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Single Recoil Cornponent Optical Rarnsey Fringes at 514.5 nrn in an 12 Supersonic Bearn

G. Camy, N. Courtier, J. Helmcke

Published Jan 1, 1987• 0 citations
OpenAlex

Abstract

In conventional saturated absorption experiments utilizing I 2 absorption cells, the acccuracy in determining the line center is ultimately limited by : unresolved recoil structure /1/ ; residual transit-time broadening ; second-order Doppler broadening and shift ; and collisions (nonlinear pres­ sure broadening and shift). These limitations can be greatly reduced by applying the method of separated field excitation /2/ in a supersonic I 2 beam. The experiments were carried out using a high-resolution I 2 -stabilized Ar'-laser spectrometer at 514.5 nm and a collimated supersonic beam. The spectral width of the Ar'-laser was estimated to be in the range of 10 kHz. From the high density and the narrow velocity distribution of the super­ sonic beam we expect to achieve good signal-to-noise ratio, reduced second order Doppler broadening and improved knowledge of the second order Doppler shift. The experimental setup is shown in Fig. 1. The I 2 molecules of the supersonic be am are excited sequentially by two pairs of travelling waves propagating parallel but in opposite directions. A coherence between the initial and the excited states is built up in the first interaction zone /4/. It then precesses freely through the field-free region. In the second zone, two of the initial state and the excited state are created in the Vz -space, depending on the phase difference between the coherence and the exciting field. The two gratings are slightly shifted relatively to each other due to the recoil splitting. The distance a bet­ ween the first two interaction zones is limited to about 3 mm to avoid excessive decay of the upper state of the selected absorption line. During the free flight between the second and third interaction zones, separated by a distance d = 35 mm, the upper state population decays completely into many ro-vibrational levels of the electronic ground state, leaving the population grating of the initial state practically unchanged. Correspon­ dingly, only the initial state population grating will contribute to the Ramsey signal generated after the final two interactions and only the recoil component associated with the initial state will contribute to the Ramsey fringes /1,3,4/.

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Physical Review A•Jun 1, 1981•1 citation
In conventional saturated absorption experiments utilizing I 2 absorption cells, the acccuracy in determining the line center is ultimately limited by : unresolved recoil structure /1/ ; residual transit-time broadening ; second-order Doppler broadening and shift ; and collisions (nonlinear pres­ sure broadening and shift). These limitations can be greatly reduced by applying the method of separated field excitation /2/ in a supersonic I 2 beam. The experiments were carried out using a high-resolution I 2 -stabilized Ar'-laser spectrometer at 514.5 nm and a collimated supersonic beam. The spectral width of the Ar'-laser was estimated to be in the range of 10 kHz. From the high density and the narrow velocity distribution of the super­ sonic beam we expect to achieve good signal-to-noise ratio, reduced second order Doppler broadening and improved knowledge of the second order Doppler shift. The experimental setup is shown in Fig. 1. The I 2 molecules of the supersonic be am are excited sequentially by two pairs of travelling waves propagating parallel but in opposite directions. A coherence between the initial and the excited states is built up in the first interaction zone /4/. It then precesses freely through the field-free region. In the second zone, two of the initial state and the excited state are created in the Vz -space, depending on the phase difference between the coherence and the exciting field. The two gratings are slightly shifted relatively to each other due to the recoil splitting. The distance a bet­ ween the first two interaction zones is limited to about 3 mm to avoid excessive decay of the upper state of the selected absorption line. During the free flight between the second and third interaction zones, separated by a distance d = 35 mm, the upper state population decays completely into many ro-vibrational levels of the electronic ground state, leaving the population grating of the initial state practically unchanged. Correspon­ dingly, only the initial state population grating will contribute to the Ramsey signal generated after the final two interactions and only the recoil component associated with the initial state will contribute to the Ramsey fringes /1,3,4/.
Abstract : The goal of our work has been to investigate a few well-chosen effects in laser spectroscopy under clean experimental conditions which facilitate a sharp and meaningful contact with theory. The initially-defined task was to study large pulse-area effects in single upward transitions from metastable atoms in a fast, monovelocity Ne* beam. It proved possible as well to investigate multiple-pulse transitions on a cw basis. Ramsey fringe resonance signatures were clearly observed and for the first time in the optical domain, using three pulse excitation. Basically the first two zones of light provide initially a coherence and secondly an atomic population in the excited state. The counter-running fields in the second zone and third zones convert excited state population into a coherence again and finally into a nonlinear (with intensity) change in the excited and ground state populations. In view of the time gap tau in the preparation phase, one could anticipate and did observe narrow Ramsey fringes which vary as cos (delta, omega, tau) in the middle of the single zone excitation linewidth function ( Rabbi pedestal ). The neon beam employed was rather fast (v/c approx. .001) and, being produced by charge transfer had an exceptionally an narrow velocity distribution, delta, approx .0001. These characteristics permitted unambiguous measurements of the fringe shapes and facilitated useful contact with theory. A second extremely successful experiment was the combination of laser excitation of two-photon Doppler-free transitions in the 5S yields nS series in Rubidium with precise Lambdameter interferometer techniques to accurately measure the transition wavelength.
Previously derived equations describing recoil effects in saturated absorption by a gas of two-level atoms or molecules are solved numerically for the case of standing plane-wave excitation and large Doppler broadening. The numerical techniques and accuracy estimates are discussed briefly. The lineshapes obtained are well approximated by the superposition of two Lorentzian functions whose width, heights, and separation parametrise the lineshape. The dependence of these parameters on saturation, recoil splitting, and relaxation rates is described and compared with corresponding results in the rate equation approximation and in the weak-probe-beam approximation. Saturation broadening depends strongly on these latter approximations, but very little on recoil splitting. When multiphoton processes are retained in the equations, increasing saturation causes a slight decrease in the fitted separation of the peaks. The ratio of the heights of the two Lorentzians is closely related to the ratio of relaxation rates for the level populations.
Well-resolved Ramsey fringes have been demonstrated using laser Stark spectroscopy of CH3{\mathrm{CH}}_{3}CH3​F in a molecular beam with two interaction regions. Fringe separations as low as 155 kHz were obtained. The interaction regions were produced by masks containing slits placed in the Gaussian-profile beam of a cw CO2{\mathrm{CO}}_{2}CO2​ laser. Population inversion resulting from a frequency sweep associated with the curvature of the slit-diffracted wave fronts has limited the observations at high laser-power values. An imaging system has been shown to eliminate this effect. Experiments with three and five laser-molecular beam crossings were also conducted to demonstrate the feasibility of using multiple interaction regions as a tool for high-resolution spectroscopy. The results have been modeled using the optical Bloch equation for a two-level system. Good agreement with the experiment has been obtained.
The excited state population distribution created by 6S1/2 --> 6P3/2 laser excitation in room temperature cesium vapor mixed with Ar was quantitatively analyzed applying absorption and saturation spectroscopy. A simple method for the determination of the excited state population in a single excitation step based on the measurements of the saturated and unsaturated absorption coefficients was tested. When the line profile is nearly pure Doppler and the laser linewidth is much smaller than the inhomogeneous linewidth but comparable with homogeneous linewidth, the fraction N(v(z)) of the atoms in the ground state that are able to absorb under inhomogeneous broadening conditions can be determined. The transmission of the medium in the center of the Doppler envelope of the strong h. f. component of the CsD2 line due to hyperfine pumping alone amounts to approximately 5%. The assumption that has been made is that the lower-state hyperfine levels are populated in a statistical ratio. The absorption coefficients were measured for a series of P852 powers between 20 microW and 2. 5 mW. The velocity selective population density in the 6P3/2 state was obtained. The population in the 6P3/2 level obtained from the saturation measurements was also determined by the absorption measurement of narrow spectral line from a Cs hollow cathode lamp. The agreement between the results obtained in these two ways is very good. It was shown that 5% of the ground state population could be transferred to the first excited state by pumping the Doppler broadened line with a single-mode narrow-band laser. The argon caused line broadening and therefore increased the effective pumping rate in the first excitation step. The dependence of the 852 nm line fluorescence intensity was plotted against the population in the 6P3/2 level determined from the saturation absorption. This can serve as confirmation of the reliability of the method used for the determination of the excited state population.
Separated-beam experiments for both the Doppler-free two-photon and saturated absorptions are under consideration. In the Doppler-free two-photon absorption, the experimental arrangement is important. Such important has not been recognized before. Three different arrangements are discussed here. In the first one, the separated-beam experiment is not coherent and the Ramsey fringe variation cannot be observed. In the second, the transit-time broadening is present and the Ramsey fringe variation depends on the atom's time of flight as well as the transit time. Finally, in the third arrangement, the transit-time broadening is eliminated and the Ramsey fringe variation depends on the atom's time of flight alone. These examples have shown that in the Doppler-free two-photon absorption, the elimination of the transit-time broadening is through a mechanism other than the beam separation. In the saturated absorption the situation is quite different. Even in the presence of transit-time broadening, the Ramsey fringe spacing depends only on the atom's time of flight.