Ultrafast X-ray Science Laboratory, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
J Phys Chem A. 2011 Jul 14;115(27):7891-900. doi: 10.1021/jp2004216. Epub 2011 Jun 20.
Ultrafast relaxation of electronically excited pure He droplets is investigated by femtosecond time-resolved photoelectron imaging. Droplets are excited by extreme ultraviolet (EUV) pulses with photon energies below 24 eV. Excited states and relaxation products are probed by ionization with an infrared (IR) pulse with 1.6 eV photon energy. An initially excited droplet state decays on a time scale of 220 fs, leading predominantly to the emission of unaligned 1s3d Rydberg atoms. In a second relaxation channel, electronically aligned 1s4p Rydberg atoms are emitted from the droplet within less than 120 fs. The experimental results are described within a model that approximates electronically excited droplet states by localized, atomic Rydberg states perturbed by the local droplet environment in which the atom is embedded. The model suggests that, below 24 eV, EUV excitation preferentially leads to states that are localized in the surface region of the droplet. Electronically aligned 1s4p Rydberg atoms are expected to originate from excitations in the outermost surface regions, while nonaligned 1s3d Rydberg atoms emerge from a deeper surface region with higher local densities. The model is used to simulate the He droplet EUV absorption spectrum in good agreement with previously reported fluorescence excitation measurements.
通过飞秒时间分辨光电子成像研究了纯氦液滴中电子激发的超快弛豫。液滴通过光子能量低于 24 eV 的极紫外 (EUV) 脉冲激发。通过具有 1.6 eV 光子能量的红外 (IR) 脉冲进行电离来探测激发态和弛豫产物。初始激发的液滴状态在 220 fs 的时间尺度上衰减，主要导致未对准的 1s3d 里德伯原子的发射。在第二个弛豫通道中，电子对准的 1s4p 里德伯原子在不到 120 fs 的时间内从液滴中发射出来。实验结果在一个模型中进行了描述，该模型通过局部原子里德伯态来近似电子激发的液滴态，这些态被嵌入原子的局部液滴环境所干扰。该模型表明，在 24 eV 以下，EUV 激发优先导致局域在液滴表面区域的态。电子对准的 1s4p 里德伯原子预计源自最外层表面区域的激发，而非对准的 1s3d 里德伯原子则源自具有更高局部密度的更深表面区域。该模型用于模拟氦液滴的 EUV 吸收光谱，与先前报道的荧光激发测量结果吻合良好。
