Photodissociation Dynamics of the Simplest Diazirine: Cyclo-CH₂N₂ → CH₂ + N₂

The photodissociation of the simplest diazirine, 3H-diazirine (cyclo-CH₂N₂), was studied in the gas phase following excitation of S₁ levels with one quantum of C-N symmetric stretching (ν₆, 802 cm^-1) or two quanta of C-N asymmetric stretching (2ν₉, 216 cm^-1). The angular and velocity distributions of the products were measured using 9.9 eV single photon vacuum ultraviolet photoionization and 70 eV electron impact ionization. Preferential scattering of products perpendicular to the laser polarization axis indicates that the transition is ¹B₁ ← ¹A₁ with dissociation occurring on subpicosecond time scales. From photofragment anisotropy measurements, initial asymmetric parent vibrational excitation results in shorter dissociation time scales as compared to symmetric stretching, suggesting that dissociation is initiated by asymmetric stepwise ring-opening. However, the final product translational energy distributions were nearly identical for each level, suggesting similar later-time dissociation dynamics. We observed no evidence for formation of ground state CH₂ (X̃ ³B₁). The CH₂ + N₂ products are formed with a most probable total internal energy of ∼2 eV. Although the translational energy distributions are consistent with production of highly vibrationally excited CH₂ (ã ¹A₁) following passage through a conical intersection, a significant yield of CH₂ (b̃ ¹B₁) is possible.