The ν1 vibrational predissociation lifetime of (HCN)2 determined from upperstate microwave-infrared double-resonance measurements

We have successfully applied upper state microwave-infrared double-resonance spectroscopy, with optothermal molecular beam detection, to the determination of the (HCN)2 vibrational predissociation lifetime upon excitation of the outside CH stretching mode (ν1). The measured lifetime is 1.7(5) μs. At this time this is the longest lifetime for a hydrogen bonded complex determined with such precision. We have also been able to accurately measure several upper state transition frequencies, therewith revealing the presence of a perturbation in the spectrum that shifts at least one of the rotational levels by an amount on the order of 1 MHz, and appears to decrease the lifetime of the level measurably. A rate equation approach predicts that this experiment will have a negligible signal to noise ratio for upper state lifetimes much shorter than the infrared (IR) laser–molecular beam interaction time, such as is the case here. In order to explain the presence of a double resonance signal, we have proposed a new mechanism that is based upon a coherent interaction model. The technique of microwave–IR double resonance should be widely applicable to the measurement of dissociation lifetimes in the presently difficult-to-access range from about 100 ns to 100 μs. This is a range in which a significant number of complexes are known to have vibrational predissociation lifetimes.