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Velocity distributions of In atoms in their ground (^Pi/2) and excited i^P^n) states ejected from In foils by bombardment with 5-keV Ar"*" ions are measured with a multiphoton resonanceionization technique. We show for the first time that the ^P^fi level is not significantly populated during sputtering and that the only path for population of this state is via laser-induced photodissociation of Ina during the detection process. A new deexcitation model for sputtered excited neutral atoms is proposed which is based on the electronic localization of the fine-structure orbital.PACS numbers: 79.20.Nc, 32.80.Rm, 33.80.Gj, 36.40.+d The measurement of velocity distributions of excited atoms desorbed from ion-bombarded solids has been of long-standing interest since these distributions hold the key to unraveling the physical mechanisms which lead to the excitation. Early studies suggested that the average kinetic energy of excited atoms was many hundreds of electronvolts larger than the ground state and was characterized by a kinetic-energy threshold.^ Later, more sophisticated laser Doppler-shifted fluorescence techniques^ showed that the velocity distribution of the fine-structure excited levels of transition elements such as Zr^ and Fe^ with excitation energies between 0.1 and 1.0 eV are similar to the ground-state distributions. On the other hand, Ba ^D and ^D,^ Cdi^Pi,^ and Fe ^^5^ metastable excited levels were found to exhibit velocity distributions whose peak intensities occur at significantly larger values than those of the corresponding ground states. In none of these later studies was there found to be evidence for a kinetic energy threshold. The shift in the excitedatom velocity distribution relative to the ground state has been commonly ascribed to an exponential velocity-dependent nonradiative deexcitation."* The total population of these excited fine-structure levels has been explained by use of a Boltzmann distribution with an effective temperature of order 1000 K. With this scarcity of data, it has not been possible to formulate a general theory of neutral-atom excitation following energetic ion bombardment.In this Letter, we report velocity distributions of indium atoms ejected from polycrystalline In foil after bombardment with 5-keV Ar"*" ions. By means of a special multiphoton resonance-ionization (MPRI) technique,^ these distributions were determined for the In ^P\/2 ground state, the ^Pyi metastable finestructure excited state which lies 0.3 eV above the ^P\/2 ground state, and the In2 ^2^ ground state. The results show that the ^Pyi level is not significantly populated during sputtering, and that the only path for the population of this state is via laser-induced photodissociation of In2. These results demonstrate, for the first time, that excitation probabilities are strongly dependent on electronic structure. Further, the observations from In, together with those from Zr, Fe, Ca, and Ba, allow us to propose a new qualitative model for nonradiative deexcitation of the desorbing atom which is b...

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