Emission spectroscopy of halocarbenes: Spin-orbit mixing, the singlet-triplet gap and rovibrational structure of the singlet (X1A') and triplet (a3A") states
We recorded Single Vibronic Level (SVL) emission spectra following excitation of excited state vibrational levels in the Ã 1A" [arrow left] X 1 A' systems of the monohalocarbenes CHF, CDF, CHCl, CDCl, CHBr, and CDBr and the Ã1B 1 [arrow left] X1A1 system of CCl2 . Spectra were measured with a 0.3 m spectrograph equipped with a gated intensified CCD detector and obtained under jet-cooled conditions using a pulsed discharge source. The emission spectra reveal rich detail concerning the vibrational structure of the X state, spin-orbit mixing with the low lying triplet state, and the singlet-triplet gap. The results of a Dunham expansion fit of the ground state vibrational term energies, and comparisons with previous experimental and high level ab initio studies, are reported. The Dunham expansion works well in reproducing the ground state term energies for all the molecules except CHF, where Fermi resonances are observed, and CHBr and CDBr, which feature extensive spin-orbit interaction. In C35 Cl2 we measured [Special characters omitted.] -sorted emission spectra in order to test the previously established hypothesis by M.-L. Liu, et al , that unassigned lines lying above ∼ 5000 cm-1 belong to the ã 3B1 state. Further, we obtained high resolution spectra of singlet-singlet transitions for CHF and both singlet-singlet and singlet-triplet transitions in CHCl using Stimulated Emission Pumping (SEP) Spectroscopy. In CHF, an anharmonic effective Hamiltonian model poorly reproduces the term energies even with the improved set of data, due to the extensive interactions among levels in a given polyad (p ) related by p =2ν1 +ν2 +ν3 . The precise A rotational constants determined from the SEP data were invaluable in clarifying the assignments for these strongly perturbed levels, and the data are well reproduced using a multiresonance effective Hamiltonian model. For CHCl, spectra reveal detailed information on the rovibrational structure of the X 1A' and ã3A" states, and for the triplet state they show a pronounced vibrational state dependence of the spin-spin splitting, which is a sensitive probe of spin-orbit coupling with nearby singlet levels. For the ground state we observed forbidden Ka = 0 levels due to axis switching, which allowed us to determine the A rotational constants. The parameters derived from our spectra are in excellent agreement with recent ab initio calculations.
"Emission spectroscopy of halocarbenes: Spin-orbit mixing, the singlet-triplet gap and rovibrational structure of the singlet (X1A') and triplet (a3A") states"
(January 1, 2008).
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