arXiv subjects
Publications and source records attributed to Jan M Rost.
Non-adiabatic photo-ionization is difficult to control as it relies on the derivatives of the envelope and not on phase-details of the short ionizing pulse. Here, we introduce a catalyzing state, whose presence render non-adiabatic ionization sensitive to phase-details of tailored pulses. Since a catalyzing state is in general easy to create, this opens a perspective for coherent control of ultra-fast ionization.
The generation of a streaking spectrogram is based on energy absorption from the streaking laser. Investigating this absorption we show rigorously under which condition the measured time shift is independent of properties of the streaking light. In this case it provides the Wigner-Smith time delay. The latter is infinite for systems with a long-range potential tail, such as Coulomb systems. Here, we suggest to determine the time delay relative to pure hydrogen for meaningful results. Finite delays obtained so far for Coulombic systems without the hydrogen reference are the consequence of a finite streaking frequency and depend on its value as well as on the electron's excess energy. Our analysis also suggests a time-delay measurement technique that avoids the record of a complete streaking scan.
We propose a simple way to determine the periodicities of wave packets in quantum systems directly from the energy differences of the states involved. The resulting classical periods and revival times are more accurate than those obtained with the traditional expansion of the energies about the central quantum number n, especially when n is low. The latter type of wave packet motion occurs upon excitation of highly charged ions with short XUV or X-ray pulses. Moreover, we formulate the wave packet dynamics in such a form that it directly reveals the origin of phase shifts in the maxima of the autocorrelation function. This phenomenon, so far poorly understood since it is not prominent in the high n regime, becomes a dominant feature in low n wave packet dynamics.
We investigate dynamics of atomic and molecular systems exposed to intense, shaped chaotic fields and a weak femtosecond laser pulse theoretically. As a prototype example, the photoionization of a hydrogen atom is considered in detail. The net photoionization undergoes an optimal enhancement when a broadband chaotic field is added to the weak laser pulse. The enhanced ionization is analyzed using time-resolved wavepacket evolution and the population dynamics of the atomic levels. We elucidate the enhancement produced by spectrally-shaped chaotic fields of two different classes, one with a tunable bandwidth and another with a narrow bandwidth centered at the first atomic transition. Motivated by the large bandwidth provided in the high harmonic generation, we also demonstrate the enhancement effect exploiting chaotic fields synthesized from discrete, phase randomized, odd-order and all-order high harmonics of the driving pulse. These findings are generic and can have applications to other atomic and simple molecular systems.
The fragmentation of diatomic molecules under a stochastic force is investigated both classically and quantum mechanically, focussing on their dissociation probabilities. It is found that the quantum system is more robust than the classical one in the limit of a large number of kicks. The opposite behavior emerges for a small number of kicks. Quantum and classical dissociation probabilities do not coincide for any parameter combinations of the force. This can be attributed to a scaling property in the classical system which is broken quantum mechanically.
A two-step optimization is proposed to represent an arbitrary quantum state to a desired accuracy with the least number of gaussians in phase space. The Husimi distribution of the quantum state provides the information to determine the modulus of the weight for the gaussians. Then, the phase information contained in the Wigner distribution is used to obtain the full complex weights by considering the relative phases for pairs of gaussians, the chords. The method is exemplified with several excited states n of the harmonic and the Morse oscillators. A semiclassical interpretation of the number of gaussians needed as a function of the quantum number n is given. The representation can also be used to characterize Wigner and Husimi distributions directly which do not originate in a quantum state.
We demonstrate that a recently published quasiclassical M\oller type approach [Geyer and Rost 2002, J. Phys. B 35 1479] can be used to overcome the problem of autoionization, which arises in classical trajectory calculations for many electron targets. In this method the target is stabilized dynamically by a backward--forward propagation scheme. We illustrate this refocusing and present total cross sections for single and double ionization of helium by electron impact.
The effect of intense X-ray laser interaction on argon clusters is studied theoretically with a mixed quantum/classical approach. In comparison to a single atom we find that ionization of the cluster is suppressed, which is in striking contrast to the observed behavior of rare-gas clusters in intense optical laser pulses. We have identified two effects responsible for this phenomenon: A high space charge of the cluster in combination with a small quiver amplitude and delocalization of electrons in the cluster. We elucidate their impact for different field strengths and cluster sizes.
Elastic scattering probes directly the interaction potential. For weakly interacting condensates this potential is given by the condensate density. We investigate how the differential and total cross sections reflect the density. In particular, we have determined which signatures the Thomas Fermi approximation leaves in contrast to an exact solution for the condensate wave function within the Gross-Pitaevskii theory.
For the energy absorption of atomic clusters as a function of the laser pulse duration we find a similar behavior as it has been observed for metallic clusters [Köller et al., Phys. Rev. Lett. {\bf 82}, 3783 (1999)]. In both situations there exists an optimum radius $R_{o}$ of the cluster for energy absorption. In the metallic case the existence of $R_{o}$ has been interpreted as a consequence of the collective oscillation of a delocalized electron cloud in resonance with the laser frequency. Here, we give evidence that in the atomic cluster the origin of $R_{o}$ is very different. Based on field assisted tunneling it can be related to the phenomenon of enhanced ionization as it occurs in small molecules. The dependence of $R_{o}$ on the laser frequency turns out to be the key quantity to distinguish the processes.
A classical approximation to time dependent quantum mechanical scattering in the Møller formalism is presented. Numerically, our approach is similar to a standard Classical-Trajectory-Monte-Carlo calculation. Conceptually, however, our formulation allows one to release the restriction to stationary initial distributions. This is achieved by a classical forward-backward propagation technique. As a first application and for comparison with experiment we present fully differential cross sections for electron impact ionization of atomic hydrogen in the Erhardt geometry.
We analyze strong field atomic dynamics semiclassically, based on a full time-dependent description with the Hermann-Kluk propagator. From the properties of the exact classical trajectories, in particular the accumulation of action in time, the prominent features of above threshold ionization (ATI) and higher harmonic generation (HHG) are proven to be interference phenomena. They are reproduced quantitatively in the semiclassical approximation. Moreover, the behavior of the action of the classical trajectories supports the so called strong field approximation which has been devised and postulated for strong field dynamics.
We present a semiclassical S-matrix study of inelastic collinear electron-hydrogen scattering. A simple way to extract all necessary information from the deflection function alone without having to compute the stability matrix is described. This includes the determination of the relevant Maslov indices. Results of singlet and triplet cross sections for excitation and ionization are reported. The different levels of approximation -- classical, semiclassical, and uniform semiclassical -- are compared among each other and to the full quantum result.
It is shown that recently measured cross sections for double ionization of negative ions ($H^-, O^-$, and $C^-$) possess a universal shape when plotted in suitable dimensionless units. The shape can be represented with a simple analytical function, following the same principles as it has been done in establishing a universal shape function for single ionization [Rost and Pattard 1997 Phys. Rev. A {\bf 55} R5]. Thereby, it is demonstrated that direct double ionization dominates the cross section for the targets considered.
The spectrum of higher harmonics in atoms calculated with a uniformized semiclassical propagator is presented and it is shown that higher harmonic generation is an interference phenomenon which can be described semiclassically. This can be concluded from the good agreement with the quantum spectrum. Moreover, the formation of a plateau in the spectrum is specifically due to the interference of irregular, time delayed, trajectories with regular orbits without a time-delay. This is proven by the absence of the plateau in an artificial semiclassical spectrum generated from a sample of trajectories from which the irregular trajectories (only a few percent) have been discarded.
It is shown that the time-dependent equations (Schrödinger and Dirac) for a quantum system can be always derived from the time-independent equation for the larger object of the system interacting with its environment, in the limit that the dynamical variables of the environment can be treated semiclassically. The time which describes the quantum evolution is then provided parametrically by the classical evolution of the environment variables. The method used is a generalization of that known for a long time in the field of ion-atom collisions, where it appears as a transition from the full quantum mechanical {\it perturbed stationary states} to the {impact parameter} method in which the projectile ion beam is treated classically.