Hightlight Paper

Theoretical studies of angle-resolved ion yield spectra of core-to-valence transitions of acetylene

Conventional gas phase experiments do not allow to get information on symmetries of particular photo-excited states without additional theoretical analysis, because of random orientation of the molecules in the gas phase. Molecules dissociate after core excitation. The angle-resolved measurement of the dissociation fragments give unique opportunity to measure spectra of fixed-in-space molecules (Fig. 1). This constitutes a background of angle-resolved photoion-yield spectroscopy (ARPIS) which allows symmetry resolved study of molecular properties. In the ARPIS technique, the energetic fragment ions emitted after axial-recoil fragmentation are detected in the parallel (0°) and perpendicular (90°) directions relatively to the polarization of the incident soft x-ray photons. This allows to measure for the diatomic molecules separately the unoccupied molecular orbitals of the Σ and Π symmetry, while for polyatomic molecules the situation is changed: Time scale of the decay and fragmentation process can be comparable with some low frequency vibrational modes of molecules, like bending modes. In that case, fragmentation may occur at a bent geometry and I_0° ion yield spectra involve some Π components, and I_90° spectra – some Σ components.

Our theoretical multi-mode approach includes ab initio calculations of the potential energy surfaces of the ground and inner-shell excited states (Fig. 2), and their transition dipole moments. The theory explains quantitatively the experimental data for the acetylene molecule (Fig. 3).

Figure 1:Physical picture of the ARPIS experiment

Figure 2:Potential energy surfaces of the ground- and core-excited π^* in-plane (low) and out-of-plane (upper potential) states as functions of normal modes coordinates Q_A and Q_B.

Figure 3: (a) Energy dependence of the ion flux direction n_0z for the case of J_90° (solid line) and J_0° (dashed line). (b) Absorption cross section for stretching modes only. Position of 0 → 0 peak is matched to maximum of bending modes absorption profile. (c) Absorption cross section for C1s → π^*_g transition including all vibrational modes.

Publication reference >> V. Kimberg, N. Kosugi, and F. Gel'mukhanov, J. Chem. Phys. 130, 114302 (2009)

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