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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Adiabatic Inversion Of Infrared Transitions In A Molecular Beam: Theory And Applications

T. E. Gough, A.K. Lewin, John C. Shelley

Published Nov 25, 1986• 0 citations
Publisher

Abstract

Two level systems can interact coherently with infrared laser radiation giving rise to Rabi oscillations. The description of the interaction can in most cases, be cast in terms of the optical Bloch equations, as was shown by Feynmann et al.' Experimentally the interaction may be displayed using apparatus such as that described in reference 2. A molecular beam is generated from a supersonic expansion, crosses the output of a frequency stabilised carbon dioxide laser and impinges on a 2K bolometer. A Stark field is used to tune vibrational transitions of the beam molecules into resonance with the laser: the degree of vibrational excitation can be measured by monitoring the change in resistance of the bolometer as the molecules are tuned through resonance.

DOI
10.1117/12.938946

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Two level systems can interact coherently with infrared laser radiation giving rise to Rabi oscillations. The description of the interaction can in most cases, be cast in terms of the optical Bloch equations, as was shown by Feynmann et al.' Experimentally the interaction may be displayed using apparatus such as that described in reference 2. A molecular beam is generated from a supersonic expansion, crosses the output of a frequency stabilised carbon dioxide laser and impinges on a 2K bolometer. A Stark field is used to tune vibrational transitions of the beam molecules into resonance with the laser: the degree of vibrational excitation can be measured by monitoring the change in resistance of the bolometer as the molecules are tuned through resonance.
Rabi oscillations reflect the existence of a coherent superposition of two molecular eigenstates during the interaction of molecules with a strong, resonant electromagnetic field. This phenomenon is not restricted to two-level systems. We demonstrate its occurrence in a three-level system where the radiation induces a two-photon absorption. The intermediate level is (slightly) detuned from resonance. For a theoretical description the dressed-state picture is adopted. The two-photon excitation is performed either by using one laser, yielding a one-color two-photon process, or by using two counterpropagating, spatially overlapping laser beams, yielding a two-color two-photon process. Rabi oscillations are produced by varying the fluence, i.e., by changing either the laser power or the molecule-laser interaction time. For the one-color case the two-photon transition dipole moment for an SF6{\mathrm{SF}}_{6}SF6​ transition has been determined from the relation between laser fluence and number of Rabi oscillations. For the two-color cases the detuning with the intermediate level is varied by tuning the respective laser frequencies while keeping the sum frequency constant. A dramatic change in the degree of excitation as a function of the intermediate detuning has been observed. \textcopyright{} 1996 The American Physical Society.
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We have investigated the dynamic Stark effect in a J=0\ensuremath{\rightarrow}1\ensuremath{\rightarrow}0 three-level ladder system in an atomic beam of natural barium. An intense, actively stabilized cw dye laser was tuned to resonance between the upper two levels, and a similar laser was scanned through resonance with the transition connecting the lower two levels. Fluorescence arising from transitions out of the middle and upper levels was separately recorded as a function of probe laser frequency. This is the third and final paper in a series, of which the first two papers investigated theoretically and experimentally the ``weak-probe'' case, where the intensity of the probe laser was sufficiently low for the interaction between the atoms and the probe laser to be linear. In this paper we present an experimental investigation of the ``strong-probe'' case, where both lasers interact nonlinearly with the atoms, and we compare the results with the predictions of a theoretical model based on the optical Bloch equations.
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.
Infrared absorption spectroscopy is a highly sensitive technique for probing molecular structure, and when combined with cold molecular beam methods it provides unparalleled spectral resolution and absorption sensitivity to transitions in isolated gas-phase molecules. The first part of this dissertation investigates molecular beams of large polyatomic molecules produced via laser ablation of solid targets inside a buffer-gas cooling cell. Using matrix isolation spectroscopy to study the ablation products of graphite, we observe carbon clusters C3 to C12 produced at a rate of approximately 1011-1012 molecules for every pulse of the ablation laser. In a similar fashion, we study the production of metal oxide molecules with a buffer-gas beam source by ablating pure metal in the presence of O2 gas, and find consistent production of the WO molecule. Finally, we discuss efforts towards high resolution rotational-vibrational spectroscopy of WO in the gas phase.\nThe investigation of collective modes in ultracold atomic gases provides revealing information about the nature of interactions in these systems. Of particular interest is the interplay between a Bose-Einstein condensate and thermal excitations present at finite temperature, which effect the dynamics of collective modes by shifting mode frequencies and introducing damping. The second part of this dissertation investigates the dynamics of the monopole mode of a degenerate Bose gas in an isotropic harmonic trap recently developed at JILA. Through numerical simulations within the Zaremba-Nikuni-Griffin formalism, we identify the beating between two eigenmodes of the system, corresponding to in-phase and out-of-phase oscillations of the condensed and noncondensed portions of the gas. This beating leads to nonexponential collapse of the amplitude of the condensate oscillation, followed by a partial revival. Results of the simulations are shown to agree well with experimental data.
effects, due to the propagation of laser light in optically dense media, have been observed and studied. The use of the method of Stark switching [6] has been of much assistance in overcoming experimental difficulties. In that method, time-varying coherent effects are observed through the use of cw lasers, and the interaction between molecules and laser light can be switched on and off rapidly by changing the energy interval between levels through the use of the Stark effect. The idea of changing the energy separation was subsequently implemented in ESR [7], where the use of cw microwave radiation and pulsed changes in a polarizing magnetic field (Zeeman switching) have also made it possible to detect the basic time-varying effects. Many of the new properties of time-varying phenomena which have been studied in the field of optical resonances stem from the partial excitation of a spectral line (~o 1 a, this signal is reflected by a sinusoidal signal with a frequency specified by the generalized Rabi frequency (Wl 2 + b2)1/2.