Webb Alexander D, Nahler N Hendrik, Dixon Richard N, Ashfold Michael N R
School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom.
J Chem Phys. 2006 Nov 28;125(20):204312. doi: 10.1063/1.2390714.
DCl(+)(X (2)Pi(32),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 (2)Sigma(+) state levels with all vibrational (v(+')) quantum numbers in the range 0< or =v(+')< or =15. Image analysis yields wavelength dependent branching ratios and recoil anisotropies of the various D+Cl(+) ((3)P(J), (1)D, and (1)S) product channels. Levels with 10< or =v(+')< or =15 have sufficient energy to predissociate, forming D+Cl(+)((3)P(J)) products with perpendicular recoil anisotropies-consistent with the A (2)Sigma(+)<--X (2)Pi 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(+)((3)P(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 (2)Sigma(+)<--X (2)Pi(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 (4)Pi, (4)Sigma(-), and (2)Sigma(-) symmetries, all of which are calculated to cross the outer limb of the A (2)Sigma(+) state potential at energies close to that of the v(+')=10 level. Cl(+)((3)P(J)) fragments are detected weakly following excitation to A (2)Sigma(+) state levels with v(+')=0 or 1, Cl(+)((1)D) fragments dominate the ion yield when exciting via 2< or =v(+')< or =6 and via v(+')=9, while Cl(+)((1)S) 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 (2)Sigma(+)<--X (2)Pi 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.
通过2 + 1共振增强多光子电离制备了DCl(+)(X (2)Pi(32),v(+") = 0)阳离子，并通过对产生的Cl(+)产物进行速度成像，研究了它们在240 - 350 nm范围内多个波长激发后的后续碎片化过程。该激发波长范围允许选择性地填充A (2)Sigma(+)态能级，其所有振动(v(+'))量子数在0≤v(+')≤15范围内。图像分析得出了各种D + Cl(+) ((3)P(J), (1)D,和(1)S)产物通道的波长依赖性分支比和反冲各向异性。v(+')满足10≤v(+')≤15的能级具有足够的能量进行预解离，形成具有垂直反冲各向异性的D + Cl(+)((3)P(J))产物，这与A (2)Sigma(+)←X (2)Pi母体激发以及在与母体转动周期相比快速的时间尺度上的后续碎片化过程一致。发现Cl(+)((3)P(J))产物进入各种自旋 - 轨道态的分支对v(+')敏感，并且对于v(+') = 13能级，其随A (2)Sigma(+)←X (2)Pi(13,0)带内激发波长的精确选择而变化。根据与(4)Pi、(4)Sigma(-)和(2)Sigma(-)对称性的排斥态耦合对DCl(+)(A,v(+'))分子总预解离速率的不同贡献，对这些变化进行了定性解释，所有这些排斥态经计算在接近v(+') = 10能级能量处穿过A (2)Sigma(+)态势能的外沿。当激发到v(+') = 0或1的A (2)Sigma(+)态能级时，Cl(+)((3)P(J))碎片检测较弱；当通过2≤v(+')≤6和v(+') = 9激发时，Cl(+)((1)D)碎片主导离子产率；而当通过v(+') = 7和8能级激发时，Cl(+)((1)S)碎片主导所获得的Cl(+)图像。对形成这些Cl(+)离子的波长分辨作用光谱以及所得Cl(+)离子图像的分析表明：(i)这些离子均通过双光子吸收过程产生，在一个光子能量处通过各种A(v(+')<10)能级共振增强；(ii)在观察到显著双光子解离所需的条件下，第一个A (2)Sigma(+)←X (2)Pi吸收步骤饱和；(iii)来自共振增强A(v(+'))能级的最终吸收步骤涉及平行跃迁。
