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Journal of the Optical Society of America B

Laser temporal and spatial effects on ionization suppression

Justin B. Peatross, David D. Meyerhofer, Mikhail Vladimirovich Fedorov

Published Aug 1, 1992• 6 citations
Publisher

Abstract

Ionization suppression can be defined as the decrease of photoinduced atomic ionization rates with an increased applied field intensity. The temporal and spatial intensity distribution of a focused laser pulse introduces difficulty in observing ionization suppression. An analysis of the effect of a temporally Gaussian laser pulse on ionization suppression from Rydberg levels is given. It is found that ionization suppression by the Gaussian pulse is still apparent, even though the low-intensity tails of the pulse produce moderate ionization. Further, the spatial effects are examined for a scheme whereby a narrow atomic beam is passed through the focus of a laser at which ionization is detected. Ionization suppression in the more intense region of the laser focus is predicted to be apparent above the background ionization from the relatively large region of low intensity surrounding the laser focus.

DOI
10.1364/josab.9.001234

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We have observed suppression of ionization in Rydberg atoms irradiated by an intense short-pulse laser field. We find that single-photon ionization of the outer electron in a Ba 6snk Stark state can be severely suppressed relative to high-order (\ensuremath{\ge}3 photon) nonresonant multiphoton ionization of the tightly bound 6s core electron. This population trapping occurs in intense short laser pulses, where the bandwidth of the ionizing laser is greater than the spacings between adjacent Stark states, and may be due to a substantial modification of the initial Stark state via stimulated Raman processes through the continuum. The Ba+∗{\mathrm{Ba}}^{+\mathrm{*}}Ba+∗ ion should be left in a higly excited superposition state, or wave packet.
A single-transverse-mode QQQ-switched Nd-glass laser, which can operate over a number of longitudinal modes variable from 1 to 100, has been used to investigate laser temporal-coherence effects on the 11-photon ionization of xenon atoms. When the laser pulse has an over-all bandwidth of 4 ${\mathrm{cm}}^{\ensuremath{-}1}$, the number of ions formed is enhanced by ${10}^{6.9\ifmmode\pm\else\textpm\fi{}0.3}$ over that produced by a monomode laser pulse with the same average laser intensity. The observed enhancement in the number of ions due to the change in temporal coherence of the pulse is in good agreement with recent theoretical prediction.
We examine the suppression of 1+1 photodissociation or photoionization by laser fields. We find that atoms and molecules become more stable against photoionization or photodissociation when the intensity of the nonresonant laser field is increased above a limiting value for a two level plus continuum model system. We treat the ionization or dissociation dynamics analytically and discuss the conditions for optimum yields in a two-frequency square pulse laser field. We observe similar suppression against photodissociation for realistic sine-squared laser pulses. We demonstrate such a type of molecular suppression for 1+1 photodissociation of H2.
We show that, when an atom prepared in a high Rydberg state is one-photon ionized by a laser, a near resonance with some lower-lying state permits the formation of a nondecaying dark state of the coupled laser–atom system. This dark state traps population in an amount that increases to some upper limit as the laser radiation used becomes stronger. Thus the dark state formed causes a decrease in the Rydberg-atom ionization with increasing laser intensity, i.e., stabilization, which survives even for long times. This long-time stabilization is described by a simple zero-pole formula, and the physical meaning of the term long time is explained. We estimate, for model situations, the laser frequencies that ensure dark-state formation and maximum ionization suppression. The laser pulse intensity and duration that are required for the observation of the dark state are evaluated.
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.
One-photon ionization from the 6s27d 1^{1}1D2{\mathit{D}}_{2}D2​ state in barium is measured with short (0.25--2.7 ps), high-intensity laser pulses. Fermi's golden rule predicts that only the fluence (time-integrated intensity) determines the yield. We observed a decrease in the yield for fixed-fluence pulses shorter than the Kepler orbit time of the Rydberg electron (2.2 ps). This is explained semiclassically: The wave function of a Rydberg electron performs a Kepler-like orbit. Only the wave function near the core can be ionized. Not all of the wave function nears the core during a short pulse, and therefore the wave function far away from the core is stable against ionization. A quantum-mechanical calculation based on Raman transitions over the continuum agrees well with experimental observations and the semiclassical explanation.