Characterization of atom and ion-induced ‘‘internal’’ electron emission by thin film tunnel junctions Detlef Diesing a,⇑ , Domocos Kovacs a , Kevin Stella a , Christian Heuser b a Institut für Physikalische Chemie, Fakultät für Chemie, Universität Duisburg Essen, 45117 Essen, Germany b Fakultät für Physik, Universität Duisburg-Essen, 47048 Duisburg, Germany article info Article history: Received 25 September 2010 Received in revised form 7 December 2010 Available online 28 December 2010 Keywords: Non-adiabatic Ion-induced excitation Electron temperature Metal-insulator–metal junction abstract Highly excited charge carriers are released when single or multiply charged ions impinge on metal sur- faces. While electron emission into the adjacent vacuum phase is well investigated, one has only limited knowledge about the transport of excited electrons or holes into the bulk of a metal. This shortcoming can be reduced by studying the transport of these excited carriers over buried tunnel barriers in thin film metal-insulator–metal devices. The internal barriers can be tuned by a tunnel voltage which makes the device to a balance for excited electrons and holes. With a simple theoretical model we derive the balance function of different tunnel barriers and show their feasibility for the characterization of particle induced electronic excitations on metal surfaces. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction The interaction of ions and atoms with surfaces involves elastic as well as inelastic processes. Inelastic processes in the low energy regime ðE < 10 keVÞ were mainly discussed in the framework of nuclear stopping processes since the pioneering work of Lindhard and Scharff [1]. However, several works started also to discuss the coupling of ions kinetic energy to electronic excitations [2–4]. Furthermore, for very low kinetic energies E kin < 10 eV the transfer of particles kinetic or molecules internal energy to sub- strates electronic degrees of freedom was evidenced by a consider- able amount of experimental and theoretical works:  Experiments showed that the coupling of e.g. molecular vibra- tions on a surface to the electrons of the substrates leads to the emission of excited electrons [5,6]. Furthermore, the chem- ical excess energy of an adsorption [7] or desorption [8] may also release excited electrons with a considerable excess energy of 1 eV.  In theory of chemical surface reactions the progress in compu- tation speed and the advent of time dependent density func- tional theory methods (TDDFT) gives first insights into the dynamics of particles on surfaces, including a full picture of exchanged energy to lattice vibrations, as well as to electronic degrees of freedom [9–11]. In surface chemistry especially processes with kinetic energies lower than 10 eV are of interest since low kinetic energies are typ- ical for atom and molecular beam sources. The mentioned TDDFT works show distributions of excited electrons and holes which resemble exponential decays above E F for the electrons and below E F for the holes [9,12]. This decay in the density of excited carriers with respect to the Fermi level can be treated as a slope parameter describing the steepness of the distribution. However, for reason of simplicity we assign a value T el to an exponential decrease / e ðEE F Þ=ðk B T el Þ . This value resembles a temperature. The reciprocal value T 1 el describes the steepness of the excited carriers distribu- tion. In this paper, we will call T el an electronic temperature. But it must be noted, that this is not a temperature with a thermody- namic meaning, since these excited carriers distributions only exist for some 100 fs after the impact of the particle or after the chem- ical reaction. Afterwards the sample returns to its macroscopic thermodynamically defined sample temperature. But the short lifetime of the excited carriers is long enough, so that they can be transported through a some 10 nm thin metal film and tunnel through a some nm thick insulator film. For ion induced electronic excitations with charge states up to 9 and kinetic energies below 1 keV the spectrum of external electron emission was studied [13]. These spectra also show an exponen- tially decaying behavior up to energies of 100 eV above E F . Specific structures in the emitted electrons spectrum as Auger lines start to appear with charge states higher than 8. Thus, for both chemical and ion induced electronic excitations we will assume simple exponentially decaying distributions of excited carriers with re- spect to the Fermi level. It is the scope of the present paper to calculate this transport of excited carriers through an insulating barrier based on a simple 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.12.034 ⇑ Corresponding author. E-mail address: detlef.diesing@uni-due.de (D. Diesing). URL: http://www.phchem.uni-duisburg-essen.de/members/dedi/publist.html (D. Diesing). Nuclear Instruments and Methods in Physics Research B 269 (2011) 1185–1189 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb