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Investigations of resonantly enhanced multiphoton ionizations of atomic mercury and potassium

Kelly Robert Lim Mah

Published Jan 1, 2010• 0 citations

Abstract

Two investigations are reported on the application of resonantly enhanced multiphoton ionization (RMPI) to gaseous mixtures of an atomic vapor and a few Torr of argon. Photon fluxes large enough to ionize atoms by RMPI were produced by focussing down the light from a tunable pulsed dye laser. The irradiances generated were of the order of 500MWcm⁻². Ionization was detected by a voltage biased wire electrode that simply collected the photoelectrons either directly or after some gas multiplication. One investigation was the measurement of the dependence of RMPI processes in mercury on the polarization of the incident light for comparisons with theoretical calculations. The processes were four-photon resonant absorptions to either a ¹S₀ or ¹’³D₂ level followed by single photon ionization. Complete photoionization of all atoms excited to the resonant levels is established and the measured polarization dependences are found to agree with the calculated polarization dependence for the resonant excitation step of the RMPI process. Unexplained observations of the distortions in the polarization dependence of the ion yield and the absorption linewidth of the 6d ¹D₂ resonance are discussed. The second investigation was a study of the density dependent electric dipole forbidden two-photon resonant transition ²S → ²P in three-photon RMPI spectra of the Rydberg states of potassium. Stark interactions are shown to be unlikely and too weak. From the characteristics of the spectra, the excitation process is identified as a laser-assisted collision interaction.

DOI: 10.14288/1.0085794

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Two investigations are reported on the application of resonantly enhanced multiphoton ionization (RMPI) to gaseous mixtures of an atomic vapor and a few Torr of argon. Photon fluxes large enough to ionize atoms by RMPI were produced by focussing down the light from a tunable pulsed dye laser. The irradiances generated were of the order of 500MWcm⁻². Ionization was detected by a voltage biased wire electrode that simply collected the photoelectrons either directly or after some gas multiplication. One investigation was the measurement of the dependence of RMPI processes in mercury on the polarization of the incident light for comparisons with theoretical calculations. The processes were four-photon resonant absorptions to either a ¹S₀ or ¹’³D₂ level followed by single photon ionization. Complete photoionization of all atoms excited to the resonant levels is established and the measured polarization dependences are found to agree with the calculated polarization dependence for the resonant excitation step of the RMPI process. Unexplained observations of the distortions in the polarization dependence of the ion yield and the absorption linewidth of the 6d ¹D₂ resonance are discussed. The second investigation was a study of the density dependent electric dipole forbidden two-photon resonant transition ²S → ²P in three-photon RMPI spectra of the Rydberg states of potassium. Stark interactions are shown to be unlikely and too weak. From the characteristics of the spectra, the excitation process is identified as a laser-assisted collision interaction.

The polarization dependences of some resonant multiphoton ionizations in atomic mercury have been measured with a broadband (bandwidth ≈ 1.5 cm−1) multimode dye laser at moderate light intensities (≈ 500 MW∙cm−2). The multiphoton processes studied were the absorption of four photons to a resonant 1S0, 1D2, or 3D2 level by one-photon ionization. Complete saturation of the one-photon ionization step results in the ionization of all atoms excited to the resonant level. Because of the saturation of the ionization step, the polarization dependence of the four-photon excitation to the resonant level is measured. The theory developed by Dalby et al. is shown to give good agreement with the experiment when it is used to calculate the polarization dependence of the multiphoton transition to the resonant level. For the 6d1D2 resonance, distortions in the polarization dependence and an unusual linewidth dependence on the light polarization were observed. We relate these observations to the alternating current Stark effect and to the production of the third harmonic of the laser light in the focal volume.

This paper studies the resonant multiphoton ionization (RMPI) phenomenon, in the case of caesium atoms interacting with the intense field of a neodymium-Glass laser. In the first part, we present a recent theory of RMPI, in a simplified form in order to obtain the main informations on the physics of RMPI. Then we present two experiments of RMPI : four-photon ionization of the caesium atom in its ground state, with a three-photon resonance on the 6F level and three-photon ionization of the caesium atom is its 62P3/2 state with a two-photon resonance on the 12F level. In the first experiment, the internal structure of the resonant transition is resolved, enabling us to point out and analyse the important effect of pulse shapes and focusing conditions on our experimental results, which are discussed in detail. We also give some new interpretations of results published previously [1]. In the second experiment, we measure the 12F fine structure — which is found to be (0.016 ± 0.008) cm-1 — and show that we have reached the limits imposed by the Doppler effect. These experimental results are in good agreement with the theoretical predictions suggesting that the physical picture given of RMPI by recent works on this subject is satisfactory.

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 have measured the resonance-enhanced four-photon ionization of atomic hydrogen. The degenerate four-photon process occurs via a three-photon resonance between the 1s and the 2p levels, with subsequent one-photon ionization near threshold. We have studied the highly asymmetric resonance-enhanced profile, i.e., the photoion yield as a function of laser detuning from three-photon resonance between the 1s and 2p levels. In particular, we have determined the width, shift, peak, and asymmetry of the profile as a function of laser intensity. The experimental results are compared to theoretical models. These models involve both the properties of the atom in an intense near-resonant radiative field, and a detailed model for the multimode laser field, particularly the field fluctuations due to mode beating. The asymmetric resonance-enhanced photoionization profile is well reproduced by both a random-phase and a chaotic-light model.

Atomic strontium vapor in a thermionic diode is irradiated by the light from a tunable dye laser at two-photon resonance on the 5s5d1D2 state. At a laser intensity of ~ 108 W/cm2 and a vapor pressure of several torrs, not only three-photon ionization occurs but also the generation of new frequencies due to a nonlinear polarization is observed. Simultaneously, strong and strictly directed beams are generated which are resonantly enhanced by the 5s5p1P1 and 5s6p1P1 states, respectively. Close to the first resonance, four-wave mixing causes braodband two-photon emission, the bandwidth of which is determined by the region of negative dispersion due to phase matching. On the other hand the process via the 6p state is exactly resonant as can be shown by comparison in every case with the respective fluorescence light. At higher pressures (~ 10Torr), however, a second line appears, which is also shifted to the side of negative dispersion.