Knowledge Bank

Multiphoton ionization and dissociation processes in diazirine have been studied experimentally via 304-325 nm two-photon absorption, and theoretically by using the EOM-CCSD and B3LYP methods. The electronic structure calculations indicate the strongest one-photon absorption is to the $21A1$(3$px$$\leftarrow$n) Rydberg state. However, in two-photon absorption at comparable energies the first photon excites the low-lying $11B2(\Pi \ast$$\leftarrow$n) valence state, from which the strongest absorption is to the dissociative valence $11A2(\Pi \ast$$\leftarrow$$\sigma NN$) state. In the experimental studies, resonance enhanced multiphoton ionization (REMPI) experiments show no ions at the parent diazirine mass but only CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$ ions from dissociative photoionization. It is proposed that weak one-photon absorption to the $11B2$ state is immediately followed by more efficient absorption of another photon to reach the $11A2$ state from which competition between ionization and fast dissociation takes place. Strong signals of CH$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ ions are also detected and assigned to 2+1 of CH fragments. Velocity map CH$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ images show that CH fragments are born with substantial translational energy indicating that they arise from absorption of two photons in diazirine. It is argued that two-photon processes via the $11B2$ intermediate state are very efficient in this wavelength range, leading predominantly to dissociation of diazirine from the $11A2$ state. The most likely route to CH(X) formation is isomerization to isodiazirine followed by dissociation to CH + HN$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$.