Knowledge Bank

The rotationally resolved $S1\leftarrow S0$ fluorescence excitation spectra of para-difluorobenzene (pDFB) and its $N2$ complex have been observed. The high resolution spectrum of pDFB is pure b-type and spans an unusually wide energy region of over $20cm-1$. The $N2$ complex of pDFB exhibits two absorption bands shifted to the red of the bare molecule origin, at -27 and $-25cm-1$. Each of these bands is a superposition of the two pure c-type spectra that result from the internal rotation of the $N2$ molecule in the complex. Analyses of these bands show that the $N2$ molecule lies $\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}3.59$ {\AA} above the center of the ring plane in the $Sa$ state; this distance decreases to $\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}3.47$ {\AA} in the $S1$ state. The barriers to internal rotation in the two electronic states are also different. A full discussion of the factors responsible for the difference will be given.