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Physical Review A

Hyper-Raman scattering and three-photon resonant ionization: Competitive effects

Ángela M. Guzmán, C. Ramírez

Published May 1, 1993• 1 citation
Publisher

Abstract

A semiclassical theory of hyper-Raman scattering and three-photon resonant ionization via the coupled density-matrix and Maxwell equations is presented. A simplified three-level atom model is obtained, which includes two-photon resonant pumping and time-dependent photoionization rates. We consider conditions typically encountered in atomic vapors to simulate numerically the pulse propagation. A strong depletion of the photoionization probability in the hyper-Raman field-saturation regime is predicted.

DOI
10.1103/physreva.47.4270

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A semiclassical theory of hyper-Raman scattering and three-photon resonant ionization via the coupled density-matrix and Maxwell equations is presented. A simplified three-level atom model is obtained, which includes two-photon resonant pumping and time-dependent photoionization rates. We consider conditions typically encountered in atomic vapors to simulate numerically the pulse propagation. A strong depletion of the photoionization probability in the hyper-Raman field-saturation regime is predicted.
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Investigates the dynamics of the transient, simulated hyper-Raman effect, subject to a number of simplifying assumptions. The density matrix and probability amplitude formalisms have both been utilised for this purpose. Soluble cases of this three-photon process, involving coherent, ultrashort-pulse propagation in a non-linear medium, have been analysed in detail. In particular the authors find for the system which is far from saturation, and which allows for laser pump depletion, a solution free from ringing. This is in marked contrast to its ordinary Raman counterpart.
This chapter introduces a three-level atomic model and discusses two other coherent atomic phenomena; namely, coherent Raman scattering and lasing without inversion. Lasing without inversion may take place in three-level resonant atomic systems and, like super-radiance, results from atomic coherence. Super-radiation is initiated by spontaneous fluctuations of the polarization in the medium. The semiclassical method is a powerful tool that is useful for the treatment of propagation effects in super-radiance as well as in such coherent emission processes as self-induced transparency, photon echo and others. The use of the Maxwell—Bloch equations allows one to consider these phenomena in elongated systems. Fortunately, the semiclassical approach is readily applicable to low-density systems that are to gases. However, one has to exercise greater care when applying it to coherent emission in solids. In high-density systems there is a strong mixing of the field and atomic states in the vicinity of resonance.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
We theoretically study intensity-saturation effects in resonant harmonic generation and multiphoton ionization. A first laser excites a two-photon resonance which is coupled by a second laser to an autoionizing state. Starting from an effective Hamiltonian for the three resonant atomic states, we derive a set of equations for the density matrix of the gaseous medium and the electromagnetic field within a semiclassical framework. We present and discuss analytical and numerical solutions of these equations which show a variety of line profiles depending critically on the intensities of the incident laser pulses.
The manifestation of spontaneous Raman scattering upon the three-photon ionization of a samarium atom has been investigated. The dependence of the Sm+ yield on the laser frequency, along with the peaks due to the two-photon excitation of bound states from different levels of the 4f 66s 2 7 F ground term, exhibits a strong peak due to single-photon excitation of the 4f 55d6s 2 7 D°3 state from the 4f 65d6s 9 H 2 excited state. The 4f 65d6s 9 H 2 state was populated as a result of spontaneous Raman scattering. The large amplitude of the observed peak indicates a high efficiency of this process. The estimations performed indicate that the probability of resonant ionization through an excited state populated due to spontaneous Raman scattering can be comparable with the probability of three-photon ionization through the two-photon intermediate resonance. The necessary conditions for implementing this channel of resonant three-photon ionization are formulated.