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Physical Review A

Collision-induced three-photon ionization

Munir Hasan Nayfeh

Published Nov 1, 1979• 11 citations
Publisher

Abstract

A semiclassical treatment of collision-induced three-photon ionization of atoms with a near two-photon resonance is presented. The system is described by an effective two-level system. The wavelength dependence of the ionization yield does not obey a power law. When the first excited state is far from resonance, and with a $\frac{{C}_{6}}{{R}^{6}}$ interatomic potential, a first-order correction to the expected $\ensuremath{-}\frac{3}{2}$ power law obeys a - \textonehalf{} power law. This three-photon process promises to be an extremely sensitive method for the study of collisional effects in the far wing of two-photon $S\ensuremath{-}S$ and $S\ensuremath{-}D$ transitions, and without the requirement of high densities.

DOI
10.1103/physreva.20.1927

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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.

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Physical Review A•Sep 1, 1981•10 citations
A semiclassical treatment of collision-induced three-photon ionization of atoms with a near two-photon resonance is presented. The system is described by an effective two-level system. The wavelength dependence of the ionization yield does not obey a power law. When the first excited state is far from resonance, and with a C6R6\frac{{C}_{6}}{{R}^{6}}R6C6​​ interatomic potential, a first-order correction to the expected \ensuremath−32\ensuremath{-}\frac{3}{2}\ensuremath−23​ power law obeys a - \textonehalf{} power law. This three-photon process promises to be an extremely sensitive method for the study of collisional effects in the far wing of two-photon S\ensuremath−SS\ensuremath{-}SS\ensuremath−S and S\ensuremath−DS\ensuremath{-}DS\ensuremath−D transitions, and without the requirement of high densities.
Semiclassical expressions of two-photon ionization of atoms induced by radiative collisions are derived. The dependence of the ionization yield on the atomic forces, field intensity, and energy gap is derived. Although absorption tends to decrease as the field intensity rises due to stimulated emission at the second crossing, the two-photon ionization yield can be nearly saturated at the first crossing, thus enhancing the absorption. Both regions, ionization in single collisions and ionization between collisions, are treated. In the latter we find that saturation of the ionization can be achieved at much reduced intensities. This process promises an extremely sensitive method for studying radiative collisions, especially when absorption or fluorescence becomes extremely weak.
The relation between the spectral character of a completely off-resonant two-photon absorption and the non-impact collision dynamics of a three-level atomic system is studied. A collisionally induced resonance character in the spectral dependence of the absorption probability is shown theoretically. Measured results about the dependence of fluorescence intensity on the incident light frequencies are ingood agreement with the theory. The interatomic potential between the sodium 4D state and argon is also estimated from experimental results.
Within the framework of a three-level model, the process of three-photon ionization with one-photon resonance between two excited levels (with the lower one being initially unpopulated) is considered using the density matrix method. It is shown that such resonance can result in the appearance of a maximum in the three-photon ionization spectrum when detuning between the resonance wavenumber and the wavenumber of the transition responsible for the lower excited level being populated exceeds the laser radiation linewidth by more than three orders of magnitude.
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.