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Journal of Physics B Atomic Molecular and Optical Physics

Spectroscopy of autoionizing states of the strontium atom in photoprocesses

S. Danzan, С. И. Страхова

Published Oct 28, 1990• 1 citation
Publisher

Abstract

The characteristics of the lowest 1 P autoionizing states of the Sr atom, which are excited by a weak electromagnetic field, and also the parameters, which determine the electronic and radiative decay (harmonic generation) of these states under multistep laser excitation through the intermediate discrete states 5s5d 1 D and 5p 2 1 D, are determined. The line profiles of the resonances in the ejected photon and electron spectra are obtained. The calculation is carried out in the diagonalization approximation, the state classification is based on the LS-coupling scheme and the coupling of photon and electron decay channels is allowed for. The theoretical results agree with the experimental data on the excitation of the 1 P autoionizing states obtained using linear and non-linear optics.

DOI
10.1088/0953-4075/23/20/017

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The characteristics of the lowest 1 P autoionizing states of the Sr atom, which are excited by a weak electromagnetic field, and also the parameters, which determine the electronic and radiative decay (harmonic generation) of these states under multistep laser excitation through the intermediate discrete states 5s5d 1 D and 5p 2 1 D, are determined. The line profiles of the resonances in the ejected photon and electron spectra are obtained. The calculation is carried out in the diagonalization approximation, the state classification is based on the LS-coupling scheme and the coupling of photon and electron decay channels is allowed for. The theoretical results agree with the experimental data on the excitation of the 1 P autoionizing states obtained using linear and non-linear optics.
We present experimental photoelectron energy spectra and angular distributions from multiphoton ionization of Sr by dye laser pulses of ∼ns duration in the wavelength range of 715-737 nm and relatively low intensity (≈4 × 10 11 W·cm -2 ). The selected wavelength range comprises the highly correlated 5p 2 1 S 0 autoionizing state, excited by four photons. Our results reveal the dominant (single and double) ionization pathways that were obscured in earlier experiments, based solely on the recording of ion and ionic-fluorescence yields.
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 5p6s1P5p6s^{1}P5p6s1P autoionizing state. The involvement of the 5s5d5s5d5s5d intermediate state includes the effects of ac Stark shifts on the ionization rate, the line shape, and the photoelectron angular distributions.
In the work, the main results on excitation and decay processes of autoionizing states in alkali atoms are considered. As follows from the data on excitation dynamics of subvalence shells in an impact energy region from the lowest excitation threshold up to 600 eV, at threshold energies their electron excitation results exclusively from the formation of negative-ion resonances. At mean impact energies, the cascade population processes due to resonance excitation and subsequent radiative decay of the upper states play an essential role for the low-lying autoionizing states. By comparing with theoretical data, the absolute excitation cross sections for autoionizing states and negative-ion resonances were determined. The absolute autoionization cross sections as well as the autoionization contribution to the total single ionization cross section were also determined. Keywords: atom, ion, autoionization, resonances, ionization, effective cross section.
A theory describing the radiative decay of an autoionizing state under strong pumping by a coherent field is developed. The theory systematically takes into account the radiative decay of the unperturbed continuum. The problem at hand corresponds to the case of strongly coupled bound states decaying to electron and photon continua with the two continua also weakly coupled to each other. A master equation describing the time evolution of the atomic system is derived, and its exact solution under arbitrary initial conditions is given. The effect of radiative decay on the Fano profiles and photoelectron spectra is analyzed in detail. The time development of the system is also examined. The radiative decay of the autoionizing state and the unperturbed continuum changes the spectra in a significant way. The characteristics of the spectra are correlated with the dressed states (with complex energies) of the system. The changes in the structure of the dressed states as a function of the system parameters such as the spontaneous-emission rate and laser intensity are discussed in detail.
The fluorescence produced by an autoionizing state under strong pumping by a coherent laser field is considered. It is shown how the spectrum of emitted photons and the total number of photons are related to the two-time and single-time atomic expectation values, and explicit expressions are derived for them. Numerical results for the time development of the number of photons ejected are presented for finite times and in the long-time limit. The photon spectra are shown to exhibit ac Stark splitting of the states involved. The spectrum is a doublet for transitions to a stable final state and a triplet for transitions connected back to the initial, strongly driven state. The decay of an autoionizing state in the absence of a laser driving field is considered, and the square of the Fano qqq value is interpreted as the ratio of two decay rates.