Knowledge Bank

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 A center-of-mass separation between the HF and HCl subunits. The Dj distortion constants 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.