arXiv subjects

Christian Buth

Publications and source records attributed to Christian Buth.

At least 19 recordsLinked to original sources

arXiv2017

Nonlinearity in the sequential absorption of multiple photons

I classify multiphoton absorption into separable, linked, and simultaneous processes. The first and second types can be distinguished when the rate-equation approximation is valid whereas the third type refers to the case when the full description of multiphoton absorption is essential. For this purpose, rate equations are solved analytically without decay processes which shows that even if many photons are absorbed the interaction with the light field is linear and one has the case of separable multiphoton absorption. Next a short-pulse approximation is investigated in which I first solve the rate equations without decay processes and then solve only rate equations for the ensuing decay. Finally, the full rate equations are examined and a successive approximation of the underlying Volterra integral equation of the second kind is derived leading to linked multiphoton absorption by the involved decay widths. The three methods are applied to a nitrogen atom in intense and ultrafast x rays from free-electron lasers (FELs). The linearity theorem barely approximates the results in the presence of decay processes which is also not satisfactorily corrected for by the short-pulse approximation. The successive approximation gives excellent agreement with the numerically-exact solution of the rate equations.

physics.atom-ph↗

arXiv2017

Dissociating diatomic molecules in ultrafast and intense light

An $\mathit{ab\ initio}$ theory is devised for the quantum dynamics of molecules undergoing (multiple) ionization in ultrafast and intense light. Specifically, the intertwined problem of photoionization, radiative, and electronic transitions in the course of dissociation is addressed which arises, e.g., when molecules are exposed to XUV light or x rays from free electron lasers or attosecond light sources, but the approach is equally useful in optical strong-field physics. The coherent interaction of the molecule with the light in a specific charge state is also treated. I set out from an abstract formulation in terms of the quantum optical notion of system-reservoir interaction using a master equation in Lindblad form and analyze its short-time approximation. First, I express it in a direct sum rigged Hilbert space for an efficient solution with numerical methods for systems of differential equations. Second, I derive a treatment via quantum Monte Carlo wave packet (MCWP) propagation. The formalism is concretized to diatomic molecules in Born-Oppenheimer approximation whereby molecular rotation is disregarded. The numerical integration of the master equation is carried out with a suitably factored density matrix that exploits the locality of the Hamiltonian and the Lindblad superoperator with respect to the internuclear distance. The formulation of the MCWP for molecules requires a thorough analysis of the quantum jump process; namely, the dependence on the continuous distance renders a straight wave packet promotion useless and, instead, a projected outer product needs to be employed involving an integrated quantum jump operator.

physics.chem-ph↗

arXiv2017

LCLS in - photon out: fluorescence measurement of neon using soft x-rays

We measured the fluorescence photon yield of neon upon soft x-ray ionization (~1200 eV) from the x-ray free electron laser at Linac Coherent Light Source, and demonstrated the usage of a grazing incidence spectrometer with a variable linespacing grating to perform x-ray fluorescence spectroscopy on a gas phase system. Our measurements also allowed us to estimate the focal size of the beam from the theoretical description developed, in terms of the rate equation approximation accounting for photoionization shake-off of neutral neon and double Auger decay of single core holes.

physics.atom-ph↗

arXiv2017

Neon in ultrashort and intense x rays from free electron lasers

We theoretically examine neon atoms in ultrashort and intense x rays from free electron lasers and compare our results with data from experiments conducted at the Linac Coherent Light Source (LCLS). For this purpose, we treat in detail the electronic structure in all possible nonrelativistic cationic configurations using a relativistic multiconfiguration approach. The interaction with the x rays is described in rate-equation approximation. To understand the mechanisms of the interaction, a path analysis is devised which allows us to investigate what sequences of photoionization and decay processes lead to a specific configuration and with what probability. Thereby, we uncover a connection to the mathematics of graph theory and formal languages. In detail, we study the ion yields and find that plain rate equations do not provide a satisfactory description. We need to extend the rate equations for neon to incorporate double Auger decay of a $K$-shell vacancy and photoionization shake off for neutral neon. Shake off is included for valence and core ionization; the former has hitherto been overlooked but has important consequences for the ion yields from an x-ray energy below the core ionization threshold. Furthermore, we predict the photon yields from XUV and x-ray fluorescence; these allow one insights into the configurations populated by the interaction with the x rays. Finally, we discover that inaccuracies in those Auger decay widths employed in previous studies have only a minor influence on ion and photon yields.

physics.atom-ph↗

arXiv2017

Rate equations for nitrogen molecules in ultrashort and intense x-ray pulses

We study theoretically the quantum dynamics of nitrogen molecules (N$_2$) exposed to intense and ultrafast x-rays at a wavelength of 1.1 nm (1100 eV photon energy) from the Linac Coherent Light Source (LCLS) free electron laser. Molecular rate equations are derived to describe the intertwined photoionization, decay, and dissociation processes occurring for N$_2$. This model complements our earlier phenomenological approaches, the single-atom, symmetric-sharing, and fragmentation-matrix models of J. Chem. Phys. $\mathbf{136}$, 214310 (2012). Our rate-equations are used to obtain the effective pulse energy at the sample and the time scale for the dissociation of the metastable dication N$_2^{2+}$. This leads to a very good agreement between the theoretically and experimentally obtained ion yields and, consequently, the average charge states. The effective pulse energy is found to decrease with shortening pulse duration. This variation together with a change in the molecular fragmentation pattern and frustrated absorption---an effect that reduces absorption of x-rays due to (double) core hole formation---are the causes for the drop of the average charge state with shortening LCLS pulse duration discovered previously.

physics.chem-ph↗

arXiv2015

High-order harmonic generation with resonant core excitation by ultraintense x rays

High-order harmonic generation (HHG) is combined with resonant x-ray excitation of a core electron into the transient valence vacancy that is created in the course of the HHG process. To describe this setting, I develop a two-active-electron quantum theory for a single atom assuming no Coulomb interaction among the electrons; one electron performs a typical HHG three-step process whereas another electron is excited (or even Rabi flops) by intense x rays from the core shell into the valence hole after the first electron has left the atom. Depending on the amplitude to find a vacancy in the valence and the core, the returning continuum electron recombines with the valence and the core, respectively, emitting high-order harmonic (HH) radiation that is characteristic of the combined process. After presenting the theory of x-ray boosted HHG for continuous-wave light fields, I develop a description for x-ray pulses with a time-varying amplitude and phase. My prediction offers novel prospects for nonlinear x-ray physics, attosecond x rays, and HHG-based time-dependent chemical imaging involving core orbitals.

physics.atom-ph↗

arXiv2015

Ramsey method for Auger-electron interference induced by an attosecond twin pulse

We examine the archetype of an interference experiment for Auger electrons: two electron wave packets are launched by inner-shell ionizing a krypton atom using two attosecond light pulses with a variable time delay. This setting is an attosecond realization of the Ramsey method of separated oscillatory fields. Interference of the two ejected Auger-electron wave packets is predicted, indicating that the coherence between the two pulses is passed to the Auger electrons. For the detection of the interference pattern an accurate coincidence measurement of photo- and Auger electrons is necessary. The method allows one to control inner-shell electron dynamics on an attosecond timescale and represents a sensitive indicator for decoherence.

physics.atom-ph↗

arXiv2014

Attosecond pulses at kiloelectronvolt photon energies from high-order harmonic generation with core electrons

High-order harmonic generation (HHG) in simultaneous intense near-infrared (NIR) laser light and brilliant x rays above an inner-shell absorption edge is examined. A tightly bound inner-shell electron is transferred into the continuum. Then, NIR light takes over and drives the liberated electron through the continuum until it eventually returns to the cation leading in some cases to recombination and emission of a high-order harmonic (HH) photon that is upshifted by the x-ray photon energy. We develop a theory of this scenario and apply it to $1s$ electrons of neon atoms. The boosted HH light is used to generate a single attosecond pulse in the kiloelectronvolt regime. Prospects for nonlinear x-ray physics and HHG-based spectroscopy involving core orbitals are discussed.

physics.atom-ph↗

arXiv2014

Optical control of an atomic inner-shell x-ray laser

X-ray free-electron lasers have had an enormous impact on x-ray science by achieving femtosecond pulses with unprecedented intensities. However, present-day facilities operating by the self-amplified spontaneous emission (SASE) principle have a number of shortcomings, namely, their radiation has a chaotic pulse profile and short coherence times. We put forward a scheme for a neon-based atomic inner-shell x-ray laser (XRL) which produces temporally and spatially coherent subfemtosecond pulses that are controlled by and synchronized to an optical laser with femtosecond precision. We envision that such an XRL will allow for numerous applications such as nuclear quantum optics and the study of ultrafast quantum dynamics of atoms, molecules, and condensed matter.

physics.atom-ph↗

arXiv2014

Broadband high-resolution x-ray frequency combs

Optical frequency combs have had a remarkable impact on precision spectroscopy. Enabling this technology in the x-ray domain is expected to result in wide-ranging applications, such as stringent tests of astrophysical models and quantum electrodynamics, a more sensitive search for the variability of fundamental constants, and precision studies of nuclear structure. Ultraprecise x-ray atomic clocks may also be envisaged. In this work, an x-ray pulse-shaping method is put forward to generate a comb in the absorption spectrum of an ultrashort high-frequency pulse. The method employs an optical-frequency-comb laser, manipulating the system's dipole response to imprint a comb on an excited transition with a high photon energy. The described scheme provides higher comb frequencies and requires lower optical-comb peak intensities than currently explored methods, preserves the overall width of the optical comb, and may be implemented by presently available x-ray technology.

physics.atom-ph↗

arXiv2013

X-ray frequency combs from optically controlled resonance fluorescence

An x-ray pulse-shaping scheme is put forward for imprinting an optical frequency comb onto the radiation emitted on a driven x-ray transition, thus producing an x-ray frequency comb. A four-level system is used to describe the level structure of N ions driven by narrow-bandwidth x rays, an optical auxiliary laser, and an optical frequency comb. By including many-particle enhancement of the emitted resonance fluorescence, a spectrum is predicted consisting of equally spaced narrow lines which are centered on an x-ray transition energy and separated by the same tooth spacing as the driving optical frequency comb. Given a known x-ray reference frequency, our comb could be employed to determine an unknown x-ray frequency. While relying on the quality of the light fields used to drive the ensemble of ions, the model has validity at energies from the 100 eV to the keV range.

physics.atom-ph↗

arXiv2013

Optical control of resonant Auger processes

We theoretically show that core-excited state populations can be efficiently manipulated with strong optical fields during their decay, which takes place in a few femtoseconds. We focus on the $1s^{-1}3p$ resonant excitation in neon, where the $1s^{-1}3p$ and $1s^{-1}3s$ core-excited states are coupled by an optical field. By analyzing the Auger electron spectrum we observe the inner-shell population transfer induced by the optical coupling. We also show that the angular anisotropy of the Auger electron is imprinted in the multipeak structure induced by the optical-dressed continuum, namely sidebands.

physics.optics↗

arXiv2012

Ultrafast absorption of intense x rays by nitrogen molecules

We devise a theoretical description for the response of nitrogen molecules (N2) to ultrashort and intense x rays from the free electron laser (FEL) Linac Coherent Light Source (LCLS). We set out from a rate-equation description for the x-ray absorption by a nitrogen atom. The equations are formulated using all one-x-ray-photon absorption cross sections and the Auger and radiative decay widths of multiply-ionized nitrogen atoms. Cross sections are obtained with a one-electron theory and decay widths are determined from ab initio computations using the Dirac-Hartree-Slater (DHS) method. We also calculate all binding and transition energies of nitrogen atoms in all charge states with the DHS method as the difference of two self-consistent field calculations (Delta SCF method). To describe the interaction with N2, a detailed investigation of intense x-ray-induced ionization and molecular fragmentation are carried out. As a figure of merit, we calculate ion yields and the average charge state measured in recent experiments at the LCLS. We use a series of phenomenological models of increasing sophistication to unravel the mechanisms of the interaction of x rays with N2: a single atom, a symmetric-sharing model, and a fragmentation-matrix model are developed. The role of the formation and decay of single and double core holes, the metastable states of N_2^2+, and molecular fragmentation are explained.

physics.chem-ph↗

arXiv2011

Electron correlation and interference effects in strong-field processes

Several correlation and interference effects in strong-field physics are investigated. We show that the interference of continuum wave packets can be the dominant mechanism of high-harmonic generation (HHG) in the over-the-barrier regime. Next, we combine HHG with resonant x-ray excitation to force the recolliding continuum electron to recombine with a core hole rather than the valence hole from that it was previously tunnel ionized. The scheme opens up perspectives for nonlinear xuv physics, attosecond x-ray pulses, and spectroscopy of core orbitals. Then, a method is proposed to generate attochirp-free harmonic pulses by engineering the appropriate electron wave packet. Finally, resonant photoionization mechanisms involving two atoms are discussed which can dominate over the direct single-atom ionization channel at interatomic distances in the nanometer range.

physics.atom-ph↗

arXiv2011

Transient absorption and reshaping of ultrafast XUV light by laser-dressed helium

We present a theoretical study of transient absorption and reshaping of extreme ultraviolet (XUV) pulses by helium atoms dressed with a moderately strong infrared (IR) laser field. We formulate the atomic response using both the frequency-dependent absorption cross section and a time-frequency approach based on the time-dependent dipole induced by the light fields. The latter approach can be used in cases when an ultrafast dressing pulse induces transient effects, and/or when the atom exchanges energy with multiple frequency components of the XUV field. We first characterize the dressed atom response by calculating the frequency-dependent absorption cross section for XUV energies between 20 and 24 eV for several dressing wavelengths between 400 and 2000 nm and intensities up to 10^12 W/cm^2. We find that for dressing wavelengths near 1600 nm, there is an Autler-Townes splitting of the 1s ---> 2p transition that can potentially lead to transparency for absorption of XUV light tuned to this transition. We study the effect of this XUV transparency in a macroscopic helium gas by incorporating the time-frequency approach into a solution of the coupled Maxwell-Schrödinger equations. We find rich temporal reshaping dynamics when a 61 fs XUV pulse resonant with the 1s ---> 2p transition propagates through a helium gas dressed by an 11 fs, 1600 nm laser pulse.

physics.atom-ph↗

arXiv2010

High-order harmonic generation enhanced by XUV light

The combination of high-order harmonic generation (HHG) with resonant XUV excitation of a core electron into the transient valence vacancy that is created in the course of the HHG process is investigated theoretically. In this setup, the first electron performs a HHG three-step process whereas, the second electron Rabi flops between the core and the valence vacancy. The modified HHG spectrum due to recombination with the valence and the core is determined and analyzed for krypton on the 3d -- > 4p resonance in the ion. We assume an 800 nm laser with an intensity of about 10^14 W/cm^2 and XUV radiation from the Free Electron Laser in Hamburg (FLASH) with an intensity in the range 10^13--10^16 W/cm^2. Our prediction opens perspectives for nonlinear XUV physics, attosecond x rays, and HHG-based spectroscopy involving core orbitals.

physics.atom-ph↗

arXiv2010

Refraction and absorption of x rays by laser-dressed atoms

X-ray refraction and absorption by neon atoms under the influence of an 800 nm laser with an intensity of 10^13 W/cm^2 is investigated. For this purpose, we use an ab initio theory suitable for optical strong-field problems. Its results are interpreted in terms of a three-level model. On the Ne 1s --> 3p resonance, we find electromagnetically induced transparency (EIT) for x rays. Our work opens novel perspectives for ultrafast x-ray pulse shaping.

physics.atom-ph↗

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↗