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Physical Review A

New Laser Technique for the Identification of Molecular Transitions

N. Skribanowitz, M. J. Kelly, Michael S. Feld

Published Dec 1, 1972• 51 citations
Publisher

Abstract

A laser technique is proposed which may be useful for the assignment of molecular spectra in the visible and infrared regions. The method is based on the resonant interaction of two monochromatic fields with a Doppler-broadened three-level system. Under the appropriate conditions the absorption line shape of one of the transitions shows a complex structure over a narrow section of the Doppler profile, and for sufficiently high laser power the line shape splits into a number of narrow peaks. Analysis of the resulting intensity pattern leads to unambiguous assignment of the angular momentum quantum numbers of the three levels involved. A simple set of rules is given to facilitate interpretation of spectra. The line shapes discussed are also relevant to monochromatic optical pumping of gases and unidirectional laser amplifiers.

DOI
10.1103/physreva.6.2302

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A laser technique is proposed which may be useful for the assignment of molecular spectra in the visible and infrared regions. The method is based on the resonant interaction of two monochromatic fields with a Doppler-broadened three-level system. Under the appropriate conditions the absorption line shape of one of the transitions shows a complex structure over a narrow section of the Doppler profile, and for sufficiently high laser power the line shape splits into a number of narrow peaks. Analysis of the resulting intensity pattern leads to unambiguous assignment of the angular momentum quantum numbers of the three levels involved. A simple set of rules is given to facilitate interpretation of spectra. The line shapes discussed are also relevant to monochromatic optical pumping of gases and unidirectional laser amplifiers.
A method is introduced to calculate for a model set of molecular levels the spectral line shapes expected for a variety of conventional laser experiments including absorption, hole burning, fluorescence line narrowing, and Raman scattering. The method allows the incident laser field to have arbitrary intensity. Furthermore, the effects of model Gaussian or Lorenzian inhomogeneous distributions are readily incorporated. Earlier results for a two-level system are easily obtained and new results are presented for inhomogeneously broadened two and three level systems, and for the effects of pure dephasing on the strong field spectra. In particular we have described the differences between fluorescence and Raman in strong fields, and the effect of strong fields on the spontaneous emission of inhomogeneously broadened transitions. Some predictions are made regarding line narrowing experiments in the strong-field limit.
The line shape of a Doppler-broadened transition is dramatically altered by the presence of a laser field resonating with a second Doppler-broadened transition sharing a common level: Two narrow resonances of different widths appear superimposed upon the broad background signal at frequencies symmetrically located about the corresponding line center. The effect has already found application in a number of seemingly different though intimately related studies, including high-resolution hfs and isotope-shift determinations. The theory of the effect is developed with reference to these applications. The treatment is formulated in terms of transition rates induced by two classical fields resonantly interacting with a pair of coupled Doppler-broadened transitions of arbitrary frequencies. The perturbation approach adopted is valid for one field fully saturating its transition; the resulting line-shape expression exhibits important power-broadening effects. This approach is equivalent to the familiar density-matrix formulation, which is also presented. Various features of the resulting expression are discussed in detail as they apply to two precision spectroscopic applications, mode crossing and spontaneous emission line narrowing. The connection with previous work is also discussed.
This paper describes a new class of techniques, called time-delayed laser saturation spectroscopy, which combine frequency- and time-domain methods of laser spectroscopy to provide a way of studying a molecular system as it evolves from an initially prepared stationary state to a second, final state. The specific example analyzed here is three-level free induction decay, in which the time-dependent gain of a Doppler-broadened molecular transition is probed after the sudden termination of an intense field resonating with a coupled transition. The theoretical calculation is based on the coupled density-matrix equations of motion in the slowly-varying envelope approximation. The time-delayed line shapes, which may be studied in either transmission or side fluorescence, exhibit linewidth asymmetries, line-shape deformations, Ramsey-type fringes, power broadening and dephasing, and dynamic Stark splittings and oscillatory decays. The technique provides a unique way of distinguishing the influence of Raman-type processes from that of population saturation and a means to separately measure the associated decay rates. The relationship of the present work to other studies is also discussed.
A new method of molecular beam spectroscopy with the ultrahigh resolution is theoretically suggested. This method is based on the phenomenon of the coherent population trapping of molecular levels and allows the identification of the overlapping optical lines in complex spectra (typical for polyatomic molecules), even when frequency intervals between the centers of given lines are less than their natural line widths. The theoretical analysis is carried out on the basis of the Schrödinger equation for the three-level molecular A-system interacting with the two-frequency laser radiation. The conditions of the practical realization and possible applications of the suggested spectroscopy method are discussed.
This chapter shows that several non-linear features can be demonstrated and the line shape of sub-natural line width can be obtained by manipulation of the powers of the two interacting laser radiations. The interaction of the laser radiation can produce a coherent superposition of the atomic states such that atoms are trapped in one state so that the atomic system may be transparent even in the presence of radiation frequency that is on resonance. Saturation spectroscopy is more conveniently performed in three level atomic systems interacting with two laser radiations. One of the radiations, called pump beam, has higher intensity and frequency on resonance or close to resonance with the transition frequency across two levels of the atom. The second laser radiation, called probe beam, is weaker and interacts with two other energy levels, one of which is common to the pump transition. A few hyperfine transitions are within the envelope of one Doppler profile.