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Topical Meeting on Multiple Excitations of Atoms

Multiphoton Excitation and Ionization of Atoms

Hernan J. Jara, Ulrich A. Johann, T. S. Luk, Iain A. McIntyre, A. McPherson, A. P. Schwarzenbach, Kefneth G. Boyer, C. K. Rhodes

Published Jan 1, 1986• 0 citations

Abstract

It has been conjectured that atomic motions fundamentally different from those presently observed may be driven under extreme conditions of irradiation. Indeed, it has been suggested1-4 that ordered many-electron motions of outer-shell electrons could lead to enhanced rates of coupling from the radiation field to an atom. In order to study the conditions necessary for new atomic responses to occur and to determine the influence of possible damping mechanisms, the properties of ion charge state distributions,5 electron energy spectra,6 and harmonic radiation7 produced by irradiation of atoms with ultra-violet radiation with different pulse lengths, wavelengths, and intensities have been investigated.

DOI: 10.1364/mea.1986.wa1

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Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence leads to the hypothesis that the basic character of the atomic response involves highly organized, coherent motions of entire atomic shells. The important regime, for which the radiative field strength is greater than an atomic unit (e/a(2)(0)), can be viewed in approximate correspondence with the physics of fast (approximately 10 MeV per atomic mass unit) atom-atom scattering. This physical picture provides a basis for the expectation that stimulated emission in the x-ray range can be produced by direct, highly nonlinear coupling of ultraviolet radiation to atoms.

In recent years a strong interest has developed in the study of the basic coupling mechanisms governing high order multiphoton processes in atomic systems.1 Such interest has been stimulated by the rapid advances in short pulse generation techniques which allow atomic interactions at an intense field strength of the order of atomic field to be studied. Furthermore, good understanding of the highly nonlinear photon interactions is necessary to evaluate the possibility of using multiphoton processes2 as a means to couple energy into excited states emitting in the x-ray region.

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.

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

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

ion should be left in a higly excited superposition state, or wave packet.