Knowledge Bank

Pyrazine (1,4-diazabenzene), when expanded in a seeded and skimmed supersonic jet of argon and crossed with a 200 kHz wide laser beam resonant with the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B3u\leftarrow 1Ag$ electronic transition in the near UV, exhibits a laser-induced fluorescence spectrum which consists of many irregularly spaced rovibronic lines lying under each P- and R-branch $transition.2$ This structure is believed to have its origin in the coupling between a single zero-order singlet level and several nearly isoenergetic levels of a zero-order triplet state. Proof that this is indeed the case has now been provided by studies of the magnetic field dependence of the spectrum, at low fields between 0-100 G, for several different $J\prime$ values. The $J\prime =0$ spectrum exhibits Doppler-broadened lines of width $\sim 10$ MHz, whereas $Jprime\ne 0$ spectra show significantly broader lines. An interpretation of these differences in terms of the composition of the molecular eigenstates (ME's), or the ``selection rules for intersystem crossing'', will be given, and the connection between these spectra and the decay properties of coherently prepared $ME\prime s3$ will be established. Similar results for pyrimidine (1,3-diazabenzene), which exhibits qualitatively different behavior in both the frequency and time domains, will be presented.