Thermionic emission in small carbon cluster anions B. Baguenard, J.C. Pinar  e, F. L epine, C. Bordas * , M. Broyer Laboratoire de Spectrom etrie Ionique et Mol eculaire, Universit e Claude Bernard Lyon 1 and CNRS, B^ at. A. Kastler 43, Boulevard du 11 Novembre 1918, F69622 Villeurbanne Cedex, France Received 30 October 2001 Abstract Photodetachment of mass-selected small C  n clusters has been studied by photoelectron imaging spectroscopy. As in small refractory metal cluster anions, the low kinetic energy spectrum is dominated by thermionic emission. However, the effective temperatures measured from the electron kinetic energy spectra are sensibly larger than expected assuming complete thermal equilibrium. This discrepancy, more significant than in metal clusters, is attributed to the presence of a large gap in the density-of-states, which precludes a complete thermal equilibrium in the excited species before de- cay. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction Bridging the gap between simple atoms or molecules and bulk matter is one of the major goals of cluster physics. From this point of view, atomic clusters may be considered as model sys- tems to study the decay of energy-rich microscopic systems where the excitation energy exceeds the threshold for the emission of an electron. In that case, the behavior of the clusters allows establish- ing a connection between photoionization or photodetachment of simple systems and their an- alogs in bulk matter like photoelectric effect or thermionic emission. In energy-rich clusters, a complete breakdown of the Born–Oppenheimer approximation occurs and the dynamics of the system is governed by the exchange of energy be- tween the electronic and vibronic degrees of free- dom. As a result, the electron does not carry away all the residual energy and may take a long time before being ejected. This phenomenon of delayed ionization [1,2] has been observed mainly from metal [2–7] or carbon [8–11] clusters. It is de- scribed as thermionic emission when the excitation energy is equipartitioned between the nuclear and electronic degrees of freedom. By extension the term of thermionic emission is also used when thermal equilibrium is only partially reached, which is often the case in microscopic systems where one has simultaneously a fast electron emission process and presence of many bottlenecks complicating the electron–vibration coupling. The large density-of-states in metal clusters is a favorable condition for a fast internal energy re- distribution allowing thermalization of the system. Also, in most metal cluster anions, electron emis- sion is an open decay channel at moderate internal energy where fragmentation is not energetically 30 January 2002 Chemical Physics Letters 352 (2002) 147–153 www.elsevier.com/locate/cplett * Correspondig author. Fax: +33-4-72-43-15-07. E-mail address: bordas@lasim.univ-lyon1.fr (C. Bordas). 0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0009-2614(01)01449-X