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 EOMCCSD 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 21A1(n3px) Rydberg state, whereas in two-photon absorption at comparable energies the first photon excites the low-lying 11B2 (npi*) valence state, from which the strongest absorption is to the dissociative valence 11A2 (sigmaNNpi*) 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 CH2+...N2 complex. In the experimental studies, resonance enhanced multiphoton ionization (REMPI) experiments show no ions at the parent diazirine mass but only CH2+ 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+ ions are also detected and assigned to 2+1 REMPI via the D2Pi(v'=2)-X2Pi (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 proposed that two photon processes via the 11B2 intermediate state are very efficient in this wavelength range, leading predominantly to dissociation of diazirine from the 21A1 state. The most likely route to CH(X) formation is isomerization to isodiazirine followed by dissociation to CH + HN2. In agreement with other theoretical papers, we recommend revisions of the heats of formation of diazirine and diazomethane.