MULTIPHOTON IONIZATION AND DISSOCIATION OF DIAZIRINE
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Date
2009
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Publisher
Ohio State University
Abstract
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 $2^{1}A_{1}$(3$p_{x}$$\leftarrow$n) Rydberg state. However, in two-photon absorption at comparable energies the first photon excites the low-lying $1^{1}B_{2} (\Pi^{*}$$\leftarrow$n) valence state, from which the strongest absorption is to the dissociative valence $1^{1}A_{2} (\Pi^{*}$$\leftarrow$$\sigma_{NN}$) state. In the experimental studies, resonance enhanced multiphoton ionization (REMPI) experiments show no ions at the parent diazirine mass but only CH$_{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 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 $1^{1}B_{2}$ 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$_{2}$.
Description
Author Institution: Department of Chemistry, University of Southern California, Los Angeles, CA 90089