Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003, USA.
J Chem Phys. 2011 Dec 28;135(24):244306. doi: 10.1063/1.3672161.
We report zero kinetic energy (ZEKE) photoelectron spectroscopy of benzo[a]pyrene (BaP) via resonantly enhanced multiphoton ionization (REMPI). Our analysis concentrates on the vibrational modes of the first excited state (S(1)) and those of the ground cationic state (D(0)). Similar to pyrene, another peri-condensed polycyclic aromatic hydrocarbon we have investigated, the first two electronically excited states of BaP exhibit extensive configuration interactions. However, the two electronic states are of the same symmetry, hence vibronic coupling does not introduce any out-of-plane modes in the REMPI spectrum, and Franck-Condon analysis is qualitatively satisfactory. The ZEKE spectra from the in-plane modes observed in the REMPI spectrum demonstrate strong propensity in preserving the vibrational excitation of the intermediate state. Although several additional bands in combination with the vibrational mode of the intermediate state are identifiable, they are much lower in intensity. This observation implies that the molecular structure of BaP has a tremendous capability to accommodate changes in charge density. All observed bands of the cation are IR active, establishing the role of ZEKE spectroscopy in mapping out far infrared bands for astrophysical applications.
我们通过共振增强多光子电离（REMPI）报告了苯并[a]芘（BaP）的零动能（ZEKE）光电子能谱。我们的分析集中在第一激发态（S(1)）和基态阳离子态（D(0)）的振动模式上。与我们研究的另一种稠合多环芳烃芘类似，BaP 的前两个电子激发态表现出广泛的构型相互作用。然而，这两个电子态具有相同的对称性，因此振子耦合不会在 REMPI 光谱中引入任何非平面模式，Franck-Condon 分析在定性上是令人满意的。在 REMPI 光谱中观察到的平面模式的 ZEKE 光谱表明强烈倾向于保持中间态的振动激发。尽管可以识别与中间态振动模式相结合的几个附加带，但它们的强度要低得多。这一观察结果表明，BaP 的分子结构具有巨大的能力来适应电荷密度的变化。阳离子的所有观察到的带都是红外活性的，这为天体物理应用中的远红外带的测绘确立了 ZEKE 光谱的作用。
