Dmitry S. Efremenko

Photoelectron Emission for Layers of Finite Thickness

Basic tools that describe the energy and angular distributions of photoelectrons emitted by inhomogeneous targets are presented. These are the functions of reflection and transmission, the function of photoelectron emission, the equation for photoelectron flow, and the equation for the transmission function, which describes the dynamics of the angular and energy spectra of photoelectrons when they move in the tar-get. The determining influence of the effects of multiple elastic scattering on the angular distribution of photoelectrons is shown. It is demonstrated in which way and by how much the effect of brightness body rotation, well known in optics, influences the X-ray photoelectron spectroscopy signal. Along with analytical solutions, the results of simulation of the X-ray photoelectron spectroscopy signals are presented.

Extraction of cross-sections of inelastic scattering from energy spectra of reflected atomic particles

REELS spectra of the electrons reflected off niobium are measured with energy resolution <0.5 eV within the 5–40 eV energy range of the probing beam. The measurements were performed for the scattering angles θ = 45° and θ = 120° by means of two electron guns. The process of energy losses is described within the framework of a model with three different energy loss laws: surface, intermediate, and bulk layers are considered. Differential cross-sections of inelastic scattering are represented in the form of simple equations.

Multi-core-CPU and GPU-accelerated radiative transfer models based on the discrete ordinate method

The operational processing of remote sensing data usually requires high-performance radiative transfer model (RTM) simulations. To date, multi-core CPUs and also Graphical Processing Units (GPUs) have been used for highly intensive parallel computations. In this paper, we have compared multi-core and GPU implementations of an RTM based on the discrete ordinate solution method. To implement GPUs, the original CPU code has been redesigned using the C-oriented Compute Unified Device Architecture (CUDA) developed by NVIDIA. GPU memory management is a crucial issue regarding the performance. To cope with limitations of GPU registers, we have adapted an RTM based on the matrix operator technique together with the interaction principle for multilayer atmospheric systems. The speed-up of such an implementation depends on the number of discrete ordinates used in the RTM. To reduce the CPU/GPU communication overhead, we have exploited the asynchronous data transfer between host and device. To obtain optimal performance, we have also used overlapping of CPU and GPU computations by distributing the workload between them. With GPUs, we have achieved a 20x–40x speed-up for the multi-stream RTM, and 50x speed-up for the two-stream RTM with respect to the original single-threaded CPU codes. Based on these performance tests, an optimal workload distribution scheme between GPU and CPU is proposed. Additionally, CPU/GPU benchmark tests regarding basic matrix operations are given. Finally, we discuss the performance obtained with the multi-core-CPU and GPU implementations of the RTM.

On the application of the invariant embedding method and the radiative transfer equation codes for surface state analysis

The equivalence of equations, describing the various physical phenomena, always provides a mutual enrichment of theories. For example, the analogy of the force lines of the electric field and the current lines of a viscous incompressible fluid created the Ostrogradsky–Gauss theorem. The authors of this chapter have background in the theory of electron transfer [1–3] and light ion transfer [4]. Atmospheric remote sensing and electron spectroscopy have much in common in the principle of their methodology. In both cases the inverse problem is solved on the base of spectra of the reflected beam of photons or particles. The spectra are formed due to the interaction between the beam and the investigated medium.

Efficiency of algorithm for solution of vector radiative transfer equation in turbid medium slab

The numerical solution of the vectorial radiative transfer equation (VRTE) is possible only by its discretization, which requires elimination of the solution anisotropic part including all the singularities. Discretized VRTE for the turbid medium slab has the unique analytical solution in the matrix form. Modern packages of matrix (linear) algebra allow only one possible algorithm of VRTE solution by computer. Various realizations of such an algorithm differ by the method of the elimination of the solution anisotropic part. Methods of the solution anisotropic part elimination are analysed in the paper. The codes created by the authors of these methods are analysed in simple situations in order to define its influence on the code efficiency. It is shown that the most effective method is based on the small angle modification of the spherical harmonics method (MSH). The code based on MSH is investigated in details by the influence of different properties of hard and software.

Direct numerical reconstruction of inelastic cross sections from REELS and ISS spectra

The reconstruction of inelastic scattering cross sections faces two problems: the measured signal (energy spectrum) is a multiple scattering signal; the inelastic energy loss is nonuniform over the target depth. In this paper, we present a method for numerical reconstruction of cross sections from characteristic energy loss spectra, which efficiently solves both problems within a multilayer model. It is shown that the inverse problem of cross section extraction in the three-layer model is ill-conditioned, and the method is practically inapplicable to the three-layer model. The direct numerical reconstruction method yields a strongly “noised” result and can be applied only to obtain a priori information on the inelastic cross section form for further fitting. Using a combination of two methods, inelastic scattering cross sections were reconstructed for aluminum from characteristic energy loss spectra at probe beam energies of 5 and 40 keV. It is shown that ionization in solids should be described as a local process and as a collective one using the dispersion formula similarly to the case of excitation plasmons.

Photoelectron spectra of finite-thickness layers

A method of computing x-ray photoemission spectra in the wide range of energy losses and different sighting angles is presented. Photoemission spectra for layers of finite thickness are investigated. Angular and energy spectra are analyzed using the invariant imbedding principle. They are computed using small-angle approximation and the exact numerical solution of the multiple photoelectron scattering events in solids. The presented methods of x-ray photoemission spectra analysis are compared regarding their efficiencies. Comparison of the exact numerical solution to those based on straight line approximation and small-angle approximation reveals an error in straight line approximation of about 50%. Numerical solutions are compared with the experimental data and Monte-Carlo simulations.

Volcanic SO2 plume height retrieval from UV sensors using a full-physics inverse learning machine algorithm

ABSTRACT Precise knowledge of the location and height of the volcanic sulphur dioxide (SO2) plume is essential for accurate determination of SO2 emitted by volcanic eruptions. Current SO2 plume height retrieval algorithms based on ultraviolet (UV) satellite measurements are very time-consuming and therefore not suitable for near-real-time applications. In this work we present a novel method called the full-physics inverse learning machine (FP-ILM) algorithm for extremely fast and accurate retrieval of the SO2 plume height. FP-ILM creates a mapping between the spectral radiance and the geophysical parameters of interest using supervised learning methods. The FP-ILM combines smart sampling methods, dimensionality reduction techniques, and various linear and non-linear regression analysis schemes based on principal component analysis and neural networks. The computationally expensive operations in FP-ILM are the radiative transfer model computations of a training dataset and the determination of the inversion operator – these operations are performed off-line. The application of the resulting inversion operator to real measurements is extremely fast since it is based on calculations of simple regression functions. Retrieval of the SO2 plume height is demonstrated for the volcanic eruptions of Mt. Kasatochi (in 2008) and Eyjafjallajökull (in 2010), measured by the GOME-2 (Global Ozone Monitoring Instrument – 2) UV instrument on-board MetOp-A.

Influence of the processes of multiple elastic scattering on the angular distributions of X-ray photoelectrons

Based on the boundary-value problem for the transport equation, the angular distributions of photoelectrons are analyzed using the method of invariant immersion. Monte Carlo (MC) simulation of the process of photoelectron emission is carried out. The determining influence of the process of multiple elastic scattering on the photoelectron spectra is shown. Simple analytical formulas for the angular distributions of photoelectrons are obtained in the small-angle approximation. The results of MC simulation and those of other authors are compared.

Experimental verification of the technique for calculating light scattering in turbid media and determination of the single-scattering albedo based on the spectroscopy of elastically reflected electrons

The equations of light transfer in a turbid medium and transfer equations for electrons undergoing only elastic collisions in a solid body are shown to be equivalent. The angular distributions of elastically reflected electrons were calculated using the DISORT and MDOM optical codes. The calculation data are compared with the results of experimental measurements for the angular distributions of elastically reflected electrons and with calculations according to the Monte-Carlo method. Optical codes are shown to be highly efficient in calculating the angular distributions of electrons elastically reflected from multilayer media.