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Journal of the Optical Society of America B

Ac-Stark broadening of three-photon resonances in four-photon ionization of iodine atoms with broadband laser radiation

Victor N. Bagratashvili, Anatoly Viktorovich Masalov, S. I. Ionov, G. V. Mishakov, Vladimir A. Semchishen

Published Feb 1, 1987• 11 citations
Publisher

Abstract

Ac-Stark-broadened three-photon resonances in four-photon ionization of iodine atoms are investigated experimentally and analyzed theoretically, including the effect of laser intensity fluctuations and spatiotemporal distribution of average intensity. It is shown that the resonance maximum cannot be used in estimating the hyperpolarizabilities of atomic levels. By applying the properly designed procedure, the difference in hyperpolarizabilities of 5p(2P1/2) and nd(…)73/2 levels is found to be α12 = 20 ± 10 Å3.

DOI
10.1364/josab.4.000129

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Ac-Stark-broadened three-photon resonances in four-photon ionization of iodine atoms are investigated experimentally and analyzed theoretically, including the effect of laser intensity fluctuations and spatiotemporal distribution of average intensity. It is shown that the resonance maximum cannot be used in estimating the hyperpolarizabilities of atomic levels. By applying the properly designed procedure, the difference in hyperpolarizabilities of 5p(2P1/2) and nd(…)73/2 levels is found to be α12 = 20 ± 10 Å3.
We present a theory of Stark splitting in doubly resonant three-photon ionization (three-level system) with nonmonochromatic fields. The asymmetry of the peaks of the resonance curve due to Stark splitting is shown to be reversed when the laser bandwidth is larger than the width of the atomic transitions of the resonant states. It is further shown that ionization as well as the probe field have a significant effect on the features of the splitting. Similar effects are found with monochromatic fields in the presence of an additional level near the initial level (four-level system). The results of the analysis are in general qualitative agreement with existing observations.
The influence of ac Stark shifts on the three-photon ionization of cesium in a moderate-intensity laser beam is investigated. The ionization process involves a two-photon resonance with the 8d8d8d fine-structure levels. The work consists of two related parts. In the first part, atomic parameters, such as the two-photon Rabi rate and the ac Stark shift, are calculated in the nonrelativistic Hartree-Fock approximation. In the second part, the atomic parameters thus obtained are used as coefficients in a system of coupled differential equations representing the time evolution of the density matrix. Ionization line shapes are generated for a transform-limited pulsed laser of moderate intensity with a single-mode transverse structure. As the time history of the intensity profile is varied from square to smooth, the ionization line shape becomes asymmetric due to the time dependence of the ac Stark shift. The effects of varying the spatial intensity distribution of the laser light to include transverse centering and longitudinal focusing are also studied.
An intense laser and a probe laser in near resonance with the sodium $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ and $3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{3}{2}}$ transitions, respectively, produce a three-photon ionization current used to study the broadening and Rabi splitting of the 3P123{P}_{\frac{1}{2}}3P21​​ intermediate state as a function of probe-laser tuning. We find a narrow, intense-laser detuning region where the peak-heights ratio of the Rabi-split 3P123{P}_{\frac{1}{2}}3P21​​ doublet is reversed from that predicted for a monochromatic laser field. This is attributed to the finite (\ensuremath{\sim} GHz) laser linewidth.
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.