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Annales de Physique

Expérience "deux couleurs" dans le magnésium. Excitation et couplage d'états autoionisants par des sources UV

Natalia E. Karapanagioti, Y. L. Shao, O. Faucher, Dimitrios Charalambidis, H. Bachau, E. Cormier

Published Oct 1, 1994• 0 citations
Publisher

Abstract

We study double-resonance processes in Magnesium.One laser is used to "pump" the 3p2 resonance through two-photon absorption from the 3s^ fundamental state of Mg.A second laser couples the 3p^ l se resonance to the 3p3d lp° narrow autoionizing state.In the experiment the ionization rate is measured in two different situations; with a fixed frequency for the pumping (coupling) laser and varying the frequency of the coupling (pumping) laser.In agreement with the experimental findings, a theoretical study shows that, for particular frequencies of the laser fields, a situation of coherent trapping occurs with a partial stabilization of the system.

DOI
10.1051/anphys/1994020

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Physical Review Letters•Sep 21, 1992•7 citations
We study double-resonance processes in Magnesium.One laser is used to "pump" the 3p2 resonance through two-photon absorption from the 3s^ fundamental state of Mg.A second laser couples the 3p^ l se resonance to the 3p3d lp° narrow autoionizing state.In the experiment the ionization rate is measured in two different situations; with a fixed frequency for the pumping (coupling) laser and varying the frequency of the coupling (pumping) laser.In agreement with the experimental findings, a theoretical study shows that, for particular frequencies of the laser fields, a situation of coherent trapping occurs with a partial stabilization of the system.
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.
Using the R -matrix Floquet theory we have carried out non-perturbative, ab initio one- and two-colour calculations of the multiphoton ionization of magnesium with the laser frequencies chosen such that the initial state of the atom is resonantly coupled with autoionizing resonances of the atom. Good agreement is obtained with previous calculations in the low-intensity regimes. The single-photon ionization from the excited state of magnesium has been studied in the vicinity of the autoionizing resonance at non-perturbative laser intensities. Laser-induced degenerate states (LIDS) are observed for modest laser intensities. By adding a second laser which resonantly couples the and autoionizing levels, we show that, due to the small width of the state, LIDS occur between this state and the state at intensities of the first laser below . We next investigate the case in which the first laser induces a resonant two-photon coupling between the ground state and the autoionizing state, while the second laser again resonantly couples the respective and autoionizing states. At weak intensities, our calculations compare favourably with recent experimental data and calculations. We show that when the intensity of the first laser is increased, the effect of an additional autoionizing state, the state, becomes significant. This state is coupled to the autoionizing level by one photon, inducing a triply resonant processes. We show that LIDS occur among the three autoionizing levels and we discuss their effect on the decay rate of the ground state. We consider dressed two- and three-level atoms which can be used to model the results of our calculations.
An anomalous resonant dip of two-photon ionisation with 3s3p 1 P as an intermediate state and a stimulated emission by the 3s4s 3 S-3s3p 3 P transition, at 518.37 nm following the pulsed laser excitation near the 3s 2 1 S-3s3p 1 P resonance at 285.21 nm in magnesium vapour are observed and studied. The mechanism of the ionisation dip is attributed to formation and predissociation of the A 1 Sigma + u Mg 2 dimer, which transfer the population from the laser-pumped 3s3p 1 P level to 3s3p 3 P levels. Re-excitation of the produced 3s3p 3 P atoms and the subsequent radiative processes are presumed to explain the generation of stimulated emission. A theoretical model is employed to interpret the experimental phenomena.
We present results from perturbative calculations on a scheme of coherent control in Mg. The scheme involves the excitation of Mg in the vicinity of an autoionizing resonance lying above the first two ionization thresholds by a two-color field composed of a fundamental frequency and its third harmonic, whose relative phase can be continuously controlled. Four as well as two-photon coherent excitation pathways, involving appropriate combinations of the two frequencies, proceed through the atomic continuum simultaneously contributing to the excitation, while three-photon ionization by the fundamental is also energetically possible. As a result, some of the excitation processes involve above-threshold ionization (ATI). The calculated ionization yields for the various groups of photoelectrons reveal significant modulation as the relative phase of the field component frequencies is varied. The modulation patterns for the different photoelectron groups are mutually shifted due to the multiphoton matrix element phases associated both with the ATI process and the multichannel nature of the final state. The effect is particularly pronounced when the autoionizing state is in near four-photon resonance with the fundamental frequency. The role of an autoionizing state as a final state as well as that of the continuum as an intermediate state in coherent control processes is discussed.
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.
Observations of a new dynamical process, absorption of two optical photons during a binary collision, have been made. Measurements on magnesium-rare-gas collisions display a rich phenomenology associated with the collisions, the relative linear polarization directions of the exciting light beams, and their separate detunings from atomic Mg 3s 1^{1}1S0{\mathit{S}}_{0}S0​\ensuremath{\rightarrow}3p 1^{1}1P1{\mathit{P}}_{1}P1​\ensuremath{\rightarrow}5s 1^{1}1S0{\mathit{S}}_{0}S0​ stepwise resonance. Polarization spectra for Mg-Ne and Mg-Ar fractional collisions are reported, and are seen to contain information on short-range collision dynamics and on evolution of alignment produced in the process.