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Journal of Physics B Atomic Molecular and Optical Physics

Laser avoided crossing in the ionization continuum

Yun Shao, O. Faucher, Jian Zhang, Dimitrios Charalambidis

Published Mar 14, 1995• 6 citations
Publisher

Abstract

We present experimental results in AC-Stark splitting of an autoionizing state probed by a multiphoton process. In the experiment performed in calcium, the 3d4f 1 P 1 autoionizing state is single-photon coupled with the 4s6s 1 S 0 excited state. The splitting caused by this coupling is probed by three-photon excitation from the ground state. Detuning and laser intensity dependent measurements have been carried out in a collision-free environment. The experimental results can be interpreted and modelled in terms of a few levels system.

DOI
10.1088/0953-4075/28/5/008

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Physical Review Letters•Jul 24, 1978•71 citations
We present experimental results in AC-Stark splitting of an autoionizing state probed by a multiphoton process. In the experiment performed in calcium, the 3d4f 1 P 1 autoionizing state is single-photon coupled with the 4s6s 1 S 0 excited state. The splitting caused by this coupling is probed by three-photon excitation from the ground state. Detuning and laser intensity dependent measurements have been carried out in a collision-free environment. The experimental results can be interpreted and modelled in terms of a few levels system.
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.
We present the experimental and theoretical investigation of the effects of the strong electromagnetic coupling of two autoionizing states (AIS's) on the photoionization properties of an atomic system. These effects are observed by probing the ladder system from the ground state of the atom through a weak two-photon transition to the region of the lower AIS. The experimental observation of the modification of the total ionization and partial photoelectron spectra at different wavelengths of the coupling and probe lasers are accurately reproduced by a theoretical analysis which uses a density-matrix approach. Both theory and experiment indicate a strong inhibition of the ionization of the system in the case when both lasers are resonant with the two atomic transitions, while displaying a rapidly varying asymmetry induced as the detuning between the two states and the fields increases. In the extreme case of a large detuning, two separate structures appear as distinct total ionization and photoelectron peaks. The above findings combined with the full theoretical treatment indicate that the phenomena observed are an experimental demonstration of ac Stark splitting between two AIS's. \textcopyright{} 1996 The American Physical Society.
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.
An experimental study of doubly resonant multiphoton ionization of Na has been carried out. Coherence effects, such as AC - Stark splitting and population trapping, have been manifested in different coupling schemes, i.e. ladder, lambda, vee configurations. These effects have been investigated in terms of their dependence on the laser intensity and resonance detuning, as well as the state of the laser polarization. The experimental results are qualitatively interpreted in terms of a few level system.
The coherent resonant coupling of two autoionizing states of magnesium by an intense electromagnetic field is investigated experimentally by means of ionization and energy-selected photoelectron spectra probed from the ground state of the atom. The resulting effect of a drastic reduction in the ionization of the system is interpreted as the splitting of the two states achieved through their mixing by the field. A complete theoretical treatment of the problem employing the density matrix formalism is presented, reproducing the effects observed and explaining the phenomenon in terms of the relative populations of the states in the system.
An intense laser and a probe laser in near resonance with the sodium $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ and $3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{3}{2}}$ transitions, respectively, produce a three-photon ionization current used to study the broadening and Rabi splitting of the 3P123{P}_{\frac{1}{2}}3P21​​ intermediate state as a function of probe-laser tuning. We find a narrow, intense-laser detuning region where the peak-heights ratio of the Rabi-split 3P123{P}_{\frac{1}{2}}3P21​​ doublet is reversed from that predicted for a monochromatic laser field. This is attributed to the finite (\ensuremath{\sim} GHz) laser linewidth.