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Dynamics of Excess Electrons in Atomic and Molecular Clusters

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Dynamics of Excess Electrons in Atomic and Molecular Clusters Dynamics of excess electrons in atomic and molecular clusters by Ryan Michael Young A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Chemistry in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Daniel M. Neumark, Chair Professor Stephen R. Leone Professor Robert W. Dibble Fall 2011 Abstract Dynamics of excess electrons in atomic and molecular clusters by Ryan Michael Young Doctor of Philosophy in Chemistry University of California, Berkeley Professor Daniel M. Neumark, Chair Femtosecond time-resolved photoelectron imaging (TRPEI) is applied to the study of excess electrons in clusters as well as to microsolvated anion species. This technique can be used to perform explicit time-resolved as well as one-color (single- or multiphoton) studies on gas phase species. The first part of this dissertation details time-resolved studies done on atomic clusters with an excess electron, the excited-state dynamics of solvated molecular anions, and charge-transfer dynamics to solvent clusters. The second part summarizes various one-color photoelectron imaging studies on tetrahydrofuran clusters with an excess electron or doped with an iodide ion in order to probe the solvent structure of these clusters. Finally, a mixed study is presented exploring the effect of warmer cluster conditions on both the binding energies and relaxation times of excess electrons in water clusters. Time-resolved studies on mercury cluster anions (Hg)n¯ (7 ≤ n ≤ 20) demonstrate the different timescales of electron-phonon and electron-electron scattering in small systems. Low- energy (1.0-1.5 eV) excitation of the excess electron to a higher-lying electronic state decays via a cascade through the conduction band on a 10-40 ps timescale. Conversely, high-energy (4.7 eV) excitation of an electron from the valence band into the conduction band opens a second relaxation pathway: emission of the excess electron via Auger decay. The larger number of charge carriers and the geometrical changes to the cluster following the creation of the valence band hole state increase the relaxation rate, causing relaxation to occur on a 100s of fs timescale. The size dependence of both relaxation timescales becomes much less significant around n = 13 near the van der Waals-to-covalent bonding transition seen in other studies of mercury clusters. The solvated acetonitrile dimer anion, (CH3CN)n¯ (20 ≤ n ≤ 50) is also studied using TRPEI. The dimer anion is selectively excited with 790 nm (1.57 eV) pulses and probed with 395 nm (3.14 eV) pulses, detaching both the ground and excited states. The excited clusters are observed to autodetach on a timescale of ~200-300 fs with no size dependence. The excited- state autodetachment shows a direct link for the first time between the two different binding motifs observed in the gas phase with the two isomers observed in solution from their absorption 1 profiles. Electron solvation dynamics following charge-transfer-to-solvent excitation from iodide to small methanol clusters, I¯(CH3OH)n (4 ≤ n ≤ 11) are also examined with TRPEI. After electron transfer, the excited state spectrum undergoes significant evolution in both its position and shape. Considerations of the geometries of the initial iodide-doped methanol cluster as well as the intermediate bare methanol anion cluster and final neutral clusters suggest the electron is solvated, as at least one methanol molecule rotates to bring its hydroxyl group inward toward the cluster center, maximizing the hydrogen bond network. The observed relaxation timescales for both the vertical detachment energies and the spectral width (5-30 ps) are consistent with this type of motion. An autodetachment feature is also observed at all pump-probe delays, indicating that this is the primary decay pathway for these clusters, which is consistent with the lack of observed stable methanol cluster anions in this size range. One-color, one photon photoelectron imaging is applied to study tetrahydrofuran cluster anions (THF)n¯ (1 ≤ n ≤ 100) to probe the nature of the solvated electron in that solvent. An anion at the same mass-to-charge ratio as the THF anion is observed, though THF¯ is not expected due to its closed shell electronic structure, high HOMO-LUMO gap and dipole moment. Two peaks are observed in the photoelectron spectrum for this species, one of which is attributed to a long-chain C4H8O¯ anion formed after ring-opening from the secondary electron attachment. The other peak is likely due to a metastable THF transient negative ion arising from fragmentation of the larger clusters. These features persist until n = 5. By n = 6, the photoelectron spectra change shape, becoming much larger, and maintain that shape through n = 100. This transition is accompanied by an abrupt change in the photoelectron angular distribution. These changes are attributed to onset of the solvated electron state in THF clusters. The binding energy for the smallest cluster of this species is 1.96 eV, much higher than that for other solvated electron clusters at onset. Extrapolation to infinite cluster sizes yields a bulk value of 3.10 ± 0.03 eV. The energetics are analyzed in the frameworks of dielectric continuum theory and the proposed cavity structure for bulk THF. Iodide-doped THF clusters, I¯(THF)n (1 ≤ n ≤ 30), are also studied using ultraviolet photoelectron imaging in order to understand the nature of their solvation in THF and in attempt to define their structures. A substantial decrease in the stabilization energy is seen by n = 9, indicating the coordination number is maximized. However, the iodide ion continues to be significantly stabilized with addition of THF molecules, suggesting that the solvation shell is not completely closed. Larger sizes are stabilized in a manner similar to the bare cluster anions. Ab initio calculations suggest the iodide is at least partially embedded in the solvent cluster near the surface, surrounded by a sub-structure of 7-9 solvent molecules. The effect of warmer clustering conditions on electron binding energies and relaxation times in water clusters is investigated by using neon instead of argon as the carrier gas in the adiabatic expansion. Only isomer I water cluster anions are observed, with their binding energies only slightly perturbed by the change in cluster internal energy. The relaxation dynamics following p ← s excitation is monitored using time-resolved photoelectron imaging. Internal conversion lifetimes are seen to be shorter for anions formed in neon compared to those formed in argon, though they appear to converge to the same bulk limit. 2 Dedication This dissertation is dedicated in memory of my grandparents who taught me the value of hard work and to always push myself to do my best. i Contents Abstract ........................................................................................................................................... 1 Dedication ........................................................................................................................................ i Contents .......................................................................................................................................... ii Acknowledgements ........................................................................................................................ vi Chapter 1. Introduction .................................................................................................................. 1 1.1 Anion Clusters as Model Systems ........................................................................................ 1 1.2 Femtosecond Time-Resolved Photoelectron Spectroscopy ................................................. 2 1.2.1 Photodetachment ........................................................................................................... 2 1.2.2 Photoelectron imaging .................................................................................................. 6 1.3 Summary of systems studied ................................................................................................ 8 1.3.1 Semiconducting mercury cluster anions ........................................................................ 8 1.3.2 Acetonitrile cluster anions ............................................................................................. 9 1.3.3 Iodide-doped methanol clusters................................................................................... 10 1.3.4 Tetrahydrofuran cluster anions ................................................................................... 12 1.3.5 Iodide-doped tetrahydrofuran clusters ........................................................................ 13 1.3.6 Water cluster anions ..................................................................................................... 14 Chapter 2. Experimental Apparatus ............................................................................................. 17 2.1 Atomic, Molecular and Cluster Ion Source ........................................................................ 17 2.2 Femtosecond laser system .................................................................................................. 19 2.2.1 Dispersion compensating prisms ................................................................................. 22 2.2.2 Optical parametric amplifier ......................................................................................
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