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Physical Review A

Multiphoton ionization under sequential excitation by coherent pulses

Jean‐Claude Diels, James P. Stone

Published Apr 1, 1985• 21 citations
Publisher

Abstract

The use of pulse sequences offers a solution to the conflicting requirements of spectral selectivity and high peak intensities needed to study multiphoton processes. In addition, it is shown that random---rather than regular---pulse sequences can provide unique information on the influence of coherence on a resonant multiphoton process, because the coherence can be adjusted independently of the resonance condition.

DOI
10.1103/physreva.31.2397

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The use of pulse sequences offers a solution to the conflicting requirements of spectral selectivity and high peak intensities needed to study multiphoton processes. In addition, it is shown that random---rather than regular---pulse sequences can provide unique information on the influence of coherence on a resonant multiphoton process, because the coherence can be adjusted independently of the resonance condition.
In this paper we present novel and simple pulse sequences for enhancing the intensity of forbidden or highly nonresonant optical transitions. These sequences provide a straightforward approach to circumventing the most serious limitation of optical coherent transient spectroscopy: Available laser intensities are often insufficient to excite a significant fraction of the ground state population into desired excited states, either because of large inhomogeneous broadening or, in the case of multiphoton absorption, large anharmonicities. Optical phase modulation (which can be produced by an acousto-optic modulator) or amplitude modulation (which can be produced by an interferometer) with carefully chosen pulse flip angles and delays can effectively remove even very large energy mismatches, thus permitting essentially complete population inversions arbitrarily far from resonance. Coherent averaging theory and computer calculations are used to derive particularly valuable sequences. Pumping enhancement of 103–105 (depending on individual molecular parameters) for these modulated pulse sequences are predicted. Specific applications to multiphoton pumping of local vibrational modes are discussed.
The non linear interaction between an intense laser radiation and atoms leads to ionization through the absorption of N photons from the laser radiation via laser-induced virtual states. The multiphoton ionization rate varies as a function of the laser intensity I as IN. We discuss the two most important effects which govern multiphoton ionization processes : resonance effects and laser-coherence effects. In a moderate laser intensity range (107 - 109 W cm-2) corresponding to the two, three or four-photon ionization of atoms, resonance effects play a dominant role while the photon statistics of the laser radiation are relatively unimportant. On the contrary, in the very high intensity range (1012 - 1013 W cm-2) required to observe large Nth - order ionization processes, coherence effects play a dominant role through N! enhancement in the N-photon ionization rate, while resonance effects are completely damped due to the high laser intensity. We conclude by reviewing the possible applications of multiphoton absorption processes in other fields.
Multiphoton coherent interaction effects are investigated in atoms and molecules. It is demonstrated theoretically as well as experimentally that, in the presence of a two photon resonance, maximum conversion efficiency can be achieved using energy preserving pulse sequences, in condition of coherent interaction. However, highest conversion efficiencies are obtained with weak two photon resonances. A study of four photon resonant coherent interaction in mercury points to a peculiar quenching of the interaction by the generated third harmonic. Four photon resonant coherent interaction is also investigated in molecules, where it is shown that an input signal can be found that results in selective excitation of the upper levels.
As a succession of two pulses propagates through a medium which exhibits a resonance (single or multiphoton), the resonant parts of the susceptibility will depend on the relative phase of the two pulses, provided the delay between the pulses is shorter than the phase relaxation time of the interaction. As a result of this "coherent interaction", the reradiation of the sample (the fluorescence, the absorption and the generation of harmonics) will be a periodic function of the phase difference and delay between the two pulses of the sequence 1. The study of these coherent effects is important, because one expect to find conditions under which the maximum harmonic conversion to much shorter wavelengths can be greatly enhanced 2. The presence of the multiphoton resonance enhances the nonlinear susceptibility responsible for the harmonic generation, but reduces also the maximum achievable conversion efficiency because of resonant losses (depletion of the fundamental by multiphoton absorption). The latter effect can be eliminated by a judicious choice of the pulse sequencing and phasing. In addition to demonstrating, for the first time, coherent interaction enhancement in harmonic generation, the data of third harmonic versus relative phase of the pulse sequences enable us to measure the phase angle of the third order susceptibility (hence the contribution of the continuum relative to that of the resonance).