arXiv subjects

Christian Buth

Publications and source records attributed to Christian Buth.

At least 37 records · Page 2Linked to original sources

arXiv2010

Molecular double core-hole electron spectroscopy for chemical analysis

We explore the potential of double core hole electron spectroscopy for chemical analysis in terms of x-ray two-photon photoelectron spectroscopy (XTPPS). The creation of deep single and double core vacancies induces significant reorganization of valence electrons. The corresponding relaxation energies and the interatomic relaxation energies are evaluated by CASSCF calculations. We propose a method how to experimentally extract these quantities by the measurement of single and double core-hole ionization potentials (IPs and DIPs). The influence of the chemical environment on these DIPs is also discussed for states with two holes at the same atomic site and states with two holes at two different atomic sites. Electron density difference between the ground and double core-hole states clearly shows the relaxations accompanying the double core-hole ionization. The effect is also compared with the sensitivity of single core hole ionization potentials (IPs) arising in single core hole electron spectroscopy. We have demonstrated the method for a representative set of small molecules LiF, BeO, BF, CO, N2, C2H2, C2H4, C2H6, CO2 and N2O. The scalar relativistic effect on IPs and on DIPs are briefly addressed.

physics.chem-ph↗

arXiv2009

Theory of Auger decay by laser-dressed atoms

We devise an ab initio formalism for the quantum dynamics of Auger decay by laser-dressed atoms which are inner-shell ionized by extreme ultraviolet (XUV) light. The optical dressing laser is assumed to be sufficiently weak such that ground-state electrons are neither excited nor ionized by it. However, the laser has a strong effect on continuum electrons which we describe in strong-field approximation with Volkov waves. The XUV light pulse has a low peak intensity and its interaction is treated as a one-photon process. The quantum dynamics of the inner-shell hole creation with subsequent Auger decay is given by equations of motion (EOMs). For this paper, the EOMs are simplified in terms of an essential-states model which is solved analytically and averaged over magnetic subshells. We apply our theory to the M_4,5 N_1 N_2,3 Auger decay of a 3d hole in a krypton atom. The orbitals are approximated by scaled hydrogenic wave functions. A single attosecond pulse produces 3d vacancies which Auger decay in the presence of an 800nm laser with an intensity of 10^13 W / cm^2. We compute the Auger electron spectrum and assess the convergence of the various quantities involved.

physics.atom-ph↗

arXiv2009

Auger decay in krypton induced by attosecond pulse trains and twin pulses

Using attoscience, we study the electron correlations responsible for Auger decay in krypton atoms. The Auger decay is induced by a pulse train or a twin pulse composed of subpulses of attosecond duration. During the Auger decay an optical dressing laser may be present. Interference effects between the ejected Auger electron wave packets are predicted.

physics.atom-ph↗

arXiv2008

Rotational molecular dynamics of laser-manipulated bromotrifluoromethane studied by x-ray absorption

We present a computational study of the rotational molecular dynamics of bromotrifluoromethane (CF3Br) molecules in gas phase. The rotation is manipulated with an offresonant, 800nm laser. The molecules are treated as rigid rotors. Frequently, we use a computationally efficient linear rotor model for CF3Br which we compare with selected results for full symmetric-rotor computations. The expectation value (t) is discussed. Especially, the transition from impulsive to adiabatic alignment, the temperature dependence of the maximally achievable alignment and its intensity dependence are investigated. In a next step, we examine resonant x-ray absorption as an accurate tool to study laser manipulation of molecular rotation. Specifically, we investigate the impact of the x-ray pulse duration on the signal (particularly its temporal resolution), and study the temperature dependence of the achievable absorption. Most importantly, we demonstrated that using picosecond x-ray pulses, one can accurately measure the expectation value (t) for impulsively aligned CF3Br molecules. We point out that a control of the rotational dynamics opens up a novel way to imprint shapes onto long x-ray pulses on a picosecond time scale. For our computations, we determine the dynamic polarizability tensor of CF3Br using ab initio molecular linear-response theory in conjunction with wave function models of increasing sophistication: coupled-cluster singles (CCS), second-order approximate coupled-cluster singles and doubles (CC2), and coupled-cluster singles and doubles (CCSD).

physics.chem-ph↗

arXiv2008

X-ray refractive index of laser-dressed atoms

We investigated the complex index of refraction in the x-ray regime of atoms in laser light. The laser (intensity up to 10^13 W/cm^2, wavelength 800nm) modifies the atomic states but, by assumption, does not excite or ionize the atoms in their electronic ground state. Using quantum electrodynamics, we devise an ab initio theory to calculate the dynamic dipole polarizability and the photoabsorption cross section, which are subsequently used to determine the real and imaginary part, respectively, of the refractive index. The interaction with the laser is treated nonperturbatively; the x-ray interaction is described in terms of a one-photon process. We numerically solve the resolvents involved using a single-vector Lanczos algorithm. Finally, we formulate rate equations to copropagate a laser and an x-ray pulse through a gas cell. Our theory is applied to argon. We study the x-ray polarizability and absorption near the argon K edge over a large range of dressing-laser intensities. We find electromagnetically induced transparency (EIT) for x rays on the Ar 1s --> 4p pre-edge resonance. We demonstrate that EIT in Ar allows one to imprint the shape of an ultrafast laser pulse on a broader x-ray pulse (duration 100ps, photon energy 3.2keV). Our work thus opens new opportunities for research with hard x-ray sources.

physics.atom-ph↗

arXiv2008

Using strong electromagnetic fields to control x-ray processes

Exploration of a new ultrafast-ultrasmall frontier in atomic and molecular physics has begun. Not only is is possible to control outer-shell electron dynamics with intense ultrafast optical lasers, but now control of inner-shell processes has become possible by combining intense infrared/optical lasers with tunable sources of x-ray radiation. This marriage of strong-field laser and x-ray physics has led to the discovery of methods to control reversibly resonant x-ray absorption in atoms and molecules on ultrafast timescales. Using a strong optical dressing field, resonant x-ray absorption in atoms can be markedly suppressed, yielding an example of electromagnetically induced transparency for x rays. Resonant x-ray absorption can also be controlled in molecules using strong non-resonant, polarized laser fields to align the framework of a molecule, and therefore its unoccupied molecular orbitals to which resonant absorption occurs. At higher laser intensities, ultrafast field ionization produces an irreversible change in x-ray absorption. Finally, the advent of x-ray free electron lasers enables first exploration of non-linear x-ray processes.

physics.atom-ph↗

arXiv2008

An x-ray probe of laser-aligned molecules

We demonstrate a hard x-ray probe of laser-aligned small molecules. To align small molecules with optical lasers, high intensities at nonresonant wavelengths are necessary. We use 95 ps pulses focused to 40 mum from an 800 nm Ti:sapphire laser at a peak intensity of 10^12 W/cm^2 to create an ensemble of aligned bromotrifluoromethane (CF3Br) molecules. Linearly polarized, 120 ps x-ray pulses, focused to 10 mum, tuned to the Br 1s --> sigma* pre-edge resonance at 13.476 keV, probe the ensemble of laser-aligned molecules. The demonstrated methodology has a variety of applications and can enable ultrafast imaging of laser-controlled molecular motions with Angstrom-level resolution.

physics.chem-ph↗

arXiv2008

Theory of x-ray absorption by laser-aligned symmetric-top molecules

We devise a theory of x-ray absorption by symmetric-top molecules which are aligned by an intense optical laser. Initially, the density matrix of the system is composed of the electronic ground state of the molecules and a thermal ensemble of rigid-rotor eigenstates. We formulate equations of motion of the two-color (laser plus x rays) rotational-electronic problem. The interaction with the laser is assumed to be nonresonant; it is described by an electric dipole polarizability tensor. X-ray absorption is approximated as a one-photon process. It is shown that the equations can be separated such that the interaction with the laser can be treated independently of the x rays. The laser-only density matrix is propagated numerically. After each time step, the x-ray absorption is calculated. We apply our theory to study adiabatic alignment of bromine molecules (Br2). The required dynamic polarizabilities are determined using the ab initio linear response methods coupled-cluster singles (CCS), second-order approximate coupled-cluster singles and doubles (CC2), and coupled-cluster singles and doubles (CCSD). For the description of x-ray absorption on the sigma_g 1s --> sigma_u 4p resonance, a parameter-free two-level model is used for the electronic structure of the molecules. Our theory opens up novel perspectives for the quantum control of x-ray radiation.

physics.chem-ph↗

arXiv2007

Strong-field control of x-ray absorption

Strong optical laser fields modify the way x rays interact with matter. This allows us to use x rays to gain deeper insight into strong-field processes. Alternatively, optical lasers may be utilized to control the propagation of x rays through a medium. Gas-phase systems are particularly suitable for illustrating the basic principles underlying combined x-ray and laser interactions. Topics addressed include the impact of spin-orbit interaction on the alignment of atomic ions produced in a strong laser field, electromagnetically induced transparency in the x-ray regime, and laser-induced alignment of molecules.

physics.atom-ph↗

arXiv2007

Electromagnetically induced transparency for x rays

Electromagnetically induced transparency (EIT) is predicted for x rays in laser-dressed neon gas. The x-ray photoabsorption cross section and polarizability near the Ne K edge are calculated using an ab initio theory suitable for optical strong-field problems. The laser wavelength is tuned close to the transition between 1s^-1 3s and 1s^-1 3p (approximately 800nm). The minimum laser intensity required to observe EIT is of the order of 10^12 W/cm^2. The ab initio results are discussed in terms of an exactly solvable three-level model. This work opens new opportunities for research with ultrafast x-ray sources.

physics.atom-ph↗

arXiv2007

Quantum state-resolved probing of strong-field-ionized xenon atoms using femtosecond high-order harmonic transient absorption spectroscopy

Femtosecond high-order harmonic transient absorption spectroscopy is used to resolve the complete |j,m> quantum state distribution of Xe+ produced by optical strong-field ionization of Xe atoms at 800nm. Probing at the Xe N_4/5 edge yields a population distribution rho_j,|m| of rho_3/2,1/2 : rho_1/2,1/2 : rho_3/2,3/2 = 75 +- 6 : 12 +- 3 : 13 +- 6 %. The result is compared to a tunnel ionization calculation with the inclusion of spin-orbit coupling, revealing nonadiabatic ionization behavior. The sub-50-fs time resolution paves the way for table-top extreme ultraviolet absorption probing of ultrafast dynamics.

physics.atom-ph↗

arXiv2007

Theory of x-ray absorption by laser-dressed atoms

An ab initio theory is devised for the x-ray photoabsorption cross section of atoms in the field of a moderately intense optical laser (800nm, 10^13 W/cm^2). The laser dresses the core-excited atomic states, which introduces a dependence of the cross section on the angle between the polarization vectors of the two linearly polarized radiation sources. We use the Hartree-Fock-Slater approximation to describe the atomic many-particle problem in conjunction with a nonrelativistic quantum-electrodynamic approach to treat the photon-electron interaction. The continuum wave functions of ejected electrons are treated with a complex absorbing potential that is derived from smooth exterior complex scaling. The solution to the two-color (x-ray plus laser) problem is discussed in terms of a direct diagonalization of the complex symmetric matrix representation of the Hamiltonian. Alternative treatments with time-independent and time-dependent non-Hermitian perturbation theories are presented that exploit the weak interaction strength between x rays and atoms. We apply the theory to study the photoabsorption cross section of krypton atoms near the K edge. A pronounced modification of the cross section is found in the presence of the optical laser.

physics.atom-ph↗

arXiv2007

A priori Wannier functions from modified Hartree-Fock and Kohn-Sham equations

The Hartree-Fock equations are modified to directly yield Wannier functions following a proposal of Shukla et al. [Chem. Phys. Lett. 262, 213-218 (1996)]. This approach circumvents the a posteriori application of the Wannier transformation to Bloch functions. I give a novel and rigorous derivation of the relevant equations by introducing an orthogonalizing potential to ensure the orthogonality among the resulting functions. The properties of these, so-called a priori Wannier functions, are analyzed and the relation of the modified Hartree-Fock equations to the conventional, Bloch-function-based equations is elucidated. It is pointed out that the modified equations offer a different route to maximally localized Wannier functions. Their computational solution is found to involve an effort that is comparable to the effort for the solution of the conventional equations. Above all, I show how a priori Wannier functions can be obtained by a modification of the Kohn-Sham equations of density-functional theory.

cond-mat.other↗

arXiv2006

Quasiparticle band structure of infinite hydrogen fluoride and hydrogen chloride chains

We study the quasiparticle band structure of isolated, infinite HF and HCl bent (zigzag) chains and examine the effect of the crystal field on the energy levels of the constituent monomers. The chains are one of the simplest but realistic models of the corresponding three-dimensional crystalline solids. To describe the isolated monomers and the chains, we set out from the Hartree-Fock approximation, harnessing the advanced Green's function methods "local molecular orbital algebraic diagrammatic construction" (ADC) scheme and "local crystal orbital ADC" (CO-ADC) in a strict second order approximation, ADC(2,2) and CO-ADC(2,2), respectively, to account for electron correlations. The configuration space of the periodic correlation calculations is found to converge rapidly only requiring nearest-neighbor contributions to be regarded. Although electron correlations cause a pronounced shift of the quasiparticle band structure of the chains with respect to the Hartree-Fock result, the bandwidth essentially remains unaltered in contrast to, e.g., covalently bound compounds.

cond-mat.other↗

arXiv2006

Hydrogen bonding in infinite hydrogen fluoride and hydrogen chloride chains

Hydrogen bonding in infinite HF and HCl bent (zigzag) chains is studied using the ab initio coupled-cluster singles and doubles (CCSD) correlation method. The correlation contribution to the binding energy is decomposed in terms of nonadditive many-body interactions between the monomers in the chains, the so-called energy increments. Van der Waals constants for the two-body dispersion interaction between distant monomers in the infinite chains are extracted from this decomposition. They allow a partitioning of the correlation contribution to the binding energy into short- and long-range terms. This finding affords a significant reduction in the computational effort of ab initio calculations for solids as only the short-range part requires a sophisticated treatment whereas the long-range part can be summed immediately to infinite distances.

cond-mat.mtrl-sci↗

arXiv2005

Basis set convergence in extended systems: infinite hydrogen fluoride and hydrogen chloride chains

Basis set convergence of the Hartree-Fock and the correlation energy is examined for the hydrogen bonded infinite bent chains (HF)_infinity and (HCl)_infinity. We employ series of correlation consistent basis sets up to quintuple zeta quality together with a coupled cluster method (CCSD) to describe electron correlation on ab initio level. The Hartree-Fock energy converges rapidly with increasing basis set quality whereas the correlation energy is found to be slowly convergent for the same series of basis sets. We study basis set extrapolation for (HF)_infinity and (HCl)_infinity and show that it substantially enhances the accuracy of both the Hartree-Fock and the correlation energy in extended systems.

cond-mat.other↗

arXiv2005

Ab initio Green's function formalism for band structures

Using the Green's function formalism, an ab initio theory for band structures of crystals is derived starting from the Hartree-Fock approximation. It is based on the algebraic diagrammatic construction scheme for the self-energy which is formulated for crystal orbitals (CO-ADC). In this approach, the poles of the Green's function are determined by solving a suitable Hermitian eigenvalue problem. The method is not only applicable to the outer valence and conduction bands, it is also stable for inner valence bands where strong electron correlations are effective. The key to the proposed scheme is to evaluate the self-energy in terms of Wannier orbitals before transforming it to a crystal momentum representation. Exploiting the fact that electron correlations are mainly local, one can truncate the lattice summations by an appropriate configuration selection scheme. This yields a flat configuration space; i.e., its size scales only linearly with the number of atoms per unit cell for large systems and, under certain conditions, the computational effort to determine band structures also scales linearly. As a first application of the new formalism, a lithium fluoride crystal has been chosen. A minimal basis set description is studied, and a satisfactory agreement with previous theoretical and experimental results for the fundamental band gap and the width of the F 2p valence band complex is obtained.

cond-mat.other↗

arXiv2004

Impact of Interatomic Electronic Decay Processes on Xe 4d Hole Decay in the Xenon Fluorides

A hole in a 4d orbital of atomic xenon relaxes through Auger decay after a lifetime of 3 fs. Adding electronegative fluorine ligands to form xenon fluoride molecules, results in withdrawal of valence-electron density from Xe. Thus, within the one-center picture of Auger decay, a lowered Xe 4d Auger width would be expected, in contradiction, however, with experiment. Employing extensive ab initio calculations within the framework of many-body Green's functions, we determine all available decay channels in XeFn and characterize these channels by means of a two-hole population analysis. We derive a relation between two-hole population numbers and partial Auger widths. On this basis, interatomic electronic decay processes are demonstrated to be so strong in the xenon fluorides that they overcompensate the reduction in intra-atomic Auger width and lead to the experimentally observed trend. The nature of the relevant processes is discussed. These processes presumably underlie Auger decay in a variety of systems.

physics.chem-ph↗