Time-resolved Photoelectron Spectroscopy and Imaging Studies of Anion Dynamics PDF Download
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Author: Alison Virginia Davis
Publisher:
ISBN:
Category :
Languages : en
Pages : 362
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Author: Alison Virginia Davis
Publisher:
ISBN:
Category :
Languages : en
Pages : 362
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Author: Gareth Michael Roberts
Publisher:
ISBN:
Category : Anions
Languages : en
Pages :
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We present a detailed account of the development, construction, and commissioning of a new experiment for studying the spectroscopy and ultrafast dynamics of molecular anions in the gas phase. The new instrument incorporates: an electrospray ionisation source, which is capable of generating a vast class of molecular anions; a Wiley-McLaren time-of-flight mass spectrometer; and a compact photoelectron imaging arrangement for anions, which negates the use of pulsed high voltages. We use this instrument in conjunction with a femtosecond laser system to perform the first ultrafast time-resolved photoelectron imaging experiments on molecular anions generated through electrospray ionisation. A method for reconstructing three dimensional charged particle distributions from their associated two dimensional projections on an imaging detector plane is described. This new method utilises: (1) onion-peeling in polar co-ordinates (POP) to perform the reconstruction; and (2) basis set concepts to significantly enhance the algorithms computational speed. We compare this new POP algorithm with other reconstruction algorithms, which shows that the method is as good as the benchmark pBASEX method in terms of accuracy. Importantly, we show that it is also computationally fast, allowing images to be reconstructed as they are acquired in a typical imaging experiment. Original work is presented which investigates the spectroscopy and ultrafast excited dynamics of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion. The photoelectron spectrum of TCNQ? is measured at 3.1 eV, which is used to gain insight into the electronic structure and geometries of both the anion and neutral states. Time-resolved photoelectron imaging experiments explore the relaxation dynamics of its first excited 1 2B3u state, which we show undergoes internal conversion back to the 2B2g ground state on a timescale of 650 fs. Results also provide evidence of a wave packet motion on the excited state, which exhibits a characteristic frequency of 30 cm?1. Finally, we describe, for the first time, a formulism which allows ultrafast relaxation timescales to be extracted from the photoelectron angular distributions of isoenergetic photoelectron features. As an example, we use the time-resolved photoelectron angular distributions of a nearly isoenergetic feature in the photoelectron images of TCNQ?. From this model we extract a relaxation time for the 1 2B3u state, which quantitatively agrees with those extracted from fits to the features in the photoelectron spectra derived from the images.
Author: Daniel Alfred Horke
Publisher:
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Category :
Languages : en
Pages :
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Several recent results of a time-resolved photoelectron imaging experiment are presented. Following a broad introduction into the area of femtochemistry and time-resolved photoelectron spectroscopy, a detailed description of the spec- trometer is given. This utilises an electrospray ionisation source, coupled to an electrostatic ion trap. Ions are mass selected using time-of-flight methods and investigated using photoelectron imaging in a velocity-mapping geometry. Ultrafast dynamics are investigated by pump-probe spectroscopy with femtosec- ond laser pulses. Recent results are separated into three distinct projects: (i) The investigation of electron acceptor radical anions based on the quinone backbone. These commonly exhibit electron transfer rates exceeding those pre- dicted by Marcus theory by orders of magnitude. We show that an alternative pathway to electron transfer could involve the participation of electronic excited states, as these couple strongly to the anion ground state. Specifically, for p- Benzoquinone we show that electronic resonances located in the detachment continuum primarily undergo internal conversion via a number of conical inter- sections. (ii) Several polyanions have been investigated in the gas-phase. These systems exhibit unusual electronic properties, due to the presence of multiple excess charges, leading to the formation of a repulsive Coulomb barrier to photode- tachment. We investigate the effect of excess internal energy on this barrier and how it affects outgoing photoelectrons. We show that the trajectories of electrons are strongly influenced by this potential and demonstrate its use as a probe for large amplitude structural dynamics in polyanions. (iii) The isolated chromophore of the green fluorescent protein (GFP) has been studied, and the vertical and adiabatic detachment energies determined for the first time. Using time-resolved spectroscopy the excited state dynamics are in- vestigated. We show that the first singlet excited state of the anion primarily decays through internal conversion, explaining the absence of fluorescence in the gas-phase. Using high level quantum chemistry calculations we show the specific motion involved and hence confirm the function of the protein back- bone in GFP. This thesis is concluded with a few suggested experimental improvements and ideas for future studies of anions using the presented spectrometer.
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: Arthur Edward Bragg
Publisher:
ISBN:
Category :
Languages : en
Pages : 628
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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:
Publisher:
ISBN:
Category :
Languages : en
Pages : 513
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A new experimental technique for the time-resolved study of anion reactions is presented. Using femtosecond laser pulses, which provide extremely fast (~100 fs) time resolution, in conjunction with photoelectron spectroscopy, which reveals differences between anion and neutral potential energy surfaces, a complex anion reaction can be followed from its inception through the formation of asymptotic products. Experimental data can be modeled quantitatively using established theoretical approaches, allowing for the refinement of potential energy surfaces as well as dynamical models. After a brief overview, a detailed account of the construction of the experimental apparatus is presented. Documentation of the data acquisition program is contained in the Appendix. The first experimental demonstration of the technique is then presented for I2- photodissociation, modeled using a simulation program which is also detailed in the Appendix. The investigation of I2- photodissociation in several size-selected I2-(Ar)n (n = 6-20) and I2-(CO2)n (n = 4-16) clusters forms the heart of the dissertation. In a series of chapters, the numerous effects of solvation on this fundamental bond-breaking reaction are explored, the most notable of which is the recombination of I2- on the ground $\tilde{X}$(2[Sigma]u+) state in sufficiently large clusters. Recombination and trapping of I2- on the excited $\tilde{A}$(2[pi]3/2,g) state is also observed in both types of clusters. The studies have revealed electronic state transitions, the first step in recombination, on a ~500 fs to ~10 ps timescale. Accompanying the changes in electronic state is solvent reorganization, which occurs on a similar timescale. Over longer periods (~1 ps to>200 ps), energy is transferred from vibrationally excite d I2- to modes of the solvent, which in turn leads to solvent evaporation. These effects become more important as cluster size increases. In addition, differences in timescale and mechanism are observed between clusters of Ar, which binds to I- and I2- rather weakly, and CO2, whose large quadruple moment allows substantially stronger binding to these anions.
Author: Benjamin Jefferys Greenblatt
Publisher:
ISBN:
Category :
Languages : en
Pages : 554
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Author: I-Ren Lee
Publisher:
ISBN:
Category : Femtochemistry
Languages : en
Pages : 286
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This thesis presents the studies of ultrafast dynamics of negatively charged molecules and clusters in the gas phase using femtosecond photoelectron spectroscopy. The core motifs of two distinct complex systems - solvated electrons and protein chromophores - were studied in the gas phase. For the solvated electron systems - hydrated electrons and ammoniated electrons - were studied in finite-sized clusters in the gas phase. Interestingly, the results show a significant difference. In the hydrated electron, ground-state vibrational cooling is evident by the transient photoelectron spectra, while, in the ammoniated electron, a coherent motion with a 500-fs relaxation is observed. The difference is attributed to the cage rigidity, which results in different solvent motions for the electron's interaction with water (libration) or ammonia (phonon-like). The photocycle of the photoactive yellow protein (PYP) has been studied extensively, but the dynamics of the isolated chromophore responsible for the transduction of phototacticity is less known. The anionic chromophore model molecule was investigated in the gas phase using femtosecond photoelectron spectroscopy and the results indicate that the protein function is in directing efficient conversion to the cis-structure and in impeding radical formation within the protein. Finally, a classic system of conformational twisting, stilbene, was studied in its anionic radical state. Ultrafast conversions from both trans- and cis- isomers are accompanied with coherent oscillation, in contrast to observations in the solution phase, and this suggests that a major solvent retardation take place. Dynamic studies of the photochemistry of gas-phase anions are very scarce due to the experimental difficulties. However, our results successfully resolve the photophysics and photochemistry of the isolated species and, thereby, elucidate the effect of solution.
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.
