Physics

We present ab initio study of the electronic stopping power of protons and helium ions in an insulating material, HfO 2 . The calculations are carried out in channeling conditions with different impact parameters by employing Ehrenfest dynamics and real--time, time--dependent density functional theory. The satisfactory comparison with available experiments demonstrates that this approach provides an accurate description of electronic stopping power. The velocity-proportional stopping power is predicted for protons and helium ions in the low energy region, which conforms the linear response theory. Due to the existence of wide band gap, a threshold effect in extremely low velocity regime below excitation is expected. For protons, the threshold velocity is observable, while it does not appear in helium ions case. This indicates the existence of extra energy loss channels beyond the electron--hole pair excitation when helium ions are moving through the crystal. To analyze it, we checked the charge state of the moving projectiles and an explicit charge exchange behavior between the ions and host atoms is found. The missing threshold effect for helium ions is attributed to the charge transfer, which also contributes to energy loss of the ion.

We have performed systematic large-scale all-electron correlated calculations on boron Bn, aluminum Aln and magnesium Mgn clusters (n=2--5), to study their linear optical absorption spectra. Several possible isomers of each cluster were considered, and their geometries were optimized at the coupled-cluster singles doubles (CCSD) level of theory. Using the optimized ground-state geometries, excited states of different clusters were computed using the multi-reference singles-doubles configuration interaction (MRSDCI) approach, which includes electron correlation effects at a sophisticated level. These CI wavefunctions were used to compute the transition dipole matrix elements connecting the ground and various excited states of different clusters, eventually leading to their linear absorption spectra. The convergence of our results with respect to the basis sets, and the size of the CI expansion was carefully examined. Isomers of a given cluster show a distinct signature spectrum, indicating a strong structure property relationship. This fact can be used in experiments to distinguish between different isomers of a cluster. Owing to the sophistication of our calculations, our results can be used for benchmarking of the absorption spectra and be used to design superior time-dependent density functional theoretical (TDDFT) approaches. The contribution of configurations to many-body wavefunction of various excited states suggests that in most cases optical excitations involved are collective, and plasmonic in nature. Optical absorption in planar boron clusters in wheel shape, B7, B8 and B9 computed using EOM-CCSD approach, have been compared to the results obtained from TDDFT approach with a number of functionals. This benchmarking reveals that range-separated functionals such as wB97xD and CAM-B3LYP give qualitatively as well as quantitatively the same results as that of EOM-CCSD.

A pseudopotential of C − 60 has been constructed from ab-initio quantum-mechanical calculations. Since the obtained pseudopotential can be easily fitted by rather simple analytical approximation it can be effectively used both in classical and quantum molecular dynamics of fullerene-based compounds.

Dissociative electron attachment (DEA) and ion-pair dissociation (IPD) processes of Difluoromethane (CH 2 F 2 ) have been studied in the incident electron energy range 0 to 45 eV. Three different fragment anions (F − , CHF − and F − 2 ) are detected in the DEA range and two anions (F − and CHF − ) are detected in IPD range. Absolute cross-section of the F − fragment ion is measured for the first time. Three different resonances for both F − and CHF − ions and one single resonance peak for the F − 2 ions are observed. Constant increase in ion counts above 8 eV incident electron energy indicates the involvement of IPD process. From the experimental observation, it is speculated that near 11 eV incident electron energy both DEA and IPD processes occur simultaneously.

The absorption-ablation-excitation mechanism in laser-cluster interactions is investigated by measuring Rayleigh scattering of aerosol clusters along with atomic emission from phase-selective laser-induced breakdown spectroscopy (PS-LIBS). As the excitation laser intensity is increased beyond 0.16GW/cm2, the scattering cross-section of TiO_2 clusters begins to decrease, concurrent with the onset of atomic emission of Ti, indicating a scattering-to-ablation transition and the formation of nanoplasmas. To better clarify the process, time-resolved measurements of scattering signals are examined for different excitation laser intensities. For increasing laser intensities, the cross-sections of clusters decrease during a single pulse, evincing the shorter ablation delay time and larger ratios of ablation clusters. Assessment of the electron energy distribution during the ablation process is conducted by non-dimensionalizing the Fokker-Planck equation, with analogous Strouhal Sl_E, Peclet Pe_E, and Damkohler Da_E numbers defined to characterize the laser-induced aerothermochemical environment. For conditions of Sl_E>>1, Pe_E>>1, and Da_E<<1, the electrons are excited to the conduction band by two-photon absorption, then relax to bottom of the conduction band by collisional electron energy loss to the lattice, and finally serve as the energy transfer media between laser field and lattice. The relation between delay time and excitation intensity is well predicted by this simplified model with quasi-steady assumption.

Results of accurate computations of bound states in three- and four-electron atomic systems are discussed. Bound state properties of the four-electron lithium ion Li − in its ground 2 2 S− state are determined from the results of accurate, variational computations. We also consider a closely related problem of accurate numerical evaluation of the half-life of the beryllium-7 isotope. This problem is of paramount importance for modern radiochemistry.

The optical properties of a weak probe field in a duplicated two-level system are investigated in multiphoton resonance (MPR) condition and beyond it. It is shown that, by changing the relative phase of applied fields, the absorption switches to the amplification without inversion in MPR condition. By applying the Floquet decomposition to the equation of motion beyond MPR condition, it is shown that the phase-dependent behavior is valid only in MPR condition. Moreover, it is demonstrated that the group velocity of light pulse can be controlled by the intensity of applied fields and the gain-assisted superluminal light propagation (fast light) is obtained in this system. In addition, the optical bistability (OB) behavior of the system is studied beyond MPR condition. We apply an indirect incoherent pumping field to the system and it is found that the group velocity and OB behavior of the system can be controlled by incoherent pumping rate.

This paper presents a new explanation of the width of gamma-ray spectra based on Rahm's electronegativity scale. This quantitatively proves, for the first time, that positrophilic electrons in the positron-electron annihilation process are exactly the valence electrons. This suggests the replacement of Full Width at Half Maximum (FWHM) of the gamma-ray spectra with the newly defined physical quantity Average Doppler Shift (ADS). Both FWHM and ADS of the gamma-ray spectra in light elements agree well with the corresponding Rahm's electronegativity values, respectively. Further, ADS provides strong evidence in favor of Rahm's electronegativity scale. It is expected that this will be useful in understanding the mechanism of the positron-electron annihilation process.

We have used information theory analogue of entropy, Shannon entropy, for estimating the variations during the isotropic and anisotropic AuNP fractal growth process. We have firstly applied the Shannon entropy on the simulated fractal aggregates obtained from DLA model with noise reduction scheme. In conventional noise reduction scheme used in past, the growth process of identical particles was performed and no effect of the evolving cluster on the incoming particle was considered, hence the noise is reduced in discrete amount and do not account for the noise fluctuations present during the morphological evolution of the fractals. The Shannon entropy is shown to capture the emergence of the anisotropic morphological evolution. The imaging tool was further found to be promising for capturing the readjustment effects during cluster-cluster aggregation.

We propose a new approach to analysis of cathodoluminescence spectra of substrate-free nanoclusters of argon produced in a supersonic jet expanding into a vacuum. It is employed to analyze intensities of the luminescence bands of neutral and charged excimer complexes (Ar2)* and (Ar4+)* measured for clusters with an average size of 500 to 8900 atoms per cluster and diameters ranging from 32 to 87 Å. Concentration of the jet substance condensed into clusters, which determines the absolute values of the integrated band intensities, is shown to be proportional to the logarithm of the average cluster size. Analysis of reduced intensities of the (Ar2)* and (Ar4+)* bands in the spectra of crystalline clusters with an fcc structure allows us to conclude that emission of the neutral molecules (Ar2)* comes from within the whole volume of the cluster, while the charged complexes (Ar4+)* radiate from its near-surface layers. We find the cluster size range in which the jet is dominated by quasicrystalline clusters with an icosahedral structure and demonstrate that the transition from icosahedral to fcc clusters occurs when the average cluster size is N = 1400+-400 atoms per cluster.