Electron Thermalization and Relaxation in Laser-Heated Nickel by Few-Femtosecond Core-Level Transient Absorption Spectroscopy

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel M$_{2,3}$ edge. It is observed that the core-level absorption lineshape of photoexcited nickel can be described by a Gaussian broadening ($\sigma$) and a red shift ($\omega_{s}$) of the ground state absorption spectrum. Theory predicts, and the experimental results verify that after initial rapid carrier thermalization, the electron temperature increase ($\Delta T$) is linearly proportional to the Gaussian broadening factor $\sigma$, providing quantitative real-time tracking of the relaxation of the electron temperature. Measurements reveal an electron cooling time for 50 nm thick polycrystalline nickel films of 640$\pm$80 fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of $\omega_{s}\propto\Delta T^{1.5}$. Rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4 fs photoexcitation pulse until the carriers reach a quasi-thermal equilibrium. Entwined with a <6 fs instrument response function, carrier thermalization times ranging from 34 fs to 13 fs are estimated from experimental data acquired at different pump fluences and it is observed that the electron thermalization time decreases with increasing pump fluence. The study provides an initial example of measuring electron temperature and thermalization in metals in real time with XUV light, and it lays a foundation for further investigation of photoinduced phase transitions and carrier transport in metals with core-level absorption spectroscopy.Comment: 20 pages, 8 figure

From Excited Charge Dynamics to Cluster Diffusion: Development and Application of Techniques Beyond DFT and KMC

This dissertation focuses on developing reliable and accurate computational techniques which enable the examination of static and dynamic properties of various activated phenomena using deterministic and stochastic approaches. To explore ultrafast electron dynamics in materials with strong electron-electron correlation, under the influence of a laser pulse, an ab initio electronic structure method based on time-dependent density functional theory (TDDFT) in combination with dynamical mean field theory (DMFT) is developed and applied to: 1) single-band Hubbard model; 2) multi-band metal Ni; and 3) multi-band insulator MnO. The ultrafast demagnetization in Ni reveal the importance of memory and correlation effects, leading to much better agreement with experimental data than previously obtained, while for MnO the main channels of charge response are identified. Furthermore, an analytical form of the exchange-correlation kernel is obtained for future applications, saving tremendous computational cost. In another project, size-dependent temporal and spatial evolution of homo- and hetero-epitaxial adatom islands on fcc(111) transition metals surfaces are investigated using the self-learning kinetic Monte Carlo (SLKMC) method that explores long-time dynamics unbiased by apriori selected diffusion processes. Novel multi-atom diffusion processes are revealed. Trends in the diffusion coefficients point to the relative role of adatom lateral interaction and island-substrate binding energy in determining island diffusivity. Moreover, analysis of the large data-base of the activation energy barriers generated for multitude of diffusion processes for variety of systems allows extraction of a set of descriptors that in turn generate predictive models for energy barrier evaluation. Finally, the kinetics of the industrially important methanol partial oxidation reaction on a model nanocatalyst is explored using KMC supplemented by DFT energetics. Calculated thermodynamics explores the active surface sites for reaction components including different intermediates and energetics of competing probable reaction pathways, while kinetic study attends to the selectivity of products and its variation with external factors

Toward Multiscale Modeling Of Thin-Film Growth Processes Using Slkmc

The self-learning kinetic Monte Carlo method has been shown to be suitable for examining the temporal and spatial evolution of adatom islands on the (111) surface of several fcc metals, unbiased by diffusion processes chosen a priori. A pattern-recognition scheme and a diffusion path finder scheme enable collection of a large database of diffusion processes and their energetics. A variety of mechanisms involving single and multiple atoms, and concerted island motion are uncovered in long-time simulations. In this contribution, after reviewing the methodology, we present results comparing the diffusion kinetics of two sets of homo-epitaxial and hetero-epitaxial systems: small (2-8 atom) Pd and Ag islands on the respective (111) surfaces and small Cu islands on Ni(111) and Ni islands on Cu(111). We trace the dominance of concerted motion in Pd/Pd(111) and Ni/Cu(111) and competition among concerted, multiatom and single-atom processes in Ag/Ag(111) and Cu/Ni(111) to the strength of the lateral interaction among adatoms in these systems

Towards TDDFT for Strongly Correlated Materials

We present some details of our recently-proposed Time-Dependent Density-Functional Theory (TDDFT) for strongly-correlated materials in which the exchange-correlation (XC) kernel is derived from the charge susceptibility obtained using Dynamical Mean-Field Theory (the TDDFT + DMFT approach). We proceed with deriving the expression for the XC kernel for the one-band Hubbard model by solving DMFT equations via two approaches, the Hirsch–Fye Quantum Monte Carlo (HF-QMC) and an approximate low-cost perturbation theory approach, and demonstrate that the latter gives results that are comparable to the exact HF-QMC solution. Furthermore, through a variety of applications, we propose a simple analytical formula for the XC kernel. Additionally, we use the exact and approximate kernels to examine the nonhomogeneous ultrafast response of two systems: a one-band Hubbard model and a Mott insulator YTiO3. We show that the frequency dependence of the kernel, i.e., memory effects, is important for dynamics at the femtosecond timescale. We also conclude that strong correlations lead to the presence of beats in the time-dependent electric conductivity in YTiO3, a feature that could be tested experimentally and that could help validate the few approximations used in our formulation. We conclude by proposing an algorithm for the generalization of the theory to non-linear response

Diffusion Of Small Cu Islands On The Ni(111) Surface: A Self-Learning Kinetic Monte Carlo Study

We elucidate the diffusion kinetics of a heteroepitaxial system consisting of two-dimensional small (1–8 atoms) Cu islands on the Ni(111) surface at (100–600) K using the Self-Learning Kinetic Monte Carlo (SLKMC-II) method. Study of the statics of the system shows that compact CuN (3≤N≤8) clusters made up of triangular units on fcc occupancy sites are the energetically most stable structures of those clusters. Interestingly, we find a correlation between the height of the activation energy barrier (Ea) and the location of the transition state (TS). The Ea of processes for Cu islands on the Ni(111) surface are in general smaller than those of their counterpart Ni islands on the same surface. We find this difference to correlate with the relative strength of the lateral interaction of the island atoms in the two systems. While our database consists of hundreds of possible processes, we identify and discuss the energetics of those that are the most dominant, or are rate-limiting, or most contributory to the diffusion of the islands. Since the Ea of single- and multi-atom processes that convert compact island shapes into non-compact ones are larger (with a significantly smaller Ea for their reverse processes) than that for the collective (concerted) motion of the island, the later dominate in the system kinetics – except for the cases of the dimer, pentamer and octamer. Short-jump involving one atom, long jump dimer-shearing, and long-jump corner shearing (via a single-atom) are, respectively, the dominating processes in the diffusion of the dimer, pentamer and octamer. Furthermore single-atom corner-rounding are the rate-limiting processes for the pentamer and octamer islands. Comparison of the energetics of selected processes and lateral interactions obtained from semi-empirical interatomic potentials with those from density functional theory show minor quantitative differences and overall qualitative agreement

High Catalytic Activity Of Pd1/Zno(1010) Toward Methanol Partial Oxidation: A Dft+Kmc Study

We perform density functional theory (DFT) calculations of the energetics for several pathways associated with methanol partial oxidation (MPO) reaction on singly distributed Pd on ZnO (Pd1/ZnO) and use them in kinetic Monte Carlo (KMC) simulations for elucidating reaction mechanism. We compare these results for Pd1/ZnO with those obtained for the same set of reactions on a 32-atom Pd16Zn16 nanocluster. Our KMC simulations show that Pd1/ZnO offers high, temperature-dependent selectivity (∼93%) for H2 production and a moderate one (∼76%) for CO2, in good agreement with experiment (which reports 90 and 85%, respectively). On the other hand, Pd16Zn16 yields no selectivity for H2 but almost perfect, temperature-independent selectivity (∼100%) for CO2 and H2O, leading to full oxidation of methanol. The high activity of Pd1/ZnO for MPO can be credited to the singly distributed Pd sites and to the Pd-modified geometric and electronic structures of the neighboring Zn sites, and its high H2 selectivity may be related to the abundant supply of H atoms resulting from methanol decomposition on the surface. Pd loading has a decisive impact on adsorption and dissociation of methanol and oxygen. With higher Pd loadings, the activity of the Zn site alters in such a way that it provides weaker binding to methanol and stronger binding to O2, thereby resulting in facile O2 dissociation. Singly distributed Pd atoms not only serve as a more stable binding site for methanol than does Pd in Pd16Zn16 but also induce spontaneous CO2 formation and nearly spontaneous dissociation of H2O. In an alternate but slower pathway for production of CO2 involving HCOO∗ intermediate on Pd1/ZnO, the rate-limiting step is dissociation of H2COO∗, followed by decomposition of HCOO∗ into CO2∗ and H