NEAR-INFRARED ELECTRONIC SPECTROSCOPY OF THE HCCl $\tilde{A}^{1}A" \leftarrow \tilde{X}^{1}A'$ TRANSITION

Abstract

Rotationally resolved near-infrared spectra of the chloromethylene (HCCl) radical $\tilde{A}^{1}A^{\prime\prime} \leftarrow \tilde{X}^{1}A^{\prime}$ transition were obtained at Doppler-limited resolution using a combination of laser transient absorption and radio frequency modulation (FM) techniques. Merer and $Travis^{1}$ first reported the observation of the HCCl $\tilde{A} \leftarrow \tilde{X}$ bands, but were unable to rotationally resolve the $\tilde{A}(0,1,0)\leftarrow \tilde{X}(0,0,0)$ band and did not observe any other near-infrared bands due to their extreme weakness. $Kakimoto et al.^{2}$ reported several laser-induced fluorescence (LIF) studies on the HCCl $\tilde{A} \leftarrow \tilde{X}$ bands at visible wavelengths, which correspond to the transitions terminating in the levels well above the barrier to linearity in the $\tilde{A}$ and $\tilde{X}$ states. In the present study, we have identified the transitions from the $\tilde{X}(0,0,0)$ level to the $\tilde{A}(0,0,0), \tilde{A}(0,1,0)$, and $\tilde{A}(0,0,1)$ levels. The $\tilde{A}(0,0,0)\leftarrow \tilde{X}(0,0,0)^{3}$ and $\tilde{A}(0,0,1)\leftarrow \tilde{X}(0,0,0)$ bands had not been previously observed. These observed $\tilde{A}$ state levels are located below the barrier to linearity; i.e., the geometry in these levels is bent. Analysis has determined the geometry for the $\tilde{A}(0,0,0)$ level: $\angle HCC1=134.7^{\circ}$ and $R(C-Cl)=1.623 {\AA}$. We have also found that the rotational structure is still perturbed by Renner-Teller effects resulting from interactions between the $\tilde{X}$ and $\tilde{A}$ states which become degenerate at the linear configuration in a combination with spin-orbit interactions. A dispersed fluorescence experiment following excitation of a strong $\tilde{A} \leftarrow \tilde{X}$ transition in the visible is also in progress. The data from dispersed fluorescence spectra will shed light on the vibrational structure in the $\tilde{X}$ state and probably will help to determine the singlet-triplet $(\tilde{X}^{1}A^{\prime}-\tilde{a}^{3}A^{\prime\prime})$ separation. The details of our experimental data and their analysis will be presented. Acknowledgment: This work was carried out under Contract No. DE-AC02-76CH00016 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences.