Theoretical description of the electron continuum spectrum in polyatomic molecules: Application to angular-resolved electron and fluorescence spectroscopy

• Applicant: Professor Dr. Philipp Demekhin
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2014 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 259904497

Single Center (SC) method for the calculation of the electron continuum spectrum in polyatomic molecules is developed and implemented in computer codes. In the method, a one-particle molecular orbital is represented with respect to a single center of the molecule via an expansion in terms of spherical harmonics. We developed two formulations of the method. In the stationary SC method, wave functions of a photoelectron are sought as the numerical solutions of the system of coupled Hartree-Fock equations for the radial partial waves. The time-dependent formulation of the SC method (TDSC) accounts for the interaction of electrons with external field nonperturbatively and consists in the direct propagation of single-active-electron wave packets in the effective molecular potentials in the presence of intense laser pulses. The method was tested by computing the differential photoionization cross sections of small Liclusters and applied to interpret several angular-resolved photoionization experiments with different polyatomic molecules, which were performed by our cooperation partners. In particular, we studied different types of interference effects in the inner-shell excitation followed by the resonant Auger decay (and subsequent fluorescence emission) in N2O, CO2, and CH3Cl molecules. It was also used to uncover a novel ‘decay rate borrowing’ mechanism via vibronic coupling in the core-to-Rydberg excited CH4 molecule. Finally, SC method enabled theoretical interpretation of experiments on the circular dichroism in angular distribution of photoelectrons emitted after inner-shell ionization of uniaxially oriented methyloxirane C3H6O and randomly-oriented trifluoromethyloxirane C3H3F3O molecules. In the second period, we plan to apply SC method to the theoretical interpretation of new angularresolved experiments on electron and fluorescence emission in molecules. The proposed theoretical studies will be performed in cooperation with our experimental partners. Together, we will address several fundamental physical questions, which can be clarified with the angular resolution. In particular, these are: (i) localization of the O 1s electron in inner-shell photoionization of fixed-in-space CO2 molecule; (ii) circular dichroism in the fluorescence spectra after excitation and cascade decay of CO molecule; (iii) electronic state interference in the O K-edge resonant Auger decay of oriented N2O molecule, and in excitation and decay of freely rotating CH3Cl molecule below its Cl L2,3-edges. We also plan a systematic theoretical study of the angular-resolved photoionization of different electronic orbitals of H2O and N2O molecules.

• DFG Programme: Research Grants