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45076-G6
Electron Transfer in Cluster Anions: Dissociative Attachment Processes in Real Time
Our aim is to develop an experimental methodology capable of probing the dynamics of electron transfer initiated chemistry on the timescale of molecular motion. Since the excess electron in cluster anions is often localized on a particular moiety (with the surrounding solvent molecules retaining a largely neutral and unperturbed character) our strategy is to use cluster anions as molecular scale electron beam instruments and to initiate the chemistry with an intracluster electron transfer using an ultrafast laser pulse. In the first year of this project we have made considerable progress in the production of cluster anion species and with the implementation of a photoelectron imaging detection scheme. Initial experiments have focused on atomic, molecular and cluster anions based on O2- and O-. We have produced several species in the anion source chamber including O-, O2- and O2-.(H2O)n (integer n = 1-18) . This mixture is separated by time of flight mass spectrometry and synchronization of the arrival of a laser pulse with a particular peak in the time of flight mass spectrum allows selective recording of a specific anion's photoelectron image. The molecular superoxide and singly charged oxygen atomic anions have well characterized spectra and serve as excellent calibration systems for the imaging detector. Furthermore, by collecting images of the atomic anion in an event counting mode we have been able to develop teaching aids to demonstrate the dual wave-particle nature of the electron to undergraduate freshman chemists in a novel and interesting manner. Subsequent to detector calibration we have embarked upon a study of the effects of sequential solvation in superoxide-water cluster anions. Extraction of photoelectron spectra from preliminary images of O2-.(H2O)n at a photodetachment wavelength of 400 nm (fig. 1) shows a marked decrease in electron kinetic energy. The images are shown in the illustration with n=0-3 corresponding to (a-d). The changing size of the images corresponds to a shifting of the peak in the photoelectron spectrum, a consequence of the increased stabilization of the anion by the presence of one or more solvent molecules. Our initial data are in agreement with previous results.1,2 Interestingly the image recorded for the O2-.(H2O)3 cluster anion contains a small central feature corresponding to a low energy peak in the photoelectron spectrum. Two possibilities exist for the origin of this feature. The first is direct detachment (where the photon is ejected from the cluster at the instant of excitation without any further interaction with the other cluster constituents). Detachment occurs near the cluster threshold and there is little excess energy. A second possibility is an intracluster electron transfer to an temporary state of the cluster anion followed by autodetachment. The image features are strikingly similar to those previously recorded in CO2-.(H2O)5,6 clusters3 where photoexcitation induces an intracluster electron transfer to the water network (charge transfer to solvent, CTTS).4 The CTTS state autodetaches with a signature isotropic central spot in the photoelectron images. It is stressed that our images of the above systems are preliminary observations and any conclusions as to their meaning are tentative at this stage. However, work improving the signal-to-noise ratio in the images and allowing a thorough analysis of the photoelectron angular distributions and spectra, will soon be complete. These new results will provide a better demonstration of the nature of the parent electronic structure and of the location of the electron within these clusters. In addition to further single photon detachment studies, the possible presence of an autodetachment pathway in the O2-.(H2O)3 cluster opens up the opportunity of probing the relatively long lived state created using an ultrafast pump-probe photoelectron spectroscopic scheme allowing time resolved probing of details of the dissociative detachment process identified by Luong et al.2 We will also be turning our attention to the production of halide anion-methyl halide cluster anions and intracluster electron transfer induced processes within these. Experiments to generate the necessary precursor clusters will begin in the next month, subsequent to the completion of the superoxide-water cluster anion experiments. 1 Akin, F. A.; Schirra, L. K.; Sanov, A. J.Phys.Chem.A, 110, 8031 (2006). 2 Luong, A.K.; Clements, T.G.;Sowa Resat, M.;Continetti, R.E. J.Chem.Phys., 114, 3449 (2001) 3 Surber, E.; Mabbs, R.; Habteyes, T.; Sanov, A. J.Phys.Chem.A, 109, 4452 (2005). 4 Lehr, L.; Zanni, M. T.; Frischkorn, C.; Weinkauf, R.; Neumark, D. M. Science, 284, 635 (1999).
