Knowledge Bank

The use of laser excitation to eliminate inhomogeneous broadening effects has made it possible to obtain sharp-line ($0.5-5.0cm-1$) vibronic spectra of compounds in low-temperature hosts which normally yield only broad bands ($\sim 200-300cm-1$). This technique, called site selection spectroscopy is not only useful as a new tool for studying organic compounds, but has provided insights into the nature of the broad conventional spectrum itself. We describe the use of a cw, tunable dye laser of variable bandwidth to probe the broad absorption spectra of a number of compounds. Our results show that the broad conventional low-temperature absorption and emission spectra can be considered the result of adding large inhomogeneous broadening to the behavior of individual guest molecules in identical host sites. If this view is adopted, such features of the conventional spectra as the band positions and shapes and the Stokes shift follow as a natural consequence. The Stokes shifts predicted from site selection fluorescence spectra are shown to be in qualitative agreement with those determined from conventional broadband absorption and fluorescence spectra. We can further predict correctly the features and behavior of site selection fluorescence spectra as well as fluorescence excitation spectra and certain hole-burning experiments.