Vibronic coupling and valence mixing in the 1s→Rydberg excited states of C2H2 in comparison with N2 and CO (original) (raw)
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Journal of Electron Spectroscopy and Related Phenomena, 2005
High resolution angle-resolved ion-yield spectra are reported for the C1s → Rydberg excitations of acetylene. Vibronic coupling features are found in the energy regions of 3sσ g /3σ * u , 3pσ u , and near threshold. By increasing retarding potentials for ion detectors to select more energetic fragmentation channels, the feature observed in the 90 • direction is assigned to the C1s → 3σ * u valence state coupled with the C1s → 1π * g excited state via cis bending (π u) vibrational mode.
Vibronic Couplings in the C 1s → n sσ g Rydberg Excited States of CO 2
The Journal of Physical Chemistry, 1996
Fragment ion yields in the C 1s f Rydberg excitation region of CO 2 were measured in the 90°and 0°d irections relative to the electric vector of the linearly polarized light. The C 1s f ns (n) 3, 4), npπ and npσ (n) 3-7), and nd (n) 3, 4) Rydberg transitions are clearly observed and show some vibrational structures. The dipole-forbidden C 1s(σ g) f 3sσ g Rydberg transition is the strongest of all the Rydberg transitions, and the ion yield in the 90°direction is dominant. This indicates that the bending vibration is predominantly coupled with the 3sσ g Rydberg state and the intensity-lending dipole-allowed state is a very strong π* resonance, only 2 eV lower than the 3sσ g state. On the other hand, in the 4sσ g Rydberg state the vibronic coupling through the antisymmetric stretching mode is strongly observed in the 0°direction. This is probably because the 4sσ g state approaches another intensity-lending state with Σ u + symmetry and goes away from the π* resonance. The angle-resolved ion-yield technique is very powerful for elucidating the vibronic coupling mechanism.
Comptes Rendus Physique, 2004
The ungerade ns + nd Rydberg states of C 2 H 2 converging to the ground state of the C 2 H + 2 cation have been investigated in the energy range 74 000-88 000 cm −1 by (3 + 1)-multiphoton ionisation (REMPI) and by VUV absorption spectroscopy at the Super-ACO synchrotron radiation facility. Both methods have allowed the selective analysis of the Rydberg transitions with rotational resolution. Mulliken's semi-united atom model, in which predissociation has been taken into account, was used to understand the relative three-photon intensities among the different electronic transitions within the same Rydberg supercomplex. Lifetimes have been evaluated and illustrate very different behaviours towards predissociation for the observed Rydberg states. To cite this article: S. Boyé et al., C. R. Physique 5 (2004). 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.
Vibrational modes in excited Rydberg states of acetone: A computational study
Computational studies of electronically excited states of the acetone molecule [(CH 3) 2 CO] and its fully deuterated isotopologue [(CD 3) 2 CO] are performed using the time dependent density functional (TDDFT) methodology. In addition to vertical excitation energies for singlet and triplet states, equilibrium geometries and vibrational frequencies of the n ¼ 3 Rydberg states (3s, 3p and 3d) are obtained. This is the first report of geometry optimization and frequency calculations for the 3p x , 3p z , 3d yz , 3d xy , 3d xz , 3d x2–y2 and 3d z2 Rydberg states. Results of the geometry optimization indicate that the molecule retains approximate C 2V geometry in most of these excited Rydberg states, with the most significant structural change seen in the CCO bond angle which is found to be reduced from the ground state value. Detailed comparison of the computationally predicted vibrational wavenumbers with experimental studies helps to confirm several of the earlier vibronic assignments while leading to revised/new assignments for some of the bands. The important role of hot bands in analysis of the room temperature photoabsorption spectra of acetone is corroborated by this study. While the vibrational frequencies in excited Rydberg states are overall found to be close to those of the ionic ground state, geometry optimization and vibrational frequency computation for each excited state proves to be very useful to arrive at a consistent set of vibronic assignments. Isotopic substitution helps in consolidating and confirming assignments. An offshoot of this study is the interpretation of the band at $ 8.47 eV as the π–3s Rydberg transition converging to the second ionization potential.
Vacuum–ultraviolet photodissociation of C2H2 via Rydberg states: a study of the fluorescent pathways
Chemical Physics Letters, 1999
The visible fluorescence produced by vacuum-ultraviolet photodissociation of acetylene, particularly through the Rydberg states, has been studied in a gas-flow system using synchrotron radiation as a light source between 154 and 60 nm and an original light collection device. Excitation of the Rydberg states below the first IP reveals only the C H product by 2ĩ ts A-X continuum emission spectrum between 330 and 900 nm. Evolution in the emission spectral profile as a function of excitation wavelength has been observed, indicative of specific internal distribution favoring the bending levels of the à 2 state of C H. A new assignment of the Rydberg series converging to the first excited state A A of the cation is proposed 2 g on the basis of recent experimental and theoretical characterization of this state and leads to an approximate value of the Ž y1 . C-C stretching frequency of the cation n f 1500 cm . q
The Journal of Chemical Physics, 2004
Rotationally cold absorption and two-photon ionization spectra of CO in the 90-100 nm region have been recorded at a resolution of 0.3-1.0 cm Ϫ1 . The analyses of up to four isotopomers seek to clarify the observations in regions where the Rydberg levels built on the ground state X 2 ⌺ ϩ of the ion interact with valence states of 1 ⌺ ϩ and 1 ⌸ symmetry. Previous observations of the 3s, B 1 ⌺ ϩ Rydberg state, reviewed by Tchang-Brillet et al. ͓J. Chem. Phys. 96, 6735 ͑1992͔͒, have been extended to energies above its avoided crossing with the repulsive part of the DЈ 1 ⌺ ϩ valence state where resonances of varying intensities and widths have been attributed to the fully coupled 3s or 4s and DЈ potentials, and where the B state approaches a second avoided crossing with the CЈ 1 ⌺ ϩ valence state ͓Cooper and Kirby, J. Chem. Phys. 87, 424 ͑1987͒; 90, 4895 ͑1989͒; Chem. Phys. Lett. 152, 393 ͑1988͔͒. Fragments of a progression of weak and mostly diffuse bands, observed for all four isotopomers, have been assigned to the CЈ←X transition. The least-squares modeling of the 4p and 5p complexes reveals the 3p, E 1 ⌸ Rydberg state to be one of the perturbers, violating the ⌬vϭ0 selection rule for Rydberg-Rydberg interactions on account of its rapid transition with increasing v from Rydberg to valence state. A second 1 ⌸ perturber, very loosely bound and clearly of valence type, contributes to the confusion in the published literature surrounding the 5p, vϭ0 complex.
Excited states of acetylene: a CASPT2 study
Theoretical Chemistry Accounts: Theory, Computation, and Modeling (Theoretica Chimica Acta), 1998
Valence and low-lying Rydberg states of acetylene (C 2 H 2 ) are reexamined in the singlet as well as in the triplet manifold. The major goal of this work is a better understanding of the valence states that contribute to the low-energy electron-energy-loss spectrum recorded under conditions where transitions to triplet states are enhanced. An appropriate theoretical treatment of these states has to include the low-lying Rydberg states because of their energetic proximity to some of the valence states. The CASSCF/CASPT2 method provides a suitable framework for such a task. For some important states the geometry was optimized at the CASPT2 level to allow a comparison with the results of other highly accurate methods that have been applied to acetylene in the past.
The Journal of Chemical Physics, 2004
Rotationally cold absorption and two-photon ionization spectra of CO in the 90–100 nm region have been recorded at a resolution of 0.3–1.0 cm−1. The analyses of up to four isotopomers seek to clarify the observations in regions where the Rydberg levels built on the ground state X 2Σ+ of the ion interact with valence states of 1Σ+ and 1Π symmetry. Previous observations of the 3sσ, B 1Σ+ Rydberg state, reviewed by Tchang-Brillet et al. [J. Chem. Phys. 96, 6735 (1992)], have been extended to energies above its avoided crossing with the repulsive part of the D′ 1Σ+ valence state where resonances of varying intensities and widths have been attributed to the fully coupled 3sσ or 4sσ and D′ potentials, and where the B state approaches a second avoided crossing with the C′ 1Σ+ valence state [Cooper and Kirby, J. Chem. Phys. 87, 424 (1987); 90, 4895 (1989); Chem. Phys. Lett. 152, 393 (1988)]. Fragments of a progression of weak and mostly diffuse bands, observed for all four isotopomers, have...
Vibrational assignments in the 3p Rydberg states of acetone
The Journal of Chemical Physics, 1993
The 2 and 3 photon resonantly enhanced multiphoton ionization spectra of the 3p Rydberg + 2 transitions of acetone and acetone-& were remeasured and reanalyzed. Seven of the eight al and one bl vibrational modes were assigned. On the basis of these assigned fundamental frequencies the anomalous isotope shift of the origin of the A, transition was rationalized, the contribution of the nonbonding electrons to the scissors force constant was demonstrated, and a location for the A, ?r# valence excited transition was proposed. through a pulsed 500 pm nozzle (Newport BY-100) into a