Eur. Phys. J. D 43, 201–204 (2007) DOI: 10.1140/epjd/e2007-00113-x T HE EUROPEAN P HYSICAL JOURNAL D Dynamical aspects and the role of IVR for the reactivity of noble metal clusters towards molecular oxygen R. Mitri´ c, U. Werner, C. B¨ urgel, and V. Bonaˇ ci´ c-Kouteck´ y a Humboldt-Universit¨at zu Berlin, Institut f¨ ur Chemie, Brook-Taylor-Straße 2, 12489 Berlin, Germany Received 8 September 2006 / Received in final form 10 October 2006 Published online 24 May 2007 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2007 Abstract. Here, we present the dynamical aspects and the role of internal energy redistribution (IVR) in the reactivity of noble metal clusters towards O2. We show on the example of Ag3O - 2 / Ag3O2 / Ag3O + 2 that NeNePo spectroscopy carried out under zero electron kinetic energy (ZEKE) conditions can be a powerful tool to investigate the geometry relaxation and IVR induced by photodetachment in real time. Furthermore, we demonstrate that difference in the reactivity of Ag - 6 and Au - 6 towards O2 can be attributed to different nature of the IVR process. Dissipative IVR in Ag - 6 favors fast complex stabilization, whereas resonant IVR found for Au - 6 might be an important factor determining the catalytic activity of Au - 6 cluster in the CO oxidation. PACS. 31.15.Qg Molecular dynamics and other numerical methods – 31.15.Ar Ab initio calculations 1 Introduction Noble metal clusters exhibit fascinating reactive proper- ties such as strongly size and charge dependent reactiv- ity towards small molecules such as O 2 , CO etc. [1–3]. Moreover, gold clusters have attracted significant atten- tion due to their ability to catalyze the oxidation of CO at low temperatures [4–6]. In this context, the understand- ing of the mechanism for the activation of molecular oxy- gen is of fundamental importance. This has stimulated a number of theoretical studies in which specific geomet- ric as well as electronic properties which are responsible for cooperative adsorption of multiple adsorbate molecules on small noble metal clusters, have been identified re- cently [2,6,7]. In particular, the cooperative effects are re- garded essential to the catalytic activity of the gas-phase noble metal clusters in the CO combustion reaction [8,9]. Moreover, since both experimental and theoretical stud- ies have shown that the clusters alone are not capable to break the O–O bond [1,10], the question can be raised whether exploration of reactivity exclusively based on the stationary energetics is sufficient or whether dynamical processes, e.g. internal energy redistribution (IVR) [11], might become the key issue for full understanding of the catalytic properties. Here, we wish to address two complementary aspects of reaction dynamics of silver and gold clusters towards O 2 . One is to study reaction dynamics in neutral clusters in the frame of the negative-to-neutral-to-positive (NeNePo) spectroscopy [12–16]. After an initial ensemble is gener- ated in the anionic electronic ground state, photodetach- ment of the excess electron occurs and subsequently, dy- a e-mail: vbk@chemie.hu-berlin.de namics in the neutral state is monitored by probing the cationic state with a laser pulse of sufficiently high energy for ionization. The goal of theoretical fs-NeNePo spec- troscopy is to provide conditions under which different processes, such as bond breaking/formation, isomerization and IVR, and their time scales can be observed [17]. The potential prospects of the NeNePo spectroscopy for real time laser spectroscopic investigations of the activation of molecular O 2 will be illustrated on the prototype example of Ag 3 O 2 . The second aspect we would like to address concerns differences in reactivity of gold and silver clusters towards O 2 which might be caused by inherently different dy- namical properties. It has been found experimentally that Au 6 O - 2 is catalyzing the oxidation of CO [18] in contrast to Ag 6 O - 2 [19] which is inactive. However, structural and binding properties of Ag 6 O - 2 and Au 6 O - 2 do not differ sub- stantially. Therefore, the question can be raised whether the pronounced relativistic effects in gold [20–22] might be responsible for the found catalytic activity by invoking a special type of IVR which is reflected in the reaction dynamics and energy redistribution. Therefore, we inves- tigated the differences in the reaction dynamics between silver and gold clusters by studying the collision dynamics of Ag - 6 and Au - 6 with O 2 . In particular, we will highlight the role of resonant vs. dissipative IVR in the stabilization of complexes with O 2 . 2 Computational The time-resolved NeNePo spectra were simulated in the framework of our Wigner distribution approach [13] based