Theoretical study of angular-resolved two-photon ionization of H2

Abstract

Cross sections and angular distribution parameters of electrons ejected via two-photon ionization of the hydrogen molecule by linearly and circularly polarized light are computed in the exciting-photon energy range of 8-14 eV, which covers the first four optical resonant states of the molecule. Photoelectron partial waves in the continuous spectrum are obtained within the single center method in precise numerical potential of the molecular ion field. The correlation function technique is used to calculate the two-photon transition matrix element. The presently computed angular-resolved spectra are in good agreement with the theoretical results available for the photon energy of 8.854 eV and can be considered as reliable predictions in the respective energy range.

Multi-photon above threshold ionization of multi-electron atoms and molecules using the R-matrix approach

“…We see a good agreement with the calculation of Apalategui and Saenz [41]; the small systematic difference in magnitude of the cross section before the first resonance is discussed in Appendix D, where we show that it probably comes from a typo in the codes of Apalategui and Saenz evaluating their formula for the orientational average. We also observe a qualitative agreement with the calculations of Ritchie and McGuire [39] and Demekhin et al [42] Here, the quantitative differences also follow from very different ionization thresholds used; whereas Demekhin et al employ the experimental value of the adiabatic first ionization threshold between ground vibronic states (15.43 eV), in the present calculation we use our calculated value for the vertical threshold, which-for the employed model-comes out as 16.43 eV.…”

“…The results below the one-photon ionization thresholds are compared to other published calculations [39,41,42]. We see a good agreement with the calculation of Apalategui and Saenz [41]; the small systematic difference in magnitude of the cross section before the first resonance is discussed in Appendix D, where we show that it probably comes from a typo in the codes of Apalategui and Saenz evaluating their formula for the orientational average.…”

“…The large 1 eV difference is responsible for most of the relative shift of the resonance features between the datasets. Furthermore, the quantitatively different behaviour is related to the choice of different quantum chemical models: The models used in [39] and [42] appear to provide results much closer to the static exchange model discussed earlier, see Fig. 5d-f, where also our calculated one-photon vertical first ionization threshold (15.55 eV) accidentally agrees better with the adiabatic experimental value.…”

“…Figure 5a-c shows the molecular-orientation-averaged data: the total isotropic cross section σ 0 and the asymmetry parameters β 2 and β 4 , respectively, calculated by transforming the matrix elements (7) to the spherical basis and performing averaging over molecular orientations parametrized by Euler angles as in [42,44]; see Appendix D. Alternatively, the isotropic cross section σ 0 can be obtained directly in the real spherical harmonic basis and molecular frame by averaging only over polarizations of the photons and disregarding the dependence on the direction of the photoelectron momentum, by use of the formula [56,57]…”

“…An exception to this rule is the multi-electron molecular R-matrix Floquet (RMF) approach [36] based on an earlier atomic RMF theory [37] and applied to MPI of H 2 [38]. With the exception of two-photon cross-sections for molecular hydrogen [39][40][41][42] and nitrogen [43] and general (N + 0) MPI of molecules in DFT approach [44], multi-photon processes in molecules are studied using time-dependent approaches for dynamics in arbitrary external fields, see e.g. [23,24] and references in [45]; this is computationally very demanding and doesn't provide direct access to the multi-photon transition matrix elements.…”