Laser-radiofrequency double-resonance spectroscopy of formic acid: Assignment and analysis of the ν6 and ν8 bands

The highly sensitive method of infrered-radiofrequency double resonance inside a carbon dioxide laser cavity has been used to observe direct K-type doubling transitions in formic acid. Radiofrequency transitions corresponding to both ν₆ and ν₈ bands were observed. In the light of these new data some earlier infrared laser measurements are now assigned unambiguously. This results in greatly improved band parameters and a $\text{ν}{}_6\text{ν}{}_8$a-type Coriolis coupling constant that is 10 times better defined.