Selectivity in electron attachment to water clusters.

Electron attachment onto water clusters to form water cluster anions is studied by varying the point of electron attachment along a molecular beam axis and probing the produced cluster anions using photoelectron spectroscopy. The results show that the point of electron attachment has a clear effect on the final distribution of isomers for a cluster containing 78 water molecules, with isomer I formed preferentially near the start of the expansion and isomer II formed preferentially once the molecular beam has progressed for several millimetres. These changes can be accounted for by the cluster growth rate along the beam. Near the start of the expansion, cluster growth is proceeding rapidly with condensing water molecules solvating the electron, while further along the expansion, the growth has terminated and electrons are attached to large and cold preformed clusters, leading to the isomer associated with a loosely bound surface state

Photoelectron spectroscopy of the deprotonated tryptophan anion: the contribution of deprotomers to its photodetachment channels †

Photoelectron spectroscopy and electronic structure calculations are used to investigate the electronic structure of the deprotonated anionic form of the aromatic amino acid tryptophan, and its chromophore, indole. The photoelectron spectra of tryptophan, recorded at different wavelengths across the UV, consist of two direct detachment channels and thermionic emission, whereas the hν = 4.66 eV spectrum of indole consists of two direct detachment features. Electronic structure calculations indicate that two deprotomers of tryptophan are present in the ion beam; deprotonation of the carboxylic acid group (Trp(i)−) or the N atom on the indole ring (Trp(ii)−). Strong similarities are observed between the direct detachment channels in the photoelectron spectra of tryptophan and indole, which in conjunction with electronic structure calculations, indicate that electron loss from Trp(ii)− dominates this portion of the spectra. However, there is some evidence that direct detachment of Trp(i)− is also observed. Thermionic emission is determined to predominantly arise from the decarboxylation of Trp(i)−, mediated by the ππ* excited state near λ = 300 nm, which results in an anionic fragment with a negative electron affinity that readily autodetaches

Charge Transfer to Solvent Dynamics at the Ambient Water/Air Interface

Electron-transfer reactions at ambient aqueous interfaces represent one of the most fundamental and ubiquitous chemical reactions. Here the dynamics of the charge transfer to solvent (CTTS) reaction from iodide was probed at the ambient water/air interface by phase-sensitive transient second-harmonic generation. Using the three allowed polarization combinations, distinctive dynamics assigned to the CTTS state evolution and to the subsequent solvating electron-iodine contact pair have been resolved. The CTTS state is asymmetrically solvated in the plane of the surface, while the subsequent electron solvation dynamics are very similar to those observed in the bulk, although slightly faster. Between 3 and 30 ps, a small phase shift distinguishes an electron bound in a contact pair with iodine and a free hydrated electron at the water/air interface. Our results suggest that the hydrated electron is fully solvated in a region of reduced water density at the interface

Spectroscopy and dynamics of the hydrated electron at the water/air interface

The hydrated electron, e–(aq), has attracted much attention as a central species in radiation chemistry. However, much less is known about e–(aq) at the water/air surface, despite its fundamental role in electron transfer processes at interfaces. Using time-resolved electronic sum-frequency generation spectroscopy, the electronic spectrum of e–(aq) at the water/air interface and its dynamics are measured here, following photo-oxidation of the phenoxide anion. The spectral maximum agrees with that for bulk e–(aq) and shows that the orbital density resides predominantly within the aqueous phase, in agreement with supporting calculations. In contrast, the chemistry of the interfacial hydrated electron differs from that in bulk water, with e–(aq) diffusing into the bulk and leaving the phenoxyl radical at the surface. Our work resolves long-standing questions about e–(aq) at the water/air interface and highlights its potential role in chemistry at the ubiquitous aqueous interface

Dynamics of Charge-Transfer-to-Solvent Precursor States in I-(water)n(n= 3−10) Clusters Studied with Photoelectron Imaging†

The dynamics of charge-transfer-to-solvent states are studied in I-(H2O)n)3-10 clusters and their deuterated counterparts using time-resolved photoelectron imaging. The photoelectron spectra for clusters with n g 5 reveal multiple time scales for dynamics after their electronic excitation. An increase in the vertical detachment energy (VDE) by several hundred millielectronvolts on a time scale of 1 ps is attributed to stabilization of the excess electron, primarily through rearrangement of the solvent molecules, but a contribution to this stabilization from motion of the I atom cannot be ruled out. The VDE drops by 50 meV on a time scale of tens of picoseconds; this is attributed to loss of the neutral iodine atom. Finally, the pump-probe signal decays with a time constant of 60 ps-3 ns, increasing with cluster size. This decay is commensurate with the growth of very slow electrons and is attributed to autodetachment. Smaller clusters (n) 3, 4) display simpler dynamics. Anisotropy parameters are reported for clusters n) 4-9. 1