Ion Velocity Imaging Studies On Photodissociation Dynamics Of Small Molecules

This dissertation presents the experimental studies on the following two aspects: (1)Rotational Spectrum and Photodissociation Dynamics of N2O+(A2E+). (2)Photodissociation Dynamics of HNCO molecules at 210nm.Rotational Spectrum and Photodissociation Dynamics of N2O+(A2Σ+)The rotational structures of the photo fragment excitation spectrum of N2O+(A2E+) at high vibrational levels have been studied experimentally. The parent N2O+(X2Π) ions were prepared by [3+1] resonance-enhanced multiphoton ionization of jet-cooled N2O molecules at 360.55nm, and were excited by another laser to the predissociative A2E+state in the range of 280-320nm. Two types of rotational transition, i.e., 2Σ+←2Πand 2Π←2Π, have been clearly observed for a series of vibronic transitions for the first time. The rotational constants and spin splitting constants have been obtained from the spectral analysis.The photofragment NO+velocity imagings have been studied at high vibrational levels of N2O+(A2E+). And both angular and translational energy distribution have been obtained by integrating the imagings. It indicate the predissociation process on N2O+(A2Σ+) with the isotropy of NO+angular distribution. And it is found that there are three compositions in the total translational energy distribution. The conclusion is made that NO+fragments come from the NO++N(4S) and NO++N(2D) channels, However, the produce NO+from N(2D) channel pass through two pathways, and induce high and low rotational excitation.Photodissociation Dynamics of HNCO molecules at 210nmThe photodissociation of isocyanic acid (HNCO) on the first excited singlet state (S1) following the excitation at 210 nm has been investigated with ion velocity slice imaging technique by probing the CO fragment. It was found from the (2+1) resonance-enhanced multi-photon ionization (REMPI) spectrum that the CO fragment is rotationally hot with population up to Jmax=50. The velocity imagings of CO fragment at Jco=30 and 35 indicate that formation of NH(a1Δ)+CO(X1Σ+,v=0) is the predominant dissociation channel at 210nm. From analysis of the CO fragment translational energy distributions, the NH(a1Δ) fragment was observed to be rotationally cold, about half of the available energy was partitioned into the translational motion of fragments after dissociation, and the NH(a1Δ)+CO(X'Σ+) dissociation threshold was determined at 42738±30cm-1. From analysis of the CO fragment angular distributions, the dissociation anisotropy parametersβwere found to be negative, and increase with the rotational quantum number of NH fragment, i.e., from-0.75 at JNH=2～4 to -0.17 at JNH=11. Impulsive direct and vertical dissociation process of HNCO on the S1 state at 210nm was confirmed. A classical impact dissociation model was employed to explain the dependence ofβvalue on the rotational levels of NH fragment.