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The Journal of Chemical Physics

Hyperfine and Zeeman quantum beats of the Na(3 2P3/2–3 2S1/2) transition, and the decay of coherence

Masaaki Baba, Kazumasa Namba, Hajime Katô

Published May 15, 1989• 0 citations
Publisher

Abstract

Quantum beats of the Na(3 2P3/2–3 2S1/2) transition and their change with the magnetic field were observed in pure Na vapor and Na+He gas by using a picosecond laser system. The energies of the hyperfine and Zeeman sublevels of the 3 2P3/2 and 3 2S1/2 states were calculated by using the reported hyperfine constants and g factors, and the theoretical curve of the quantum beats was calculated. The results were in good agreement with the observed profile of the quantum beats in phase of modulation, but the difference of the amplitudes increased with time. By comparing the amplitude of the theoretical curve of the quantum beats with that of the experimental curve, we determined the relaxation time of coherence. The decay of amplitude of the modulation was found to become faster as the pressure of the foreign gas (He) was increased. The decay of amplitude, i.e., the decay of coherence arises from the dephasing, which occurs from the fluctuation of the level energies induced by the interatomic interaction.

DOI
10.1063/1.456477

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Quantum beats of the Na(3 2P3/2–3 2S1/2) transition and their change with the magnetic field were observed in pure Na vapor and Na+He gas by using a picosecond laser system. The energies of the hyperfine and Zeeman sublevels of the 3 2P3/2 and 3 2S1/2 states were calculated by using the reported hyperfine constants and g factors, and the theoretical curve of the quantum beats was calculated. The results were in good agreement with the observed profile of the quantum beats in phase of modulation, but the difference of the amplitudes increased with time. By comparing the amplitude of the theoretical curve of the quantum beats with that of the experimental curve, we determined the relaxation time of coherence. The decay of amplitude of the modulation was found to become faster as the pressure of the foreign gas (He) was increased. The decay of amplitude, i.e., the decay of coherence arises from the dephasing, which occurs from the fluctuation of the level energies induced by the interatomic interaction.
Quantum beats of the Na(3 /sup 2/P/sub 3//sub ///sub 2/--3 /sup 2/S/sub 1//sub ///sub 2/) transition and their change with the magnetic field were observed in pure Na vapor and Na+He gas by using a picosecond laser system. The energies of the hyperfine and Zeeman sublevels of the 3 /sup 2/P/sub 3//sub ///sub 2/ and 3 /sup 2/S/sub 1//sub ///sub 2/ states were calculated by using the reported hyperfine constants and g factors, and the theoretical curve of the quantum beats was calculated. The results were in good agreement with the observed profile of the quantum beats in phase of modulation, but the difference of the amplitudes increased with time. By comparing the amplitude of the theoretical curve of the quantum beats with that of the experimental curve, we determined the relaxation time of coherence. The decay of amplitude of the modulation was found to become faster as the pressure of the foreign gas (He) was increased. The decay of amplitude, i.e., the decay of coherence arises from the dephasing, which occurs from the fluctuation of the level energies induced by the interatomic interaction.
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.
The population gratings were created by two intersecting resonant light pulses in the ground state 3 2 S 1/2 and the excited state 3 2 P 1/2 in sodium vapor, and their developements in a static magnetic field were monitored by the diffraction of the resonant probe pulse. Intensity modulations of the diffraction signals at the Zeeman frequencies (Zeeman beats) were observed, which are due to the oscillation of the LIPG's. The detailed analysis of the creation, detection and the time development of the LIPG was made in a simple model system with total angular momentum J =1/2 in both the ground and the excited states. The observed beat patterns were compared with the theoretical calculations including the effects of hyperfine interaction in both states. Polarization characteristics were also examined. The agreement between theoretical predictions and experimental results was quite satisfactory.
Pressure shifts of the hyperfine splitting in Na 3(2)S((1/2)) that are due to buffer gases have been measured for the first time to our knowledge by all-optical means using the method of synchronized-quantum-beat spectroscopy reported previously [Y. Fukuda et al., Opt. Commun. 38, 357 (1981)]. The substate coherence is excited and detected with amplitude-modulated mode-locked laser pulses tuned to the D(1) line (pulse-repetition frequency, f(p); modulation frequency, f(m)), and the resonances observed at f(0) = nf(p) + ńf(m), (n and n' are integers) are used for the measurement. The pressure-shift coefficients obtained at 210 degrees C are 82, 61, -1, and -47 Hz/Torr for He, N(2), Ar, and Kr, respectively, within an accuracy of +/-3 Hz/Torr.
The method of Synchronized Quantum Beat Spectroscopy has been improved by introducing the amplitude modulation on the excitation pulse train. By using an amplitude-modulated train of picosecond pulses from a cw mode-locked dye laser, we obtain resonance enhancement of the substate coherence (quantum beat) at f 0 = n f p + n ' f m ( n , n ' : integer, f 0 : sublevel splitting, f p : pulse repetition frequency, f m : modulation frequency), which can be detected by monitoring the optical anisotropy due to the substate coherence. Resonances can be obtained in the frequency domain by sweeping f m . Pressure shift coefficients of the hyperfine splitting in Na 3 2 S 1/2 are measured to be 82, 61, -1 and -47 Hz/Torr for He, N 2 , Ar and Kr, respectively, within an accuracy of ±3 Hz/Torr.
We report detailed studies of freely evolving Zeeman coherence (Zeeman quantum beats) observed after optically pumping a gas of Na atoms and suddenly turning off the light field. This Zeeman coherence corresponds to a macroscopic magnetization that precesses in an external magnetic field; it is detected with a cw optical probe beam by using polarization-selective detection of the transmitted light. The amplitude and the phase of the signal show a pronounced variation with laser intensity, laser detuning, and the strength of the magnetic field. This dependence was measured on the 3s2S1/2 ground state of sodium, using the D1 line for optical excitation and detection. The experimental data are compared with theoretical predictions based on a Bloch-type equation of motion for an optically driven spin system in a J = 1/2 ground state.