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The Excited Helium Photoionization

O Lhagva, S. Danzan

Published Feb 24, 2022• 0 citations
Publisher

Abstract

The photoionization characteristic behaviour of the excited He atom has been studied by using the parametrical approach, and the physical nature of the processes is cleared up. It is shown that in the excited He photoionization, the profiles of the resonance structures appear far enough from the positions of autoionization states at energies comparable hundred times of their natural widths. The mentioned peculiarities in the recombination of the He+ ioninto excited states have been observed too. It is also shown that such "extended" resonance structures are well described by "width" and "profile" functions of the energy.The competition and interference of phototransitions to open and closed (autoionization resonance) channels are found to be due to the broadening and shift in the resonance structure profile formed in the photoprocesses with the excited He. The ways to revealexperimentally the influence of closed channels in the resonance structure forming the photoprocesses with excited He atoms are discussed.

DOI
10.22353/physics.v3i132.264

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The photoionization characteristic behaviour of the excited He atom has been studied by using the parametrical approach, and the physical nature of the processes is cleared up. It is shown that in the excited He photoionization, the profiles of the resonance structures appear far enough from the positions of autoionization states at energies comparable hundred times of their natural widths. The mentioned peculiarities in the recombination of the He+ ioninto excited states have been observed too. It is also shown that such "extended" resonance structures are well described by "width" and "profile" functions of the energy.The competition and interference of phototransitions to open and closed (autoionization resonance) channels are found to be due to the broadening and shift in the resonance structure profile formed in the photoprocesses with the excited He. The ways to revealexperimentally the influence of closed channels in the resonance structure forming the photoprocesses with excited He atoms are discussed.
Single photoionization of neutral atoms is examined theoretically for a situation in which the photoionizing laser is tuned close to the resonant frequency for transitions between the ground and first-excited states of the positive ion. The resonant coupling in the positive ion is shown to have a dramatic peak-splitting effect on the kinetic-energy spectrum of the ionized electron. An essential-states analysis of the photoionization process is presented, and a comparison is made with numerical results for one-dimensional helium. The agreement between the results establishes the validity of the essential-states approach. The possibility of superposing this core-coherence effect with laser-induced autoionization effects is considered.
Abstract We present an extended theoretical study of two-colour doubly resonant photoionization of helium involving two doubly excited states below the N = 2 ionization threshold. We model the effects of a strong laser coupling between the lowest 1 P o and the selected 1 S e , 1 P e and 1 D e doubly excited states on the photoelectron (photoion) yield by taking into account the spatio-temporal structure of the pulses and by considering the effects of the neighbouring resonance states. We show that the laser coupling results in a measurable modification of the line profiles and in a reduction of the photoelectron yield under resonant or near-resonant conditions.
A projection-operator formalism is employed in studying photorecombination at energies close to an autoionizing resonance. A resonance profile is defined, and is presented without approximation for a model system consisting of one electron continuum, one autoionizing state, and one final radiatively stabilized atomic state. Effective line-profile parameters \ensuremath{\varepsilon}^ and q^ are defined, and the photorecombination resonance profile is shown to be the same as the photoionization resonance profile (for the process in which initial and final atomic states are interchanged from the photorecombination process), provided one-photon spontaneous radiative decay effects are included. The effective line-profile parameters are examined, and are shown to reduce to the standard autoionization line-profile parameters in the limit of no spontaneous radiative decay. Effective line-profile parameters are also defined in the pole approximation for two model systems featuring an isolated autoionizing state. Comparison is made with effective line-profile parameters that have been defined elsewhere in the context of photoionization into coupled electron-continuum channels. The similarity in mathematical formalism for systems supporting multiple continua, regardless of whether these continua are electron or photon continua, is discussed.
In the present work it is shown that in atomic photoionization the Fano-like behaviour of vector correlation parameters in the region of an isolated autoionizing resonance possesses two universal values: the width and the energy shift with respect to the energy of the autoionizing state. This property is confirmed by fluorescence polarimetry studies, which provide, in the particular case of Xe 4d−15/26p (J = 1) photoexcitation, the experimental determination of the alignment and the orientation of the final photoion across the resonance. 1.
In the present work, the influence of an alternating circularly polarized electric field on the energy spectrum of the He atom is studied. The calculations are performed by the method of the energy matrix diagonalization of an atom in the electric field. This method has allowed us to study the behaviour of the helium energy spectrum from the same numerical procedure under resonant and non-resonant excitations by the electric field. Based on the calculation results, we have found that the resonance effects take place not only in the vicinity of resonance, but they influence the shift directions of the Stark states even under non-resonant excitation. Additionally, we have established that the helium energy spectrum behaves consistently in the electric field. The results obtained have allowed us to clarify mechanisms of the influence of the resonance effects on the radiative characteristics of helium plasma.
We study two-color photoionization of helium in the region of the 3s3d1D3s3d{}^{1}D3s3d1D autoionizing state with resonant excitation of the intermediate 2s2p1P2s2p{}^{1}P2s2p1P autoionizing state. We show that the corresponding time-dependent Schr\"odinger equation may be solved by expanding the total wave function on a few complex energy states. Perimetric coordinates are used to describe correlation effects accurately. Populations at the end of the interaction as well as photoionization probabilities are studied as a function of the laser pulse duration and both frequencies. Our calculations show that many continua play a crucial role at high intensities.