High resolution absorption, excitation and microwave-UV double resonance spectroscopy on a molecular beam: S1 aniline

A high-resolution, molecular beam UV spectrometer was constructed that uses both traditional laser induced fluorescence and optothermal detection of the energy content of the molecular beam. Microwave-UV double resonance capabilities were implemented to assist in the assignment of the UV spectra of medium-sized molecules with the ultimate goal to unravel their S-1/T-1 dynamics. It is shown that high quality optothermal spectra can be obtained in the 300 nm region, by recording the high-resolution, optothermally detected, S-1 <-- S-0 0-0 absorption spectrum of aniline with a signal-to-noise ratio comparable to that of the LIF spectrum of the same band. The latter is used to obtain accurate rotational constants for the vibrationless S-1 state of this molecule. Double resonance is demonstrated by confirming several rotational assignments in this way.