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Journal of Applied Physics

Absolute excited state and ion densities from two- and three-photon processes in some 6p levels of atomic krypton

Noshir M. Khambatta, John A. Oertel, Robert Silk, Leon J. Radziemski, Joseph M. Mack

Published Nov 15, 1988• 8 citations
Publisher

Abstract

Neutral krypton atoms are excited from the ground state to the 4p56p[3/2]2 state by two-photon absorption from an ArF excimer laser operating at 193.41 nm. The third photon ionizes the krypton atoms. Excitation and ionization yields are theoretically computed employing the standard laser rate-equations analysis. Measurements are made on both the excitation and ionization yields. The experimental results are in good agreement with the theoretical predictions of our formulations, as well as with those of other authors.

DOI
10.1063/1.341226

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Neutral krypton atoms are excited from the ground state to the 4p56p[3/2]2 state by two-photon absorption from an ArF excimer laser operating at 193.41 nm. The third photon ionizes the krypton atoms. Excitation and ionization yields are theoretically computed employing the standard laser rate-equations analysis. Measurements are made on both the excitation and ionization yields. The experimental results are in good agreement with the theoretical predictions of our formulations, as well as with those of other authors.
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We have observed fluorescence and ionization signals resulting from two-photon resonant excitation of thallium in an atomic beam. The output of a pulsed dye laser is used to excite the resonance between the ground state (6('2)P(,1/2)) and an excited n('2)P state of thallium. Both two-photon absorption and three-photon absorption occur simultaneously; the latter results in photoionization of the atom. The excited states populated by two-photon absorption decay via cascade fluorescence back to the ground state; one or more of these spectral components constitute our fluorescence signal. The lifetimes of the excited n('2)P states of thallium have been measured directly by observing the exponential decay of this fluorescence. Calculations are included to ascertain the time required for the fluorescence signal to reach within 2% of a simple exponential decay. This decay is a measure of the lifetime of the initial state, under the condition that none of the fluorescence components arise from states which are populated by cascade through longer-lived states. The lifetimes of the n('2)P(,1/2) and n('2)P(,3/2) states of thallium for n = 7 to 11 are determined in this manner and compared with the values deduced from semi-empirical calculations of oscillator strengths by various authors. For calculations employing the effects of core polarizability in the electron wavefunctions and in the transition matrix elements, the agreement is very good--to within our experimental uncertainty, which is typically(, )<5%. Our measurements are also compared to the four other known experimental values for such excited states of thallium. Some of these previous experiments and calculations were carried out in conjunction with parity non-conservation experiments in thallium. Our measurements may provide useful checks on the accuracy of some of the calculations performed in the evaluation of such experiments. The ionization and fluorescence signals are also studied under conditions of strong saturation of the two-photon resonance with the 7('2)P states. The manifestation of saturation in the ionization signal is a broadening of the excitation spectrum, which is found to greatly exceed that of the fluorescence signal. Furthermore, the ratio of the ionization signal to the fluorescence signal at the peak of the resonance is found to vary linearly at intensities below about 10('9) W/cm('2); this too saturates at higher intensities. Various models are employed to simulate the observed behavior, encompassing both rate equations as well as density matrix formulations. The numerical solutions to these models are compared with the observed behavior; the agreement is found to depend strongly on the assumed experi- mental conditions to which the ensemble of atoms is subjected.
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We demonstrate the generation of metastable krypton in the long-lived 1s^{5} state using laser excitation. The atoms are excited through a two-photon absorption process into the 2p^{6} state using a pulsed optical parametric oscillator laser operating near 215 nm, after which the atoms decay quickly into the metastable state with a branching ratio of 75%. The interaction dynamics are modeled using density matrix formalism and, by combining this with experimental observations, we are able to calculate photoionization and two-photon absorption cross sections. When compared to traditional approaches to metastable production, this approach shows great potential for high-density metastable krypton production with minimal heating of the sample. Here, we show metastable production efficiencies of up to 2% per pulse. The new experimental results gained here, when combined with the density matrix model we have developed, suggest that fractional efficiencies up to 30% are possible under optimal conditions.
At low pressures, excitation is due to the absorption of three photons and causes emission of the resonance line and the related first continuum ( lambda approximately 125 nm). At higher pressures, excitation mainly arises from a photon of the third harmonic of the laser wave and the second continuum ( lambda approximately 148 nm) is detected. Luminescence attenuations in the excitation spectra can be explained by the absorption of a fourth photon. The natural lifetime of the 3 P 1 is determined using Holstein's theory. The role of two and three-body collisions is demonstrated in the decay of the 3 P 1 state as in the role of three-body collisions for the 3 P 2 state. The corresponding rate constants are determined and the lifetime of the 1 u ( 3 P 2 ) state evaluated.