Quantum Mechanical Model Calculations Research Articles

Computer Simulation of the Nonlinear Optical Properties of Langmuir–Blodgett Films of a Squaraine Derivative

The linear and nonlinear optical properties (L&NLO) of a monolayer of (4-(N-methyl-N-(carboxypropyl)amino)-phenyl-4′-(N,N-dibutylamino)phenylsquaraine (SBA) were computed by a combination of molecular dynamics simulations, molecular quantum-mechanical, and discrete local-field model calculations to compute the macroscopic optical susceptibilities. Langmuir films on water and graphite were simulated and several structural properties investigated. In addition, SBA in water and dimethylsulfoxide (DMSO) solutions were simulated, and the aggregation behavior of SBA in these solvents was analyzed. The molecular (hyper)polarizabilities of SBA were calculated with density functional theory and validated with MP2 correlated ab initio calculations. An unusual large component perpendicular to the molecular dipole moment is obtained for the molecular first hyperpolarizability, which is a main contributor to the large in-plane anisotropy of the computed macroscopic second-harmonic generation signal of the Langmuir–Blo...

Ultrafast Photoinduced Electron Transfer at Electrodes: The General Case of a Heterogeneous Electron‐Transfer Reaction

The general case of a heterogeneous electron-transfer reaction is realized by ultrafast electron transfer from a light-absorbing molecule to a wide continuum of electronic acceptor states, realizing the so-called wide band limit. Experimental data obtained for perylene dye/TiO(2) systems confirm the predictions of fully quantum mechanical model calculations of the dynamics. The energy distribution of the injected electron shows an energy loss due to excitations of high-energy (quantum) vibrational modes in the ionized perylene moiety. The electron-transfer mechanism is non-adiabatic and the reaction is ultrafast, for example, with a time constant of 9 fs for the COOH anchor-bridge group. The underlying strong coupling of the electronic states to high-energy vibrational modes is a characteristic feature of sensitizer molecules.

Enhanced photoluminescence of colloidal nanocrystals embedded in epitaxially grown semiconductor microstructures

Colloidal nanocrystals (NCs) integrated in epitaxially grown semiconductors provide a flexible alternative to Stranski-Krastanow quantum dots for many different optoelectronic applications such as low-threshold lasers or single-photon emitters. We studied the optical properties of various CdSe core-only and core/shell NCs integrated in ZnSe epitaxial-chemical hybrid structures. Depending on the type and environment of the NCs, a blueshift of the photoluminescence energy and a strong reduction of the emission intensity are observed after overgrowth. We demonstrate that these effects are caused by a potential barrier at the interface between NCs and ZnSe. The application of an appropriate postgrowth laser annealing procedure removes this potential barrier and enhances charge transfer from the ZnSe into the NCs. Consequently, we observe a significant increase of the emission intensity and a redshift of the photoluminescence energy of the NCs after annealing. Three-dimensional quantum-mechanical model calculations of the band diagram of the NCs as synthesized in solution, after overgrowth with ZnSe and after laser annealing confirm the presence of this potential barrier and its reduction due to the annealing.

Channeling, superfocusing, and nuclear reactions

A highly collimated beam of protons (≈1 MeV) entering the channel of a monocrystal film forms at a certain depth an extremely sharp (<0.005 nm) and relatively long (some monolayers of the crystal) focusing area where the increase of the flux density can reach hundreds times. Impinging atoms in this focusing area can undergo nuclear or atomic reactions with proper foreign dopants which disappear if the crystal is tilted from this position by only 10−3 rad. This effect can be called channeling superfocusing, in contrast to the ordinary flux peaking where the increase of flux density reaches only few times. Results are predicted by the exactly solvable quantum mechanical model calculations and confirmed by channeling Monte Carlo simulations accounting for several properties of the real lattice. Unfortunately, the mosaic structure of the film and statistical spread of the film and statistical spread of the axes of channels prevent the observation. Special technologies and materials choice are needed to minimize this effect.

Why do Electromagnetic Pulses Enhance Bone Growth?

The excitation probability of substrate molecules involved in the production of growth factors influencing the division of chondrocytes in the growth layer of bone under the influence of pulsed electromagnetic fields is studied theoretically in a quantum mechanical model calculation. In this model matrix elements and anti-bonding energy levels are assumed known and the dynamics of the interaction with pulsed electromagnetic fields is derived. The derivation makes it clear that continuous pulsing or large driving currents can overwhelm local diffusive transport to the growth plane resulting in a loss of its enhancement properties. Optimal locations within a pair of Helmholtz coils for enhancement of bone growth are also investigated and found to be close to the coils. The work presented here is believed to be the first derivation in a model calculation of a physical basis for the effects of pulsed electromagnetic fields on bone growth and fusion.

Single attosecond pulse generation in the multicycle-driver regime by adding a weak second-harmonic field

We present a method of producing single attosecond pulses by high-harmonic generation with multicycle driver laser pulses. This can be achieved by tailoring the driving pulse so that attosecond pulses are produced only every full cycle of the oscillating laser field rather than every half-cycle. It is shown by classical and quantum-mechanical model calculations that even a minor addition (1%) of phase-locked second-harmonic light to the 800 nm fundamental driver pulse for high-harmonic generation leads to a major (15%) difference in the maximum kinetic energies of the recombining electrons in adjacent half-cycles.

One- and two-electron processes in collisions between hydrogen molecules and slow highly charged ions

A coincidence time-of-flight technique coupled with projectile charge state analysis was used to study electron capture in collisions between slow highly charged ions and hydrogen molecules. We found single electron capture with no target excitation to be the dominant process for both C6+ projectiles at a velocity of 0.8 atomic units and Ar11+ projectiles at v=0.63a.u. Double electron capture and transfer excitation, however, were found to be comparable and occur about 30% of the time relative to single capture. Most projectiles (96%) auto-ionize quickly following double capture into doubly excited states. The data are compared to classical and quantum mechanical model calculations.

A sub-atomic microscope, superfocusing in channeling and close encounter atomic and nuclear reactions

A highly collimated beam of protons (\(\approx\)1 MeV) entering the channel of a monocrystal film forms at a certain depth an extremely sharp (<0.005 nm) and relatively long (some monolayers of the crystal) focusing area where the increase of the flux density can reach thousand times. Impinging atoms in this focusing area can undergo nuclear or atomic reactions with proper foreign dopants which disappear if the crystal is tilted from this position by only 10-3 radians. This effect can be called channeling superfocusing, in contrast to the ordinary fluxpeaking where the increase of flux density reaches only few times. Results are predicted by quantum mechanical model calculations and confirmed by channeling Monte Carlo simulations accounting for several properties of the real lattice.

Population of excited erbium levels in Er3+:Na0.4Y0.6F2.2 (Er:NYF) laser crystals

We have studied theoretically and experimentally the population dynamics of six radiative erbium levels in concentration series of Er3+:NYF crystals. The radiative transition probabilities were calculated by the Judd–Ofelt method; the nonradiative transition probabilities were estimated; energy transfer (ET) microparameters (for migration, selfquenching (SQ), and upconversion (UC) processes) in studied crystals were estimated theoretically using the method of quantum–mechanical model calculation. The rate equations for the six lowest energy levels of erbium in Er3+:NYF crystals, including the 3μm laser transition levels, were numerically solved for the case of excitation by nanosecond pulse at λpump=1.53μm. The theoretical and experimental results for Er3+:NYF crystals are in good agreement. Single mode CW laser action was achieved in Er:NYF crystal at 2.8μm with InGaAs LD pumping of the 4I11/2 level. Output power about 72mW with slope efficiency ∼4% was achieved in longitudinal pump scheme.

Analytical investigations of an electron–water molecule pseudopotential. II. Development of a new pair potential and molecular dynamics simulations

A new electron–water molecule pseudopotential is developed and tested in the present paper. The formal development of the potential is based on our earlier quantum mechanical model calculations of the excess electronic states of the electron-water molecule system [Turi et al., J. Chem. Phys. 114, 7805 (2001)]. Although the new pseudopotential has a very simple analytical form containing only nine adjustable parameters, it reproduces the exact eigenvalue of the excess state and the electron density of the smooth pseudo-wave function in the static-exchange limit. Of the individual potential energy terms, one can extract the exact electrostatic, the local repulsion and, as the remaining part, the local exchange potentials. The polarization term is added to the potential a posteriori. The most important feature of the potential is that the repulsive core region of the potential is finite and relatively narrow. This property leads to the non-negligible penetration of the excess electron in the core. The attractive wells of the potential also appear significantly closer to the nuclei than in previous pseudopotentials. The new pseudopotential is tested in quantum molecular dynamics simulations of a ground-state excess electron in a water bath. Whereas the basic features of the equilibrium hydrated electron are similar to those predicted in earlier simulations, important quantitative details are significantly improved relative to available experimental data. In particular, the simulations reproduce the equilibrium ground state energy and the optical absorption spectrum quite well. The differences of the present pseudopotential from previous works are also manifested in the more diffuse ground-state electron distribution and the more compact solvation structure. Further structural and dynamical consequences of the application of the new pseudopotential are analyzed in detail.

Asymmetric nonlinear conductance of quantum dots with broken inversion symmetry

Coherent electron transport in open, asymmetric (triangular) quantum dots is studied experimentally and theoretically in the nonlinear response regime. The nonlinear dot conductance is found to be asymmetric with respect to zero bias voltage. This conductance asymmetry is related to the nonsymmetric effect of an applied electric field on the quantum electron states inside the dot and on their coupling to the states in the electron reservoirs. The direction of the asymmetry depends sensitively on the amplitude of an applied ac voltage, on the Fermi energy and on the magnetic field, and is suppressed at temperatures above a few Kelvin. Quantum dots can therefore be viewed as ratchets, that is, devices in which directed particle flow is induced by nonequilibrium fluctuations, in the absence of (time-averaged) external net forces and gradients. A quantum mechanical model calculation reproduces the key experimental observations. The magnitude of the conductance asymmetry is found to depend strongly on the electric field distribution inside the dot. In addition to exact calculations, an approximation is presented which makes it possible to qualitatively predict the nonlinear behavior from the energy dependence of the conductance in the linear response regime. We also discuss a semiclassical explanation for our observations and comment on limits of quantum-interference induced rectification.

Delayed fluorescence detected magnetic resonance study on the spin dynamics of the transient radical-ion pair formed in the photolysis of carbazole in 2-propanol

A delayed fluorescence detected magnetic resonance method is applied to the study of a transient radical-ion pair (RIP) formed in the photolysis of carbazole in 2-propanol. The spectra under different microwave powers are analyzed with a simple quantum-mechanical model calculation. The experimental and calculated data prove that the formation of the RIP (carbazole cation and solvated electron) is by way of a biphotonic process and the initial spin state of the RIP is the singlet. It is shown that no strong interaction such as exchange between the component ion radicals exists, which implies the large separation of the ion radicals immediately after their birth. The spectrum of the RIP having a solvated electron as the counter radical shows the characteristic shape of spin locking at high microwave power.

A Simulation of the Catalytic Mechanism of Aspartylglucosaminidase Using ab Initio Quantum Mechanics and Molecular Dynamics

Ab initio quantum mechanical (QM) and molecular dynamics (MD) methods have been used to study the catalytic mechanism of aspartylglucosaminidase (AGA)-catalyzed hydrolysis of an amide bond. QM active-site models were used to calculate the reaction sequences of the acylation and deacylation parts of the catalytic reaction at the MP2/6-31G*//HF/6-31G* level. MD simulations, in which the structures of the active sites were constrained to the geometries obtained from the QM model calculations, were carried out for all the species of the reaction sequence. Structural information from the MD simulations was used in the subsequent QM calculations in which the charge distribution of the protein environment and solvent was included as a set of atomic point charges. These QM calculations provided an estimate of the effects of the protein environment on the energetics of the catalytic reaction. The reaction mechanism was also simulated using a continuum model for the aqueous solvent, in order to differentiate genera...

Ab Initio Quantum Mechanical Model Calculations on the Catalytic Mechanism of Aspartylglucosaminidase (AGA): A Serine Protease‐Like Mechanism with an N‐terminal Threonine and Substrate‐Assisted Catalysis

AbstractAb initio quantum mechanical model calculations were used in studying the acylation and deacylation steps of the catalytic mechanism of aspartylglucosaminidase (AGA). AGA catalyses the hydrolysis of an amide linkage between oligosaccharide and asparagine by utilising an N‐terminal threonine as a catalytic amino acid. Results are reported for the model enzyme reaction at the MP2/6–31+G*//HF/6–31 + G* + ΔZPE level. Contribution of aqueous solvation to the relative energies was estimated by using the continuum solvation model of Tomasi. The serine protease‐like catalytic mechanism was found to be feasible for AGA. The protonated α‐amino group of the substrate of AGA was suggested to enhance the catalysis by stabilising the anionic oxygen of the substrate, which is formed in the reaction, and by lowering the pKa of the nucleophilic oxygen of the N‐terminal threonine. Finally, the similarities in the catalytic mechanisms of AGA and other amidohydrolases were discussed.

Percolation of carriers through low potential channels in thick AlxGa1-xAs (x

We study by photoluminescence excitation the heretofore unsolved puzzle of a significant charge transfer over a thick (100 to 1500 \AA{}) ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As barrier in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As asymmetric double quantum wells, which the normally considered tunneling cannot account for. This phenomenon is completely general, observed in all the samples grown under standard growth conditions (\ensuremath{\sim}600 \ifmmode^\circ\else\textdegree\fi{}C), that originated from many different sources. The existence of such leakage is also confirmed by time-resolved photoluminescence experiments. However, when the alloy barrier is replaced by an equivalent GaAs/AlAs digital alloy, or by AlAs, the leak largely disappears. In addition, a GaAs barrier separating two shallow ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells permits only relatively small transfer. The leak has a weak dependence on the barrier thickness at x=0.3, but is a very strong function of x around x=0.3. We argue that there is no way to explain all of the observed phenomena simultaneously other than by the existence of intrinsic structural inhomogeneities in the alloy. Essentially, there may exist low potential channels in the alloy barrier created by microscopic clustering of like molecules, through which percolationlike transport occurs. This picture is supported by a three-dimensional quantum-mechanical model calculation. Our work pins down the dynamical implications of the partial ordering and clustering in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As and related semiconductor ternary alloys. The scope of this paper is exclusively for the transport over thick ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As barriers (x0.35) and does not include the relatively small but still non-negligible transport over thick homogeneous barriers such as GaAs, AlAs, and AlAs/GaAs digital alloys, and ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As (x&gt;0.35). We contend that transport over these thick, homogeneous barriers may be owing to other mechanisms such as dipole-dipole transfer, photon reabsorption, nonequilibrium distribution of carriers, and polariton transport that are also unrelated to conventional tunneling. \textcopyright{} 1996 The American Physical Society.

Effects of Rotational State Excitations on the Dissociative Adsorption Dynamics of D2/Cu(111)

Recent time-of-flight experiments show a non-monotonous dependence of the dissociative adsorption/sticking * probability of D 2 (H 2 )/Cu(111) on the initial rotational state. There is an observed decrease in the sticking probability with a slight increase in the initial rotational state quantum number j. As j is increased further, the sticking probability eventually increases. We show that, to account for this interesting behaviour, there are two opposing factors working for and against the adsorption process: STEERING EFFECT and ENERGY TRANSFER EFFECT. The first is due to a dynamical reorientation of the molecule, and the latter is due to the coupling of the rotational motion to the translational motion along the reaction path. To illustrate these two effects, we performed quantum mechanical model calculations using a variation of the coupled-channel method, a physically realistic model potential based on available potential energy (hyper-) surface plots for D 2 (H 2 )/Cu for two different orientations-...

The dynamics of desorption induced by atomic hydrogen: HD/Cu(111)

Recent observations of a direct reaction between adsorbates and hydrogen atoms incident from the gas phase are interpreted in terms of an Eley-Rideal reaction. A detailed comparison of the experimental data for the HD/Cu(111) system with quantum mechanical model calculations corroborates such an interpretation. The peculiar isotope effect observed can be understood from the different dynamical implications of appropriately rescaled potential energy surfaces. The width of the measured time-of-flight spectrum is explained from the overlapping contributions of the populated vibrational levels. The angular distributions are rationalized by contributions both from ‘indirect’ events, where the incident atoms make several bounces in the surface well prior to reaction, and ‘direct’ reactive events.

Angle-dependent electron energy spectra resulting from autoionizing Ne? (3s 3 P 2) +H(12 S) collisions

Experimental angle-dependent electron energy spectra for the autoionization complex Ne*(3s3P2)+H(12S), leading to Penning and associative ionization, are reported. The data, measured at thermal collision energies (Ērel∼51 meV), clearly show an angular variation of the spectral shape, indicating that electrons with angular momentuml>0 participate in the autoionization process. The corresponding non-isotropic electron emission leads to a correlation between the impact parameter-dependent heavy-particle dynamics and the observed electron energy spectrum at a certain detection angle. The experimental results are qualitatively discussed in connection with previous work on the system He*(23S)+H(12S). Furthermore, we present quantum mechanical model-calculations for the electron energy spectrum on the basis of available potential data.

Effects of geometry and potential profile on nonlinear transport in narrow ballistic constrictions

The authors present quantum-mechanical model calculations of nonlinear transport in a ballistic quasi-one-dimensional (1D) channel. The effects of the geometry and potential profile on the quantum conductance for 1D ballistic transport through a constriction in a quasi-two-dimensional electron gas (2DEG) are investigated. It is found that the effects of the confining electrostatic potential in the channel on conductance are significant for large biases. To determine the nonlinear transport properties it is essential to consider how the voltage is dropped along the constriction. The appearance of new plateaux in the differential conductance of a ballistic 1D channel in the high-bias regime is possible, but depends on the form of the constriction.

State-selective studies of the associative desorption of hydrogen from Pd(100) and Cu(100)

Vibrational-state populations and velocity distributions of H2, HD, and D2 desorbing from Pd(100) are measured with rotational state selectivity over a wide temperature range from Ts= 325 to 825 K. At all surface temperatures the vibrational populations, increasing exponentially with Ts, are found to be significantly higher than those expected for thermally equilibrated molecules. The slopes of Boltzmann plots are considerably lower than expected for a thermal excitation mechanism of the vibrational states at the corresponding gas-phase energies. They also show a non-trivial isotope effect. The velocity distributions from clean Pd(100) are found to be Maxwell–Boltzmann-like. The translational energies of H2 molecules are accommodated at the surface temperatures, whereas those of D2 are higher than kTs by ca. 10–30 meV. Both the vibrational excitation and the isotope effect in translation can be understood with a quantum-mechanical model calculation on a two-dimensional potential-energy surface. The vibrational-state selective angular distributions for desorption from Cu(100) display a different behaviour for ground and excited states. The distributions for the vibrationally excited states are broader than that of ground-state molecules. They also show an isotope effect.