Wang Chia C, Kornilov Oleg, Gessner Oliver, Kim Jeong Hyun, Peterka Darcy S, Neumark Daniel M
Department of Chemistry, University of California, Berkeley, California 94720, USA.
J Phys Chem A. 2008 Oct 2;112(39):9356-65. doi: 10.1021/jp802332f. Epub 2008 Aug 9.
Helium droplets doped with Xe and Kr atoms were photoionized by using VUV synchrotron radiation from the Advanced Light Source and the resulting photoelectron images were measured. A wide range of He droplet sizes, photon energies, and dopant pick-up conditions was investigated. Significant ionization of dopants was observed at 21.6 eV, the absorption maximum of 2p (1)P1 electronic excited state of He droplets, indicating an indirect ionization mechanism via excitation transfer. The photoelectron images and spectra reveal multiple photoionization mechanisms and pathways for the photoelectrons to escape the droplet. Specifically, they show sets of sharp peaks assigned to two mechanisms for Penning ionization of the dopant by He* in which the photoelectrons leave the droplet with no detectable energy loss, a broad, intense feature representing electrons that undergo significant energy loss, and a small amount of ultraslow electrons that may result from electron trapping at the droplet surface. The droplet-size dependence of the broad, intense feature suggests the development of the conduction band edge in the largest droplets seen here ((N) approximately 250,000).
掺杂有Xe和Kr原子的氦液滴通过使用来自先进光源的真空紫外同步辐射进行光电离，并测量了产生的光电子图像。研究了广泛的氦液滴尺寸、光子能量和掺杂剂俘获条件。在21.6 eV处观察到掺杂剂的显著电离，这是氦液滴2p(1)P1电子激发态的吸收最大值，表明通过激发转移的间接电离机制。光电子图像和光谱揭示了多种光电离机制以及光电子逃离液滴的途径。具体而言，它们显示了与He*对掺杂剂进行彭宁电离的两种机制相关的尖锐峰组，其中光电子离开液滴时没有可检测到的能量损失，一个宽的、强烈的特征代表经历显著能量损失的电子，以及少量可能由液滴表面电子俘获产生的超慢电子。宽的、强烈特征对液滴尺寸的依赖性表明在此处看到的最大液滴（(N)约为250,000）中导带边缘的发展。
