Surface Trapped Excess Electrons on Ice Francesca Baletto, 1 Carlo Cavazzoni, 2 and Sandro Scandolo 1 1 The Abdus Salam International Centre of Theoretical Physics (ICTP) and INFM/Democritos National Simulation Center, Strada Costiera 11, 34100 Trieste, Italy 2 CINECA and INFM/S3 nanoStructures and bioSystems at Surfaces (Modena), Via Magnanelli 6, I-40033 Casalecchio di Reno, Bologna, Italy (Received 16 June 2005; published 18 October 2005) Local trapping of excess electrons at the surface of solid water systems has recently been observed in large water clusters and at the ice/vacuum interface. The existence of stable surface-bound states for the excess electron may have important implications in atmospheric chemistry, electrochemistry, and radiation physics. By means of first-principles molecular dynamics we find that excess electrons induce a structural reconstruction of the ice surface on a time scale of a fraction of a picosecond. The surface molecular rearrangement leads to an increase of the number of dangling OH bonds pointing towards the vacuum and to the appearance of an electrostatic barrier preventing the penetration of the electron in the bulk. Both factors imply a remarkable stability for the surface-bound excess electron, with respect to its decay into the bulk solvated state. DOI: 10.1103/PhysRevLett.95.176801 PACS numbers: 73.20.2r, 36.40.Wa, 68.35.Bs, 71.15.Pd Excess electrons (EEs) in water systems are a subject of widespread interest, with implications in biology, atmos- pheric science, astrophysics, and cluster science [1– 6]. The dipole moment of the isolated water molecule is too small to bind an EE [7], but negatively charged states have been observed in the water dimer [6] and in many other larger water systems. Stabilization of the EE in larger systems is a consequence of cooperative molecular rearrangements [8]. Excess electrons in liquid bulk water are known to form a solvated complex characterized by a 4–6 A ˚ electron pocket, surrounded by a solvation shell of six H 2 O mole- cules [9,10]. In water clusters, the location of the EE depends on the cluster size and structure [11,12]. For sizes smaller than a few tens of molecules, the EE resides out- side the cluster, in a surface state, while for larger clusters it prefers a solvated state presumably similar to that found in bulk liquid water [5]. By varying the cluster formation pressure, however, Verlet et al. have recently found evi- dence for surface states even in large but cold clusters [11]. The observation of surface states was attributed to the fact that bulk solvation is kinetically hindered in cold clusters by the large energy barriers required to form the bulk solvation pocket [11]. Similar conclusions have been reached for the behavior of EEs at the surface of ice: while relaxation into a bulk solvated state is likely to be the ultimate fate of EEs initially attached to the ice surface, the kinetics of bulk solvation, at least below 150 K, is believed to be very slow (>1 ms) [3,13]. Based on the long lifetime of excess electrons at ice surfaces, it has been hypothesized that, in ice stratospheric clouds, excess elec- trons produced by cosmic rays, may catalyze some of the chemical reactions that lead to the formation of the radical halogen species responsible for the decomposition of ozone [2,3]. The relevance of this process to the depletion of the antarctic ozone is still under dispute [14–16]; but its broader impact onto the rich electron-driven chemistry of condensed halogenated compounds is discussed, e.g., in [17]. Stabilization of the EE in a surface-bound state is likely to be associated with a structural reorganization of the surface molecular layers. Accurate theoretical studies in very small clusters, up to H 2 O 12 , indicate that the pres- ence of the EE modifies substantially the molecular struc- ture of the cluster with respect to the neutral case [12]. Attachment of an EE induces a reorganization of the cluster bonding topology such that one of the molecules exposes two dangling protons towards the EE. Such a structural reorganization increases the total dipole moment of the cluster and decreases the exchange repulsion be- tween the EE and electrons in molecule [12]. Evidence for short-lived metastable surface trap states has been reported in a recent theoretical study of an EE at the surface of liq- uid water [18]. At the ice surface, the formation of ‘‘pre- solvated’’ surface states of the excess electron has been observed in experiments studying electron attachment to electronegative species [2,3]. Similarly, time-resolved spectroscopic studies on very thin ice films (up to four molecular layers) indicate that photoexcited electrons in- duce a local relaxation of the surface molecular structure with a characteristic time of less than 1 ps [19–21]. Generally speaking, attachment of an EE at the ice surface can be expected to be followed by one of the following processes. (a) Decay of the EE into a conduction band state of bulk ice. This mechanism is suggested by the observation that the conduction band of bulk ice lies below the vacuum level by about 1 eV [22]; however, it has not been reported in experiments so far. (b) Decay of the EE into a bulk solvated state. Kinetics hinders such process, as already noted, so we do not take it into account in this study. (c) The surface responds to the attachment with PRL 95, 176801 (2005) PHYSICAL REVIEW LETTERS week ending 21 OCTOBER 2005 0031-9007= 05=95(17)=176801(4)$23.00 176801-1  2005 The American Physical Society