Cis-trans isomerization in the S1 state of acetylene: Identification of cis-well vibrational levels
- Inst. of Atomic and Molecular Sciences, Taipei (Taiwan); Univ. of British Columbia, Vancouver, BC (Canada)
- Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States); Univ. of California at San Francisco, San Francisco, CA (United States)
- Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
- Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
A systematic analysis of the S1-trans (Ã1Au) state of acetylene, using IR-UV double resonance along with one-photon fluorescence excitation spectra, has allowed assignment of at least part of every single vibrational state or polyad up to a vibrational energy of 4200 cm–1. Four observed vibrational levels remain unassigned, for which no place can be found in the level structure of the trans-well. The most prominent of these lies at 46 175 cm-1. Its 13C isotope shift, exceptionally long radiative lifetime, unexpected rotational selection rules, and lack of significant Zeeman effect, combined with the fact that no other singlet electronic states are expected at this energy, indicate that it is a vibrational level of the S1-cis isomer (ÃA1A2). Guided by ab initio calculations [J. H. Baraban, A. R. Beck, A. H. Steeves, J. F. Stanton, and R. W. Field, J. Chem. Phys. 134, 244311 (2011)] of the cis-well vibrational frequencies, the vibrational assignments of these four levels can be established from their vibrational symmetries together with the 13C isotope shift of the 46 175 cm-1 level (assigned here as cis-3161). The S1-cis zero-point level is deduced to lie near 44 900 cm-1, and the ν6 vibrational frequency of the S1-cis well is found to be roughly 565 cm-1; these values are in remarkably good agreement with the results of recent ab initio calculations. The 46 175 cm-1 vibrational level is found to have a 3.9 cm-1 staggering of its K-rotational structure as a result of quantum mechanical tunneling through the isomerization barrier. Such tunneling does not give rise to ammonia-type inversion doubling, because the cis and trans isomers are not equivalent; instead the odd-K rotational levels of a given vibrational level are systematically shifted relative to the even-K rotational levels, leading to a staggering of the K-structure. Furthermore, these various observations represent the first definite assignment of an isomer of acetylene that was previously thought to be unobservable, as well as the first high resolution spectroscopic results describing cis-trans isomerization.
- Research Organization:
- Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- Grant/Contract Number:
- FG02-87ER13671
- OSTI ID:
- 1557653
- Journal Information:
- Journal of Chemical Physics, Vol. 134, Issue 24; ISSN 0021-9606
- Publisher:
- American Institute of Physics (AIP)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
Web of Science
Probing the predissociated levels of the S 1 state of acetylene via H-atom fluorescence and photofragment fluorescence action spectroscopy
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journal | November 2018 |
One-colour (∼220 nm) resonance-enhanced (S 1 − S 0 ) multi-photon dissociation of acetylene: probe of the C 2 A 1 Π u − X 1 Σ + g band by frequency-modulation spectroscopy
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journal | February 2020 |
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