The photodissociation spectroscopy and dynamics of the HNCN free radical have been investigated by fast beam photofragment translational spectroscopy. Predissociative transitions for both the B 2 AЈ←X 2 AЉ band and a higher-energy band system assigned to the C 2 AЉ←X 2 AЉ band were observed. Photofragment mass distributions indicate that N 2 loss is the primary dissociation pathway. Translational energy distributions reveal a resolved vibrational structure of the N 2 fragment, suggesting that the HNCN radical first isomerizes to a cyclic HCN 2 intermediate. A dissociation mechanism is proposed in which electronically excited HNCN undergoes internal conversion to the ground state, followed by isomerization to cyclic HCN 2 and dissociation through a tight three-center transition state. The HNCN bond dissociation energy D 0 and heat of formation ⌬ f H 0 ͑HNCN͒ were determined to be 2.80Ϯ0.03 eV and 3.35Ϯ0.03 eV, respectively.

The photodissociation of mass-selected linear carbon clusters (C n , n ) 4-6) is studied using fast beam photofragment translational spectroscopy. The photofragment yield (PFY) spectra consist of several continua spanning the whole visible and ultraviolet region. The product mass distributions for dissociation of C n clusters are dominated by C 3 and its partner fragment C n-3 , although some minor channels are also identified for dissociation of C 4 and C 5 clusters. Translational energy P(E T ) distributions for the C 3 + C n-3 channel were measured at several photolysis energies. The PFY spectra and P(E T ) distributions indicate that multiphoton dissociation occurs at photon energies below the dissociation threshold and that both single-photon and multiphoton dissociation occur above the threshold. The one-photon components of the P(E T ) distributions can be modeled by phase space theory (PST), suggesting that photoexcitation is followed by internal conversion to the ground state. The PST analysis yields dissociation energies for C n f C 3 + C n-3 in reasonable agreement with recent Knudsen effusion mass spectrometry measurements.

The photodissociation dynamics of bare I