Photoionization Dynamics in Pure Helium Droplets

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• 作者：Darcy S. Peterka ; Jeong Hyun Kim ; Chia C. Wang ; Lionel Poisson ; Daniel M. Neumark
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：August 9, 2007
• 年：2007
• 卷：111
• 期：31
• 页码：7449 - 7459
• 全文大小：1301K
• 年卷期：v.111,no.31(August 9, 2007)
• ISSN：1520-5215

文摘

The photoionization and photoelectron spectroscopy of pure He droplets were investigated at photon energiesbetween 24.6 eV (the ionization energy of He) and 28.0 eV. Time-of-flight mass spectra and photoelectronimages were obtained at a series of molecular beam source temperatures and pressures to assess the effect ofdroplet size on the photoionization dynamics. At source temperatures below 16 K, where there is significantproduction of clusters with more than 10⁴ atoms, the photoelectron images are dominated by fast electronsproduced via direct ionization, with a small contribution from very slow electrons with kinetic energies below1 meV arising from an indirect mechanism. The fast photoelectrons from the droplets have as much as 0.5eV more kinetic energy than those from atomic He at the same photon energy. This result is interpreted andsimulated within the context of a "dimer model", in which one assumes vertical ionization from two nearest-neighbor He atoms to the attractive region of the He₂⁺ potential energy curve. Possible mechanisms for theslow electrons, which were also seen at energies below IE(He), are discussed, including vibrational autoionizatonof Rydberg states comprising an electron weakly bound to the surface of a large He_N⁺ core.