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Journal of Physics B Atomic Molecular and Optical Physics

Strong laser-induced coupling between autoionizing states: the case of the four-photon-excited 3p2 1S0state of magnesium

Anastasios Dimitriou, S. Cohen, A. Lyras, Ioannis Liontos

Published Sep 18, 2012• 5 citations

Abstract

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.

DOI: 10.1088/0953-4075/45/20/205003

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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.

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.

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.

We report on the single and double multiphoton ionization of ground state Sr atoms observed in an atomic beam experiment with laser pulses of ∼5 ns duration, maximum intensity ∼4 × 10 11 W cm −2 and within the 710–740 nm wavelength range. The Sr + spectrum consists of two strong lines originating from three-photon resonant four-photon ionization of bound states, a number of weak autoionizing resonances and a broad line due to four-photon excitation of the doubly excited 5p 2 1 S 0 state. The latter, along with a strong, broad and structured spectral feature, is also evident in the wavelength dependence of the doubly charged Sr 2+ ion. A weakly evident but reproducible inflection point ('knee' structure) appears in the intensity dependence of the Sr 2+ yield at the location of the 5p 2 1 S 0 resonance. A complementary fluorescence experiment revealed the accumulation of population in the 5p 1/2 , 3/2 , 6s 1/2 and 5d 3/2 , 5/2 excited Sr + states during the laser pulse. All fluorescence signals depend on laser wavelength in a manner similar to the recorded Sr + spectrum. The population accumulation in the 5p 1/2 , 3/2 ionic states unambiguously proves the absorption of two photons above the first 5s 1/2 atomic threshold while that of the 5d 3/2 , 5/2 and 6s 1/2 ones suggest the absorption of at least two more. Since under our laser pulse duration and intensity the absorption of such a number of photons in an unstructured continuum is highly improbable, it may be concluded that the process is mediated by dense manifolds of near- resonant doubly excited states, their role in the dynamics of laser–atom interaction under our conditions being far more crucial than in studies conducted using intense ultrashort pulses.

A theory of two- and three-photon autoionization is presented for intermediate resonant and non-resonant cases, stressing the laser-intensity effect on the configuration mixing of the final state. Sample calculations are performed for the

5p6s^{1}P

autoionizing state. The involvement of the

5s5d

intermediate state includes the effects of ac Stark shifts on the ionization rate, the line shape, and the photoelectron angular distributions.

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.

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.