Search

We report a theoretical study of the angular distribution of core-level photoemission time delay in a molecular frame, which is converting into possible measure with the development of attosecond pulsed lasers and metrology, focusing on homonuclear diatomic molecules. Two-center interference patterns in gerade and ungerade core-level molecular-frame photoelectron angular distributions (MFPADs) of homonuclear diatomic molecules exhibit symmetry-broken with respect to the direction of light propagation due to the non-dipole (multipole) effect. In this study, we investigate the photoemission time delay due to the non-dipole effect by introducing a theoretical model. We show that in incoherent sums, considering the average of the contributions from the gerade and ungerade delocalized core states, two-center interference terms cancel each other out in both of the MFPADs and photoemission time delays; however, there remains a residual term exhibiting the non-dipole effect for the photoemission time delays. By expanding the scattering state of photoelectrons with Multiple Scattering theory, we also demonstrate that the scattering effect at the molecular potential plays a crucial role in the descriptions of the photoemission time delays of homonuclear diatomic molecules. Furthermore, we demonstrated that the direct wave contribution does not show angular dependence in the photoemission time delays. We then apply the theoretical model to a nitrogen molecule to show the energy- and angular dependence of the MFPADs and photoemission time delays both analytically and numerically. While the incoherent sums of the MFPADs in a given and opposite direction have the same intensity, the incoherent sums of the photoemission time delays depicts a few hundred zeptosecond difference from numerical calculations by multiple scattering code.

We present a new variation of Young's double-slit formula for polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) of hetero-diatomic molecules, which may be used to extract the bond length. So far, empirical analysis of the PA-MFPADs has often been carried out employing Young's formula in which each of the two atomic centers emits a $s$-photoelectron wave. The PA-MFPADs, on the other hand, can consist of an interference between the $p$-wave from the X-ray absorbing atom emitted along the molecular axis and the $s$-wave scattered by neighboring atom, within the framework of Multiple Scattering theory. The difference of this $p$-$s$ wave interference from the commonly used $s$-$s$ wave interference causes a dramatic change in the interference pattern, especially near the angles perpendicular to the molecular axis. This change involves an additional fringe, urging us to caution when using the conventional Young's formula for retrieving the bond length. We have derived a new formula analogous to Young's formula but for the $p$-$s$ wave interference. The bond lengths retrieved from the PA-MFPADs via the new formula reproduce the original C-O bond lengths used in the reference $ab$-$initio$ PA-MFPADs within the relative error of 5 %. In the high energy regime, this new formula for $p$-$s$ wave interference converges to the ordinary Young's formula for the $s$-$s$ wave interference. We expect it to be used to retrieve the bond length for time-resolved PA-MFPADs instead of the conventional Young's formula., Comment: 12 pages, 9 figures

We present an experimental and theoretical study of core-level ionization of small hetero- and homo-nuclear molecules employing circularly polarized light and address molecular-frame photoelectron angular distributions in the light's polarization plane (CP-MFPADs). We find that the main forward-scattering peaks of CP-MFPADs are slightly tilted with respect to the molecular axis. We show that this tilt angle can be directly connected to the molecular bond length by a simple, universal formula. The extraction of the bond length becomes more accurate as the photoelectron energy is increased. We apply the derived formula to several examples of CP-MFPADs of C 1s and O 1s photoelectrons of CO, which have been measured experimentally or obtained by means of ab initio modeling. The photoelectron kinetic energies range from 70 to 1000~eV and the extracted bond lengths agree well with the known bond length of the CO molecule in its ground state. In addition, we discuss the influence of the back-scattering contribution that is superimposed over the analyzed forward-scattering peak in case of homo-nuclear diatomic molecules as N$_2$.

Recent developments of high-reputation-rate X-ray free electron lasers (XFELs) such as European XFEL and LSCS-II, combined with coincidence measurements at the COLTRIMS-Reaction Microscope, is now opening a door to realize a long-standing dream to create molecular movies of photo-induced chemical reactions of gas-phase molecules. In this paper, we theoretically propose a new method to experimentally visualize dissociation of diatomic molecules via time-resolved polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) measurements using the COLTRIMs--Reaction Microscope and two-color XFEL pump-probe set-up. The first and second order scattering theories with the Muffin-tin approximation give us simple EXAFS type formula for the forward and backward scattering peaks in the PA-MFPADs structure. This formula acts as an experimentally applicable "bond length ruler" by adjusting only three semi-empirical parameters from the time-resolved measurements. The accuracy and applicability of a new ruler equation are numerically examined against the PA-MFPADs of CO2+ calculated by Full-potential multiple scattering theory as a function of the C-O bond length reported in the preceding work. The bond lengths retrieved from the PA-MFPADs via the EXAFS formula well reproduce the original C-O bond lengths used in the reference ab-initio PA-MFPADs with accuracy of 0.1 {\AA}. We expect that time-resolved PA-MFPADs will be a new attractive tool to make molecular movies visualizing intramolecular reactions.

We present a theoretical study on polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) emitted from $1s$ orbital of oxygen atom of dissociating dicationic carbon monoxide CO$^{2+}$. Due to the polarization-average, contribution of direct wave of photoelectron which has the biggest contribution to MFPADs is removed, so that PA-MFPADs clearly show the detail of scattering image of the photoelectron. As a result, it is necessary to employ well precise theory for the continuum state for the theoretical analysis. In this study, we applied our Full-potential multiple scattering theory, where the space is partitioned by using Voronoi polyhedra and truncated spheres to take into account the electron charge density outside the physical atomic spheres. We did not use spherical harmonic expansion of the cell shape functions to avoid convergence problems.The potentials in scattering cells are prepared employing Multiconfigurational Second-Order Perturbation Theory Restricted Active Space (RASPT2) method in order to take into account the influence of core hole in the electron charge density in the final state to realize realistic relaxation. We showed that the Full-potential treatment plays an important role for the PA-MFPADs at 100 eV of kinetic energy of photoelectron. Instead, the PA-MFPADs are not sensitive to type of major excited state in the Auger final state.We also studied the dynamics of CO$^{2+}$ dissociation. We found that the PA-MFPADs dramatically change its shape as a function of C-O bond length.

The stretched exponential function, $\exp[-(t/\tau_{K})^{\beta}]$, describes various relaxation processes while it has been suggested that the power exponent, $\beta$ is derived from the non-uniformity of the process. In this paper, we attempted to estimate this non-uniformity by introducing Shannon entropy. Shannon entropy evaluates the average information contents of the distribution function, which reflects statistical homogeneity. We investigated the relaxation process of shear induced multilamellar system, which is described with the stretched exponential function. Three types of shear (constant, square, sine) at different frequencies are attempted in order to determine their effects on the relaxation process. We found that the Shannon entropy to which the first moment was introduced is maximized at $\beta~=~1$ : a single exponential. The Shannon entropy of sine shear experiments exhibited the frequency dependence. Thus it is interpreted that the increase of the intensity of shearing and the thermodynamic entropy are reflected on the Shannon entropy. We discussed the meaning of the maximum Shannon entropy in terms of various points of view, it was found that it corresponds to the diffusion process free from the restriction such as geometrical constraints. The constraints and the non-uniformity of process were successfully estimated by Shannon entropy. This study gives a new insight of entropy in general.

We present a numerically efficient and accurate Multiple Scattering formalism, which is a generalization of the Multiple Scattering method with a truncated basis set [X. -G. Zhang and W. H. Butler, Phys. Rev. B 46,7433 (1992)]. Compared to the latter method, we keep the phase shifts of high angular momenta but apply approximations in the elements of the scattering matrix which is the subtraction of the unit matrix and the product of transition operator matrix and structure constant matrix. The detailed behaviour of our formalism for different types of calculations, where not full information of Green's function is needed, are discussed. We apply our formalism to study density of states of fcc Cu and silicon and C K-edge X-ray absorption spectra of graphene, in order to check the efficiency and accuracy of our formalism. We find that compared to Zhang's method, the accuracy is greatly improved by our method., Comment: 14 pages, 7 figures

The occurrence of scissors modes in crystals that have deformed ions in their cells has been predicted some time ago. The theoretical value of their energy is rather uncertain, however, ranging between 10 and a few tenths of eV, with the corresponding widths of 10^-7, 10^-6 eV. Their observation by resonance fluorescence experiments therefore requires a photon spectrometer covering a wide energy range with a very high resolving power. We propose and discuss a new experiment in which such difficulties are overcome by measuring with a SQUID the variation of the magnetic field associated with the excitation of scissors modes., Comment: 8 pages, 2 figures

We argue that for some species of magnetic nanoparticles the macrospin can have a nonvanishing moment of inertia and then an orbital angular momentum. We represent such nanoparticles by two interacting rigid rotors one of which has a large spin attached to the body, namely by a Two Rotor Model with spin. By this model we can describe in a unified way the cases of nanoparticles free and stuck in an elastic or rigid matrix. We evaluate the magnetic susceptibilities for the latter case and under some realistic assumptions we get results in closed form. A crossover between thermal and purely quantum hopping occurs at a temperature much larger than that at which purely quantum tunneling becomes important. A comparison with some experimental data is outlined., Comment: 5 pages, 2 figures

The Two-Rotor Model predicts two levels above the Scissors Modes with degenerate intrinsic energy. They have $J^{\pi}= 0^+,2^+$ and are referred to as overtones. Their energy is below threshold for nucleon emission, which should make them observable. The $J^{\pi}=0^+$ overtone, that has the structure of an isovector breathing mode, has vanishing $E0$ amplitude so that cannot be directly excited, but it could be reached in the decay of the $J=2^+$ overtone. We discuss such a process and evaluate the $B(E2)$ strength, which, however, turns out to be very small., Comment: 4 pages, no figure

Scissors modes were predicted in the framework of the Two-Rotor Model. This model has an intrinsic harmonic spectrum, so that the level above the Scissors Mode, the first overtone, has excitation energy twice that of the Scissors Mode. Since the latter is of the order of 3 MeV in the rare earth region, the energy of the overtone is below threshold for nucleon emission, and its width should remain small enough for the overtone to be observable. We find that $B(E2)\uparrow_{overtone} = {3 /over 64 \theta_0^{2}}B(E2)\uparrow_{scissors}$, where $\theta_0$ is the zero-point oscillation amplitude, which in the rare earth region is of order $ 10^{-1}$., Comment: 5 pages, Rapid Communications

A recent experiment has questioned the standard relative value of spin-orbit and crystal-field strengths in rare-earth $4f$ electron systems, according to which the first should be one order of magnitude larger that the second. We find it difficult to reconcile the standard values of crystal field strength with the Single Ion Model of magnetic anisotropy. If in rare-earth systems the spin-orbit force is much larger than the crystal field, however, spin and orbit of $4f$ electrons should be locked to each other. For rare earths with non-vanishing spin, an applied magnetic field should rotate both spin and charge density profile. We suggest experiments to investigate the possible occurrence of such Spin-Orbit Locking, thus making a test of the standard picture, by studying the Scissors Modes in such systems., Comment: 8 pages, 3 figures

We recently suggested that the Scissors Mode (a collective excitation in which one system rotates with respect another one conserving its shape) can occur in crystals with axially symmetric atoms as a precession of these atoms around the anisotropy axis of their cells, giving rise to a form of dichroism. In the present paper we investigate how the Scissors Mode can be realized in crystals with cubic symmetry and evaluate its photo-absorption cross-section. This turns out to be of the same order of magnitude as that for crystals with axially symmetric atoms, but does not exhibit any correlation between the direction of the photon and the axes of the cell., Comment: 4 pages, 2 figures

In a recent study of the magnetic properties of rare-earth systems the two extreme situations have been considered in which the crystalline electrostatic field is large or small with respect to the spin-orbit interaction. In the first case the orbitals of localized electrons are firmly coupled to the lattice so that while an applied magnetic field rotates the spin, the charge profile remains fixed to the lattice. In the second case an applied magnetic field rotates both spin and density profile, even though through a very small angle. We show that in this second case we have, in addition to the use of photons or electrons, the possibility of exciting and observing the Scissors Mode in crystals in a way analogous to that used in Bose-Einstein Condensates., Comment: This paper has been withdrawn by the authors. It is replaced by arXiv:1004.2220

We present a rigorous derivation of a real space Full-Potential Multiple-Scattering-Theory (FP-MST), valid both for continuum and bound states, that is free from the drawbacks that up to now have impaired its development, in particular the need to use cell shape functions and rectangular matrices. In this connection we give a new scheme to generate local basis functions for the truncated potential cells that is simple, fast, efficient, valid for any shape of the cell and reduces to the minimum the number of spherical harmonics in the expansion of the scattering wave function. The method also avoids the need for saturating 'internal sums' due to the re-expansion of the spherical Hankel functions around another point in space (usually another cell center). Thus this approach, provides a straightforward extension of MST in the Muffin-Tin (MT) approximation, with only one truncation parameter given by the classical relation $l_{\rm max} = kR_b$, where $k$ is the excited (or ground state) electron wave vector and $R_b$ the radius of the bounding sphere of the scattering cell. It is shown that the theory converges absolutely in the $l_{\rm max} \to \infty$ limit. As a consequence it provides a firm ground to the use of FP-MST as a viable method for electronic structure calculations and makes possible the computation of x-ray spectroscopies, notably photo-electron diffraction, absorption and anomalous scattering among others, with the ease and versatility of the corresponding MT theory. Some numerical applications of the theory are presented, both for continuum and bound states., Comment: 48 pages, 6 figures, 2 tables

This work reports a theoretical study of the x-ray absorption near-edge structure spectra at both the Cu and the Mn K-edge in several Cu$_2$MnM (M= Al, Sn and In) Heusler alloys. Our results show that {\it ab-initio} single-channel multiple-scattering calculations are able of reproducing the experimental spectra. Moreover, an extensive discussion is presented concerning the role of the final state potential needed to reproduce the experimental data of these half-metallic alloys. In particular, the effects of the cluster-size and of the exchange and correlation potential needed in reproducing all the experimental XANES features are discussed., Comment: 15 pages, 5 figures

We present a Full Potential Multiple Scattering (FP-MS) scheme for the interpretation of several X-ray spectroscopies that is a straightforward generalization of the more conventional Muffin-Tin (MT) version. Like this latter, it preserves the intuitive description of the physical process under consideration and overcomes some of the limitations of the existing FP-MS codes. It hinges on a fast and efficient method for solving the single cell scattering problem that avoids the convergence drawbacks of the angular momentum (AM) expansion of the cell shape function and relies on an alternative derivation of the multiple scattering equations (MSE) that allows us to work reliably with only one truncation parameter, {\it i.e.} the number of local basis functions in the expansion of the global scattering function determined by the classical relation $l_{\rm max} \sim k R$., Comment: 4 pages 2 figures

We suggest that in an anisotropic crystal there should be a new mechanism of dichroism related to a scissors mode, a kind of excitation observed in several other many-body systems. Such an effect should be found in crystals, amorphous systems and also metallo-proteins. Its signature is a strong magnetic dipole transition amplitude, which is a function of the angle between the momentum of the photon and the anisotropy axis of the cell., Comment: 5 pages, 2 figures