Vibronic Couplings in the C 1s → nsσ_g Rydberg Excited States of CO₂

Abstract

Fragment ion yields in the C 1s f Rydberg excitation region of CO 2 were measured in the 90°and 0°d irections relative to the electric vector of the linearly polarized light. The C 1s f ns (n ) 3, 4), npπ and npσ (n ) 3-7), and nd (n ) 3, 4) Rydberg transitions are clearly observed and show some vibrational structures. The dipole-forbidden C 1s(σ g ) f 3sσ g Rydberg transition is the strongest of all the Rydberg transitions, and the ion yield in the 90°direction is dominant. This indicates that the bending vibration is predominantly coupled with the 3sσ g Rydberg state and the intensity-lending dipole-allowed state is a very strong π* resonance, only 2 eV lower than the 3sσ g state. On the other hand, in the 4sσ g Rydberg state the vibronic coupling through the antisymmetric stretching mode is strongly observed in the 0°direction. This is probably because the 4sσ g state approaches another intensity-lending state with Σ u + symmetry and goes away from the π* resonance. The angle-resolved ion-yield technique is very powerful for elucidating the vibronic coupling mechanism.

Vibrationally resolved partial cross sections and asymmetry parameters for carbon K-shell photoionization of the CO2molecule

“…12.6 eV above the C 1s ionization threshold [30]. These spectra have been normalized for the photon flux and gas pressure and corrected for the transmission function.…”

Orbital picture in molecular inner-shell excited states of Rydberg-valence mixed character

“…In the C1s (2σ g ) excitation spectrum of CO 2 , the strongest Rydberg state is of 3sσ g symmetry [8,9]. However, the C1s (2σ g ) →3sσ g Rydberg excitation is dipole-forbidden.…”

“…This bending geometry, or bend-ing motion in terms of molecular dynamics, is related to the vibronically induced 3sσ g Rydberg transition. In highresolution ARPIS spectra, bending vibrational fine structures are resolved in the C1s (2σ g ) →3sσ g and 4sσ g Rydberg bands [8,9].…”

O 1s excitation and ionization processes in the CO2molecule studied via detection of low-energy fluorescence emission

“…The resulting ions and/or neutrals are then pushed apart along the molecular axis in a linear molecule. However, this so-called axial recoil approximation breaks down in polyatomic linear molecules if a transition to a core-excited state induces bending vibrations in the molecule, which is the case for the core-to-π * excited states of CO 2 [49,50]. A partial preferentiality of the ionic expulsion along the perpendicular direction is nevertheless kept at the O 1s → π * excitation [23], and this is also reflected in our experiment by the appearance of a single peak in the VUV -O + coincidence spectrum.…”