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

Photon flux optimization in three-color multiphoton ionization of uranium atoms

David Levron, Avi Bar-Shalom, Z. Burshtein, R. David, G. Hazak, Joshua J. Ivry, Lawrence A. Levin, J. Oreg, Moshe Strauss

Published May 28, 1993• 0 citations
Publisher

Abstract

Ionization probability in three color excitation of uranium atoms depends on the photon flux which drives the transition to an autoionizing level. For a proper choice of the flux ratio, the ionization probability is insensitive of the actual fluxes over a considerable range. Thus, the laser intensities used may be lowered by a factor of about 3 as compared to values predicted on the basis of cross-sections of the individual transitions. A quantum-mechanical model, based on a detailed calculation of population dynamics of the levels in the ionization scheme, predicts the ionization probability as a function of the three-laser intensities. Ionization saturation measurements provide parameters for the model. The theoretical predictions were partially consistent with the experimental results. Ionization probabilities as high as 93% are achievable.

DOI
10.1117/12.145511

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Ionization probability in three color excitation of uranium atoms depends on the photon flux which drives the transition to an autoionizing level. For a proper choice of the flux ratio, the ionization probability is insensitive of the actual fluxes over a considerable range. Thus, the laser intensities used may be lowered by a factor of about 3 as compared to values predicted on the basis of cross-sections of the individual transitions. A quantum-mechanical model, based on a detailed calculation of population dynamics of the levels in the ionization scheme, predicts the ionization probability as a function of the three-laser intensities. Ionization saturation measurements provide parameters for the model. The theoretical predictions were partially consistent with the experimental results. Ionization probabilities as high as 93% are achievable.
Laser-induced photoionization and fluorescence signals were simultaneously observed in atomic uranium using a single Nd:YAG-pumped dye laser. These signals were recorded in two specific cases. In the first case, the dye laser was resonant to the first-step transition (0–16900.38 cm−1). In the second case, the laser was near-resonant to the first-step transition with a slight detuning (0.15 cm−1) so that it became two-photon resonant at 33801.06 cm−1. The uranium atoms in the ground state were ionized by a single-color, three-photon photoionization technique resulting in the photoionization signal, and the fluorescence signal was simultaneously obtained from the first excited state involved in the photoionization process. The photoionization and the fluorescence signals in the above-mentioned cases were also estimated theoretically for several values of the photoionization cross section for the transition between the second excited state at 33801.06 cm−1 and the autoionization state at 50701.59 cm−1 using density matrix formalism. From the comparison of theoretically calculated ratios of fluorescence signals in the two specific cases with the experimentally obtained values, the photoionization cross section for the 33801.06–50701.59 cm−1 transition has been obtained, which is found to be (5±1)×10−16 cm2.
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.
Orer a small range of light intensity the three-photon ionization rate of caesium atoms by ruby laser light decreases when the incident photon flux is increased. This result may be quantitatively explained by an induced energy shift of the 913 levels of the caesium atom proportional to the square of the light intensity. Thus perturbations of bound electron states by an intense light beam can be of importance in determining the multiphoton ionization rate. In this paper the functional dependence of the ionization rate on the photon flux is calculated for caesium, using a perturbation theory somewhat different from that usually employed.
In the present study we have investigated three-photon ionization in Ca in which autoionizing states are engaged. The two-photon resonant process (from the Ca ground state 4s 2 1 S 0 ) occurred through or at least in the vicinity of one of the following states: 4s4d 1 D 2 , 4p 2 3 P 2 , 4s6s 1 S 0 , 4p 2 1 D 2 and 4p 2 1 S 0 , with the third photon either reaching the continuum directly or one of the autoionizing states. The three-photon resonant transitions to 3d m p, m f: 1 P 1 , 3 P 1 and 3 D 1 autoionizing states for m up to 21 have been observed. Some of the autoionizing resonances which we have found had not been observed before in a high-resolution one-photon absorption experiment (for J = 1) and in multiphoton experiments (for J = 3). We have compared the ionization signal as a function of the laser detuning and the laser intensity with theoretical curves obtained within a simple model (three-level atom + one-mode laser field). This gives information about the order of magnitude of the three-photon ionization probability through autoionizing states.
In this paper we analyze two-photon-resonant three-photon ionization of atomic oxygen via the 2p3{\mathit{p}}^{3}p3(4{(}^{4}(4S\ifmmode^\circ\else\textdegree\fi{})np3{\mathit{np}}^{3}np3P0{\mathit{P}}_{0}P0​,1,2, and the 2p3{\mathit{p}}^{3}p3(4{(}^{4}(4S\ifmmode^\circ\else\textdegree\fi{})nf 3^{3}3F2{\mathit{F}}_{2}F2​,3,4 states. The various atomic parameters required for calculating the resonant enhanced multiphoton ionization (REMPI) probabilities are obtained using quantum-defect theory. The infinite sums over nonresonant states are truncated at a finite number of terms. Our calculated two-photon excitation cross sections agree well with the results of other recent calculations and of experimental measurements. The photoionization cross section is calculated for various electron kinetic energies. The REMPI dynamics is analyzed by solving the density-matrix equations. This framework consistently takes into account the effects of saturation and ac Stark shifts. REMPI probability is seen to be quite sensitive to the initial detuning, the intensity, and the particular resonant state accessed. The photoionization cross sections also imply that due to the rapid falloff of the cross section, two-color REMPI schemes with a lower-frequency ionizing photon would increase the effective REMPI probability.
The probability of two-photon excitation of an atom by the intense laser light is calculated formally and the probability per unit time of the three-photon ionization of an atom by the laser light with finite spectral line width is obtained for the case where a two-photon resonance occurs in an intermediate atomic level and the line shape of the laser light is Lorentzian. It is shown that the probability of three-photon ionization is proportional to the square of photon flux in the case of low photon flux and large spectral line width.