Knowledge Bank

The laser induced fluorescence excitation spectrum of the lowest $n\pi \ast$ transition of pyrimidine has been measured in a free jet expansion. A complex system of vibronic bands is observed over a range of $3100cm-1$. The previously noted irregular vibrational spacings of most of the major features are reproduced in this study, and some additional closely spaced bands are clearly resolved. The broad background underlying the entire room temperature vapor absorption spectrum is absent in the free jet. The origin and other low energy bands are laser limited in their width, but some of the higher ones are clearly broadened. We also note a dramatic decrease in fluorescence quantum yield which begins just above the origin just as it does at room temperature. We will discuss several alternative approaches we are attempting to use to analyze this complex vibronic spectrum.