Knowledge Bank

The $F1\Pi g$ and $f3\Pi g$ states of $F2$ are excited from the ground $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma g+$ by two photons near 207 nm and detected by vuv fluorescence or by ionization by a third photon. The laser source for these measurements is an excimer-pumped dye laser operating with PBBO dye at 415 nm. This light is doubled in a $\beta -BaB2O4$ crystal and focused into a cell containing a mixture of $F2$ in He. The uv wavelengths were calibrated against the $B2\Pi -X2\Pi$ (3.0) band in NO, which was calibrated against $I2$ in the visible. Vibrational levels $v\prime =0,1,2$ were observed in the $F1\Pi g$ state and $v\prime =3$ in the $f3\Pi g$ state, based on the previous electron-impact $assignments1$, and partially resolved rotationally (the effective excitation linewidth is $1cm-1$). These assignments are supported by simulations of the two-photon excitation spectra. Although the fluorescence has not yet been spectrally resolved, we believe that it arises predominantly from the triplet state even when the singlet is initially excited. In the latter case, the fluorescence is temporally delayed, and increases in intensity as the He density is increased. The two-phonon excitation scheme we have developed should be useful in investigating the kinetics of the 158 nm $F2$ laser, which is believed to arise from a transition from the outer well of the $f3\Pi g$ state to a weakly bound $3\Pi u$ state correlating to ground state atoms.$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$