Bulletin of the American Physical Society
2005 APS April Meeting
Saturday–Tuesday, April 16–19, 2005; Tampa, FL
Session R6: Few Body Collisions |
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Sponsoring Units: GFB Chair: H.R.J. Walters, Queens University, Belfast Room: Marriott Tampa Waterside Grand Salon I/J |
Monday, April 18, 2005 10:45AM - 11:21AM |
R6.00001: Collisions with molecular targets Invited Speaker: Something interesting and often unexpected happens when you excite a reaction close to its threshold. In threshold photoionisation the target is ionised just above a reaction onset. Photoelectrons are produced with energies that are typically in the range from zero to a few meV and these are detected by analysers tuned to these low energies. The studies are valuable because they give both dynamic and spectroscopic information. They also have a number of experimental advantages. Close to threshold, the low energy photoelectrons move only slowly away from the ion core allowing ample time for electron correlation effects to become dominant. These correlation effects are particularly important for double photoionisation Also, close to threshold, indirect processes can become dominant. In the case of molecules this can lead to the observation of vibrational excitation well outside the Frank-Condon region. On the experimental side, the threshold technique offers the advantages of very high resolution and simultaneously very high detection efficiency. This means that they can give spectroscopic information at the rotational level. The very high resolution is well matched to the high photon resolution provided by current synchrotron radiation sources. The very high detection efficiency facilitates the use of coincidence techniques that are required for the study of double ionisation, where both photoelectrons are detected simultaneously. [Preview Abstract] |
Monday, April 18, 2005 11:21AM - 11:57AM |
R6.00002: Ionization Processes by Ion Impact: the Role of Multiple Electron Scattering Invited Speaker: Ionization of atoms by charged particle impact is a fundamental process. It is not exotic, it does not need high energies, it is a common process with many practical aspects. Still it is a challenge for theory, representing a (minimum) three-body problem with long-range forces. The measured double (or more) differential spectra of the emitted electrons (and outgoing ions) provide detailed information about the ionization process. These spectra are often rich in characteristic structures, which can be associated with different collision mechanisms. In the present talk, we provide a brief sketch of the most common structures and the underlying mechanisms, and focus our attention to a higher-order process. This is the so-called Fermi-shuttle acceleration [1,2], where the electron is scattered forward and backward by the incoming heavy projectile ion and the target core before being ejected. Due to the repeated collisions, the electron can be accelerated to high energies. In recent works [3,4], evidence has been provided for double (projectile-target, P-T), triple (P-T-P) and quadruple (P-T-P-T) scattering sequences in ion-atom collisions. Surprisingly, our latest measurements and the corresponding CTMC calculations have shown that accelerating multiple electron scattering can even dominate electron emission for slow ion impact, providing a large amount of unexpectedly high-energy electrons. This finding may get relevance in many cases, where ions are stopped in solids or in biological tissues. \textit{Supported by the Hungarian OTKA Fund (T045905)} [1] S. Suarez \textit{et al}, Phys. Rev. Lett. \textbf{77,} 474 (1996). [2] U. Bechthold \textit{et al}, Phys. Rev. Lett. \textbf{79,} 2034 (1997). [3] B. Sulik \textit{et al}, Phys. Rev. Lett. \textbf{88,} 073201 (2002). [4] B. Sulik \textit{et al}, Nucl. Inst. and Meth. B \textbf{212}, 32 (2003). [Preview Abstract] |
Monday, April 18, 2005 11:57AM - 12:33PM |
R6.00003: Collision with surfaces Invited Speaker: New materials are increasingly complex and to gain the insight necessary for their intelligent engineering, entirely new experimental approaches are needed. In the past 20 years, a steadily increasing number of experiments on atoms and molecules have demonstrated the capability of few body collisions to investigate complex systems with sensitivity and specificity well beyond the limits imposed by conventional electron spectroscopies. Over the past decade or so, electron-electron coincidence spectroscopies, an experimental tool originally developed to study few body collision dynamics, have emerged as a powerful technique for obtaining detailed information on surfaces and overlayers. The class of one photon IN two electrons OUT experiments will be discussed with an emphasis on grazing incidence geometry that is expected to be particularly suited for studying system with reduced dimensionality. The crucial question of which is the dominant mechanism that leads to ejection of correlated electron pairs from the surface will be addressed. By the help of selected examples, it will be shown that, depending on the kinematics chosen, different kind of sensitivity and specificity can be exploited. [Preview Abstract] |
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