| 1. |
- Borne, Kurtis D., et al.
(författare)
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Ultrafast electronic relaxation pathways of the molecular photoswitch quadricyclane
- 2024
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Ingår i: NATURE CHEMISTRY. - 1755-4330 .- 1755-4349. ; 16, s. 499-505
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Tidskriftsartikel (refereegranskat)abstract
- The light-induced ultrafast switching between molecular isomers norbornadiene and quadricyclane can reversibly store and release a substantial amount of chemical energy. Prior work observed signatures of ultrafast molecular dynamics in both isomers upon ultraviolet excitation but could not follow the electronic relaxation all the way back to the ground state experimentally. Here we study the electronic relaxation of quadricyclane after exciting in the ultraviolet (201 nanometres) using time-resolved gas-phase extreme ultraviolet photoelectron spectroscopy combined with non-adiabatic molecular dynamics simulations. We identify two competing pathways by which electronically excited quadricyclane molecules relax to the electronic ground state. The fast pathway (<100 femtoseconds) is distinguished by effective coupling to valence electronic states, while the slow pathway involves initial motions across Rydberg states and takes several hundred femtoseconds. Both pathways facilitate interconversion between the two isomers, albeit on different timescales, and we predict that the branching ratio of norbornadiene/quadricyclane products immediately after returning to the electronic ground state is approximately 3:2.
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| 2. |
- Hama, Tetsuya, et al.
(författare)
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A desorption mechanism of water following vacuum-ultraviolet irradiation on amorphous solid water at 90 K
- 2010
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Ingår i: JOURNAL OF CHEMICAL PHYSICS. - 0021-9606. ; 132:16
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Tidskriftsartikel (refereegranskat)abstract
- Following 157 nm photoexcitation of amorphous solid water and polycrystalline water ice, photodesorbed water molecules (H2O and D2O), in the ground vibrational state, have been observed using resonance-enhanced multiphoton ionization detection methods. Time-of-flight and rotationally resolved spectra of the photodesorbed water molecules were measured, and the kinetic and internal energy distributions were obtained. The measured energy distributions are in good accord with those predicted by classical molecular dynamics calculations for the kick-out mechanism of a water molecule from the ice surface by a hot hydrogen (deuterium) atom formed by photodissociation of a neighboring water molecule. Desorption of D2O following 193 nm photoirradiation of a D2O/H2S mixed ice was also investigated to provide further direct evidence for the operation of a kick-out mechanism.
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| 3. |
- Sage, Rebecca S., et al.
(författare)
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Quadrupole mass spectrometry and time-of-flight analysis of ions resulting from 532 nm pulsed laser ablation of Ni, Al, and ZnO targets
- 2008
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 103:9
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Tidskriftsartikel (refereegranskat)abstract
- This work describes the design and validation of an instrument to measure the kinetic energies of ions ejected by the pulsed laser ablation (PLA) of a solid target. Mass spectra show that the PLA of Ni, Al, and ZnO targets, in vacuum, using the second harmonic of a Nd:YAG laser (532 nm, pulse duration similar to 10 ns) generates abundant X(n+) ions (n <= 3 for Ni, <= 2 for Al, <= 3 and <= 2 for Zn and O respectively from ZnO). Ions are selected by their mass/charge (m/z) ratio prior to the determination of their times of flight. PLA of Ni has been studied in most detail. The mean velocities of ablated Ni(n+) ions are shown to follow the trend v(Ni(3+))>v(Ni(2+))>v(Ni(+)). Data from Ni(2+) and Ni(3+) are fitted to shifted Maxwellian functions and agree well with a model which assumes both thermal and Coulombic contributions to ion velocities. The dependence of ion velocities on laser pulse energy (and fluence) is investigated, and the high energy data are shown to be consistent with an effective accelerating voltage of similar to 90 V within the plume. The distribution of velocities associated with Ni(3+) indicates a population at cooler temperature than Ni(2+).
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| 4. |
- Andersson, Stefan, 1973, et al.
(författare)
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A theoretical and experimental study on translational and internal energies of H2O and OH from the 157 nm irradiation of amorphous solid water at 90 K.
- 2011
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Ingår i: Physical chemistry chemical physics : PCCP. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 13:35, s. 15810-20
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Tidskriftsartikel (refereegranskat)abstract
- The photodesorption of H(2)O in its vibrational ground state, and of OH radicals in their ground and first excited vibrational states, following 157 nm photoexcitation of amorphous solid water has been studied using molecular dynamics simulations and detected experimentally by resonance-enhanced multiphoton ionization techniques. There is good agreement between the simulated and measured energy distributions. In addition, signals of H(+) and OH(+) were detected in the experiments. These are inferred to originate from vibrationally excited H(2)O molecules that are ejected from the surface by two distinct mechanisms: a direct desorption mechanism and desorption induced by secondary recombination of photoproducts at the ice surface. This is the first reported experimental evidence of photodesorption of vibrationally excited H(2)O molecules from water ice.
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| 5. |
- Pilkington, Georgia A., et al.
(författare)
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Amontonian frictional behaviour of nanostructured surfaces
- 2011
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 13, s. 9318-9326
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Tidskriftsartikel (refereegranskat)abstract
- With nanotextured surfaces and interfaces increasingly being encountered in technological and biomedical applications, there is a need for a better understanding of frictional properties involving such surfaces. Here we report friction measurements of several nanostructured surfaces using an Atomic Force Microscope (AFM). These nanostructured surfaces provide well defined model systems on which we have tested the applicability of Amontons' laws of friction. Our results show that Amontonian behaviour is observed with each of the surfaces studied. However, no correlation has been found between measured friction and various surface roughness parameters such as average surface roughness (Ra) and root mean squared (rms) roughness. Instead, we propose that the friction coefficient may be decomposed into two contributions, i.e., μ = μ0 + μg, with the intrinsic friction coefficient μ0 accounting for the chemical nature of the surfaces and the geometric friction coefficient μg for the presence of nanotextures. We have found a possible correlation between μg and the average local slope of the surface nanotextures.
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| 6. |
- Quignon, Benoit, et al.
(författare)
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Sustained Frictional Instabilities on Nanodomed Surfaces : Stick Slip Amplitude Coefficient
- 2013
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 7:12, s. 10850-10862
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the frictional properties of nanostructured surfaces is important because of their increasing application in modem minlaturized devices. In this work, lateral force microscopy was used to study the frictional properties between AFM nanotip and surfaces bearing well-defined ranging from tens to hundreds of nanometers. Our results show that the average lateral force varied linearly with applied load, as described by Amontons' first law of friction, although no direct correlation between the sample topographic properties and their measured friction coeffidents was identified. Furthermore, all the nanodomed textures exhibited pronounced osdllations in the shear traces, similar to the dassic stick slip behavior, under all the shear velocities and load regimes studied. That is, the nanotextured topography led to sustained frictional instabilities, effectively with no contact frictional sliding. The amplitude of the stick slip oscillations, ab was found to correlate with the topographic properties of the surfaces and scale linearly with the applied load. In line with the friction coefficient, we define the slope of this linear plot as the stick slip amplitude coeffident (SSAC). We suggest that such stick slip behaviors are characteristics of surfaces with nanotextures and that such local frictional instabilities have important implications to surface damage and wear. We thus propose that the shear characteristics of the nanodomed surfaces cannot be fully described by the framework of Amontons' laws of friction and that additional parameters (e.g., a, and SSAQ are required, when their friction, lubrication, and wear properties are important considerations in related nanodevices.
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| 7. |
- Wu, Guorong, et al.
(författare)
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Excited state non-adiabatic dynamics of N-methylpyrrole : A time-resolved photoelectron spectroscopy and quantum dynamics study
- 2016
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 144:1
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of N-methylpyrrole following excitation at wavelengths in the range 241.5-217.0 nm were studied using a combination of time-resolved photoelectron spectroscopy (TRPES), ab initio quantum dynamics calculations using the multi-layer multi-configurational time-dependent Hartree method, as well as high-level photoionization cross section calculations. Excitation at 241.5 and 236.2 nm results in population of the A(2)(pi sigma*) state, in agreement with previous studies. Excitation at 217.0 nm prepares the previously neglected B-1(pi 3p(y)) Rydberg state, followed by prompt internal conversion to the A(2)(pi sigma*) state. In contrast with the photoinduced dynamics of pyrrole, the lifetime of the wavepacket in the A(2)(pi sigma*) state was found to vary with excitation wavelength, decreasing by one order of magnitude upon tuning from 241.5 nm to 236.2 nm and by more than three orders of magnitude when excited at 217.0 nm. The order of magnitude difference in lifetimes measured at the longer excitation wavelengths is attributed to vibrational excitation in the A(2)(pi sigma*) state, facilitating wavepacket motion around the potential barrier in the N-CH3 dissociation coordinate.
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| 8. |
- Wu, Guorong, et al.
(författare)
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Excited state non-adiabatic dynamics of pyrrole : A time-resolved photoelectron spectroscopy and quantum dynamics study
- 2015
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 142:7
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of pyrrole excited at wavelengths in the range 242-217 nm are studied using a combination of time-resolved photoelectron spectroscopy and wavepacket propagations performed using the multi-configurational time-dependent Hartree method. Excitation close to the origin of pyrrole's electronic spectrum, at 242 and 236 nm, is found to result in an ultrafast decay of the system from the ionization window on a single timescale of less than 20 fs. This behaviour is explained fully by assuming the system to be excited to the A(2)(pi sigma*) state, in accord with previous experimental and theoretical studies. Excitation at shorter wavelengths has previously been assumed to result predominantly in population of the bright A(1)(pi pi*) and B-2(pi pi*) states. We here present time-resolved photoelectron spectra at a pump wavelength of 217 nm alongside detailed quantum dynamics calculations that, together with a recent reinterpretation of pyrrole's electronic spectrum [S. P. Neville and G. A. Worth, J. Chem. Phys. 140, 034317 (2014)], suggest that population of the B-1(pi sigma*) state (hitherto assumed to be optically dark) may occur directly when pyrrole is excited at energies in the near UV part of its electronic spectrum. The B-1(pi sigma*) state is found to decay on a timescale of less than 20 fs by both N-H dissociation and internal conversion to the A(2)(pi sigma*) state.
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