Hideo HATTORI, Yasumasa HIKOSAKA (Tokyo Inst. Tech. and IMS), Takumi HIKIDA (Tokyo Inst. Tech.) and Koichiro MITSUKE
[J. Chem. Phys. 106, 4902 (1997)]
Two-dimensional photoelectron spectroscopy is performed for studying autoionization of acetylene in the Franck-Condon gap between the X 2(PI)u and A 2Ag 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 (nu)1 of C2H2+(X 2(PI)u), which results from autoionization of the valence state (3(sigma)g)-1(3(sigma)u)1. Vibrational frequencies with anharmonicities of the (nu)1 and (nu)2 (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 (3(sigma)g)-1(3p(pi)u)1 leads to a complicated photoelectron spectrum where probably the trans-bending mode (nu)4 of C2H2+(X 2(PI)u) as well as (nu)1 is excited, reflecting a substantial geometrical change during autoionization. Furthermore, a similar excitation of the (nu)4 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 (3(sigma)g)-1(3p(sigma)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 (nu)1 = 0 - 4 levels of C2H2+(X 2(PI)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 (3(sigma)g)-1(3p(sigma)u)1 and valence (3(sigma)g)-1(3(sigma)u)1 states.