A¹Φ_uRydberg state of acetylene revealed by gas phase multiphoton ionization spectroscopy

Abstract

A new Rydberg state of acetylene has been identified as a three photon resonance in the four photon ionization spectra of both C2H 2 and C2D 2 . In both isotopic species the zero point level of this new electronic state is sufficiently long lived to support rotational structure. Spectral analysis, including use of the appropriate three photon rotational linestrength theory, enables characterization of the excited state symmetry as 10 u. Quantum defect considerations, and comparisons with recent ab initio calculations, encourage assignment of this 10 u state in terms of the electronic promotion n 3 3d69 ~-n u .~ Possible reasons for the dominance of this 10u-aY+g transition in the 3 + 1 MPI spectrum of acetylene are considered.

Observation of the lowest 1Δu Rydberg state of acetylene by multiphoton ionization spectroscopy

“…13͒ were also found very close to those of the ion ground state in both isotopomers. As for the vibrational analysis of these bands, only the 2 progression was observed in these systems, 8,13 and again, the 2 stretching frequency was found very close to that of the ground state of the ion. 19 The molecular parameters of the ground state of C 2 H 2 ϩ and C 2 D 2 ϩ were determined either experimentally or theoretically [18][19][20][21][22][23] and are summarized in Table II.…”

“…1͑a͒. For comparison, observed energies ͓by one photon absorption 5,7,8 and by ͑3ϩ1͒-photon REMPI 13,16 ͔ are displayed on Fig. 1͑b͒.…”

“…6 More recently, resonantly enhanced multiphoton ionization ͑REMPI͒ studies have been carried out on the ungerade Rydberg states of acetylene, by using resonant three-photon excitation followed by one-photon ionization. They allowed for the observation of new electronic symmetries which are inaccessible in one photon absorption [13][14][15][16] ͑see Table I͒. In particular, Ashfold et al 13 reported the discovery of the lowest Rydberg state of 1 ⌽ u symmetry belonging to the 3d complex, by ͑3ϩ1͒ REMPI.…”

“…They allowed for the observation of new electronic symmetries which are inaccessible in one photon absorption [13][14][15][16] ͑see Table I͒. In particular, Ashfold et al 13 reported the discovery of the lowest Rydberg state of 1 ⌽ u symmetry belonging to the 3d complex, by ͑3ϩ1͒ REMPI. Later on, Orlando et al 16 performed low resolution REMPI photoelectron spectroscopy of the higher ungerade states and identified higher members of the ns g , 1 ⌸ u , nd␦ g , 1 ⌸ u series up to nϭ5 and of the nd␦ g , 1 ⌽ u Rydberg series up to nϭ4.…”

Excited states of acetylene: a CASPT2 study

“…The energy obtained for 2 1 R u from the state-averaged calculation (8.74 eV) is not reliable because of the low weight of the reference wavefunction (see Table 3) and an extremely large PT2 correction of 5.43 eV. The in- [62] vestigation of the level shift (See Table 3) also shows that the excitation energy increases with increasing reference weight. The calculation with the changed active space predicts an excitation energy of 10.65 eV and a fairly large TDM.…”