Ultrafast X-ray Science Laboratory, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
J Phys Chem A. 2010 Jan 28;114(3):1437-45. doi: 10.1021/jp907312t.
The dynamics of electronically excited helium nanodroplets are studied by femtosecond time-resolved photoelectron imaging. EUV excitation into a broad absorption band centered around 23.8 eV leads to an indirect photoemission process that generates ultraslow photoelectrons. A 1.58 eV probe pulse transiently depletes the indirect photoemission signal for pump-probe time delays <200 fs and enhances the signal beyond this delay. The depletion is due to suppression of the indirect ionization process by the probe photon, which generates a broad, isotropically emitted photoelectron band. Similar time scales in the decay of the high energy photoelectron signal and the enhancement of the indirect photoemission signal suggest an internal relaxation process that populates states in the range of a lower energy droplet absorption band located just below the droplet ionization potential (IP approximately 23.0 eV). A nearly 70% enhancement of the ultraslow photoelectron signal indicates that interband relaxation plays a more dominant role for the droplet de-excitation mechanism than photoemission.
通过飞秒时间分辨光电子成像研究了电子激发的氦纳米液滴的动力学。EUV 激发到一个以 23.8 eV 为中心的宽吸收带导致间接光电子发射过程，产生超慢光电子。1.58 eV 的探测脉冲在泵浦-探测时间延迟 <200 fs 时暂时耗尽间接光电子发射信号，并在延迟后增强信号。这种耗散是由于探测光子抑制了间接电离过程，产生了一个宽的各向同性发射的光电子带。高能光电子信号的衰减和间接光电子发射信号的增强具有相似的时间尺度，表明存在一种内部弛豫过程，该过程填充了位于纳米液滴电离势（约 23.0 eV）下方的较低能液滴吸收带范围内的状态。超慢光电子信号增强近 70%表明，带间弛豫在液滴去激发机制中比光发射起着更主导的作用。
