Rathbone G J, Poliakoff E D, Bozek John D, Toffoli Daniele, Lucchese R R
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, USA.
J Chem Phys. 2005 Jul 1;123(1):014307. doi: 10.1063/1.1946738.
Vibrationally resolved photoelectron spectroscopy of the N2O+(A 2Sigma+) state is used to compare the dependence of the photoelectron dynamics on molecular geometry for two shape resonances in the same ionization channel. Spectra are acquired over the photon energy range of 18< or =hv< or =55 eV. There are three single-channel resonances in this range, two in the 7sigma-->ksigma channel and one in the 7sigma-->kpi channel. Vibrational branching ratio curves are determined by measuring vibrationally resolved photoelectron spectra as a function of photon energy, and theoretical branching ratio curves are generated via Schwinger variational scattering calculations. In the region 30< or =hv< or =40 eV, there are two shape resonances (ksigma and kpi). The ksigma ionization resonance is clearly visible in vibrationally resolved measurements at hv=35 eV, even though the total cross section in this channel is dwarfed by the cross section in the degenerate, more slowly varying 7sigma-->kpi channel. This ksigma resonance is manifested in non-Franck-Condon behavior in the approximately antisymmetric v3 stretching mode, but it is not visible in the branching ratio curve for the approximately symmetric v1 stretch. The behavior of the 35-eV ksigma resonance is compared to a previously studied N2O 7sigma-->ksigma shape resonance at lower energy. The mode sensitivity of the 35-eV ksigma resonance is the opposite of what was observed for the lower-energy resonance. The contrasting mode-specific behavior observed for the high- and low-energy 7sigma-->ksigma resonances can be explained on the basis of the "approximate" symmetry of the quasibound photoelectron resonant wave function, and the contrasting behavior reflects differences in the continuum electron trapping. An examination of the geometry dependence of the photoelectron dipole matrix elements shows that the ksigma resonances have qualitatively different dependences on the individual bond lengths. The low-energy resonance is influenced only by changes in the end-to-end length of the molecule, whereas the higher-energy resonance depends on the individual N-N and N-O bond lengths. Branching ratios are determined for several vibrational levels, including the symmetry-forbidden bending mode, and all of the observed behavior is explained in the context of an independent particle, Born-Oppenheimer framework.
利用N₂O⁺(A²Σ⁺)态的振动分辨光电子能谱，比较了同一电离通道中两个形状共振的光电子动力学对分子几何结构的依赖性。在18≤hv≤55 eV的光子能量范围内采集光谱。该范围内有三个单通道共振，两个在7σ→kσ通道，一个在7σ→kπ通道。通过测量作为光子能量函数的振动分辨光电子能谱来确定振动分支比曲线，并通过施温格变分散射计算生成理论分支比曲线。在30≤hv≤40 eV区域，有两个形状共振(ksigma和kpi)。尽管该通道的总截面与简并的、变化较慢的7σ→kπ通道的截面相比相形见绌，但在hv = 35 eV的振动分辨测量中，ksigma电离共振清晰可见。这种ksigma共振表现为在近似反对称的v3伸缩模式中的非弗兰克-康登行为，但在近似对称的v1伸缩的分支比曲线中不可见。将35 eV ksigma共振的行为与先前研究的较低能量下的N₂O 7σ→ksigma形状共振进行了比较。35 eV ksigma共振的模式敏感性与较低能量共振的观察结果相反。对于高能和低能7σ→ksigma共振观察到的对比模式特异性行为，可以基于准束缚光电子共振波函数的“近似”对称性来解释，并且这种对比行为反映了连续电子俘获的差异。对光电子偶极矩矩阵元的几何依赖性的研究表明，ksigma共振对各个键长的依赖性在性质上是不同的。低能共振仅受分子端到端长度变化的影响，而高能共振则取决于各个N-N和N-O键长。确定了包括对称禁戒弯曲模式在内的几个振动能级的分支比，并在独立粒子的玻恩-奥本海默框架内解释了所有观察到的行为。
