Optothermal detection of non-radiative excited states of aromatic molecule in a molecular beam

Using a molecular beam laser spectrometer equipped with laser-induced fluorescence (LIF) and optothermal detectors, the high-resolution rovibronic spectra of large molecules, such as aniline, tetrazine and pyridine, were measured. The aim of this research was to demonstrate the ability of the optothermal detection technique to produce high-quality spectra, in particular in those cases in which the fluorescence quantum yield is so low as to render LIF detection practically impossible, e.g. pyridine. This technique reveals, in a direct manner, the energy transferred to vibrational and rotational degrees of freedom of the ground state after relaxation. In this sense, the optothermal spectrum is complementary to its LIF counterpart. The intensities of the spectral features in an optothermal spectrum are proportional to those in the absorption spectrum provided that the fluorescence quantum yield is small.