15min:

ANDREW K. MOLLNER, I. FEDOROV, L. KOZIOL, A. I. KRYLOV, H. REISLER, Department of Chemistry, University of Southern California, Los Angeles, CA 90089.

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(3p_x n) Rydberg state. However, in two-photon absorption at comparable energies the first photon excites the low-lying 1¹B₂ ( ^* n) valence state, from which the strongest absorption is to the dissociative valence 1¹A₂ ( ^* _NN) state. 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 of CH fragments. Velocity map CH⁺ 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 + HN₂.