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OBSERVATIONS OF RABI OSCILLATIONS IN THE INFRA-RED ON THE Q(1, 1) TRANSITION OF METHLYL FLUORIDE

A. G. Adam, T. E. Gough, Neil R. Isenor, A.K. Lewin, G. Scoles

Published Jan 1, 1982• 0 citations
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Abstract

We shall report on the observation of Rabi oscillations for the $\\Delta M = -1$, Q(1, 1) component of the $\\upsilon_{3}$ band of methyl fluoride $(CH_{3}F)$ in the $10 \\mu m$ spectral region. This coherence phenomenon has been observed in a beam produced by a supersonic expansion. The transition is detected by measuring, with a low temperature bolometer, the energy contents of the molecules in the beam downstream from the laser excitation region. A Stark field is used to shift the transition into resonance with a $CO_{2}$ laser line thereby circumventing the need for a tunable source. This work is preliminary to further observations of multiple excitation region coherence phenomena (e.g. Ramsey fringes) and their perturbation by molecular collisions.

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We shall report on the observation of Rabi oscillations for the DeltaM=−1\\Delta M = -1DeltaM=−1, Q(1, 1) component of the upsilon3\\upsilon_{3}upsilon3​ band of methyl fluoride (CH3F)(CH_{3}F)(CH3​F) in the 10mum10 \\mu m10mum spectral region. This coherence phenomenon has been observed in a beam produced by a supersonic expansion. The transition is detected by measuring, with a low temperature bolometer, the energy contents of the molecules in the beam downstream from the laser excitation region. A Stark field is used to shift the transition into resonance with a CO2CO_{2}CO2​ laser line thereby circumventing the need for a tunable source. This work is preliminary to further observations of multiple excitation region coherence phenomena (e.g. Ramsey fringes) and their perturbation by molecular collisions.
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.
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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.
Broadband, coherent radiation in the optical-frequency range is generated using microplasma channels in atmospheric gases in a pump-probe experiment. A microplasma medium is created in a gas by a focused intense femtosecond pump pulse. A picosecond probe pulse then interacts with this microplasma channel, producing broad, coherent sidebands that are associated with luminescence lines and are redshifted and blueshifted with respect to the laser carrier frequency. These sidebands originate from the induced Rabi oscillations between pairs of excited states that are coupled by the probe pulse. Thus the sideband radiation intensity tracks the microplasma evolution. The sidebands arise from broad and tunable Rabi shifts corresponding to varying values of the electric-field magnitude in the probe pulse. The \ensuremath{\sim}1010{}^{10}10 W cm${}^{\ensuremath{-}2}$ probe beam creates a maximum sideband shift of >90 meV from the carrier frequency, resulting in an effective bandwidth of 200 meV. The sidebands can be tuned and controlled by the intensity and temporal profile of the probe pulse. The fact that the coherence is observed in a microplasma demonstrates that Rabi cycling is possible at high temperature with moderately high laser intensities as long as transitions close to the driving frequency ($\ensuremath{\Delta}$\ensuremath{\sim}2% ${\ensuremath{\omega}}_{c}$) are available. Plasma excitation combined with Rabi-shifting measurements also serves as a means to simultaneously extract quantitative ratios for the transition-dipole moments between multiple sets of highly excited states with transitions in the optical regime.
We report on the observation of coherent excitation and spectroscopy performed using Ramsey's method of separated excitation fields on a single trapped and laser cooled ion of strontium. Single pulse Rabi excitation studies of the ion have shown linewidths approaching the transform limited value. Studies of transition rate versus pulse duration have shown a quadratic initial increase in transition rate with a duration consistent with coherent excitation and a rapid loss of Rabi oscillations, consistent with the concept of a dephasing of the thermally populated trap oscillator states as the excitation evolves. When two-pulse Ramsey excitation is applied to the single ion, fringe widths down to a linewidth of 840 Hz (FWHM) were observed with measured fringe contrasts consistent with a probe laser linewidth of 500 Hz.PACS Nos.: 32.80Qk, 32.30Jc, 06.30Ft