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Physical Review A

Observation of two-zone Ramsey fringes using laser Stark spectroscopy of a molecular beam

Allan G. Adam, T. E. Gough, Neil R. Isenor, G. Scoles, John C. Shelley

Published Dec 1, 1986• 10 citations
Publisher

Abstract

Well-resolved Ramsey fringes have been demonstrated using laser Stark spectroscopy of ${\mathrm{CH}}_{3}$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 ${\mathrm{CO}}_{2}$ 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.

DOI
10.1103/physreva.34.4803

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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.
A non-perturbative theoretical analysis of Ramsey interference lineshapes in three-level lambda systems excited by laser fields has been developed for the general case of several interaction regions. In conventional Ramsey excitation, two laser fields excite the lambda system each in two separate spatial interaction regions. Here we examine how increasing the number of interaction regions or altering the interaction times change and improve the lineshape characteristics such as fringe visibility. For short interaction times the fringe pattern is strongly dependent on the number of interaction regions, with the n = 2 sinusoidal fringes replaced by sharper fringes of greater visibility. With larger interaction times, the fringe pattern is virtually unchanged on increasing the number of interaction regions. The optimum fringe pattern is obtained with short interaction times and a larger (n a 9) number of regions.
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.
Sub-Doppler molecular-beam laser Stark spectroscopy has been employed to produce high-contrast Rabi oscillations in the ${\ensuremath{\nu}}_{3}$ band of CH3{\mathrm{CH}}_{3}CH3​F. By varying the intensity of the cw CO2{\mathrm{CO}}_{2}CO2​ laser, up to five complete oscillations were observed before the phenomenon was washed out by rapid-passage effects and damping mechanisms. Besides being useful in clarifying key features of coherent ir molecular-beam spectroscopy, the observation of Rabi oscillations provides one of the most accurate means of directly measuring transition dipole moments. Analysis of the present data on three rovibrational transitions, Q(1,1) -1\ensuremath{\leftarrow}0, P(1,0) 0\ensuremath{\leftarrow}0, and R(1,1) 0\ensuremath{\leftarrow}1, has yielded a rotationless transition dipole moment of 0.21\ifmmode\pm\else\textpm\fi{}0.01 D for the ${\ensuremath{\nu}}_{3}$=1\ensuremath{\leftarrow}0 vibration. This result is in agreement with values estimated from both band-intensity and absorption-coefficient data in the literature.