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Physical Review A

Multiphoton ionization dynamics of barium Rydberg states in intense femtosecond pulses

Duncan Tate, Thomas F. Gallagher

Published Oct 1, 1998• 5 citations
Publisher

Abstract

We have investigated multiphoton ionization of Ba $6snl$ Rydberg states by intense 200-fs laser pulses. Specifically, we have excited Ba atoms to the $6sns$ and $6snd$ 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 ${\mathrm{Ba}}^{+}$ Rydberg states and ${\mathrm{Ba}}^{2+}$ 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 ${\mathrm{Ba}}^{+}$ Rydberg states.

DOI
10.1103/physreva.58.3058

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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.
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++.
Multiphoton ionization (MPI) of barium atom using an excimer-pumped pulsed dye laser has been studied in the wavelength range 570-608 nm at a laser intensity approximately 10.0 GW cm -2 . Resonances in MPI were recorded by monitoring either the ions produced in the multiphoton process using a thermionic detector or fluorescence from the ions formed in their various excited states of Ba + . The observation of photoion and fluorescence spectra were made on Ba vapour in the heat-pipe oven in the moderate-density regime ( approximately 10 13 atoms/cm 3 ) so that the collisional effects are negligible. Fine-structure resolution of the final ionic state (6p 2 P 1/2,3/2 ), produced after four-photon autoionizing decay was obtained. The decay of the Ba atom after four-photon absorption preferentially populates the 6p 2 P 3/2 state relative to the 6p 2 P 1/2 state.
The authors have observed the production of Ba2+ from neutral Ba by intense picosecond laser pulses in the range 560-650 nm. Several new features have been observed, which were not observed with nanosecond pulses. Most of the strong resonances in Ba2+ production can be attributed to Ba+ transitions originating in the 6s1/2 or 5dj states, suggesting that the dominant process in double ionization in this intensity and wavelength regime is sequential removal of electrons. However, there are several resonances which cannot be attributed in this manner, evidence for possible alternative ionization mechanisms. Ba2+ production is strong throughout the region studied, at times comparable to that of Ba+. The authors have also looked for Ba2+ production in the range 280-325 nm, but no Ba2+ signal was observed in this part of the ultraviolet.
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.
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.
In an effort to gain more insight into the process of multiphoton multiple ionization, several experiments have recently been carried out to determine the wavelength dependence of the double ionization of Sr and Ba. 1-3 These studies were not as conclusive as one might hope, in that the observed resonances in the production of Sr++ and Ba++ did not quite match the energy levels of Sr+ and Ba+. Here we report experiments in which we have compared the spectra for the production of Ba by an intense laser with and without the population of specific states of Ba by modest secondary lasers. The results of these experiments show clearly that Ba++ is produced by first producing real intermediate states of Ba+ which are multiphoton ionized through intermediate resonances in Ba+.