Knowledge Bank

The rotationally resolved $S1\leftarrow S0$ fluorescence excitation spectra of p-difluorobenzene and its $N2$ complex have been observed. While the spectrum of pDFB is pure b-type, the spectrum of the $N2$ complex is a superposition of two pure c-type spectra that result from the internal rotation of the $N2$ molecule. The analysis reveals that $N2$ molecule lies above the center of the ring plane in the $S0$ state; this distance decreases in the $S1$ state. The preferred orientation and barrier to internal rotation in the two electronic states are also different. A full discussion of the factors responsible for the differences will be given.