Infrared spectroscopy of jet-cooled HCCl singlet chlorocarbene diradical: CH stretching and vibrational coupling...

Quantum shot noise limited laser absorption methods are used to obtain first high-resolution infrared rovibrational spectra of jet cooled chlorocarbene (HCCl) diradical in a supersonic slit-jet discharge expansion spectrometer. The rotationally resolved absorption spectra of the C-H stretch ν₁ fundamental are analyzed in the framework of a Watson non-rigid asymmetric rotor Hamiltonian model. Further analysis of the mid-infrared data reveals the additional presence of what has nominally been assigned as the $$ combination band with one quantum of the H-C-Cl bend (ν₂) and two quanta of the C-Cl stretch (2ν₃). Rovibrational constants are obtained from least squares fits for each of the four excited vibrational states built on the ν₁ fundamental $$ and the $$ combination mode for each ³⁵Cl and ³⁷Cl atom isotopologue. The four bands occur within a narrow spectral window, requiring detailed comparison of multiple spectral properties (e.g., rotational constant dependence on vibrational excitation, band types/transition dipole moment alignment in the body-fixed frame, etc.) to aid in the vibrational assignment. Indeed, the IR transition intensities arise from strong anharmonic mixing between the "bright" ν₁ C-H stretch and "dark" $$ H-C-Cl bend/C-Cl stretch combination modes, resulting in nearly equal amplitudes for the zeroth order $$ and $$ harmonic states. Finally, to aid the spectral search for HCCl in the interstellar medium, ground state two-line combination differences are combined with previous laser-induced fluorescence results to predict precision microwave transitions for HC³⁵Cl and HC³⁷Cl.