A velocity map Imaging study of the one and two photon dissociations of state-selected DCl~+ cations

摘要

DCl~+(X~2PI_(3,2),v~(+")=0) cations have been prepared by 2+1 resonance enhanced multiphoton ionization,and their subsequent fragmentation following excitation at numerous wavelengths in the range of 240-350 nm studied by velocity map imaging of the resulting Cl~+ products.This range of excitation wavelengths allows selective population of A ~2SIGMA~+ state levels with all vibrational (v~(+')) quantum numbers in the range 0<=v~(+') <= 15.Image analysis yields wavelength dependent branching ratios and recoil anisotropies of the various D+Cl~+ (~3P_J,~1D,and ~1S) product channels.Levels with 10<=v~(+')<=l5 have sufficient energy to predissociate,forming D+Cl~+(~3P_J) products with perpendicular recoil anisotropies-consistent with the A ~2SIGMA~+<- X~2PI parent excitation and subsequent fragmentation on a time scale that is fast compared with the parent rotational period.Branching into the various spin-orbit states of the Cl~+(~3P_J) product is found to depend sensitively upon v~(+') and,in the case of the v~(+') = 13 level,to vary with the precise choice of excitation wavelength within the A ~2SIGMA~+<- X~2PI(13,0) band.Such variations have been rationalized qualitatively in terms of the differing contributions made to the overall predissociation rate of DCl~+(A,v~(+')) molecules by coupling to repulsive states of ~4PI,~4SIGMA~-,and ~2SIGMA~- symmetries,all of which are calculated to cross the outer limb of the A 22+ state potential at energies close to that of the v~(+') = 10 level.CL~+(~3P_j) fragments are detected weakly following excitation to A ~2SIGMA~+ state levels with v~(+')=0 or 1,Cl~+(~1D) fragments dominate the ion yield when exciting via 2<=v~(+') <= 6 and via v~(+')=9,while Cl~+(~1S) fragments dominate the Cl~+ images obtained when exciting via levels with v~(+') =7 and 8.Analysis of wavelength resolved action spectra for forming these Cl~+ ions and of the resulting Cl~+ ion images shows that (i) these ions all arise via two photon absorption processes,resonance enhanced at the one photon energy by the various A(v~(+') < 10) levels,(ii) the first A ~2SIGMA~+<- X ~2PI absorption step is saturated under the conditions required to observe significant two photon dissociation,and (iii) the final absorption step from the resonance enhancing A(v~(+')) level involves a parallel transition.