Photodissociation dynamics of N_4~+ in the 300-670 nm range

摘要

The photodissociation of N_4~+ produced in a supersonic jet ion source is studied in an octopole ion guide at selected wavelengths in the 300-670 nm range. The N_2~+ photofragment recoil velocity distributions are measured using the guided-ion beam time-of-flight (GIB-TOF) technique. The derived kinetic energy release distributions are structureless at all photon energies. The average anisotropy parameter, <#beta#>, ranges between 0.7 and 1.2. The absolute magnitude and photon energy dependence of both average kinetic energy release, , and <#beta#(E'_T)> are inconsistent with a previously assumed impulsive mechanism. Experiments are also conducted with N_4~+ produced with approx 0.6 eV internal excitation. This results in a reduction of the observed kinetic energy release and anisotropy parameter, implying that different regions of the excited state surfaces are accessed when ion vibrational modes are excited. The observations can be rationalized by invoking excited states with bent equilibrium geometries, consistent with valence-bond calculations of Langenberg et al. [Langenberg et al., Chem. Phys. 221, 225 (1997)].