Time-resolved Photoelectron Imaging of Anionic Cluster Dynamics PDF Download

Author: Aster Ellen Kammrath
Publisher:
ISBN:
Category :
Languages : en
Pages : 320

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Author: Graham Bailey Griffin
Publisher:
ISBN:
Category :
Languages : en
Pages : 164

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Electronic relaxation dynamics are measured on a femtosecond timescale in three types of anionic clusters using time resolved photoelectron imaging. Auger relaxation timescales following interband excitation of electron-hole pairs in small Hgn- (n=9-20) are determined. Relaxation dynamics following charge transfer are investigated in I-(CH3CN)n (n=5-10). Internal conversion lifetimes of excited states of large anionic water clusters, (H2O)n- and the fully deuterated isotopolog (D2On- (n=25-200), as well as solvation dynamics in these clusters, are evaluated. A pronounced increase in the Auger lifetime of interband-excited states of Hgn - clusters with 13 or more constituent mercury atoms is revealed, indicating a shift from the van der Waals interactions typical of smaller clusters towards covalent bonding between mercury atoms in the cluster. This creates more delocalized electronic orbitals which reduce the coulomb interactions of the electron-hole pair, increasing the amount of time required for recombination and ejection of Auger electrons. Initial dynamics following charge transfer in I-(CH3CN)n clusters are associated with localization of an initially diffuse electron contained within the cluster. Later dynamics are assigned to rearrangement of the network of CH3CN molecules and ejection of neutral iodine from the cluster. Ultrafast internal conversion lifetimes of the first electronic excited state of anionic water clusters are measured at larger cluster sizes and with better time resolution than previous measurements. A marked reduction in the size dependence of the internal conversion lifetime at large sizes indicates a change in the electron water interaction for clusters larger than n0≈70. Extrapolating internal conversion lifetimes of the larger clusters towards infinite cluster size predicts a condensed phase internal conversion lifetime of ̃50 fs for the hydrated electron, supporting the nonadiabatic relaxation model. Solvation dynamics on both the ground and excited states are also observed.

Author: Arthur Edward Bragg
Publisher:
ISBN:
Category :
Languages : en
Pages : 628

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Author: Alison Virginia Davis
Publisher:
ISBN:
Category :
Languages : en
Pages : 362

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Author: Margaret Ashley Yandell
Publisher:
ISBN:
Category :
Languages : en
Pages : 119

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Time-resolved photoelectron imaging spectroscopy is used to examine the dynamics of charge accommodation by solvent species and biomolecules upon photo-initiated intracluster charge transfer. Excitation of a charge transfer state of an iodide-complexed molecule or cluster with a UV pulse and subsequent interrogation by photodetachment with a lower energy probe enables detection of changes in photoelectron signals over hundreds of femtoseconds. Velocity map imaging detection permits simultaneous collection of electron kinetic energy (eKE) and photoelectron angular distributions that provide insight into the strength and structure of the association between the cluster or molecule and the excess electron. Application of this methodology to iodide-containing clusters of small polar molecules such as water, methanol, and ethanol elucidates the stability and extent of intramolecular forces within a given cluster. In complexes of iodide with small solvent clusters (≤ 10 molecules), iodide is situated somewhat outside of the solvent network. Interaction of iodide-water clusters with a UV pulse to produce iodine and a free electron results in the partial solvation of the excess charge through hydrogen bonding interactions over hundreds of picoseconds before electron autodetachment. In contrast, methanol and ethanol cluster networks can only support the excess charge for tens of ps. Notably, stable bare water cluster anions have previously been measured with as few as two molecules, while upwards of seventy methanol molecules are necessary to stabilize an excess electron. Drawing an analogy between electron autodetachment and statistical unimolecular decay, an excited iodide-water cluster with a given number of water molecules might be expected to decay most rapidly given its significantly smaller density of states. The observation of the opposite pattern, as well as the similarity between iodide-methanol and -ethanol cluster anion lifetimes, suggests that energetics, rather than molecular structure, play a larger role in stabilizing an excess charge to autodetachment. Applying a thermionic emission model confirms this result. The dynamics of charge accommodation are also examined for small biomolecules. Radiative damage to DNA caused by low energy electrons is thought to originate in the attachment of an electron to a nucleobase unit of a nucleotide in the DNA double helix. Previous experiments have examined binding motifs and fragmentation patterns of transient negative ions (TNIs) of nucleobases using Rydberg electron transfer from excited noble gas atoms or collision of the nucleobase with a beam of electrons of defined energy. Here, nascent TNIs of the nucleobase uracil are created by intracluster charge transfer from a complexed iodide ion and their decay examined with time-resolved photoelectron imaging. Anions created with several hundred meV of excess energy appear as valence anions and are observed to decay biexponentially with time constants of hundreds of fs and tens of ps by iodine atom loss and autodetachment. Repetition of these experiments with uracil molecules methylated at the N1, N3, or C5 positions results in a dramatic reduction of the longer time constant. The addition of the methyl group may hasten the intramolecular vibrational energy redistribution process preceding autodetachment. Photoelectron spectroscopy of isolated nucleobase anions has measured only the dipole-bound state (DBS) of the anion consisting of an electron weakly associated with the molecular dipole moment and very delocalized over the molecular structure. Though the valence anion has not been directly measured, the DBS has been posited to serve as a `doorway' to the valence-bound state (VBS). Such a mechanism has also been proposed for nitromethane. In contrast, acetonitrile should only support a DB anion state. Examination of nascent acetonitrile and nitromethane anions excited near the vertical detachment energies of their corresponding iodide-molecule complexes indeed produces the DB acetonitrile anion, which then decays biexponentially with time constants of few and hundreds of ps by iodine atom loss and autodetachment. The nitromethane DB anion decays rapidly over hundreds of fs to form the valence anion, which decays biexponentially with time constants similar to those measured for the acetonitrile DB anion. This study marks the first direct observation of a transition from a dipole-bound anion to a valence anion and will inform future studies of iodide-nucleobase complexes.

Author: Ryan Michael Young
Publisher:
ISBN:
Category :
Languages : en
Pages : 304

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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)n0¯ (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)n0¯ (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 2̃00-300 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 profiles. Electron solvation dynamics following charge-transfer-to-solvent excitation from iodide to small methanol clusters, I0¯(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)n0¯ (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 THF0¯ 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 C4H8O0¯ 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, I0¯(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.

Author: Emily Rose Grumbling
Publisher:
ISBN:
Category :
Languages : en
Pages : 514

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This dissertation explores the use of anion photoelectron imaging to interrogate electronic dynamics in small chemical systems with an emphasis on photoelectron angular distributions. Experimental ion generation, mass selection, laser photodetachment and photoelectron imaging were performed in a negative-ion photoelectron imaging spectrometer described in detail. Results for photodetachment from the simplest anion, H & ndash;, are used to illustrate fundamental principles of quantum mechanics and provide basic insight into the physics behind photoelectron imaging from a pedagogical perspective. This perspective is expanded by introducing imaging results for additional, representative atomic and small molecular anions, (O & ndash;, NH2 & ndash;and N3 & ndash;) obtained at multiple photon energies to address the energy-dependence of photoelectron angular distributions both conceptually and semi & ndash;quantitatively in terms of interfering partial photoelectron waves. The effect of solvation on several of these species, (H & ndash;, O & ndash;, and NH2 & ndash;) is addressed in photoelectron imaging of several series of cluster anions. The 532 and 355 nm energy spectra for H & ndash;(NH3)nand NH2 & ndash;(NH3)n(n= 0-5) reveal that these species are accurately described as the core anion solute stabilized electrostatically bynloosely coordinated NH3molecules. The photoelectron angular distributions for solvated H & ndash;deviate strongly from those predicted for unsolvated H & ndash;as the electron kinetic energy approaches zero, indicating a shift in the partial-wave balance consistent with both solvation-induced perturbation (and symmetry-breaking) of the H & ndash;parent orbital and photoelectron-solvent scattering. The photoelectron energy spectra obtained for the cluster series [O(N2O)n] & ndash;and [NO(N2O)n] & ndash;indicate the presence of multiple structural isomers of the anion cores, the former displaying sharp core-switching atn= 4, the latter isomer coexistence over the entire range studied. The photoelectron angular distributions for detachment from the O & ndash;(N2O)nand NO & ndash;(N2O)nisomers deviate strongly from those expected for bare O & ndash;and NO & ndash;, respectively, in the region of an anionic shape resonance of N2O, suggesting resonant photoelectron-solvent scattering. Partial-wave models for two-centered photoelectron interference in photodetachment from dissociating I2 & ndash;is presented and discussed in the context of previous results. New time-resolved photoelectron imaging results for I2super & ndash;, for both parallel and perpendicular pump and probe beam polarizations, are presented and briefly discussed. Finally, new ideas and directions are proposed.

Author: Sarah Bailey King
Publisher:
ISBN:
Category :
Languages : en
Pages : 159

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Time-resolved photoelectron imaging is used to investigate the dynamics of electron attachment and electron interaction with the molecules uracil (U), thymine (T), adenine (A) and imidazole (Im). In this technique, the molecule of interest is clustered with an iodide atom, and a tunable UV photon induces ultrafast electron transfer from iodide to the molecule, forming a transient negative ion with femtosecond time resolution. After a known time delay, a second photon detaches the transient negative ion and the resulting photoelectrons are detected using velocity map imaging. This experimental method allows for insight into how biologically relevant molecules in the gas phase interact with, and accommodate, an excess electron, an important question in radiation biology. Uracil and thymine interact similarly with excess electrons. We observe two different electron attachment motifs, dependent on the pump pulse excitation energy that induces charge transfer from the iodide atom. The vertical detachment energy (VDE) of the iodide-uracil cluster is 4.11 " 0.05 eV and 4.05 " 0.05 eV for the iodide-thymine cluster. Excitation of the clusters with photon energies of approximately 500-700 meV above the I-U and I-T VDEs results in electrons with approximately 500-700 meV of kinetic energy that scatter directly into the valence-bound orbitals of uracil and thymine, forming the valence bound anion. Using lower excitation energies, between 120 meV below the VDE and 110 meV above the I-U and I-T VDEs, the I-T anion ground state is photoexcited to an anion state where the excess electron is bound in a dipole-bound (DB) anion state by the dipole moment of the cluster. Due to a changing photodetachment cross-section of the uracil and thymine DB anion from geometry relaxation at early times, the DB photoelectron signal has a rise-time longer than the cross-correlation of the pump and probe pulses. Subsequently, a small population of the uracil and thymine DB anions transition to the valence-bound (VB) anions, in agreement with theoretical predictions. However, no participation of the uracil or thymine DB anion is observed in the formation of the respective VB anion at excitation energies 500-700 meV above the I-U/T VDEs, contrary to experiments that invoked participation of the dipole-bound anions to explain features in the dissociative electron attachment spectra. The uracil and thymine DB and VB anions ultimately decay through a variety of mechanisms. In the lower excitation energy region, both the DB and VB anions of uracil and thymine decay bi-exponentially at all of the excitation energies studied. The decay lifetimes range between 2 to 25 ps for the short decay lifetime and 30-2000 ps for the long decay lifetime, depending on excitation energy and anion state. In the higher excitation energy region, the thymine VB anion signal decays completely by 10 ps, unlike uracil that has a bi-exponential long-time lifetime that persists until at least 100 ps. The bi-exponential decays for the DB and VB anions of uracil and thymine are attributed to various mechanisms depending on the molecule and excitation energy including: different rates of autodetachment prior and subsequent to iodine loss, and non-statistical autodetachment versus statistical autodetachment. Experiments investigating the electron attachment dynamics to adenine show evidence of multiple tautomers of adenine participating in the dynamics. Excitation from the ground state I-A anion cluster to the iodine-adenine DB anion, is induced with excitation energies near the 3.96 " 0.05 eV VDE of the I-A9 canonical tautomer. The DB anion of adenine is initially formed with a ~250 fs rise-time due to a changing photodetachment cross-section correlated with relaxation of the cluster geometry from the Franck-Condon region, as is observed in uracil and thymine. The DB anion undergoes a complete ultrafast transition to the VB anion at some excitation energies, and a partial transition at other excitation energies. However, electronic structure calculations do not predict a stable valence bound anion of the A9 canonical tautomer of adenine, and the relative intensities of the dipole-bound and valence-bound anions and the dipole-bound anion decay lifetimes display non-monotonic trends. These dynamics are consistent with two tautomers present in the ion beam clustered to iodide, the A9 canonical tautomer and the A3 non-canonical tautomer. The DB to VB transition is due to the A3 tautomer. The A3 tautomer is calculated to support a VB anion with an exothermic transition from the DB to VB state. The A9 canonical tautomer however only supports an excess electron in a DB orbital and the DB anion is formed in a narrower excitation energy range than the A3 tautomer, causing the non-monotonic trends in the dipole-bound and valence-bound anion intensity ratios and dipole-bound anion decay lifetimes. Imidazole, like the A9 tautomer of adenine, only supports an excess electron in a DB orbital. The VDE of the iodide-imidazole binary cluster is 3.90 " 0.05 eV. With excitation energies just below 3.90 eV, the ground state I-Im cluster is excited to the I Im-(DB) anionic excited state with an ultrafast rise-time due to geometry changes in the [I Im]- cluster. The DB state decays multi-exponentially with decay dynamics that change rapidly with small changes in the excitation energy. These dynamics suggest that the degree of vibrational excitation in the dipole-bound cluster considerably effects the decay dynamics of the transient [I Im]- ion. Overall, the systems studied provide a wide picture of the various ways that biologically relevant molecules can interact with, and accommodate, excess charge in dipole- and valence-bound anion states, and the various ways that iodide(iodine) can influence the observed dynamics, through the rise time of the dipole-bound state and the decay of both dipole- and valence-bound anions. A detailed understanding of the electron kinetic energy dependent mechanisms of electron attachment in nucleobases, and any subsequent dipole-bound anion to valence-bound anion transition, is crucial for understanding the various mechanisms of low-energy electron damage to DNA.

Author: Wolfgang Domcke
Publisher: World Scientific
ISBN: 9814313440
Category : Science
Languages : en
Pages : 769

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The concept of adiabatic electronic potential-energy surfaces, defined by the Born?Oppenheimer approximation, is fundamental to our thinking about chemical processes. Recent computational as well as experimental studies have produced ample evidence that the so-called conical intersections of electronic energy surfaces, predicted by von Neumann and Wigner in 1929, are the rule rather than the exception in polyatomic molecules. It is nowadays increasingly recognized that conical intersections play a key mechanistic role in chemical reaction dynamics. This volume provides an up-to-date overview of the multi-faceted research on the role of conical intersections in photochemistry and photobiology, including basic theoretical concepts, novel computational strategies as well as innovative experiments. The contents and discussions will be of value to advanced students and researchers in photochemistry, molecular spectroscopy and related areas.

Author: Klaus D. Sattler
Publisher: CRC Press
ISBN: 1420075551
Category : Science
Languages : en
Pages : 912

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The field of nanoscience was pioneered in the 1980s with the groundbreaking research on clusters, which later led to the discovery of fullerenes. Handbook of Nanophysics: Clusters and Fullerenes focuses on the fundamental physics of these nanoscale materials and structures. Each peer-reviewed chapter contains a broad-based introduction and enhances