Two-dimensional photoelectron spectroscopy of acetylene: Rydberg-valence interaction between the (3σg)−1(3pσu)1 and (3σg)−1(3σu)1 states

Abstract

Two-dimensional photoelectron spectroscopy is performed for studying autoionization of acetylene in the Franck–Condon gap between the X 2Πu 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 ν1 of C2H2+(X 2Πu), which results from autoionization of the valence state (3σg)−1(3σu)1. Vibrational frequencies with anharmonicities of the ν1 and ν2 (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 (3σg)−1(3pπu)1 leads to a complicated photoelectron spectrum where probably the trans-bending mode ν4 of C2H2+(X 2Πu) as well as ν1 is excited, reflecting a substantial geometrical change during autoionization. Furthermore, a similar excitation of the ν4 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 (3σg)−1(3pσu)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 ν1=0–4 levels of C2H2+(X 2Πu) show two spectral features: (a) a weak shoulder (v1=0) or a small maximum (ν1=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 (3σg)−1(3pσu)1 and valence (3σg)−1(3σu)1 states.

Ab initio study of the electronic spectrum of C2H2+: Investigation of structure of spectra involving low-lying doublet electronic states

“…A very efficient tool to study the structure of electronic states of C 2 H 2 ϩ is photoelectron spectroscopy via conveniently chosen intermediate excited states of the neutral molecule. [18][19][20] Pratt et al 18,19 reported photoelectron spectra for one-photon resonant, two-photon ionization of acetylene via the A 1 A u state. Because of the trans-bent geometry of the intermediate A 1 A u state and the linear equilibrium geometry of the a͒ final X 2 ⌸ u state of C 2 H 2 ϩ , only a long progression in the trans-bending vibration of the ion was detected.…”

The outer valence photoionization of acetylene

“…18 This shape resonance represents a 3 g →3 u valence-like transition which should prompt a reconsideration of previous works assigning the broad ϳ13.3 eV structure to such a valence transition. 6,7 The theoretical results in both Figs. 2͑a͒ and 2͑b͒ agree very well with the cross-sections predicted by Farren et al 16 notwithstanding the extra structure in the present data.…”

“…4 Rydberg series typically have vibrational frequencies which reflect that of the core ion, 4 in this case, however at the low n values prominent in Fig. 6͑b͒, the states cannot be regarded as true Rydbergs; Hattori et al, 6 have experimentally obtained a value of ϳ0.37 eV for the 3 g →3p u and 3 g →3 p u members of the series in Fig. 6͑b͒.…”

“…12,17,19 Other authors have suggested that the low energy feature has a separate origin which may arise either from a 3 g →np u ͑Ref. 11͒ or a 3 g →3 p u transition decaying into the X 2 ⌸ u 8,10,14 state, and Hattori et al 6 have suggested that two excited states, 3 g →np u , and 3 g →np u , and one valence transition, 3 g →3 u , contribute to the low energy feature in the X 2 ⌸ u channel. Also, several authors have addressed the possibility that a shape resonance contributes to the structure of the photoionization spectrum in the region of ϳ13.5 eV photon energy.…”

“…1 One example of this occurs with photoionization into the X 2 ⌸ u state of acetylene; experimental results for this system in the ϳ11.4-16.6 eV photon energy region reveal a complicated structure, about which there has existed a long-standing controversy concerning the exact assignment of experimentally observed structures and the attendant dynamics of photoionization giving rise to same. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] Concurrent with this experimentally driven controversy, there has been a significant amount of theoretical work. 2,11,12,[16][17][18][19] The theoretical work to date has contributed much to clarify the situation, but so far has not resolved all issues.…”