ИСТИНА

Multiphoton spectroscopy has been a versatile and powerful tool for probing atomic structure. It enables the probing of such continuum structures as the Cooper minimum in ionization from excited states, dipole-forbidden autoionizing states (AIS), and many others [1]. Of particular interest is the fact that some properties, such as the Fano profile index [2], are determined not by the AIS alone, but also by the state from which the AIS is excited and even by the field polarization [3]. The present report is inspired by recent progress in observation of high harmonic generation (HHG) in the vicinity of the 3d94s24p AIS of Ga+ ion [4,5]. Here we developed an approach based on a solution of the time-dependent Schrödinger equation in a limited basis of discrete states of the unperturbed ion [6]. The advantage of this approach is that a high quality spectroscopic model can be developed and implemented. Using MCHF we calculated energies and dipole matrix elements for transitions within a large group (46) of discrete states of Ga+; in addition, we calculated dipole transition amplitude from the ground and the excited states into 20 continuum channels and several autoionization matrix elements. Then the system of equations on the coefficients of wave function series was found for the two different electromagnetic pulses. First, we applied the approach to single photon ionization of Ga+ in the vicinity of the AIS under consideration and checked the sensitivity of the AIS profile to their relative configuration interaction. Having tested the method, we then applied it to seven-photon ionization in the conditions relevant for [4]: intensity 1.6 TW/cm2, wavelength 397 nm, pulse duration 128 opt. cycles (30 fs). The results reveal a significant enhancement, by nearly two orders of magnitude, in the radiation of the seventh harmonic at the corresponding energy compared to the same calculation for the continuum structure. References [1]. Mainfray G. and Manus C. Rep. Prog. Phys. 54, 1333-1372 (1991). [2]. Fano U. and Cooper J. W. Phys. Rev. A 137, 1364 (1965). [3]. Popova M.M. etal Atoms 10(4), 102 (2022). [4]. Ganeev R.A. Opt. Express 32 (24), 43571–43585 (2024). [5]. Strelkov V. Phys. Rev. Lett. 104, 123901 (2010). [6]. Magunov A.I., Strelkov V.V. and Yudin S.N. Phys. Wave Phen. 31 (6), 418–426 (2023).