In parallel with the evolution of femtosecond and attosecond laser as well as free-electron laser technology, a variety of theoretical methods have been developed to describe the behavior of atoms, ...molecules, clusters, and solids under the action of those laser pulses. Here, we review major ab initio wave-function-based numerical approaches to simulate multielectron dynamics in atoms and molecules driven by intense long-wavelength and/or ultrashort short-wavelength laser pulses. Direct solution of the time-dependent Schrödinger equation, though its applicability is limited to He, H 2 , and Li, can provide an exact description and has been greatly contributing to the understanding of dynamical electron-electron correlation. Multiconfiguration self-consistent-field (MCSCF) approach offers a flexible framework from which a variety of methods can be derived to treat both atoms and molecules, with possibility to systematically control the accuracy. The equations of motion of configuration interaction coefficients and molecular orbitals for general MCSCF ansatz have recently been derived. Time-dependent extension of the R-matrix theory, originally developed for electron-atom collision, can realistically and accurately describe laser-driven complex multielectron atoms.
We propose the use of the erfgau potential as a smooth alternative to the pure Coulomb potential between nuclei and electrons in simulating the dynamics of electrons within atoms and molecules driven ...by high-intensity laser pulses. Even without the sophistication of pseudopotentials, by utilizing a well-designed simple approximate potential, it is possible to make the simulations computationally less demanding while keeping accuracy. By employing the erfgau potential designed for the stationary state of hydrogen-like atoms, we demonstrate that it is possible to simulate not only the high harmonic generation from a hydrogen atom but also that of multielectron systems, including molecules.
Purpose
The precise assessment of the dose distribution of high linear energy transfer (LET) radiation remains a challenge, because the signal of most dosimeters will be saturated due to the high ...ionization density. Such measurements are particularly important for heavy‐ion beam cancer therapy. On this basis, the present work examined the high LET effect associated with three‐dimensional gel dosimetry based on radiation‐induced chemical reactions. The purpose of this study was to create an ion beam radio‐fluorogenic gel dosimeter with a reduced effect of LET.
Methods
Nanoclay radio‐fluorogenic gel (NC‐RFG) dosimeters were prepared, typically containing 100 μM dihydrorhodamine 123 (DHR123) and 2.0 wt% nanoclay together with catalytic additives promoting Fenton or Fenton‐like reactions. The radiological properties of NC‐RFG dosimeters having different compositions in response to a carbon‐ion beam were investigated using a fluorescence gel scanner.
Results
An NC‐RFG dosimeter capable of generating a fluorescence intensity distribution reflecting the carbon‐ion beam dose profile was obtained. It was clarified that the reduction of the unfavorable LET dependence results from an acceleration of the reactions between DHR123 and H2O2, which is a molecular radiolysis product. The effects of varying the preparation conditions on the radiological properties of these gels were also examined. The optimum H2O2 catalyst was determined to include 1 mM Fe3+ ions, and the addition of 100 mM pyridine was also found to increase the sensitivity.
Conclusions
This technique allows the first‐ever evaluation of the depth–dose profile of a carbon‐ion beam at typical therapeutic levels of several Gy without LET effect.
Recent developments of ultrafast laser pulse techniques enable us to study the subpicosecond scale dynamics out of thermal equilibrium. Multiple temperature models (MTMs) are frequently used to ...describe such dynamics where the total system is divided into subsystems each of which is in local thermal equilibrium. Typical examples include the electron-lattice two temperature model and electron-spin-phonon three temperature model. We present the exact analytical solutions of linear MTM, based on the Fourier series expansion and the Green’s function method. We then discuss their properties for the case of the two and three temperature models. We show that the general solution of MTM is expressed as a linear combinations of a spatially uniform, single-temperature stationary mode and the other non-oscillatory, decaying “eigenmodes” characterized by different wave vectors and well-defined mode lifetimes. The eigenmode picture enables us to explore the hierarchical structure of models with respect to space, time and the coupling parameter. Excitation by source term is included by the Green’s function method. As an example, we derive an analytical solution for a Gaussian type source term. We report a phenomenon “temperature inversion” where the lattice temperature exceeds electron’s temperature for ns time scale. Furthermore, we show how physical requirements such as energy conservation and equilibration are realized in the general linear MTM in terms of the eigenmode picture.
The polarization property of high harmonics from gallium selenide is investigated using linearly polarized midinfrared laser pulses. With a high electric field, the perpendicular polarization ...component of the odd harmonics emerges, which is not present with a low electric field and cannot be explained by the perturbative nonlinear optics. A two-dimensional single-band model is developed to show that the anisotropic curvature of an energy band of solids, which is pronounced in an outer part of the Brillouin zone, induces the generation of the perpendicular odd harmonics. This model is validated by three-dimensional quantum mechanical simulations, which reproduce the orientation dependence of the odd-order harmonics. The quantum mechanical simulations also reveal that the odd- and even-order harmonics are produced predominantly by the intraband current and interband polarization, respectively. These experimental and theoretical demonstrations clearly show a strong link between the band structure of a solid and the polarization property of the odd-order harmonics.
We have recently reported a study (Ishikawa 2010 Phys. Rev. B 82 201402) on a nonlinear optical response of graphene to a normally incident terahertz radiation pulse within the massless Dirac fermion ...(MDF) picture, where we have derived physically transparent graphene Bloch equations (GBE). Here we extend it to the tight-binding (TB) model and oblique incidence. The derived equations indicate that interband transitions are governed by the temporal variation of the spinor phase along the electron path in the momentum space and predominantly take place when the electron passes near the Dirac point. At normal incidence, the equations for electron dynamics within the TB model can be cast into the same form of GBE as for the MDF model. At oblique incidence, the equations automatically incorporate photon drag and satisfy the continuity equation for electron density. Single-electron dynamics strongly depend on the model and pulse parameters, but the rapid variations are averaged out after momentum-space integration. Direct current remaining after the pulse is generated in graphene irradiated by an intense monocycle terahertz pulse, even if it is linearly polarized and normally incident. The generated current depends on the carrier-envelope phase, pulse intensity and Fermi energy in a complex manner.
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