Knowledge Bank

In the fluorescence-detected infrared-ultraviolet double resonance (IR-UV DR) experiments presented here, a Ramanshifted dye laser is used to prepare gas-phase $C2H2$ molecules in the region followed by LIF probing in the $A~-X~$ vibronic band system. A complicated variety of intramolecular perturbations are found in the $\nu 2+3\nu 3$ rovibrational manifold: anharmonic, Coriolis, l-type. These perturbations reveal themselves through the occurrence of only few rovibrational states with sufficiently large Frank-Condon factors to enable detection by LIF. A novel approach to the IR-UV DR experiment is adopted, in which the sum of the IR and UV frequencies is held constant, enabling the characterisation of IR-dark (UV-bright) rovibrational eigenstates associated with these few detectable IR-bright (UV-dark) rovibrational eigenstates found in the $\nu 2+3\nu 3$ region. The appearance of unexpected $\Delta J=\pm 1$ features in collision-induced IR-UV DR spectra provide additional evidence of intramolecular perturbations.