1,720,979 research outputs found
Crystal structure of Au-pseudocarbyne(C6)
Full text linkCarbyne-related materials permit exploring the potentially extraordinary properties of this long-sought but still elusive carbon allotrope. However, accurate understanding of these materials is challenging. Here we report the crystal structure of a Au-pseudocarbyne, a representative of a possible new family of materials consisting of sp-hybridized carbon chains and stabilizing metal atoms. Au-pseudocarbyne(C6), the representative pseudocarbyne containing six-membered carbon chains, has space group P6/mmm191 and unit-cell parameters a = b = 0.60 nm, c = 0.896 nm, α = β = 90°, γ = 120°. Its long-range structure can be understood as intimately intergrown bundles, each consisting of six parallel, infinite carbon chains surrounding a column of gold atoms. This compound, together with its eight-membered counterpart Au-pseudocarbyne(C8), shows that interesting new materials resembling the carbyne structure and sharing some of its properties can be designed and developed. The current work raises serious questions regarding recent reports of carbyne synthesis
Relaxation dynamics of neutral iron oxide clusters using femtosecond pump-probe spectroscopy
Full text linkMade available in DSpace on 2019-07-15T22:16:51Z (GMT). No. of bitstreams: 2
3969.pdf: 18169 bytes, checksum: efd2d9cbb100481734782a6f9ce358c1 (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
Previous issue date: 2019-06-18Made available in DSpace on 2020-01-25T19:31:10Z (GMT). No. of bitstreams: 4
3969.pdf.txt: 1608 bytes, checksum: 4a09a793a93b9bec981e440ba4911596 (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
3969.pdf: 18169 bytes, checksum: efd2d9cbb100481734782a6f9ce358c1 (MD5)
1456623.pptx: 3916189 bytes, checksum: a93a6ea5a8a7ed6eb9b1050e282c17da (MD5)
Previous issue date: 2019-06-18Iron oxides have been used in a number of societally important catalytic processes; however, the molecular-level details behind their reaction mechanisms have been a challenge to observe. Molecular clusters have gained experimental attention due to their ability to model bulk materials, ease of production, and direct application for gaining atomic level insights. Using laser ablation combined with femtosecond pump-probe spectroscopy, the gas-phase dissociation and excitation-relaxation dynamics of neutral iron clusters in the presence of oxygen are observed. Dissociation/fragmentation time of neutral iron oxide clusters from (FeO) (n = 1-10) decreases with increased molecular size from 300 fs to 150 fs. Clusters deviating from the 1:1 stoichiometry are generally seen to increase in relaxation time, owing to an increased stability induced by a caging effect from atomic oxygen. A notable example of dissociation stability is observed with the addition of O atoms from Fe FeO which decreases in relaxation time from 650 fs to 250 fs. The molecular-level insights from these cluster studies provides a more comprehensive understanding for the design of future catalysts, leading to iron oxide materials with increased reactivity and decreased impact
Generation and study of few-femtosecond vibrational wave-packets via strong-field ionization
Full text linkMade available in DSpace on 2019-07-15T22:16:54Z (GMT). No. of bitstreams: 2
4001.pdf: 14386 bytes, checksum: 7e2e4155d3c0ee409310d48458e6b617 (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
Previous issue date: 2019-06-19Made available in DSpace on 2020-01-25T19:31:41Z (GMT). No. of bitstreams: 6
4001.pdf.txt: 1549 bytes, checksum: 2eeae957f49a229e2da61802ceb0f7b5 (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
4001.pdf: 14386 bytes, checksum: 7e2e4155d3c0ee409310d48458e6b617 (MD5)
1468367.pdf: 1086066 bytes, checksum: 6ca66b4a1546eee2007f2b8a0767c834 (MD5)
Heald_ISMS_perm.pdf: 1086066 bytes, checksum: 6ca66b4a1546eee2007f2b8a0767c834 (MD5)
1468367.pptx: 8175853 bytes, checksum: 34ee25d014b4cd1079074bc01007a516 (MD5)
Previous issue date: 2019-06-19Strong-field ionization (SFI) using ultrashort, high-intensity laser pulses presents a unique opportunity for studying dynamics of molecular systems given its capability to generate coherent vibrational and electronic motion. Furthermore, the very interaction that drives the tunneling ionization is at the root of high harmonic generation of extreme ultraviolet light (XUV). Our homebuilt XUV spectrometer is uniquely suited to utilize SFI to study coherent, vibrational wave-packets of small molecules in the gas phase given its ability to observe transitions from localized core shells to delocalized valence shells to glean information about their larger molecular systems (e.g. oxidation state, spin state, magnetic quantum number, and local bonding environment). Its temporal resolution provides the capability to observe electronic motion and vibrational coherences that occur on the few-femtosecond time period. This presentation will focus on the techniques for generating few-cycle XUV pulses and our preliminary computational and experimental results on the dynamics and mechanisms behind the SFI of simple gas phase systems of fundamental importance for understanding light-matter interactions
Femtosecond Pump-probe Spectroscopy Of Neutral Ni And Cr Oxide Clusters
Full text linkThe ultrafast electron dynamics of transition metal oxide clusters provides information about the stability and reactivity of their bulk material counterparts. Nickel and chromium oxides are widely used catalytic materials which may be improved with the detailed atomic precision of their stable conformations and low energy electron properties. In our experiments, neutral NiO and CrO clusters are produced with small amounts of oxygen and their mass spectra show a primary stable stoichiometry of m = n - 1 for Ni clusters and m = 2n - 2 for Cr clusters. I will present our ultrafast pump-probe spectroscopy measurements on the direct ionization, dissociation, formation, and excitation-relaxation properties for these metal oxide cluster systems extending up to n = 9. Nickel oxides show an increase in the lifetime with the decrease in oxygen content, owing to the change in geometry and increase in relaxation pathways. In contrast, chromium oxides generally show a decreased lifetime with less oxygen atoms from the stoichiometric (CrO) cluster. In both cluster systems, long-lived lifetimes (over 2 ps) are observed in the suboxide clusters. Overall, it is shown that the ultrafast dynamics observed for these clusters depend strongly on their metal composition, size, and oxidation state.Made available in DSpace on 2021-09-24T21:08:54Z (GMT). No. of bitstreams: 2
5069.pdf: 15941 bytes, checksum: ed9edf307025fbdb64a771e4b61b000d (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
Previous issue date: 2021-06-24Made available in DSpace on 2022-01-21T16:10:45Z (GMT). No. of bitstreams: 4
5069.pdf.txt: 1631 bytes, checksum: 10ba7f9a80d7c7ee4a5eca1171b05d36 (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
5069.pdf: 15941 bytes, checksum: ed9edf307025fbdb64a771e4b61b000d (MD5)
RJ03_5069.pdf: 394851 bytes, checksum: fe368cc92896092dfe059a8e659bb3dd (MD5)
Previous issue date: 2021-06-2
Femtosecond pump probe spectroscopy of neutral ti, fe, and ni oxide clusters
Full text linkMade available in DSpace on 2020-06-26T03:04:37Z (GMT). No. of bitstreams: 2
4631.pdf: 14789 bytes, checksum: 40058a28da75f615840045075ccda328 (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
Previous issue date: 24The dissociative electron dynamics of transition metal oxide clusters provide information about the stability and reactivity of their bulk material counterparts. Ti, Fe, and Ni oxides are widely used catalytic materials which may be improved with the detailed atomic precision of their stable conformations and low energy electron properties. In our experiments, neutral TiO, FeO, and NiO oxide clusters are produced with small amounts of oxygen and their mass spectra show a primary stable stoichiometry of m = n for Fe clusters, m = n - 1 for Ni clusters, and m = 2n - 1 for Ti clusters. I will present our ultrafast pump-probe spectroscopy measurements on the direct ionization, dissociation, formation, and excitation-relaxation properties for these metal cluster systems extending up to n = 10. The femtosecond dynamics observed for these clusters depend strongly on their metal composition, size, and oxidation state. Understanding the energy and time-resolved fragmentation patterns of large molecules may prove to produce more targeted catalysts of increased reactivity, leading to decreased cost and waste
Spectroscopic Characterization Of Pseudocarbynes: Interaction Between -c ≡ C- Chains And Metal Clusters
Full text linkPseudocarbynes are a new class of molecules/materials that we define as finite sp-hybridized carbon chains stabilized through non-covalent interactions with metal clusters. These exciting and innovative materials are expected to approach the unprecedented strength, elastic modulus, and stiffness of carbyne, which has defied isolation and characterization for over a century. We optimized a procedure to synthesize pseudocarbynes via the finely controlled laser ablation of a liquid/metal interface, where clusters and one-dimensional carbon chains self-assemble from colloidal solutions into new mesomaterials that bridge the molecular and materials realms. I will present our spectroscopic results demonstrating the non-covalent interaction between --CC-- chains and small metal clusters as characterized by strong signatures in UV-vis, Raman, and infrared spectroscopies.Made available in DSpace on 2021-09-24T21:09:32Z (GMT). No. of bitstreams: 2
5544.pdf: 13806 bytes, checksum: 14e1d3dec17784c982ed125cbb50c39d (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
Previous issue date: 2021-06-24Made available in DSpace on 2022-01-21T16:09:05Z (GMT). No. of bitstreams: 4
5544.pdf.txt: 1086 bytes, checksum: 047bfdf199fd28cd1d1fec4782d5dfa4 (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
5544.pdf: 13806 bytes, checksum: 14e1d3dec17784c982ed125cbb50c39d (MD5)
RD05_5544.pdf: 545706 bytes, checksum: 9865cf0942fe8b590e93cb2d849e1786 (MD5)
Previous issue date: 2021-06-2
Ultrafast Reaction Dynamics Of Neutral Aluminum Oxide Clusters Using Two Color Femtosecond Spectroscopy
Full text linkUltrafast relaxation dynamics of neutral aluminum oxide clusters (AlO, n,m 20) in the gas phase were studied by combining two-color femtosecond spectroscopy and time-of-flight mass spectrometry. The clusters are pumped using the second harmonic of a Ti:Sapphire femtosecond (fs) laser system (400 nm = 3.1eV) and subsequently ionized through strong field ionization of the the fundamental beam (800 nm = 1.55 eV) with a time resolution of 35 fs. The excited states explored here are high binding energy excited states, which are mainly of oxygen 2p character into the Al sp-type orbitals. Changes in the ultrafast dynamics are observed to be strongly influenced by the stoichiometry and size of these clusters. Neutral clusters are ideal mimics of the true active sites of condensed phase and surface chemistry. We have developed an approach that involves the calculation of several topological descriptors to interpret the influence of the structural differences between clusters on the excited state lifetimes. Time-dependent density functional theory (TDDFT) calculations reveal the energies and structures of the excited states at both ground and excited state structure. These studies on molecular scale clusters provide an ideal arena for probing chemical activities and mechanisms in an unperturbed environment that can assist in the analysis of condensed phase catalysis systems. Using the atomic precision of clusters, we identify preferred bonding arrangements, electronic structure, and oxidation states that lead to increased excited state lifetimes and therefore enhance photochemical transformation.Made available in DSpace on 2021-09-24T21:09:48Z (GMT). No. of bitstreams: 2
5649.pdf: 17890 bytes, checksum: 92d8583e368b64fb7a1a38b68e32968e (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
Previous issue date: 2021-06-24Made available in DSpace on 2022-01-21T16:09:12Z (GMT). No. of bitstreams: 4
5649.pdf.txt: 1949 bytes, checksum: b512d27157854db22e963b9e28074ff3 (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
5649.pdf: 17890 bytes, checksum: 92d8583e368b64fb7a1a38b68e32968e (MD5)
RJ06_5649.pdf: 950819 bytes, checksum: 6703f242df7b1367c0ae9ab0e5a4bf19 (MD5)
Previous issue date: 2021-06-2
Ultrafast coulomb explosion and proton transfer dynamics of formic acid clusters
Full text linkMade available in DSpace on 2020-06-26T03:04:42Z (GMT). No. of bitstreams: 2
4674.pdf: 17704 bytes, checksum: a2a0a45d595d0ea45ac0a72d74a76d83 (MD5)
license.txt: 4802 bytes, checksum: 58353f9dd6876860dd5221f3d7872a95 (MD5)
Previous issue date: 26With increasing concerns over the concentration of carbon dioxide in the atmosphere, new materials are being explored to capture CO for use as a feedstock. The suspected reaction sites of these materials deviates from the bulk phase, such as the quantum confinement of water droplets that happens within pores. The formic acid clusters’, (HCOOH)(HO), minimum energy structures show an evolving cage structure with each additional molecule. The n,m = (5,0) cluster exhibits a much greater stability due to its rigid cage-like structure. This cage structure then encapsulates a water molecule to make an even more stable n,m = (5,1) cluster. The interaction of formic acid clusters with 200 fs linearly polarized laser pulses of 800 and 400 nm with intensities up to 1x10 W/cm was studied using time-of-flight mass spectrometry, verifying this trend in stability. An enhanced ionization is observed in clusters, leading to the production of triply charged carbon, oxygen, and CO ions that are not observed when only the formic acid molecule is present.
Measurements of the kinetic energy release resulting from the Coulomb explosion of clusters are in excellent agreement with our simulations performed over the clusters observed in the mass spectra and suggest that almost no movement occurs during the ionization mechanism. Finally, ultrafast pump-probe spectroscopy was used to investigate how proton transfer dynamics and excited state lifetimes are influenced by the self-solvation of formic acid. These results highlight the role of microsolvation on the excited state dynamics of simple carboxyl groups, specifically formic acid, in producing or capturing carbon dioxide and will help to direct the design of the next generation of carbon capture materials
Photofragmentation Pathways Of N-butyl Bromide
Full text linkDissociation of organic halides has been use for studying ultrafast processes over the last three decades given their relative simplicity and the significance in atmospheric chemistry. Specifically, photofragmentation of alkyl bromides with UV light has attracted substantial attention because of the ozone depletion potential of Br atoms. This presentation summarizes our recent results on the ultrafast photodissociation mechanisms of n-butyl bromide resolved using femtosecond time-resolved mass spectrometry. Multiple dissociative pathways occur upon photo excitation of n-butyl bromide include C-Br scission, C-C dissociation, and hydrogen elimination leading to unsaturated carbon bonds. The dissociative A state is accessed via two UV photon adsorption of two UV pump photons. This state undergoes direct dissociation of the C-Br bond within ~160 fs. Three photon excitation reaches the n-5p Rydberg state, where several competing fragmentation pathways are monitored. The fastest relaxations occur in states which are highly excited and have C-H dissociation leading to double and triple C-C bond formation with lifetimes of ~500 fs. Dissociation on the ion-pair state occurs within 10 ps to produce the butyl radical. Additionally, elimination of HBr from the parent molecule occurs within 4 ps. The depopulation of the 5p Rydberg state through internal conversion activates vibrations along the carbon backbone and produces an the intermediate (bromopropyl radical) within 600 fs. The bromopropyl radical undergoes a concerted ring-closure and Br elimination into highly stable cyclopropane within 7.5 ps. The reaction pathways and potential energy curves were identified with the aid of density functional theory calculations. These results elucidate the elementary steps and mechanism which are fundamental in atmospheric chemistry and provide insight into how electronic photoexcitation is dissipated into the vibrational motions of the carbon backbone of simple hydrocarbons
Stability Of Neutral Manganese Oxide Clusters
Full text linkManganese oxides are among the most widely explored transition metal oxides for diverse biomedical applications and are also employed in a wide number of industrial processes. Its wide range of oxidation states provide manganese with extreme flexibility in electron occupancy that has also attracted increasing attention for use in photocatalytic processes. Neutral clusters are excellent mimics of the active sites of bulk materials, and can be employed to understand the local geometric and electronic structure properties, and oxidation states that provide the best charge carrier lifetimes and by extension optimal photochemical efficiency. Here, I will present our ongoing work on the ultrafast relaxation dynamics of neutral manganese oxide clusters, which are prepared through the laser ablation of a pure metal rod with a 532 nm Nd:YAG laser. A synchronized pulse of He seeded with 5\% oxygen enables cluster formation through supersonic expansion. The neutral clusters are then studied through by combining two-color femtosecond spectroscopy with time-of-flight mass spectrometry. Our cluster distribution shows that manganese has a large range of oxidation states. The clusters are excited by the second harmonic of a Ti:Sapphire femtosecond (fs) laser system (400 nm = 3.1eV) and subsequently ionized through strong field ionization with the fundamental laser beam (800 nm = 1.55 eV). The subtle changes in the ultrafast dynamics upon the addition/subtraction of each atom are being evaluated to provide new understanding to the flow of energy through a material
- …
