Two-dimensional photoelectron spectroscopy of acetylene: Rydberg-valence interaction between the (3σ_g)^-1(3pσ_u)¹and (3σ_g)^-1(3σ_u)¹ states

Two-dimensional photoelectron spectroscopy is performed for studying autoionization of acetylene in the Franck-Condon gap between the X ²Π_u and A ²A_g states of C₂H₂⁺. The photoelectron spectrum in the photon energy range from 12.8 to 13.6 eV shows exclusive vibrational excitation of the symmetric C-H stretching mode v₁ of C₂H₂⁺(X ²Π_u), which results from autoionization of the valence state (3σ_g)^-1(3σ_u)¹. Vibrational frequencies with anharmonicities of the v₁ and v₂ (the symmetric C-C stretch) modes are determined by a least-squares fit of the ionization energies of the observed peaks to a second order expansion. At the photon energy of 14.120 eV, autoionization of the Rydberg state (3σ_g)^-1(3pπ_u)¹ leads to a complicated photoelectron spectrum where probably the trans-bending mode v₄ of C₂H₂⁺(X ²Π_u) as well as v₁ is excited, reflecting a substantial geometrical change during autoionization. Furthermore, a similar excitation of the v₄ mode is observed at ∼13.8 eV. An excellent agreement in positions of the vibrational levels between the spectra at 13.821 and 14.120 eV suggests the presence of the Rydberg state (3σ_g)^-1(3pσ_u)¹ at ∼13.8 eV which has not been identified previously in the photoabsorption or photoionization cross section curves. The constant-ionic-state spectra for the v₁=0-4 levels of C₂H₂⁺(X ²Π_u) show two spectral features: (a) a weak shoulder (v₁=0) or a small maximum (v₁=1-4) at 13.8 eV and (b) two groups of peaks in the range of 14.0-14.4 eV. The ratio of the integrated intensity of the 13.8 eV maximum to that of the two groups differs from level to level. This observation is interpreted in terms of a strong interaction between the Rydberg (3σ_g)^-1(3pσ_u)¹ and valence (3σ_g)^-1(3σ_u)¹ states.