Infrared-Enhanced Fluorescence-Gain Spectroscopy: Conformation-Specific Excited-State Infrared Spectra of Alkylbenzenes.An ultraviolet-infrared (UV-IR) double-resonance method for recording conformation-specific excited-state infrared spectra is described. 
UV-IR
The method takes advantage of an increase in fluorescence signal in phenylalkanes produced by infrared excitation of the S1 origin levels of different conformational isomers. 
the S1 origin levels