λ 5890 Å) from a cell containing pure Na vapor, selectively excited by a flash lamp pumped dye laser was investigated. Measurements were performed both for excitation with π-and σ-light of spectral density ranging between 10-23 and 10-17 Ws2 cm-3 and with Na number densities between 2.8 X109 and 4.0 X 1010 cm-3. The results are interpreted in terms of population of the 2P3/2 state and of hyperfine and Zeeman pumping of the 2S1/2 substates. The values for the degree of linear polarization measured at the smallest excitation densities and Na pressures agree well with theoretical values.
The dependence of the circular polarization of the fluorescence of alkali atoms on the intensity and spectral composition of the σ+-polarized exciting D1 line has been investigated by considering the stationary solutions of rate equations for populations of the Zeeman sublevels. The calculations were performed for pumping-light intensities corresponding to those of classical light sources, e.g., radio frequency or microwave discharge lamps, and commonly accepted values of cross sections for collisional relaxation of ground-state and excited atoms. The systematic errors of cross sections for collisional disorientation of 2P1/2 alkali atoms due to the neglect of intensity and spectral composition of the exciting light are estimated.
Na atoms in the magnetic field have been selectively excited to each Zeeman sub-level of either the 3p2P1/2 or 3p 2P3/2 state by a tunable dye laser and the Zeeman spectra of the fluorescence from the collisional populated 3p 2P levels have been measured. From the intensities of the Zeeman lines, the selectivity in the collisional transitions between the Zeeman sub-levels of the 3p 2P1/2 and 3p 2P3/2 states has been directly specified. The time evolution of the σ-polarized fluorescence intensity of the Na(3p 2P1/2→ 3s 2S1/2) transition has been measured following the Na(3p 2P3/2â†� 3s 2S1/2) excitation by a σ-polarized short light pulse at H= 129 G. An amplitude modulation was found in the fluorescence from the collisional energy transferred state Na(3p 2P1/2). This is the first observation of quantum beats in collision-induced fluorescence. The frequency of the modulation coincides very well with that of the quantum beats of the resonance fluorescence Na(3p 2P3/2→ 3s 2S1/2), but the phases are opposite. These results are explained by assuming the conservation of coherence during collisional energy transfer.
An experimental investigation of the atomic-Na 3s 2^{2}2${\mathit{S}}_{1/2--}$5s 2^{2}2S1/2{\mathit{S}}_{1/2}S1/2​ two-color two-photon polarization spectrum has been made. Measurements of the linear polarization degree of the two-photon transition were made for one laser tuned within a \ifmmode\pm\else\textpm\fi{}100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range of the atomic-Na 3p 2^{2}2Pj{\mathit{P}}_{\mathit{j}}Pj​ levels, with the second adjusted to satisfy the two-photon resonance condition. The relatively short-pulse (\ensuremath{\sim}0.5 ns) lasers used eliminate effects on the spectrum due to atomic-Na hyperfine interactions and radiation trapping, while greatly reducing depolarization due to collisions with Ar gas. In the absence of collisions, the measured polarization spectrum is in excellent agreement with calculations. Measurements of the Ar pressure dependence of the spectrum revealed strong depolarization in the vicinity of resonant excitation through the 3p 2^{2}2P3/2{\mathit{P}}_{3/2}P3/2​ level. A collisional depolarization cross section of 200(30) \AA{} for this process was deduced from the data.
The authors have measured time-resolved fluorescence spectra of the Na D lines in an Ar-filled vapour cell, excited by a high-intensity, pulsed dye laser of narrow bandwidth, which was tuned near the D 1 line. The results were compared with a recent theory which also takes into account the polarisation of the laser and the Zeeman degeneracy of the atomic levels. In particular, they predicted and observed a splitting into two components of the D 2 line (whose upper level is populated by mixing collisions), which is a new effect. Because of experimental difficulties, however, they obtained at most a semiquantitative agreement between theory and experiment.
The sodium D2{\mathit{D}}_{2}D2​ transition is resonantly excited by \ensuremath{\pi}-polarized, narrow-band laser radiation. The line polarization of the resulting fluorescence detected normally to the plane containing the electric-field vector of the radiation and its propagation direction, as well as the atomic beam, is measured as a function of radiation intensity and tuning. The data are compared to calculations from a quantum-electrodynamic theory which includes all participating energy states in the hyperfine representation. The quantitative agreement is very good.
3 2P1/2 ↔ 3 2P3/2 excitation transfer in sodium, induced in inelastic collisions with ground-state N2, H2, HD, and D2 molecules, has been investigated in a series of sensitized fluorescence experiments. Mixtures of sodium vapor at a pressure of 5 × 10−7 Torr, and the gases, were irradiated with each NaD component in turn, and the fluorescence which contained both D components was monitored at right angles to the direction of the exciting beam. Measurements of the relative intensities of the NaD fluorescent components yielded the following collision cross sections for excitation transfer. For Na–N2 collisions: Q12(2P1/2 → P3/2) = 144 Å2, Q21(2P1,2 ← 2P3/2) = 76 Å2 for Na–H2 collisions: Q12 = 80 Å2, Q21 = 42 Å2. For Na–HD collisions: Q12 = 84 Å2, Q21 = 44 Å2. For Na–D2 collisions: Q12 = 98 Å2, Q21 = 52 Å2. The cross sections Q21 exhibit a slight resonance effect between the atomic and molecular rotational transitions.