Rotational resolution of the NeCl2 B ← X 9–1 transition

High resolution (∼0.001 cm⁻¹) laser induced fluorescence spectra of the (9–1) rovibronic band of the $B^3{{\Pi }_0}_u^{+}$ ← X ¹${\Sigma }_g^{+}$ system of Cl₂ and the associated van der Waals molecule NeCl₂ have been obtained in a free jet expansion of Ne, Cl₂, and He. The spectrum of NeCl₂ associated with the (9–1) transition of the free Cl₂ molecule was observed 5.46 cm⁻¹ to the blue of the band origin of uncomplexed Cl₂. This study confirms the previously reported observation of a metastable vibrationally excited ground electronic state of NeCl₂. Although the energy in the Cl₂ stretching vibration is an order of magnitude larger than the Ne–Cl₂ dissociation energy, no dissociation is observed on the microsecond time scale. This study also significantly refines the structural parameters obtained from the previous low resolution work. Rotational resolution was achieved and a detailed rotational assignment was made which allowed the Ne to Cl₂ center of mass distance to be determined. This distance is 3.565±0.035 Å in the ground state, and 3.536±0.036 Å in the excited state. The structure of the NeCl₂ van der Waals molecule is T‐shaped, with the dipole transition moment lying along the Cl₂ bond axis, giving a ‘‘b‐type’’ asymmetric top spectrum. The vibrational predissociation lifetime in the B(v’=9) state was found to be 258±42 ps on the basis of homogeneous broadening.