290 research outputs found
Femtosecond laser driven molecular dynamics on surfaces: Photodesorption of molecular oxygen from Ag(110)
We simulate the femtosecond laser induced desorption dynamics of a diatomic
molecule from a metal surface by including the effect of the electron and
phonon excitations created by the laser pulse. Following previous models, the
laser induced surface excitation is treated through the two temperature model,
while the multidimensional dynamics of the molecule is described by a classical
Langevin equation, in which the friction and random forces account for the
action of the heated electrons. In this work, we propose the additional use of
the generalized Langevin oscillator model to also include the effect of the
energy exchange between the molecule and the heated surface lattice in the
desorption dynamics. The model is applied to study the laser induced desorption
of O from the Ag(110) surface, making use of a six-dimensional potential
energy surface calculated within density functional theory. Our results reveal
the importance of the phonon mediated process and show that, depending on the
value of the electronic density in the surroundings of the molecule adsorption
site, its inclusion can significantly enhance or reduce the desorption
probabilities.Comment: 11 pages, 8 figure
Quantum Dynamical Aspects of Rotationally and Vibrationally Mediated Photochemistry in Matrices and at Surfaces: HCl/DCl in Ar, and NH3/ND3 at Cu(111)
In this paper, we investigate two extensions of the concept of vibrationally mediated chemistry, from small molecules in the gas phase to small molecules in matrices and at surfaces. Exemplarily, we consider the systems HCl/DCl in Ar, and NH3/ND3 at Cu (111). The transition from isolated systems to the condensed phase calls for new quantum dynamical techniques, and it allows to predict new phenomena. For the case of matrix isolation, we propagate three-dimensional wavepackets representing photodissociated H- or D-atoms which penetrate from the initial cage into the lattice provided by the matrix. The cage exit probabilities are found to depend not only on the initial vibrational, but also on the rotational states, due to the environmental (Oh) symmetry provided by the (fcc) lattice of the matrix. As a consequence, we suggest the extension from vibrationally to rotationally, or rovibrationally mediated chemistry for matrix isolated molecules. For the case of molecules at surfaces, we adopt Gadzuk's jumping wavepacket plus incoherent averaging scheme, applied to an extended two-dimensional Antoniewicz-type model for the surface-molecule bond plus the vibrational coordinate which lends itself to preferential vibrational excitation, (here the umbrella mode of ammonia). The desorption depends selectively on the initial vibrational state. As a consequence, we suggest the extension of traditional desorption induced by electronic transitions DIET to a vibrationally mediated IR+UV DIET scheme which may be used e.g. for enrichment of specific isotopomers at surfaces
Butterfly hysteresis loop at non-zero bias field in antiferromagnetic molecular rings: cooling by adiabatic magnetization
At low temperatures, the magnetization of the molecular ferric wheel NaFe
exhibits a step at a critical field due to a field-induced
level-crossing. By means of high-field torque magnetometry we observed a
hysteretic behavior at the level-crossing with a characteristic butterfly shape
which is analyzed in terms of a dissipative two-level model. Several unusual
features were found. The non-zero bias field of the level-crossing suggests the
possibility of cooling by adiabatic magnetization.Comment: 4 pages, 5 figures, REVTEX4, to appear in PR
Ferromagnetic coupling and magnetic anisotropy in molecular Ni(II) squares
We investigated the magnetic properties of two isostructural Ni(II) metal
complexes [Ni4Lb8] and [Ni4Lc8]. In each molecule the four Ni(II) centers form
almost perfect regular squares. Magnetic coupling and anisotropy of single
crystals were examined by magnetization measurements and in particular by
high-field torque magnetometry at low temperatures. The data were analyzed in
terms of an effective spin Hamiltonian appropriate for Ni(II) centers. For both
compounds, we found a weak intramolecular ferromagnetic coupling of the four
Ni(II) spins and sizable single-ion anisotropies of the easy-axis type. The
coupling strengths are roughly identical for both compounds, whereas the
zero-field-splitting parameters are significantly different. Possible reasons
for this observation are discussed.Comment: 7 pages, 7 figure
Isotope and Quantum Effects in Vibrational State Distributions of Photodesorbed Ammonia
A marked quantum effect has been observed in the vibrational state distribution of photodesorbed ammonia. Namely, for quantum numbers larger than zero, symmetric and antisymmetric levels in the ν2 mode of the desorbed ammonia molecule are unequally populated. A strong propensity for symmetric levels is observed for NH3, whereas the reverse is found for ND3. Model calculations reproduce this effect. Moreover, it is found that the actual ratios probe the binding energy in the energetically less favorable inverted geometry with the H atoms pointing towards the surface
Shaping the Laser Control Landscape of a Hydrogen Transfer Reaction by Vibrational Strong Coupling. A Direct Optimal Control Approach
Controlling molecular reactivity by shaped laser pulses is a long-standing
goal in chemistry. Here we suggest a direct optimal control approach which
combines external pulse optimization with other control parameters arising in
the upcoming field of vibro-polaritonic chemistry, for enhanced controllability
The direct optimal control approach is characterized by a simultaneous
simulation and optimization paradigm, meaning that the equations of motion are
discretized and converted into a set of holonomic constraints for a nonlinear
optimization problem given by the control functional. Compared with indirect
optimal control this procedure offers great flexibility such as final time or
Hamiltonian parameter optimization. Simultaneous direct optimal control
(SimDOC) theory will be applied to a model system describing H-atom transfer in
a lossy Fabry-P\'erot cavity under vibrational strong coupling conditions.
Specifically, optimization of the cavity coupling strength and thus of the
control landscape will be demonstrated
Structure and Reactivity of α-Al<sub>2</sub>O<sub>3</sub>(0001) Surfaces: How Do Al-I and Gibbsite-like Terminations Interconvert?
The α-Al2O3(0001) surface has been extensively studied because of its significance in both fundamental research and application. Prior work suggests that in ultra-high-vacuum (UHV), in the absence of water, the so-called Al–I termination is thermodynamically favored, while in ambient, in contact with liquid water, a Gibbsite-like layer is created. While the view of the α-Al2O3(0001)/H2O(l) interface appears relatively clear in theory, experimental characterization of this system has resulted in estimates of surface acidity, i.e., isoelectric points, that differ by 4 pH units and surface structure that in some reports has non-hydrogen-bonded surface aluminol (Al–OH) groups and in others does not. In this study, we employed vibrational sum frequency spectroscopy (VSFS) and density functional theory (DFT) simulation to study the surface phonon modes of the differently terminated α-Al2O3(0001) surfaces in both UHV and ambient. We find that, on either water dosing of the Al–I in UHV or heat-induced dehydroxylation of the Gibbsite-like in ambient, the surfaces do not interconvert. This observation offers a new explanation for disagreements in prior work on the α-Al2O3(0001)/liquid water interface─different preparation methods may create surfaces that do not interconvert─and shows that the surface phonon spectral response offers a novel probe of interfacial hydrogen bonding structure
Condensed-phase isomerization through tunnelling gateways
Quantum mechanical tunnelling describes transmission of matter waves through a barrier with height larger than the energy of the wave. Tunnelling becomes important when the de Broglie wavelength of the particle exceeds the barrier thickness; because wavelength increases with decreasing mass, lighter particles tunnel more efficiently than heavier ones. However, there exist examples in condensed-phase chemistry where increasing mass leads to increased tunnelling rates. In contrast to the textbook approach, which considers transitions between continuum states, condensed-phase reactions involve transitions between bound states of reactants and products. Here this conceptual distinction is highlighted by experimental measurements of isotopologue-specific tunnelling rates for CO rotational isomerization at an NaCl surface, showing nonmonotonic mass dependence. A quantum rate theory of isomerization is developed wherein transitions between sub-barrier reactant and product states occur through interaction with the environment. Tunnelling is fastest for specific pairs of states (gateways), the quantum mechanical details of which lead to enhanced cross-barrier coupling; the energies of these gateways arise nonsystematically, giving an erratic mass dependence. Gateways also accelerate ground-state isomerization, acting as leaky holes through the reaction barrier. This simple model provides a way to account for tunnelling in condensed-phase chemistry, and indicates that heavy-atom tunnelling may be more important than typically assumed
Quantum size effects in Pb islands on Cu(111): Electronic-structure calculations
The appearance of "magic" heights of Pb islands grown on Cu(111) is studied
by self-consistent electronic structure calculations. The Cu(111) substrate is
modeled with a one-dimensional pseudopotential reproducing the essential
features, i.e. the band gap and the work function, of the Cu band structure in
the [111] direction. Pb islands are presented as stabilized jellium overlayers.
The experimental eigenenergies of the quantum well states confined in the Pb
overlayer are well reproduced. The total energy oscillates as a continuous
function of the overlayer thickness reflecting the electronic shell structure.
The energies for completed Pb monolayers show a modulated oscillatory pattern
reminiscent of the super-shell structure of clusters and nanowires. The energy
minima correlate remarkably well with the measured most probable heights of Pb
islands. The proper modeling of the substrate is crucial to set the
quantitative agreement.Comment: 4 pages, 4 figures. Submitte
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