Dynamical core polarization of two-active-electron systems in strong laser fields

Abstract

The ionization of two-active-electron systems by intense laser fields is investigated theoretically. In comparison with time-dependent Hartree-Fock and exact two electron simulation, we show that the ionization rate is overestimated in SAE approximation. A modified single-active-electron model is formulated by taking into account of the dynamical core polarization. Applying the new approach to Ca atoms, it is found that the polarization of the core can be considered instantaneous and the large polarizability of the cation suppresses the ionization by 50% while the photoelectron cut-off energy increases slightly. The existed tunneling ionization formulation can be corrected analytically by considering core polarization.PACS numbers: 33.80. Rv, 42.50.Hz, 42.65.Re Various of non-perturbative phenomena occurring during atom-laser interactions are started with single ionization, e.g., above threshold ionization (ATI) and high harmonic generation (HHG). Although they have been successfully interpreted by the rescattering model based on single active electron (SAE) approximation (see Reviews e.g., [1,2]), detailed examination showed that multielectron effects are embedded in the photon and electron spectra [3][4][5][6][7][8][9]. It is found that high-order harmonic generation (HHG) from molecules records interference of different channels suggesting more than one molecular orbitals are involved [3] and electron rearrangement is occuring [4], which is certainly beyond the scope of the SAE theory. On the other hand, two-electron events such as non-sequential double ionization can not be explained either without considering the electron-electron interaction [10]. It is thus desirable to examine in details the multielectron effects occurring in the ionization of atomic systems beyond SAE.The single ionization of atoms in strong laser fields can be pictured as tunneling of one electron through the barrier formed by the atomic potential and the laseratom dipole interaction. The Keldysh parameter measures the ratio of tunneling time to the optical period, γ = I p /2U p , where I p = κ 2 /2 is the ionization potential and U p = E 2 /4ω 2 is the ponderomotive energy of a free electron in a laser field of strength E and frequency of ω. When γ < 1, tunnel ionization occurs so rapid that the electric field can be considered as a static field at each instant. The so-called adiabatic approximation is the root of Ammosov-Delone-Krainov (ADK) -like theories [11] for obtaining ionization rates. Based on this picture, the rate is mainly determined by the unitless quantity κ 3 /E with κ 3 representing the atomic field strength at the classical radius of the electron motion.It is obvious that the adiabatic approximation will break down if the atomic potential acting on the tunneling electron is varing sooner than the tunneling time. For more than one electron systems, the core can be polarized by the laser fields, hence the atomic potential is time-varying. In the case of absence of resonant excitation, the polarization is instantaneou...

Regulation of Continuum Harmonic Generation by Using Near-infrared Chirped Pulses

“…The second is to verify the prediction by examining the "actual" process of CO based on the HHG spectra calculated with the time-dependent wave functions within the single-active electron (SAE) approximation. It is worth mentioning that within this framework, we take dynamic coreelectron polarization (DCEP) into account because of its important role in strong-field processes not only for polar molecules such as CO [12][13][14][15][16][17], OCS [18] but also for non-polar molecules such as CO 2 [19] and even for atoms such as Ca [20].…”

Real-Time Observation of Electron-Hole Coherence Induced by Strong-Field Ionization

“…When the field reverse its direction, the electron recombines into the original hole leading to HHG. However, due to the multielectron nature, manybody effects, e.g., dynamical polarization, [24,25] will modify the dynamics of both electrons and holes. Particularly, previous work has demonstrated that multielectron information is encoded in HHG when inner shell electrons are excited by the IR lasers and participate in the ultrafast dynamics, [26][27][28][29][30][31][32][33] while the effect on HHG from coherent driving of core-valence transition by attosecond pulses remains unexplored.…”

“…[28], where the electron released from the valence orbital promotes the inner-shell electron to the vacancy it created upon recollision, and recombines into the newly formed hole to emit harmonics. This multi-electron collision plays a crucial role in strong-field ionization in the form of dynamical core polarization, [24,25,29,35] and manifest itself in high-harmonic generation. [26,28,29] However, compared to the XUV resonant excitation in this study, the probability of collision-induced rearrangement is small enough to be neglected.…”

Carrier envelope phase measurement of multi-cycle mid-infrared field and its application on attosecond pulse generation

“…Thus, several approaches have been proposed to obtain the SAPs, including the ionization and the polarization gating methods [12][13][14][15][16][17][18][19][20], the attosecond lighthouse [21][22][23] and the mix-color field method [24][25][26][27][28][29][30][31][32] etc. Recently, beyond the SAEA, Zhao's group [33,34], Feng et al [35] and Koval et al [36] investigated the HHG They found that due to the multi-electron effect, the harmonic cutoff can be further extended but with a very lower harmonic intensities in the extended region.…”