Full Quantum Calculations Research Articles

Ionization of Stark states with half-cycle pulses: Interference effects in the continuum

We study the ionization of extreme Stark states in sodium by THz electromagnetic half-cycle pulses. The results of our full-quantum calculations reveal the presence of an oscillatory pattern in the ionization spectra of extreme red (downhill) states that have been kicked away from the ion core (downhill with respect to the potential imposed by the Stark field). We find no oscillations in the spectra of extreme blue (uphill) states that have been kicked towards the ion core (also downhill with respect to the Stark potential). The oscillatory pattern in the red state ionization spectra is explained with a one-dimensional semiclassical model in terms of interferences between two classical paths. This model also predicts that the blue state ionization spectra should also show oscillatory behavior. The absence of the oscillations in the full calculations can be regarded as a breakdown of the one-dimensional model in representing the ionization dynamics of these states. We find that the one-dimensional model fails when the duration of the THz pulse is comparable to the classical Kepler orbit time.

Quantum soliton dynamics in vibrational chains: Comparison of fully correlated, mean field, and classical dynamics

The dynamics of a chain of vibrational bonds which develop a classical solitary compression wave is simulated. A converged fully correlated quantum mechanical calculation is compared with a time dependent mean field approach (TDSCF) and with a classical simulation. The dynamics were all generated from the same Hamiltonian. The TDSCF and classical calculations show a fully developed solitary wave with the expected dependence of group velocity on amplitude. The full quantum calculations show a solitary-like wave which propagates for a while but then degrades. The robustness of the compression wave depends on the initial preparation. Evidence of partial recurrence of the wave has also been observed.

Direct tunneling current model for MOS devices with ultra-thin gate oxide including quantization effect and polysilicon depletion effect

An efficient direct tunneling current model is presented for the ultra thin gate dielectric MOS structure. The tunneling current is modeled by including the inversion layer quantization effect with a finite potential barrier height as the boundary condition and the modified WKB method for calculating the transmission probability. The model is in good agreement with the full quantum calculation and the experiments. The results indicate that the finite boundary condition has to be considered for the ultra thin gate dielectric and the gate dielectric materials with lower barrier height. This model is accuracy and computational efficient and suitable to be used in characterized the sub-100 nm MOSFET with gate oxide below 2.0 nm.

Reassignment of the O2 spectrum just below dissociation threshold based on ab initio calculations

Vibrational Herzberg bands of the O2 molecule just below its first O(3P)+O(3P) dissociation limit are since long-known to be perturbed. Jenouvrier et al. [J. Mol. Spectrosc. 198, 136 (1999)] assigned the cause of the perturbations to five vibrational levels supported by the shallow minimum in the 1 3Πu potential energy curve around 5.5a0. Using ab initio potential energy curves and spin–orbit couplings from previous work [J. Chem. Phys. 116, 1954 (2002)] we present a full quantum calculation of all ungerade rotation–vibration–electronic states of oxygen just below the dissociation threshold, through a total angular momentum quantum number of J=19. This calculation shows that the original assignment, based on a Hund’s case (a) model of a regular 1 3Πu multiplet was not correct. Based on our calculation we present a new assignment of the perturbing states: 1 3Πu,Ω=2(v=0), 1 3Πu,1(0), 1 3Πu,2(1), 1 3Πu,1(1), and 1 3Πu,0−(0) in order of ascending term values. We show the new assignment to be consistent with experimental data and we also propose new spectroscopic parameters for the perturbing states.

Magnetic-field effects in ultracold molecular collisions

We investigate the collisional stability of magnetically trapped ultracold molecules, taking into account the influence of magnetic fields. We compute elastic and spin-state-changing inelastic rate constants for collisions of the prototype molecule ${}^{17}{\mathrm{O}}_{2}$ with a ${}^{3}\mathrm{He}$ buffer gas as a function of the magnetic field and the translational collision energy. We find that spin-state-changing collisions are suppressed by Wigner's threshold laws as long as the asymptotic Zeeman splitting between incident and final states does not exceed the height of the centrifugal barrier in the exit channel. In addition, we propose a useful one-parameter fitting formula that describes the threshold behavior of the inelastic rates as a function of the field and collision energy. Results show a semiquantitative agreement of this formula with the full quantum calculations, and suggest useful applications also to different systems. As an example, we predict the low-energy rate constants relevant to evaporative cooling of molecular oxygen.

Bremsstrahlung radiation as a coherent state in thermal QED

Based on fully finite temperature field theory we investigate the radiation probability in the bremsstrahlung process in thermal QED. It turns out that the infrared divergences resulting from the emission and absorption of the real photons are cancelled by the virtual photon exchange processes at finite temperature. The full quantum calculation results for soft photon radiation coincide completely with those obtained in the semiclassical approximation. In the framework of thermofield dynamics it is shown that the bremsstrahlung radiation in thermal QED is a coherent state and its quasi-classical behaviour leads to the above coincidence.

Two-channel competition of autoionizing Rydberg states in an electric field

We present experimental data on the decay of xenon Stark states converging to the upper spin limit. In an electric field the Rydberg electron has two qualitatively different decay paths. If the electron changes the core state from the upper spin state into the lower spin state, it gains sufficient energy to escape the ionic core and autoionizes. Moreover, if the electronic state is above the saddle point, created by the electric field, it can field ionize. The probability to autoionize is nearly constant around the saddle point whereas the probability to field ionize rapidly increases above the saddle point. With the velocity map imaging technique we monitor both ionization channels as a function of ~increasing! photoexcitation energy. We observe that the field ionization channel dominates the competition and gains yield at the expense of the autoionization channel. The spectra are explained both with full quantum calculations and with a relatively simple description for the overall behavior. These experiments show that the field ionization can be used in general as a clock for total core-dependent decay. Decay of a highly excited electron most often occurs through interaction with the ionic core. The rate of, e.g., rotational and vibrational autoionization, fluorescence and predissociation is proportional to the frequency with which an electron returns to the vicinity of the ionic core. Through- out the Rydberg series the ratio between these decay chan- nels remains constant. In this respect field ionization is a special decay channel. A static electric field distorts the Cou- lomb potential by raising the potential in the direction of the field and lowering it on the other side. The maximum on the down-potential side is called the saddle point. An electron that is excited above the saddle point is energetically allowed to escape from the ionic core. The rate for electron emission depends strongly on the opening angle at which the electron can escape. This opening angle rapidly increases above the saddle point making the field ionization change from zero to the dominant pathway of decay in a small energy range. How rapidly the field ionization dominates depends on the com- petition with the core-dependent decay. This competition is reflected in the overall behavior of the ionization spectra, allowing one to retrieve the rate of one channel with the spectrum of the other. In a recent paper we investigated the field ionization yield above the saddle point energy of nitric oxide in a strong electric field @1#. A competition between the field ionization and predissociation of the NO molecule is observed. The field ionization yield just above the saddle point was strongly reduced by the other decay channel and the ionization thresh- old appeared to have shifted to higher energy. Computing the field ionization spectrum yielded the rate of predissociation. In that experiment only the field ionization channel was monitored. To investigate a two-channel competition prop- erly, however, one wants to monitor the two channels simul- taneously. An ideal system of investigation is the autoioniz- ation from the Rydberg states converging to the upper spin state of a noble gas in a strong electric field. The field ion- ized electrons have nearly zero kinetic energy whereas the electrons that autoionize by changing the core spin-orbit state, carry a kinetic energy of 1-2 eV, which makes the two ionization channels separable. The two channels are distin- guished by velocity map imaging. The detected spectra, as presented in Sec. III, are simulated by full quantum calcula- tions based on multichannel Stark theory @1-3#. Moreover, we show that the overall behavior of the spectra can already be simulated by relative simple rate equations. In Sec. IV we derive these equations and compare with the experimental observations.

Full dimensional quantum calculations of the CH4+H→CH3+H2 reaction rate

Accurate full-dimensional quantum mechanical calculations are reported for the CH4+H→CH3+H2 reaction employing the Jordan–Gilbert potential energy surface. Benchmark results for the thermal rate constant and the cumulative reaction probability are presented and compared to classical transition state theory as well as reduced dimensionality quantum scattering calculations. The importance of quantum effects in this system is highlighted.

Isolated-core excitations in strong electric fields. II. Experimental results in magnesium

We report on the electron yield spectra of doubly excited magnesium atoms in electric fields in the region 50--1600 V/cm. One electron is excited to a stationary Rydberg state around $n=16,$ either a nonmixing state with low angular momentum or a Stark state. Subsequently the second isolated-core electron is excited with a narrow-band nanosecond laser pulse. The isolated-core excitation induces a shake-up which is measured as a function of excitation frequency. An initial low-angular-momentum state leads to a much broader shakeup than an initial Stark state, i.e., many more autoionization peaks are observed for an initial low-angular-momentum state than for an initial Stark state. Full quantum calculations introduced in the preceding paper [F. Robicheaux, preceding paper Phys. Rev. A 61, 033406 (2000)] show excellent agreement with the recorded spectra. We discuss the implications of this experiment for dielectronic recombination and compare the frequency spectra to previously reported time-dependent measurements on magnesium [J.B.M. Warntjes et al., Phys. Rev. Lett. 83, 512 (1999)].

Partitioned densities of states in a resonant tunnelling structure

We analyse the properties of the partitioned densities of states inmesoscopic transport problems. In terms of an imaginary-potentialmodel, a concise derivation of the local partial density of states (LPDOS)is given. Using a numerical method to calculate the functionalderivative of the scattering matrix, a detailed study of different kindsof decomposed densities of states in double-barrier structures iscarried out. We find that the dynamical local density of statesdnαα(x)/dE and the global partialdensity of states (GPDOS) dNαα/dE in the nonclassical regime cannot be neglected in full quantumcalculations.

A transition state real wave packet approach for obtaining the cumulative reaction probability

We show how the transition state wave packet method of Zhang and Light can be applied within a real wave packet formalism. We also implement random superpositions into the approach, as in the recent work of Matzkies and Manthe, which can significantly reduce the number of propagations at higher temperatures. The net result is a very efficient approach for calculating the cumulative reaction probability, and hence the thermal rate constant, for bimolecular chemical reactions. Full dimensional quantum calculations, including all relevant total angular momenta, of the cumulative reaction probability and thermal rate constant for the D+H2→HD+H are used as illustration.

Vibrational Dynamics of the I3 Radical: A Semiempirical Potential Surface, and Semiclassical Calculation of the Anion Photoelectron Spectrum

The semiempirical diatomics in molecules (DIM) approach is used to model the potential surface for ground-state vibration of a linear I3 molecule. We use this system to explore semiclassical methods for treating quantal nuclear vibrations by computing the photoelectron spectrum of I3- which produces vibrationally excited I3 . We compare semiclassical results with full quantum calculations and experimental results recently reported by Neumark and co-workers. (Taylor, T. R.; Asmis, K.R.; Zanni, M. T.; Neumark, D. M. J. Chem. Phys. 1999, 110, 7607.)

Recovery Temperature for Nonclassical Energy Transfer in Atom-Surface Scattering

Nonperturbative expressions are derived for the angular resolved energy transfer spectra in the quantum regime of multiphonon scattering of inert gas atoms from surfaces. Application to He atom scattering from a prototype heat bath Xe/Cu(111) shows good agreement with experiments. This enables a full quantum calculation of the total energy transfer $\ensuremath{\mu}$ and from this the much debated recovery or equilibrium temperature ${T}_{r}$ characteristic of zero energy transfer in gas-surface collisions in the free molecular flow regime. The classical universal character of $\ensuremath{\mu}$ and ${T}_{r}$ is refuted.

What can short-pulse pump-probe spectroscopy tell us about Franck-Condon dynamics?

We examine the signal from pump-probe spectroscopy of a model system—nonrotating I2—at short time delays and compare signals calculated without approximation (a full quantum calculation), with a semiclassical Franck-Condon approximation, and with a classical simulation of the nuclear wave packet. In order to assess the complications of simulation and interpretation when the probe window lies in the spectroscopically and dynamically important Franck-Condon region, we concentrate on a case where pump and probe resonances are at the same internuclear distance. We find that the common practice of ignoring the pump-truncation effects of pulse overlap leads to an overestimate of the signal at short times. Moreover, both classical simulations and semiclassical Franck-Condon treatments can deviate significantly in form from the actual signal even with proper treatment of pulse overlap. The sources of these deviations can be seen in the evolution of the excited-state nuclear distributions calculated classically and under the semiclassical Franck-Condon approximation. Specifically, the differences in evolution of the classical and full quantum excited-state nuclear distributions are due to differing initial momentum distributions. We introduce an efficient method for calculating the pump-probe signal that takes advantage of the brevity of ultrashort pulses and can include pulse characteristics such as chirp. This short-pulse expansion method aids in the proper treatment of pulse-overlap and nonzero pulse duration and promises to simplify the incorporation of relaxation processes.

Intramolecular vibrational dephasing obeys a power law at intermediate times.

Experimental intramolecular vibrational dephasing transients for several large organic molecules are reanalyzed. Fits to the experimental data, as well as full numerical quantum calculations with a factorized potential surface for all active degrees of freedom of fluorene indicate that power law decays, not exponentials, occur at intermediate times. The results support a proposal that power law decays describe vibrational dephasing dynamics in large molecules at intermediate times because of the local nature of energy flow.

Geometry-dependent scattering through Ballistic microstructures: Semiclassical theory beyond the stationary-phase approximation

The conductance of a ballistic microstructure shows strong fluctuations as a function of the Fermi wave number. We present a semiclassical theory that describes these fluctuations in terms of bundles of short trajectories. These trajectories provide the dominant contribution to electron transport through a weakly open microstructure. For the coupling between the quantum wires and the cavity, contributions beyond the stationary phase approximation are taken into account giving rise to diffraction effects. A comparison with full quantum calculations for a rectangular billiard is made. The peak positions of the power spectrum agree very well between the quantum and semiclassical theories. Numerical evidence is found for the breakdown of the semiclassical approximation for long paths. A simple explanation in terms of the dispersion of the semiclassical wave packet in the interior of the cavity is proposed.

Atomic transport on one-dimensional optical lattices

We carry out quantum and semiclassical numerical simulations of the dynamics, particularly diffusion, of atoms in an optical lattice. We consider specifically the situation of a $J=1/2\ensuremath{\leftrightarrow}J=3/2$ atomic transition, whose experimental counterpart can be realized in ${\mathrm{Mg}}^{+}$ experiments. The semiclassical results, which cover a larger range of parameters than would be practical for fully quantum calculations, confirm and extend the work of Marksteiner et al. [Phys. Rev. A 53, 3409 (1996)], characterize the transition from normal to anomalous diffusion and the atomic trajectories which give rise to it, and provide a physical explanation for the difference between these two regimes. The full quantum calculations reveal uniquely quantum features of individual atomic trajectories, and in addition, predict significantly lower diffusion, by a factor of 1.2 to 2, than their semiclassical counterparts.

Elastic collisions and rotational excitation in positron scattering fromCO2molecules

Full quantum calculations are carried out for the elastic (rotationally summed) integral and differential cross sections in low-energy collisions of positrons with rigid rotor ${\mathrm{CO}}_{2}$ molecules. The interaction includes an expansion of the exact Coulomb potential on a single-center (SCE) and a parameter-free correlation-polarization (${\mathrm{V}}_{\mathrm{CP}}$) potential given in local form via density-functional theory (DFT) with gradient corrections. State-to-state rotationally inelastic cross sections (integral and differential) are also presented and their behavior is discussed in relation to what is known from similar experiments with electrons as projectiles. The agreement found between computed and measured elastic integral cross sections is rather good and confirms the realistic quality of a DFT treatment for short-range dynamical correlation effects.

Radiative association of LiH(X1Σ+) from electronically excited lithium atoms

Full quantum calculations are carried out for the collisional processes involving H atoms in their ground electronic state and electronically excited lithium atoms, Li(1${\mathit{s}}^{2}$2p). The channels that are being considered are those leading to the formation of bound rotovibrational states of LiH(X${\mathrm{}}^{1}$${\mathrm{\ensuremath{\Sigma}}}^{+}$). The effects of both the specific features of the involved electronic potential-energy curves and of the transition moments coupling bound and continuum states during the process are studied in relation to the low-energy behavior of the corresponding cross sections. The role of the multitude of open channel resonances on the final rate constants is also analyzed and discussed over a broad range of bath temperatures. \textcopyright{} 1996 The American Physical Society.

Time-dependent semiclassical calculation of resonance lifetimes

Spectral features of quantum systems can be extracted from dynamical quantities like wavefunctions or correlation functions. Using a time-dependent framework, we study symmetric transition state resonances of the collinear H + H2 system. We compare time-dependent semiclassical results and the corresponding spectra to full quantum calculations. The semiclassical time-domain approach, which includes phase information and therefore allows for interference effects, reproduces the quantum results to a high degree of accuracy. Particularly, it has been possible for the first time to determine resonance widths for a generic molecular system semiclassically.