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Journal of Physics B Atomic Molecular and Optical Physics

Resonant double multiphoton ionization via planetary states

S. Cohen, Pierre Camus, Agisilaos Bolovinos

Published Nov 14, 1993• 11 citations

Abstract

The authors have studied double ionization of Ba 6snl Rydberg states through resonant two-photon excitation of 8snl double-Rydberg states with n=13 and 14 and different l. The laser intensity dependence of Ba 2+ yield measured for l>or=8, when the electrons suffer long-range dipole interaction, shows a decrease from four-photon for l=8 to less than three for l=11. The four-photon dependence is due to autoionization decay populating the Ba + (7p) levels which gives an indirect signature to the Ba 2+ production. By increasing l this decay vanishes and the data show evidence of a direct double-ionization process via planetary-electron states for l>or=10. Additional intensity measurements performed for low l=6 states strongly mixed with 5fn'l' ones by short-range multipole interaction, reveal indirect Ba 2+ production and an individual case of partial stabilization due to destructive interference leading to direct double ionization.

DOI: 10.1088/0953-4075/26/21/014

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The authors have studied double ionization of Ba 6snl Rydberg states through resonant two-photon excitation of 8snl double-Rydberg states with n=13 and 14 and different l. The laser intensity dependence of Ba 2+ yield measured for l>or=8, when the electrons suffer long-range dipole interaction, shows a decrease from four-photon for l=8 to less than three for l=11. The four-photon dependence is due to autoionization decay populating the Ba + (7p) levels which gives an indirect signature to the Ba 2+ production. By increasing l this decay vanishes and the data show evidence of a direct double-ionization process via planetary-electron states for l>or=10. Additional intensity measurements performed for low l=6 states strongly mixed with 5fn'l' ones by short-range multipole interaction, reveal indirect Ba 2+ production and an individual case of partial stabilization due to destructive interference leading to direct double ionization.

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.

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+.

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

{\mathrm{Ba}}^{+}

ion in a Rydberg state. For low-n states the outer electron is ejected and the residual

{\mathrm{Ba}}^{+}

ion can subsequently be multiphoton ionized to

{\mathrm{Ba}}^{++}

.

Despite inherently complex multiphoton dynamics, our observations show that Ba double ionization with an infrared laser (8800--8920 cm${}^{\ensuremath{-}1}$) resembles a single-photon process; namely, its probability is proportional to the laser intensity. In this regime, single-electron ionization is due to a four-photon resonant transition through the highly perturbed state

6{p}^{2}\phantom{\rule{0.16em}{0ex}}{}^{1}{D}_{2}

, whereas double ionization is realized by the two-electron mechanism. Furthermore, we argue that these conclusions are valid for other alkaline-earth-metal atoms and other parameters of laser radiation.

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.