arXiv subjects

Robin Santra

Publications and source records attributed to Robin Santra.

At least 91 records · Page 5Linked to original sources

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↗

arXiv2006

Interaction of intense vuv radiation with large xenon clusters

The interaction of atomic clusters with short, intense pulses of laser light to form extremely hot, dense plasmas has attracted extensive experimental and theoretical interest. The high density of atoms within the cluster greatly enhances the atom--laser interaction, while the finite size of the cluster prevents energy from escaping the interaction region. Recent technological advances have allowed experiments to probe the laser--cluster interaction at very high photon energies, with interactions much stronger than suggested by theories for lower photon energies. We present a model of the laser--cluster interaction which uses non-perturbative R-matrix techniques to calculate inverse bremsstrahlung and photoionization cross sections for Herman-Skillman atomic potentials. We describe the evolution of the cluster under the influence of the processes of inverse bremsstrahlung heating, photoionization, collisional ionization and recombination, and expansion of the cluster. We compare charge state distribution, charge state ejection energies, and total energy absorbed with the Hamburg experiment of Wabnitz {\em et al.} [Nature {\bf 420}, 482 (2002)] and ejected electron spectra with Laarmann {\em et al.} [Phys. Rev. Lett. {\bf 95}, 063402 (2005)].

physics.atom-ph↗

arXiv2004

A high-accuracy optical clock via three-level coherence in neutral bosonic $^{88}$Sr

An optical atomic clock scheme is developed that utilizes two lasers to establish coherent coupling between the $5s^2 \phantom{}^1S_0$ ground state of $^{88}$Sr and the first excited state, $5s5p \phantom{}^3P_0$. The coupling is mediated by the broad $5s5p \phantom{}^1P_1$ state, exploiting the phenomenon of electromagnetically induced transparency. The effective linewidth of the clock transition can be chosen at will by adjusting the laser intensity. By trapping the $^{88}$Sr atoms in an optical lattice, long interaction times with the two lasers are ensured; Doppler and recoil effects are eliminated. Based on a careful analysis of systematic errors, a clock accuracy of better than $2\times 10^{-17}$ is expected.

physics.atom-ph↗

arXiv2004

Multiphoton ionization of xenon in the VUV regime

In a recent experiment at the VUV free-electron laser facility at DESY in Hamburg, the generation of multiply charged ions in a gas of atomic xenon was observed. This paper develops a theoretical description of multiphoton ionization of xenon and its ions. The numerical results lend support to the view that the experimental observation may be interpreted in terms of the nonlinear absorption of several VUV photons. The method rests on the Hartree-Fock-Slater independent-particle model. The multiphoton physics is treated within a Floquet scheme. The continuum problem of the photoelectron is solved using a complex absorbing potential. Rate equations for the ionic populations are integrated to take into account the temporal structure of the individual VUV laser pulses. The effect of the spatial profile of the free-electron laser beam on the distribution of xenon charge states is included. An Auger-type many-electron mechanism may play a role in the VUV multiphoton ionization of xenon ions.

physics.atom-ph↗

arXiv2004

Siegert pseudostates: completeness and time evolution

Within the theory of Siegert pseudostates, it is possible to accurately calculate bound states and resonances. The energy continuum is replaced by a discrete set of states. Many questions of interest in scattering theory can be addressed within the framework of this formalism, thereby avoiding the need to treat the energy continuum. For practical calculations it is important to know whether a certain subset of Siegert pseudostates comprises a basis. This is a nontrivial issue, because of the unusual orthogonality and overcompleteness properties of Siegert pseudostates. Using analytical and numerical arguments, it is shown that the subset of bound states and outgoing Siegert pseudostates forms a basis. Time evolution in the context of Siegert pseudostates is also investigated. From the Mittag-Leffler expansion of the outgoing-wave Green's function, the time-dependent expansion of a wave packet in terms of Siegert pseudostates is derived. In this expression, all Siegert pseudostates--bound, antibound, outgoing, and incoming--are employed. Each of these evolves in time in a nonexponential fashion. Numerical tests underline the accuracy of the method.

physics.atom-ph↗

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↗

arXiv2003

Ionization of the Xenon Fluorides

The noble-gas atom xenon can bind fluorine atoms and form several stable compounds. We study the electronic structure of the xenon fluorides (XeFn, n = 2,4,6) by calculating their ionization spectra using a Green's function method, which allows to treat many-body effects at a high level. Our focus is on the valence region and on the Xe 4d core hole. We observe a sensitive dependence of the spectra on the number of fluorine ligands. Systematic line shifts are uncovered and explained. In the Xe 5s and F 2s inner-valence regimes, from XeF2 to XeF6, the usefulness of the one-particle picture of ionization is found to become progressively worse. Moreover, adding the electronegative fluorine ligands seems to enhance--not suppress--the Auger decay of a Xe 4d core hole.

physics.chem-ph↗

arXiv2003

Properties of metastable alkaline-earth-metal atoms calculated using an accurate effective core potential

The first three electronically excited states in the alkaline-earth-metal atoms magnesium, calcium, and strontium comprise the (nsnp) triplet P^o_J (J=0,1,2) fine-structure manifold. All three states are metastable and are of interest for optical atomic clocks as well as for cold-collision physics. An efficient technique--based on a physically motivated potential that models the presence of the ionic core--is employed to solve the Schroedinger equation for the two-electron valence shell. In this way, radiative lifetimes, laser-induced clock shifts, and long-range interaction parameters are calculated for metastable Mg, Ca, and Sr.

physics.atom-ph↗

arXiv2003

Xenon Clusters in Intense VUV Laser Fields

A simple model is developed that quantitatively describes intense interactions of a VUV laser pulse with a xenon cluster. We find good agreement with a recent experiment [H. Wabnitz et al., Nature Vol. 420, 482 (2002)]. In particular, the large number of VUV photons absorbed per atom--at intensities significantly below 10^16 W/cm^2--is now understood.

physics.atm-clus↗

arXiv2003

Coulombic Energy Transfer and Triple Ionization in Clusters

Using neon and its dimer as a specific example, it is shown that excited Auger decay channels that are electronically stable in the isolated monomer can relax in a cluster by electron emission. The decay mechanism, leading to the formation of a tricationic cluster, is based on an efficient energy-transfer process from the excited, dicationic monomer to a neighbor. The decay is ultrafast and expected to be relevant to numerous physical phenomena involving core holes in clusters and other forms of spatially extended atomic and molecular matter.

physics.atm-clus↗

arXiv2003

Multichannel cold collisions between metastable Sr atoms

We present a multichannel scattering calculation of elastic and inelastic cold collisions between two low-field seeking, metastable ^{88}Sr [(5s5p) ^3P_2] atoms in the presence of an external magnetic field. The scattering physics is governed by strong anisotropic long-range interactions, which lead to pronounced coupling among the partial waves of relative motion. As a result, nonadiabatic transitions are shown to trigger a high rate of inelastic losses. At relatively high energies, T > 100 mkK, the total inelastic collision rate is comparable with the elastic rate. However, at lower collisional energy, the elastic rate decreases, and at T ~ 1mkK, it becomes substantially smaller than the inelastic rate. Our study suggests that magnetic trapping and evaporative cooling of ^{88}Sr [(5s5p) ^3P_2] atoms, as well as ^{40}Ca [(4s4p) ^3P_2], in low-field seeking states will prove difficult to achieve experimentally.

physics.atom-ph↗