Journal of Physical Chemistry A, Vol.123, No.13, 2780-2788, 2019
Anomalous Intensities in the 2+1 REMPI Spectrum of the E (1)Pi-X (1)Sigma(+) Transition of CO
We report on one-color experiments near 214 nm involving the photodissociation of jet-cooled OCS to produce high rotational states (40 < J < 80) of CO (X (1)Sigma(+), v = 0, 1) which were then ionized by 2+1 resonance-enhanced multiphoton ionization via the E (1)Pi state. The nominally forbidden Q-branch of the twophoton E (1)Pi-X (1)Sigma(+) transition is observed with intensity comparable to the allowed R-branch. The bright character of the high-J Q - branch lines can be described quantitatively as intensity borrowing due to mixing of the E (1)Pi and C (1)Sigma(+) states, using J-dependent mixing coefficients extrapolated from the observed Lambda-doubling in the lower rotational levels of the E state. In addition to the significant enhancement of Q-branch intensities above the values predicted by conventional two-photon line strengths for a (1)Pi-(1)Sigma(+) transition, the high-J lines of the R- and P-branches appear to be suppressed in intensity by approximately a factor of 3 compared to the unperturbed low-J line strengths, most likely due to perturbations associated with a (1)Sigma(-)state. The E-state rotational term values for J < 80, v = 0 derived from the present spectra agree within our measurement and calibration uncertainties with the extrapolations based on the molecular constants previously derived from rotational levels with J < 50. The E-X transition is attractive for future application to photodissociation dynamics and rotational polarization measurements of CO photofragments, with convenient access to state-selective probing on multiple rotational branches, which exhibit different sensitivity to fragment alignment.