Knowledge Bank

Because the pyrazine fluorescence and phosphorescence quantum yields are both ca. $10-3$, a novel method was employed to detect the two-photon excitation spectrum of pyrazine vapor (ca. 10 Torr) in the region of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B3u(n\Pi \ast )\leftarrow IAg$ transition (30 000-33 $100cm-l$). Monitoring the sensitized biacetyl phosphorescence subsequent to pyrazine excitation instead of the pyrazine emission yields a hundred-fold improvement in sensitivity. This method should have wider applicability for molecules having low quantum yields and absorptions in the near ultraviolet region. Measurements in linearly and circularly polarized light providing the anisotropy of the pyrazine two-photon excitation have been made. This information, coupled with calculations of the rotational contours using the two-photon absorption line strengths given by Bray and $Hochstrasser1$ make possible a vibrational assingnment of the spectrum. The cross-section for the pyrazine two-photon absorption is estimated to be several orders of magnitude weaker than the 1 $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Bu(n\Pi \ast )\leftarrow IAlg$ transition in benzene vapor.