Multiphoton ionization and dissociation of diazirine: a theoretical and experimental study.

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 identified two excited valence states and four Rydberg states in the region 4.0-8.5 eV. In one-photon excitation, the strongest absorption is to the 2(1)A(1)(3p(x) <-- n) Rydberg state, whereas in two-photon absorption at comparable energies the first photon excites the low-lying 1(1)B(2) (pi* <-- n) valence state, from which the strongest absorption is to the dissociative valence 1(1)A(2) (pi* <-- sigma(NN)) state. The diazirine ion is calculated to be rather unstable, with a binding energy of only 0.73 eV and a geometry that resembles a weakly bound CH(2)(+)...N(2) complex. In the experimental studies, resonance-enhanced multiphoton ionization (REMPI) experiments show no ions at the parent diazirine mass but only CH(2)(+) ions from dissociative photoionization. It is proposed that weak one-photon absorption to the 1(1)B(2) state is immediately followed by more efficient absorption of another photon to reach the 1(1)A(2) state from which competition between ionization and fast dissociation takes place. Strong signals of CH(+) ions are also detected and assigned to 2 + 1 REMPI via the D(2)Pi (v' = 2) <-- <-- X(2)Pi (v'' = 0) two-photon transition of CH fragments. Velocity map CH(+) images show that CH(X, v'' = 0, N'') 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 1(1)B(2) intermediate state are very efficient in this wavelength range, leading predominantly to dissociation of diazirine from the 1(1)A(2) state. The most likely route to CH(X) formation is isomerization to isodiazirine followed by dissociation to CH + HN(2). In agreement with other theoretical papers, we recommend revisions of the heats of formation of diazirine and diazomethane.