Shape resonances in photoionization: Correlation with STO-3G MO results

An unusual π* shape resonance in the near-threshold photoionization of S1 para-difluorobenzene

“…The correspondence between these pictorial representations of the p-DFB resonance and a * orbital is obvious, but while this seemingly corroborates a received wisdom, there are few systematic studies, to our knowledge, to validate this approach other than as a tool for post hoc rationalization in such cases as this. However, one such investigation of nine linear molecules 26 concluded that "the lowest * virtual orbital always corresponds to discrete uv transitions, and not to resonances in the continuum," in stark contrast with what is shown here for p-DFB. The more familiar -type resonances naturally concentrate electron density between nuclei, the scattering from which then readily provides a sensitivity to the internuclear spacing due to interference effects with the backscattered waves, and which can greatly influence the PADs.…”

“…25 A more qualitative approach to understanding shape resonances than the electron scattering calculation undertaken here seeks to identify them with excitations to virtual valencelike antibonding orbitals which are embedded in the continuum. 26,27 The 3b 2g * orbital of p-DFB is the lowest lying virtual orbital of gerade symmetry obtained in minimal basis calculations. This includes both the conventional HF/STO-3G ͑HF-Hartree-Fock͒ calculation for the S 0 ground state of p-DFB, and also a HF level calculation for the S 1 excited neutral state, performed using the maximum overlap method ͑MOM͒ implemented in Q-Chem 28 to maintain the …2b 2g 1 a u 1 excited intermediate configuration.…”

“…The S 0 ͑v =0͒ → S 1 excitation process allows, in principle, the formation of all totally symmetric vibrational states in S 1 , but of the totally symmetric modes only modes 3, 5, and 6 ͑CF stretch, ring breathing, CCC in plane bend, respectively, with modes labeled using Mulliken notation͒ are formed with one quantum in S 1 and dominate the observed vibrational progressions. In addition, two b 3g modes, 26 ͑CCC in plane bend͒ and 27 ͑CF in plane bend͒ are weakly active as a result of vibronic coupling. 9 Excitation of one quantum in either of these modes in S 1 from S 0 ͑v =0͒ gives rise to a vibronic transition moment that points along z ͑i.e., the F-F direction͒.…”

Photoion rotational distributions from near-threshold to deep in the continuum

“…The behavior of the rotational populations for CO between 25 and 50 eV arises from the 4 →k shape resonance, which has been observed in previous photoionization studies. 68,88,89 However, these new data enable us to study the shape resonant dynamics with greater clarity than previously possible. Rotationally resolved studies can be particularly valuable for understanding the microscopic behavior of shape resonances for two reasons.…”

Vibrational branching ratios in photoionization of CO and N2

“…The current investigation addresses this issue by determining CO ϩ (B 2 ⌺ ϩ )v ϩ ϭ1/v ϩ ϭ0 vibrational branching ratios over a wide energy range for comparison with those of N 2 ϩ (B 2 ⌺ u ϩ ). While there has been enormous effort expended in understanding the VUV spectroscopy and photoionization dynamics of CO, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] none has dealt with vibrationally resolved measurements over a wide range of energy ͑i.e., Ͼ100 eV͒. Previous work, 1,2,4,6 in conjunction with results from the current study, demonstrate that molecular aspects of the ionization dynamics emerge far from threshold.…”