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Journal of the Optical Society of America B

Unusual three-photon ionization of magnesium atoms using a XeCl excimer laser

S. L. Chin, W. Xiong, K. X. He, François Yergeau

Published Sep 1, 1987• 1 citation
Publisher

Abstract

Multiphoton ionization of Mg was studied with an excimer laser (XeCl, λ = 308 nm). The laser-intensity dependence of the number of ions indicates that the lowest-order two-photon process, which has enough energy to reach the first ionization limit, fails to dominate the ionization of Mg. The slope is 3 when a low-enough extracting field is applied and increases with the latter. A three-photon resonance with some autoionizing states is proposed as a partial explanation of this abnormal phenomenon, but the variation of the slope of the log N versus log I curve as a function of a dc electric field remains unexplained.

DOI
10.1364/josab.4.001345

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Multiphoton ionization of Mg was studied with an excimer laser (XeCl, λ = 308 nm). The laser-intensity dependence of the number of ions indicates that the lowest-order two-photon process, which has enough energy to reach the first ionization limit, fails to dominate the ionization of Mg. The slope is 3 when a low-enough extracting field is applied and increases with the latter. A three-photon resonance with some autoionizing states is proposed as a partial explanation of this abnormal phenomenon, but the variation of the slope of the log N versus log I curve as a function of a dc electric field remains unexplained.
We investigate the interaction of ground-state Mg atoms with tuneable laser radiation of ∼5 ns duration and moderate intensity (≤10 12 W cm −2 ), capable of exciting the 3p 2 1 S 0 autoionizing state by four photons. For the corresponding photon energy range, this level is strongly one-photon coupled both with bound (third photon) and, primarily, a multitude of other autoionizing states (fifth photon). The coupling is strong enough to induce the absorption of up to six photons before the atom is ionized, creating thus population into the first excited 3p j Mg + levels which subsequently decay radiatively to the 3s 1/2 ionic ground state. The earlier studies devoted to this excitation scheme were thorough but partial. Particularly, the issues concerning the five-photon excited levels were not addressed. In this work, the examined wavelength range is greatly extended to include these states. Moreover, the interaction is characterized in more detail by employing an array of experimental techniques, namely Mg + ion and photoelectron spectroscopy (including photoelectron angular distributions (PADs) from four- and five-photon ionization) as well as fluorescence spectroscopy where the 3p j →3s 1/2 decay is monitored. The experimental data show that most of these five-photon excited levels undergo blue ac Stark shifts which are occasionally nonlinear with respect to the laser intensity and comparable to or even larger in magnitude than the large redshift of the 3p 2 1 S 0 state itself. Finally, for the latter state, the wavelength dependence of four-photon PAD measurements is suggestive of a situation reminiscent of an ac Stark splitting picture.
In the present study we have investigated three-photon ionization in Ca in which autoionizing states are engaged. The two-photon resonant process (from the Ca ground state 4s 2 1 S 0 ) occurred through or at least in the vicinity of one of the following states: 4s4d 1 D 2 , 4p 2 3 P 2 , 4s6s 1 S 0 , 4p 2 1 D 2 and 4p 2 1 S 0 , with the third photon either reaching the continuum directly or one of the autoionizing states. The three-photon resonant transitions to 3d m p, m f: 1 P 1 , 3 P 1 and 3 D 1 autoionizing states for m up to 21 have been observed. Some of the autoionizing resonances which we have found had not been observed before in a high-resolution one-photon absorption experiment (for J = 1) and in multiphoton experiments (for J = 3). We have compared the ionization signal as a function of the laser detuning and the laser intensity with theoretical curves obtained within a simple model (three-level atom + one-mode laser field). This gives information about the order of magnitude of the three-photon ionization probability through autoionizing states.
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.