Stimulated Emission Pumping Spectroscopyof the [X̃]A‘ State of CHF

High-resolution infrared spectroscopy of HCF in the CH stretch region

We present the results from a high-resolution infrared study of jet-cooled singlet monofluorocarbene (HCF) in the CH stretch region near 2600 cm^-1. Absorption signals are recorded using near quantum shot noise limited laser absorption methods. The fully resolved absorption spectra of the CH stretch (ν₁) fundamental band and a partial progression of transitions of the HCF bend plus CF stretch (ν₂ + ν₃) combination band are observed and show clear evidence of a strong rovibrational coupling between the ν₁K_a ^' = 2 and ν₂ + ν₃K_a ^' = 3 manifolds, including the observation of "dark state" transitions. A detailed perturbation analysis of a c-type Coriolis interaction is carried out for these two coupled vibrational states, providing experimental determination of precise rovibrational constants. A combined ground state combination difference fit of the transitions to the ν₁ and ν₂ + ν₃ vibrational states in this study with previously reported LIF Ã(0,0,0) ← X̃(0,0,0) data has been done to increase the accuracy of the ground state rotational constants [M. Kakimoto et al., J. Mol. Spec. 88, 300-310 (1981)]. Moreover, we report, for the first time, hot band (ν₁ + ν₃ ← ν₃) transitions due to vibrationally excited HCF in the CF stretch mode, ν₃. The high-resolution results for all vibrational frequencies and rotational constants are in good agreement with and significantly extend the analysis of the rovibrational manifold of HCF. The present ground state and ν₃ spectroscopic parameters now permit improved predictions for pure rotational and ν₃ fundamental transitions to aid spectral searches for HCF in the laboratory and the interstellar medium.

Fluorescence excitation and emission spectroscopy of the X(1)A' --> A(1)A'' system of CHI and CDI

We report on the first detailed studies of the spectroscopy of an iodocarbene, measuring fluorescence excitation and emission spectra of the X1A' --> A1A'' system of :CHI and the deuterated isotopomer :CDI. Due to similar bending and C-I stretching frequencies in the upper state, fluorescence excitation spectra of :CHI show polyads composed of members of the 2(0)(n-x)3(0)x progressions with x = 0-3. For :CDI, only progressions with x = 0, 1 are observed. Extrapolation of the 20n term energies for both isotopomers to a common origin places the electronic origin of the X1A' --> A1A'' system near 10500 cm-1, in good agreement with theoretical predictions. Rotational analysis of the 16 observed bands for CHI and 13 observed bands for :CDI yields rotational constants for the upper and lower states that are also in good agreement with theory. To investigate the controversial issue of the ground state multiplicity of :CHI, we measured single vibronic level emission spectra from many A1A'' levels. These spectra show conclusively that the ground state is a singlet, as for both isotopomers the ã3A'' origin is observed, lying well above the origin of the X1A' state. At energies above the ã3A'' origin, the spin-orbit mixing is so severe that few vibrational assignments can be made. Analysis of the emission spectra provides a lower limit on the singlet-triplet gap of 4.1 kcal mol-1, in excellent agreement with theoretical predictions.

High resolution study of spin-orbit mixing and the singlet-triplet gap in chlorocarbene: stimulated emission pumping spectroscopy of CH(35)Cl and CD(35)Cl

We report on high resolution studies of spin-orbit mixing and the singlet-triplet gap in a prototypical halocarbene, CHCl, using stimulated emission pumping (SEP) spectroscopy from the A (1)A(") state. Results are reported for two isotopomers, CH(35)Cl and CD(35)Cl. We have obtained rotationally resolved spectra for the majority of X (1)A(') levels lying between 0 and 6000 cm(-1) above the zero-point level that were previously observed under low resolution in single vibronic level emission studies and several new levels that were previously unobserved or unresolved. In addition, SEP spectra were obtained for six a (3)A(") levels in CH(35)Cl and three levels in CD(35)Cl. The derived term energies and rovibrational parameters of the X (1)A(') and a (3)A(") states are in good agreement with theory. The a (3)A(") triplet spin-spin parameter is vibrational state dependent, and dominated by a second-order contribution from spin-orbit coupling with nearby X (1)A(') levels; it therefore provides a sensitive probe of spin-orbit mixing in this system. An analysis of three pairs of interactions between specific a (3)A(") and X (1)A(') levels in CH(35)Cl affords a pure electronic spin-orbit coupling element of 150 cm(-1), in good agreement with theoretical expectations. The derived singlet-triplet gaps, which are the most precise determined to date for any carbene, are compared with the predictions of high level ab initio theory.