Lu and Sanche Reply:

Surface Trapped Excess Electrons on Ice

“…Such a mechanism explains the remarkable stability of large cluster isomers with EE in surface states, observed recently by Neumark et al [11] and provides a microscopical picture of the surface sites relevant for the heterogeneous chemistry on atmospheric ice particles [2,3].…”

“…Evidence for short-lived metastable surface trap states has been reported in a recent theoretical study of an EE at the surface of liquid water [18]. At the ice surface, the formation of ''presolvated'' 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 induce a local relaxation of the surface molecular structure with a characteristic time of less than 1 ps [19][20][21].…”

“…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.Excess electrons (EEs) in water systems are a subject of widespread interest, with implications in biology, atmospheric science, astrophysics, and cluster science [1][2][3][4][5][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.…”

“…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 electrons 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].…”

“…Excess electrons (EEs) in water systems are a subject of widespread interest, with implications in biology, atmospheric science, astrophysics, and cluster science [1][2][3][4][5][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.…”