Time Resolved Photoelectron Imaging of Electronic Relaxation Dynamics in Anionic Clusters PDF Download
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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: Graham Bailey Griffin
Publisher:
ISBN:
Category :
Languages : en
Pages : 164
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Book Description
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: Aster Ellen Kammrath
Publisher:
ISBN:
Category :
Languages : en
Pages : 320
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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: Alison Virginia Davis
Publisher:
ISBN:
Category :
Languages : en
Pages : 362
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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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Book Description
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:
Publisher:
ISBN:
Category : Dissertations, Academic
Languages : en
Pages : 850
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Author: Oliver Kühn
Publisher: Springer Science & Business Media
ISBN: 3540680373
Category : Science
Languages : en
Pages : 855
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This book summarizes several years of research carried out by a collaboration of many groups on ultrafast photochemical reactions. It emphasizes the analysis and characterization of the nuclear dynamics within molecular systems in various environments induced by optical excitations and the study of the resulting molecular dynamics by further interaction with an optical field.
Author: Cheuk-yiu Ng
Publisher: World Scientific
ISBN: 9814494739
Category : Science
Languages : en
Pages : 684
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Book Description
Owing to the advances of vacuum ultraviolet and ultrafast lasers and third generation synchrotron sources, the research on photoionization, photoelectrons, and photodetachment has gained much vitality in recent years. These new light sources, together with ingenious experimental techniques, such as the coincidence imaging, molecular beam, pulsed field ionization photoelectron, mass-analyzed threshold ion, and pulsed field ion pair schemes, have allowed spectroscopic, dynamic, and energetic studies of gaseous species to a new level of detail and accuracy. Profitable applications of these methods to liquids are emerging.This invaluable two-volume review consists of twenty-two chapters, focusing on recent developments in photoionization and photodetachment studies of atoms; molecules, transient species, clusters, and liquids.
