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Physical Review A

Ionization suppression of Rydberg atoms by short laser pulses

J. H. Hoogenraad, Rutger B. Vrijen, Lambertus D. Noordam

Published Nov 1, 1994• 85 citations
Publisher

Abstract

One-photon ionization from the 6s27d $^{1}$${\mathit{D}}_{2}$ 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.

DOI
10.1103/physreva.50.4133

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Physical Review A•Sep 1, 1994•17 citations
One-photon ionization from the 6s27d ^{1}$${\mathit{D}}_{2} 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.
We have observed that the amount of photoionization from an atomic Rydberg state with a pulsed laser depends critically on the length of the pulse, whereas lowest-order perturbation theory implies that the yield depends only on the fluence (the integrated intensity) of the pulse. We have measured the yield for one-photon ionization with short (0.5–2.7-ps) high-intensity laser pulses from the 27d Rydberg state in barium. With equal fluence, the yield decreases for pulses shorter than the Kepler orbit time of the outer electron (2.2 ps) because not all of its wave function comes near the core during the length of the pulse. The observations and this classical picture are confirmed by quantum-mechanical calculations.
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.
We have studied multiphoton ionization of Ba 6snl Rydberg states by intense ps pulses and have shown that the duration of the laser pulse compared to the orbit time of the Rydberg electron determines the laser-atom interaction. For high-n states with a long orbit time, the loosely bound Rydberg electron is not affected by the intense laser pulse. Instead the inner electron is ejected by absorption of five photons, leaving the Ba+{\mathrm{Ba}}^{+}Ba+ ion in a Rydberg state. For low-n states the outer electron is ejected and the residual Ba+{\mathrm{Ba}}^{+}Ba+ ion can subsequently be multiphoton ionized to Ba++{\mathrm{Ba}}^{++}Ba++.
Laser-induced autoionization, a form of forced autoionization, is discussed. Highly excited atomic Rydberg states generally have a small cross section for photoionization due to their small overlap with the nucleus. The cross section for photoionization can be strongly enhanced if the Rydberg series is perturbed by an atomic state with a relatively large cross section for photoionization. The photoionization dynamics of such a perturbed Rydberg system shows strong similarities to that of an autoionizing Rydberg series. For intense laser pulses, which strongly couple the perturber to the continuum, the photoionization rate is limited by the coupling between the Rydberg states and the perturber or, in other words, the autoionization rate. The model describing this ionization dynamics is applied to the barium 6snd(1,3{(}^{1,3}(1,3D2{\mathit{D}}_{2}D2​) state which is perturbed by the 5d7d(1{(}^{1}(1D2{\mathit{D}}_{2}D2​) state. The calculations show good agreement with experimental results of photoionization of barium Rydberg states with subpicosecond laser pulses of light of 620 nm in wavelength.
We have investigated multiphoton ionization of Ba 6snl6snl6snl Rydberg states by intense 200-fs laser pulses. Specifically, we have excited Ba atoms to the 6sns6sns6sns and 6snd6snd6snd 15\ensuremath{\infty} states using two nanosecond lasers. The atoms were then irradiated with the fs pulses produced by a mode-locked, regeneratively amplified Ti:sapphire laser that is stepwise-tunable between 790 and 850 nm. We have used both the fundamental light and the second harmonic (395--425 nm) to investigate the ionization dynamics in these two wavelength regions. In contrast to previous experiments, in which laser pulses 5 ps or longer were used, our results indicate that excitation of Ba+{\mathrm{Ba}}^{+}Ba+ Rydberg states and Ba2+{\mathrm{Ba}}^{2+}Ba2+ photoions is relatively insensitive to the femtosecond laser wavelength. In addition, we find that, as the Kepler orbital period of the Ba Rydberg electron becomes comparable to this very small laser pulse width, the inner electron ionization process no longer appears to be instantaneous to the outer electron, and the outer electron is not projected onto Ba+{\mathrm{Ba}}^{+}Ba+ Rydberg states.
Stimulated by a recent experiment [L. D. Noordam et al., Phys. Rev. Lett. 68, 1496 (1992)], we present a model of Rydberg-atom photoionization whose essential feature is the resonance between the initially populated Rydberg state and an additional state lying below Rydberg quasicontinuum. Despite the infinite number of discrete states involved in our model we have obtained the expressions for photoionization yield and discrete-state populations in completely analytical form. These expressions cover both short- and long-time scales, i.e., when laser-pulse duration is shorter or longer than the classical round-trip time of the Rydberg electron. The short-time scale corresponds to the conditions met in the experiment of Noordam et al. and, on the basis of our expressions, we have gained at least qualitative understanding of their observations. In the case of long pulses, which in our considerations are never longer than 54 ps, near resonance is shown to be the origin of a profound ionization suppression, particularly for some specific frequencies, which we identify as those ensuring the formation of a perfectly dark dressed state of zero width and thus unionizable. This dark state is shown to be populated with a probability increasing when laser gets stronger. We discuss the connection between stabilization and formation of this perfectly dark state as well as of additional dressed states, which are dark in the sense that they have the property of becoming very narrow as laser intensity increases. The undamped perfectly dark state, created at specific laser frequencies, has given us rise to introduce what we call the zero-pole approximation for ionization yield, which, as we have shown, works well if duration and intensity of laser pulse are appropriately combined.