Microwave and infrared electric-resonance optothermal spectroscopy of HF–HCl and HCl–HF

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Abstract
Microwave and infrared spectra of HF–HCl and HCl–HF have been obtained using a molecular-beam electric-resonance optothermal spectrometer, which operates by quadrupole-field focusing of polar molecules onto a bolometer detector. The HF–HCl microwave measurements extend to Ka=1, the previous Ka=0 results of Janda, Steed, Novick, and Klemperer, allowing the determination of the Ka dependence and asymmetry of the Cl quadrupole coupling constant. For the metastable HCl–HF isomer no previous spectroscopic measurements have been reported. Here, microwave spectra are observed for the Ka=0 and 1 states and interpreted in terms of an L-shaped hydrogen-bonded structure for the complex, with a 3.235 Å center-of-mass separation between the HF and HCl subunits. The DJ distortion constant indicates that the harmonic stretching force constant for HCl–HF is ∼35% larger than that of HF–HCl. Infrared spectra of the Ka=0–0 and 1–0 subbands of the H–F stretching band for HF–HCl and of the Ka=0–0 subband of the H–F stretch for HCl–HF are also reported. The vibrational predissociation linewidths depend on vibration, Ka state, isotopic species, and isomer excited.