Electronic Line Shape Research Articles

Dephasing and Decoherence in Vibrational and Electronic Line Shapes

Absorption and emission line shapes of vibrational and electronic transitions in liquids are broadened by interactions with the "bath" (in this case, the rotational and translational degrees of freedom of all the molecules in the liquid). If these degrees of freedom are treated classically, the broadening process is often known as dephasing. If, on the other hand, the bath degrees of freedom are instead treated quantum mechanically, there is additional broadening due to what is known in the chemical-physics literature as decoherence. The question addressed in this paper is the relative importance of decoherence (bath quantum effects) and dephasing. We present general developments of this subject for absorption and emission line shapes, discover several new relationships connecting classical and quantum treatments of the bath, and also consider the Stokes shift (difference in peak frequencies in absorption and emission). We next draw some general conclusions by considering a model system whose transition-frequency time-correlation function has only one bath time scale. We then consider a realistic system of the vibrational OH stretch transition of dilute HOD in liquid D2O at room temperature. For this system, we conclude that bath quantum effects are not very important, except for the Stokes shift. More generally, we argue that this is the case for many vibrational and most electronic transitions in room-temperature liquids.

Electron pair emission from a Pb surface at room temperature

We provide a comprehensive study of the electron pair emission from a Pb surface at room temperature. We excited the sample via a primary electron beam or laboratory light source. Besides the excitation of the 6s and 6p valence states the weakly bound 5d core levels are accessible. This allows us to investigate the Auger-photoelectron pairs in coincidence. The electron pair excitation spectra can be largely explained by the underlying electronic structure. Varying the primary energy changes the relative contribution of the 6s and 6p states. The measured double photoemission intensity is dominated by the emission of 5d photoelectrons and the resulting Auger electron. The Auger electron line shape has mainly contributions due the 6p electrons, because the 6s electrons can not lead to the emission of an Auger electron due to energy conservation. From the sum energy spectra we find that the effective Coulomb interaction Ueff is close to zero. The double photoemission intensity without participation of the 5d levels displays rather featureless spectra. Among the materials which display superconductivity is Pb. The explanation of this effect requires the introduction of Cooper pairs. It was theoretically predicted that double photoemission of Cooper pairs is possible. We discuss the experimental feasibility of such measurements.

Comparing classical approaches with empirical or quantum-mechanically derived force fields for the simulation electronic lineshapes: application to coumarin dyes

The simulation of optical properties in complex and heterogeneous systems, as those involved in photovoltaic devices, requires efficient protocols able to account for the complexity of the environment. An usual procedure is based on molecular dynamics (MD) sampling and subsequent computation of the transition properties, at quantum mechanics/molecular mechanics (QM/MM) level, adopting, e.g., time-dependent density functional theory for the QM part. In this work, we adopt such protocol to simulate the spectra of two structurally related coumarin dyes employed as sensitizers in solar cells, c343 and nkx-2586, and we focus on gas-phase results with the scope to investigate in detail the impact of two key aspects of the protocol: the choice of the force field (FF) and the classical approximation of the spectra. We start from the selection of a FF able to accurately describe the intramolecular interactions in the system, comparing the predictions of a generic empirical force field (namely, GAFF) and a molecule-specific quantum-mechanically derived FF. The MD simulations generated by adopting each FF are analysed in terms of the sampled normal mode distributions, providing a rationale to understand the features (position and width) of the absorption spectra simulated from the distribution of vertical energies. By comparing those simulations with classical, semiclassical and fully quantum formulations of the vibronic lineshapes based on harmonic models, we demonstrate the necessity of high-quality FFs. Notwithstanding this, our results highlight that irrespective of the choice between the two FFs, the largest errors arise from the classical description of the fast nuclear motions (e.g. bond stretchings), thus emphasizing the need to account for quantum effects in order to achieve an accurate simulation of the spectrum. We further analyse the spectra simulated using a number of harmonic models of the initial and final states at either QM or FF levels, pointing out the occurrence of remarkable inaccuracies due to the large differences in the QM and FF normal modes. This problem is inherent to the fixed functional forms associated to any MM FF, and our results indicate a convenient route to avoid it. This knowledge is a prerequisite towards the goal of developing hybrid quantum/classical QM/MM approaches for the calculations of spectra in complex environments.

Post-collision interaction in Potassium 3s photoionization

SynopsisThe post-collision interaction (PCI) distortion of the Auger electron line shapes after K 3s photoionization is revealed in coincidence measurements and confirmed by calculations.

Determination of the solid angle and response function of a hemispherical spectrograph with injection lens for Auger electrons emitted from long lived projectile states.

We present SIMION 8.1 Monte Carlo type simulations of the response function and detection solid angle for long lived Auger states (lifetime τ ∼ 10(-9) - 10(-5) s) recorded by a hemispherical spectrograph with injection lens and position sensitive detector used for high resolution Auger spectroscopy of ion beams. Also included in these simulations for the first time are kinematic effects particular to Auger emission from fast moving projectile ions such as line broadening and solid angle limitations allowing for a more accurate and realistic line shape modeling. Our results are found to be in excellent agreement with measured electron line shapes of both long lived 1s2s2p(4)P and prompt Auger projectile states formed by electron capture in collisions of 25.3 MeV F(7+) with H2 and 12.0 MeV C(4+) with Ne recorded at 0° to the beam direction. These results are important for the accurate evaluation of the 1s2s2p (4)P/(2)P ratio of K-Auger cross sections whose observed non-statistical production by electron capture into He-like ions, recently a field of interesting interpretations, awaits further resolution.

Impact of momentum dependent matrix elements on scattering effects in quantum cascade lasers

AbstractResults for our fully frequency‐ and wavevector‐dependend self‐consistent nonequilibrium Green's function simulations of electron transport through quantum cascade lasers are compared to those from a simplified approach where scattering rates are only evaluated at representative ‘typical values’ of the in‐plane momentum due to the complexity of the underlying many‐body problem and the limited computational resources available. We find a noticeable impact on electronic lineshapes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Post-collision interaction after electron impact measured by (e,2e) coincidence technique

Auger electron line-shapes after electron impact inner-shell ionization of Argon at 350eV primary electron energy have been studied by coincidence spectrometry. Emitted Auger electrons are detected in coincidence with the scattered electrons and the energy of the very slow PCI inducer ejected electron was calculated from energy conservation. The background, caused by outer-shell electrons was measured and then removed from the coincident spectrum. The effect of PCI is studied on the background-free diagram Auger spectrum. A systematic deviation was found from the line-shape given by the semi-classical approach.

Post-collision interaction measured by coincidence spectrometry in the electron impact Auger process

Auger electron lineshapes after electron impact inner-shell ionization of argon at 500 eV primary electron energy are studied by the (e, 2e) coincidence technique. Emitted Auger electrons are detected in coincidence with the ionizing scattered electrons and the energy of the slow PCI inducer ejected electron is calculated from energy conservation. In particular, the strongly asymmetric energy sharing, and by this means the effect of the very low energy (i.e. 0–5 eV) ejected electrons, is measured and compared with calculations. The opposite case (equal energy sharing) is also studied.

Nonlinear electronic line shape determination in Yb^3+-doped phosphate glass

Nonlinear refraction spectroscopy has been performed in Yb3+-doped phosphate glass to determinate the line shape of real and imaginary parts of n2 (n2' and n2"). The n2' spectrum presented an asymmetric feature due to the interference of resonant and nonresonant contributions, where the nonresonant term arises from the polarizability difference between excited and ground states (delta alpha). The measurements were performed in the transient regime to determine population dynamics and the pump saturation intensity at 975 nm (peak of the absorption spectrum). Because of the small quantum defect of Yb3+, we estimated that the magnitude of the thermal lens effect is approximately 20 times smaller than the population lens effect, caused by n2.

Comparison of the PCI distortion effects on the Auger lineshape for electron and photon impact ionization

The distortion effects of the post-collision interaction (PCI) on the Ar LMM Auger electron lineshape for electron and photon impact ionization have been calculated. The calculations were based on the eikonal model of Kuchiev and Sheinerman [Sov. Phys. – Tech. Phys. 32 (1987) 879]. It is shown that the Auger peak asymmetry depends on the emission angle of the Auger electron relative to the primary beam (and the polarization vector of the photon beam). At a given excess energy, defined as the difference between the impact energy and the binding energy, the absolute value of the Auger peak asymmetry is always larger for electron impact ionization than for photoionization. At the same time, the angular dependence of the PCI distortion is stronger for photoionization. In both cases the Auger peak asymmetry has a maximum when the energy of the ejected electron and that of the Auger electron are nearly equal. The calculations are in good agreement with our previous experimental results.

Asymmetric line shape in the emission spectra of conjugated polymer thin films: an experimental signature of one-dimensional electronic states.

The photoluminescence (PL) properties of thin films of the conjugated polymer [poly(2,5-bis(2(')-ethyl-hexyl)-1,4-phenylenevinylene] have been investigated. At low temperatures the PL spectra show a narrow peak for the electronic transition and a series of well defined vibronic sidebands, which clearly reveal the electron coupling with two different vibronic modes. The purely electronic transition peak is observed to be very asymmetric so that it cannot be adjusted by a single Lorentzian or Gaussian function. In order to understand and explain this asymmetry we have considered a model where the purely electronic transition line shape is partially generated by a broadened square-root singularity representing one-dimensional electron states, and partially by localized (zero-dimensional) states. The localized states are assumed to be those very close to the band edges and are represented in our model by a single Gaussian function. Numerical Franck-Condon analysis was performed, resulting in a very good agreement between the theoretical and the experimental emission spectra. This procedure has confirmed the one-dimensional character of the electron states as the basis for the understanding of the purely electronic line shape asymmetry in the PL spectra of conjugated polymers at low temperatures.

An accurate test of calculated positions and lifetimes for Ne6+(1s23lnl′) [formula omitted] states (n=3 and 4) using a high-resolution electron spectroscopy

An accurate test of available calculations for the autoionizing doubly excited states belonging to the Be-like 1s23lnl′ Rydberg series of neon (positions and lifetimes) is presented in this short communication. These theoretical data are used to calculate electron line shapes which are compared, through a fitting procedure, with a high-resolution electron spectrum measured in Ne8+(1s2)+He collisional system, at 80 keV collision energy and 13.1° observation angle. Present tests concern the n=3 and n=4 singlet states. It is found that some of these calculations suffer from large discrepancies with experiment and do not allow a description of the electron spectrum. A quantitative comparison of measured and calculated post-collisional Coulomb interaction-shifted line positions is also given and briefly discussed; for one theoretical data set, the agreement with experiment is found to be generally within ±50 meV; in contrast agreement with other data often considerably scatters within ±500 meV and sometimes more.

Organic Monolayers as Mimics of Liquid/Liquid Interfaces: Molecular Dynamics Study of Electronic Spectra and Solvent Dynamics

The electronic spectra and the solvation dynamics following the electronic transition of a chromophore attached to the interface between water and a self-assembled hydrocarbon monolayer are examined by molecular dynamics computer simulations and are compared with the same chromophore undergoing the same electronic transitions at the water/nonane interface. Two different self-assembled monolayers are considered. One is made of only C18H38 molecules giving rise to a smooth surface, and one is made of a 1:1 random mixture of C18H38 and C22H46 molecules giving rise to a rough surface. Different choices of the chromophore charge distribution and its location at the interface are considered and provide insight into different microscopic factors which influence the electronic line shape and the water dynamic response. We find that the electronic spectrum of the chromophore at the interface between water and a self-assembled hydrocarbon monolayer is very similar to the spectrum calculated when the same chromophor...

Experimental Positions and Lifetimes of Be-like 1s23lnl' (n = 3 to 5) States of O4+ and Ne6+ Ions Investigated by High Resolution Electron Spectroscopy: Test of Calculations

Using high resolution electron spectroscopy, positions and lifetimes of many Be-like singlet states of the 1s23lnl' Rydberg series (n = 3 to 5) of oxygen and neon have been measured for the first time. This was achieved by a fitting procedure which takes into account an accurate definition of the post-collisional electron lineshapes. These states are produced after a double electron capture by multicharged ions has occurred in O6+(1s2) + He, H2 and Ne8+(1s2) + He collisions at about 4 keV/amu collision energy.

Electronic spectra of dipolar solutes at liquid/liquid interfaces: Effect of interface structure and polarity

Molecular dynamics computer simulations are used to elucidate the role of solvent polarity and interface structure in determining the electronic absorption and fluorescence line shapes for model dipolar solutes at the interface between water and one of four different organic liquids. The different organic liquids represent a range of molecular structure and polarity: 1-octanol, 1,2-dichloroethane, n-nonane, and carbon tetrachloride. The solute is represented by two rigidly connected Lennard-Jones spheres. The different electronic states correspond to different charges on the two Lennard-Jones centers. In each interfacial system, different choices of solute charge distribution and solute location relative to the interface (including the bulk region) are considered and provide insight into different microscopic factors that influence the electronic line shape. For the water/1,2-dichloroethane and water/CCl4 interfaces, all of the calculations are repeated while the interface is externally constrained to be smooth in order to investigate the role of surface roughness. The calculated electronic line shapes are Gaussians whose peak positions reflect solvent polarity, interface structure, and probe location. Their widths are in general agreement with the prediction of linear response theory. Although continuum electrostatic models predict qualitatively correct behavior, they miss interesting interfacial effects.

Incident-photon energy-distribution effects on radiationless resonant Raman scattering.

In the experimental observation of radiationless resonant Raman scattering, features that differ from the idealized case can arise when the spectral width of the incident photon distribution is comparable to the width of pertinent atomic energy levels. These features include nonlinear dispersion of the peak maxima and changes in the symmetry and width of the ejected-electron lines with changes in the average incident photon energy. Under certain conditions, the electron line shapes can become highly asymmetric or even double peaked. The origin of these features in actual experiments is explored through a model calculation for large spectral widths. Some possible applications of these effects are pointed out.

Investigation of double capture in Ne8++He, H2, by electron spectroscopy at 80 keV. I. Theoretical considerations

Positions, autoionization and radiative probabilities of Ne8+ (1s23l3l'), Ne8+ (1s23l4l') and some Ne8+ (1s24l4l') states have been calculated with the AUTOLSJ method and compared with other recent theoretical data. These results will be compared with the authors' experimental electron spectra in a companion paper. The theoretical basis of the electron spectra analysis discussed in the companion paper is also established. Various kinds of perturbations of the electron lineshapes and angular distributions (autoionization from a Stark state, post-collision effects and interferences) are presented in a unified fashion and discussed.

Calculation of Surface‐Induced Core‐Level Shifts for Covalent Semiconductors C, Si, Ge, and α‐Sn I. (111) 2 × 1 Surfaces

AbstractA theory is developed for the surface induced core‐level shift. Since the many body relaxation mechanism can be neglected only the initial‐state potential effect is considered. Starting from tight‐binding assumptions the shift is related to ion charges accompanying the reconstruction, the hybridization state, the polarity of chemical bonds, and the atomic configuration. The atomic configurations are taken from total energy minimizations, dynamical LEED or ion scattering studies. The other input parameters are calculated self‐consistently. The developed theory is applied to the (111) 2 × 1 surface of group IV materials. Various fundamental reconstruction models — buckling, π‐bonded chains, and π‐bonded molecules — are studied. The shifts resulting for eight atoms per surface elementray cell and the associated core electron line shapes seem to favour the chain model in the (111) 2 × 1 case. The molecule model cannot be completely ruled out only from core‐level shift calculations because of its homopolar bond character.

X-ray photoelectron and Auger electron spectroscopic analysis of surface treatments and electrochemical decomposition of CuInSe2 photoelectrodes

CuInSe2 photoanodes can be optimized for use in electrochemical photovoltaic cells, containing aqueous polyiodide as the electrolyte, by wet chemical etching in Br2MeOH and subsequent thermal treatment (air oxidation). Surface analyses show the formation of a rather clean, Cu-depleted surface with some adsorbed oxygen after Br2/MeOH etch, and the formation of indium-oxygen bonds after thermal treatment, in accordance with previous studies that show indium oxides to be the native ones on this semiconductor. Samples that underwent photoanodic decomposition in the iodide electrolyte and those that were purposely decomposed in acetonitrile showed severe to near-total Cu depletion near their surface and the presence of lower valent Se. These data complement those from solution analyses and from electrochemical studies, to characterize the preferred decomposition path of CuInSe2 sufficiently to stabilize this type of photoelectrochemical cell. Our conclusions are based on the use of Auger parameters and, to a lesser extent, on empirical comparison of x-ray induced Auger electron line shapes, because of the difficulty in extracting unambiguous conclusions from x-ray photoelectron binding energies only.

Auger electron lineshapes measured in coincidence with scattered electrons

The value of Auger electron lineshapes measured in coincidence with outgoing scattered electrons following electron impact inner-shell ionisation in free atoms is discussed. Preliminary data of this type are reported for the first time. Measurements for the L2-M2,3M2,3(3P0,1,2) line of argon confirm the feasibility of the technique for the study of post-collision interaction effects in Auger electron decay. The data are compared with the semiclassical calculations of Niehaus (1977) for photoionisation.