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Studies on the nonlinear dynamics of a modified Van der Pol oscillator based electronic circuit has been reported. It has been found that inclusion of an active RC section (or filter) in the Van der Pol oscillator is able to generate rich nonlinear behavior. Effect of a low pass filter as well as an all pass filter has been reported. The behavior of the proposed system has been explained analytically and numerically with respect to its parameters by exploiting standard nonlinear analysis tools such as bifurcation diagrams, Lyapunov exponent, phase portraits, Poincare sections, and basins of attraction. Occurrence of some fascinating features like antimonotonicity, hysteresis and coexisting attractors have been found in the modified system. The analytical and numerical studies has been verified experimentally with a prototype experiment.

The ion impact multiple ionization and subsequent dissociation of CCl4 is studied using a beam of Ar7+ ion having the energy of about 1 MeV in a linear time- of-flight mass spectrometer, coupled with a position-sensitive detector. The complete, as well as incomplete Coulomb explosion pathways, for [Formula: see text] and [Formula: see text] ions are identified and studied. The kinetic energy release distributions of channels, kinetic energies, and momentum distributions of fragmented ions, as well as neutrals, are also calculated. Possible modes of fragmentation pathways, i.e., concerted and/or sequential, for all the identified channels are studied using Newton diagrams, Dalitz plots, and kinetic energy distributions. The dynamical information and fragmentation pathways were analyzed with the Dalitz plot and Newton diagram for the three-body dissociation channel. The nature of the fragmentation process is further investigated with simulated Dalitz plots and Newton diagrams using the simple classical mechanical model.

Stabilization of different morphologies of iso-material native/non-native heterostructures is important for electron-hole separation in the context of photo-electrochemical and opto-electronic devices. In this regard, we explore the stabilities of different morphologies of rutile ("native", ground state phase) and anatase ("non-native" phase) TiO2 heterostructures through (1) seed-mediated growth and (2) a thermally induced arrested phase transition synthesis protocol. Furthermore, the experimental results are analyzed through a combination of Density Functional Tight Binding (DFTB) and Finite Element Model (FEM) methods. During the seed-mediated growth, anatase is grown over a polydispersed and polycrystalline rutile core through thermal treatment yielding core-shell, Janus and yolk-shell iso-material heterostructures as observed from HRTEM. The arrested phase transition of anatase to rutile at different annealing temperatures yields rutile crystals in the subsurface region of the anatase and rutile/core-thin anatase/shell heterostructures but does not yield a Janus structure. Small particles that can be modeled via DFTB computations suggest that: (1) a heterostructure of the rutile/core-anatase/shell is energetically more stable than the anatase/core-rutile/shell or any other Janus configuration, (2) the off-centered rutile/core-anatase shell is more favorable to the mid-centered rutile/core-anatase shell and (3) Janus heterostructures can be stabilized when the mass ratio of the rutile seed to anatase overgrowth is high. FEM simulations, performed to evaluate the importance of stress relaxation in bicrystalline materials without defects, suggest that Janus structures can be stabilized in larger particles. The present studies add to the heuristics available for synthesizing iso-material heterostructures.

For C${}_{60}$, the potential energy surface exhibits a driving force toward the ground state that allows it to be found rapidly. However, the potential energy surface of a cluster can also exhibit features that prevent us from finding its global minimum. In the case of decahedral Si${}_{20}$H${}_{20}$, which is a theoretically postulated stable ground state but was never synthesized, there is no driving force towards the global minimum. The system visits a huge number of low energy structures during its evolution without ever falling into the global minimum. This is exactly the scenario described by Levinthal in his famous paradox about protein folding.

Taking our cue from certain recent advances in experimental psychology, the authors propose a plausible theory of conflict between rationality and inherent behavioral biases of investors. In this t...

The ion impact multiple ionization and subsequent dissociation of Chloroform (CHCl3) is investigated with Ar7+ ion beam having an energy of 1 MeV. Using a linear time of flight mass spectrometer coupled with a position sensitive detector, complete as well as incomplete Coulomb explosion pathways for C H C l 3 2 + and C H C l 3 3 + ions are identified and studied. Possible modes of fragmentation pathway for all the identified channels are investigated with the help of kinetic energy release and momentum distributions of fragmented ions. Although concerted fragmentation is the most dominant pathway within the identified channels, we have discussed the possibilities of sequential processes which may occur. A positive slope between the coincidence partners of H+ and C+ probably features sequentiality in the breakup dynamics but remain inconclusive due to lack of complete information. Formation of C l 2 + from the doubly charged C H C l 3 2 + is a direct evidence of bond rearrangement reaction following a structural change during charge separation.

Having a bank account is widely regarded as the first step toward financial inclusion of the poor. Funds deposited in a bank account are also observed to lead to higher savings. Exploiting special features of the National Rural Employment Guarantee Scheme (NREGS) of India, our estimates indicate that households that receive NREGS wages through a bank do not appear to spend more on education than other comparable households that receive cash payment. The district level intention to treat specification produces similar results. The findings suggest that the rural poor face constraints in accessing their bank accounts due to financial illiteracy and other problems.

A derivative control technique has been proposed to control the chaotic behavior of a Chua’s circuit. The chaotic oscillations of the system can be controlled externally by adjusting the feedback resistance in the derivative control arm. For a proper choice of the feedback resistance the system initially in chaotic region can be driven to the steady state region. In addition to that the shifting of the chaotic attractor for different parameter set has been observed. Moreover, the same technique can be used to generate chaotic oscillations in a Chua’s circuit, which would otherwise exhibit steady behaviour. The analytical predictions along with the numerical analysis and experimental results have been reported to establish the present work.

Atomic environment fingerprints are widely used in computational materials science, from machine learning potentials to the quantification of similarities between atomic configurations. Many approaches to the construction of such fingerprints, also called structural descriptors, have been proposed. In this work, we compare the performance of fingerprints based on the Overlap Matrix(OM), the Smooth Overlap of Atomic Positions (SOAP), Behler-Parrinello atom-centered symmetry functions (ACSF), modified Behler-Parrinello symmetry functions (MBSF) used in the ANI-1ccx potential and the Faber-Christensen-Huang-Lilienfeld (FCHL) fingerprint under various aspects. We study their ability to resolve differences in local environments and in particular examine whether there are certain atomic movements that leave the fingerprints exactly or nearly invariant. For this purpose, we introduce a sensitivity matrix whose eigenvalues quantify the effect of atomic displacement modes on the fingerprint. Further, we check whether these displacements correlate with the variation of localized physical quantities such as forces. Finally, we extend our examination to the correlation between molecular fingerprints obtained from the atomic fingerprints and global quantities of entire molecules., Comment: 18 pages, 11 figures

Fingerprint distances, which measure the similarity of atomic environments, are commonly calculated from atomic environment fingerprint vectors. In this work we present the simplex method which can perform the inverse operation, i.e. calculating fingerprint vectors from fingerprint distances. The fingerprint vectors found in this way point to the corners of a simplex. For a large data set of fingerprints, we can find a particular largest volume simplex, whose dimension gives the effective dimension of the fingerprint vector space. We show that the corners of this simplex correspond to landmark environments that can by used in a fully automatic way to analyse structures. In this way we can for instance detect atoms in grain boundaries or on edges of carbon flakes without any human input about the expected environment. By projecting fingerprints on the largest volume simplex we can also obtain fingerprint vectors that are considerably shorter than the original ones but whose information content is not significantly reduced., Comment: 10 pages, 7 figures

Finding complex reaction and transformation pathways, involving many intermediate states, is in general not possible on the DFT level with existing simulation methods due to the very large number of required energy and force evaluations. This is due to a large extent to the fact that for complex reactions, it is not possible to determine which atom in the educt is mapped onto which atom in the product. Trying out all possible atomic index mappings is not feasible because of the factorial increase in the number of possible mappings. By using a penalty function that is invariant under index permutations, we can bias the potential energy surface in such a way that it obtains the characteristics of a structure seeker whose global minimum is the product. By performing a Minima Hopping based global optimization on this biased potential energy surface we can rapidly find intermediate states that lead into the global minimum. Based on this information we can then extract the full reaction pathway. We first demonstrate for a benchmark system, namely LJ38 that our method allows to preferentially find intermediate states that are relevant for the lowest energy reaction pathway and that we, therefore, need a much smaller number of intermediate states than previous methods to find the lowest energy reaction pathway. Finally, we apply the method to two real systems, C60 and C20H20 and show that the found reaction pathway contains valuable information on how the system can be synthesized.

Surface stress in solids can have profound effects in semi-infinite and nanoscale materials. The current work pertains to the simulation of surface stress, using the concept proposed by Shuttleworth [Proceedings of Physical Society 63 (1949) 444–457]. A two-scale approach is used for the simulation of surface stress. Density functional theory is used to compute the lattice parameter of a free-standing layer (or two layers in the case of (1 1 0) surface) of atoms, which is further used as an input into a finite element model. Aluminium is used as a model material for the computation of surface tension of (1 0 0), (1 1 1) and (1 1 0) planes and the results of the simulations are validated by comparison with results from literature. The utility of the model developed is highlighted by demonstrating the effect of surface tension on the: (i) stress variation in a thin slab & (ii) lattice parameter of nanoscale free-standing crystals.

The absolute L subshell specific electron impact ionization cross sections near the ionization threshold ( 16 E 45 keV ) of lead and thorium are obtained from the measured L X-ray production cross sections. Monte Carlo simulation is done to account for the effect of the backscattered electrons, and the final experimental results are compared with calculations performed using distorted wave Born approximation and the modified relativistic binary encounter Bethe model. The sensitivity of the results on the atomic parameters is explored. Observed agreements and discrepancies between the experimental results and theoretical estimates, and their dependence on the specific atomic parameters are reported.

Using fingerprints used mainly in machine learning schemes of the potential energy surface, we detect in a fully algorithmic way long range effects on local physical properties in a simple covalent system of carbon atoms. The fact that these long range effects exist for many configurations implies that atomistic simulation methods, such as force fields or modern machine learning schemes, that are based on locality assumptions, are limited in accuracy. We show that the basic driving mechanism for the long range effects is charge transfer. If the charge transfer is known, locality can be recovered for certain quantities such as the band structure energy.

The effect of a vector magnetic field was studied on electromagnetically induced transparency (EIT) with linear ⊥ linear polarization of the probe and pump beams in 87 R b − D 2 transition with a vapor cell at room temperature. The dependence of EIT on the direction of the quantization axis and the relative orientation of the polarizations of the applied electric fields was studied experimentally. We show that from the relative strengths of σ and π EITs, the direction of the magnetic field can be found. Moreover, from the relative separation between σ and π EITs, the strength of the magnetic field can be calculated. We also demonstrate that the EIT peak amplitudes show oscillatory behavior depending upon the orientation of the laser polarization relative to the magnetic field direction. To understand the experimental observations, a theoretical study was done numerically considering all 13 Zeeman sub-levels. Apart from the numerical simulation, a toy model was also constructed to obtain an analytical response of the medium considering the velocity distribution. The dependencies of the magnetic field direction and polarization direction of the electric field were explicitly derived with the analytical model.

The dependencies of the polarization rotation on the probe ellipticity and the longitudinal magnetic field have been studied both experimentally and theoretically in a V-type electromagnetically induced transparency for 87Rb vapour. The angle of rotation varies periodically with the change in the probe ellipticity and non-linearly with the variation of the magnetic field. We have observed that the plane of polarization is rotated maximum for linearly polarized light and have obtained angle of rotation 0.32° ± 0.01° for B = 0.036 ± 0.001 mT while it was 0.0267° ± 0.0002° without magnetic field. Thus our measurement becomes sensitive to the low magnetic field. A four-level system is considered and the corresponding density matrix equations have been solved analytically to explain these observations theoretically with the help of degenerate and non-degenerate magnetic sub-levels.

High power SiC MOSFET technologies are critical for energy saving in, e.g., distribution of electrical power. They suffer, however, from low near-interface mobility, the origin of which has not yet been conclusively determined. Here, we present unique concerting evidence for the presence of interface defects in the form of carbon clusters at native thermally processed oxides of SiC. These clusters, with a diameter of 2–5 nm, are HF-etch resistant and possess a mixture of graphitic (sp2) and amorphous (sp3 mixed in sp2) carbon bonds different from the normal sp3 carbon present in 4H-SiC. The nucleation of such defects during thermal oxidation as well as their atomic structure is elucidated by state-of-the-art atomistic and electronic structure calculations. In addition, our property prediction techniques show the impact of the simulated carbon accumulates on the electronic structure at the interface.

We present a fully ab-initio, unbiased structure search of the configurational space of decorated C60 fullerenes in the presence of an electric field. We observed that the potential energy surface (PES) is significantly perturbed by an external electric field and that the energetic ordering of low energy isomers differs with and without electric field. We identify the energetically lowest configuration for a varying number of decorating atoms (1 ≤ n ≤ 12) for Li and (1 ≤ n ≤ 6) for K on the C60 surface at different electric field strengths. Using the correct geometric ground state in the electric field for the calculation of the dipole we obtain better agreement with the experimentally measured values than previous calculations based on the ground state in absence of an electric field. Since the lowest energy structures are typically nearly degenerate in energy, a combination of different structures is expected to be found at room temperature. The experimentally measured dipole is therefore also expected to contain significant contributions from several low energy structures.

We performed an experimental and theoretical study on saturation absorption spectroscopy on Rb 87 and Rb 85 atoms in D2 transition using a Laguerre–Gaussian (LG) beam with higher orders. We observed narrow line shapes of the Doppler and hyperfine absorption profiles due to the introduction of the LG beam in comparison to the fundamental Gaussian beam. Narrowing of the line shape is dependent on the azimuthal mode index of the LG field. It is observed that the spatially dependent Rabi frequency plays a significant role behind these narrowing phenomenon.

Electromagnetically induced transparency (EIT) is experimentally studied in a rubidium vapour cell (without buffer gas, both \(^{87}\)Rb and \(^{85}\)Rb present according to their natural abundance) kept within two-layers of \(\mu(\mu)\)-metal shields to avoid the effect of Earth's magnetic field on the energy levels of atomic rubidium. An external cavity diode laser (ECDL), used as a low power probe laser, is locked to the hyperfine cross-over peak of \(F = 2 \rightarrow F' = 2, 3\) transitions of \(^{87}\)Rb. The frequency of another ECDL, the pump laser, is set to scan the \(F=1 \rightarrow F' = 0, 1, 2\) transitions of \(^{87}\)Rb. These form the \(\Lambda\)-type six level system. In the V-type system, both the pump and the probe lasers share the same \(F = 2\) ground level. The probe beam coming out of the cell is detected by a low noise fast photodetector. The resulting spectra show signature of EIT in the "peak" for the \(\Lambda\)-type system and in the "dip" for the V-type system. Numerical calculation based simulated spectra are also compared with the experimental spectra. In both the cases very narrow EIT linewidth \((\Gamma_{t} < \Gamma)\) is observed even at high value of pump Rabi-frequency \((\Omega_{c}\gg \Gamma)\). Narrower value of EIT linewidth is due to Doppler averaging phenomena.

In this article we have shown that the atomic states can be engineered by tunning the coupling Rabi frequency for a system with $\mathcal{N}$-type configuration. Electromagnetically induced transparency (EIT), Electromagnetically induced absorption (EIA) and Autler-Townes (AT) splitting has been observed experimentally in a four level $\mathcal{N}$-type atomic vapor of $^{85}Rb$ atoms in the hyperfine levels of $D_2$ transition. It has been shown that the response of the atomic medium can be tunned from highly transparent to highly absorptive in our case. The evolution of the atomic states from the dark state |D> to the non-coupled state |NC> has been studied with the partial dressed state approach which makes the backbone of the modification of the atomic response. In addition, transient solutions in the time domain and steady state solution in the frequency domain has been studied. The population dynamics and the coherence contribution in each case has been analyzed by the time dependent solutions. The experimentally observed steady line-shape profiles has been supported by the steady state solution of optical-Bloch equations considering the Maxwell Boltzmann velocity distributions of the atoms. It has been observed that the crossover between the EIT and the AT splitting has been replaced by the interference contribution of the EIA in this $\mathcal{N}$-type system.

We report here simultaneous experimental observation of Electromagnetically Induced Transparency (EIT) and Electromagnetically Induced Absorption (EIA) in a multi-level V-type system in D

The authors examine the legal and business environments, financing channels, and governance mechanisms of various types of firms in India and compare them to those from other countries. Despite its English commonlaw origin, strong legal protection provided by the law, and a democratic government, corruption within India's legal system and government significantly weakens investor protection in practice. External financing of firms has been dominated by nonmarket sources of financing, while the characteristics of listed firms are similar to those from countries with weak investor protection. The evidence, including results based on a survey of small and medium-scale private firms, shows that alternative financing channels provide the most important source of funds. The authors also find that informal governance mechanisms, such as those based on reputation, trust, and relationships are more important than formal mechanisms (such as courts) in resolving disputes, overcoming corruption, and supporting growth.

Pulse delay with the group velocity dispersion (GVD) characteristics was studied in the V-type electromagnetically induced transparency in the hyperfine levels of $^{85}Rb$ atoms with a closed system configuration. The phase coherency between the pump and the probe laser beams was maintained. We studied the pulse delay and the group velocity dispersion characteristics with the variation of the pump Rabi frequency taking temperature as a parameter. We observed a maximum of $268$ $ns$ pulse delay for $21.24 MHz$ pump Rabi frequency at $55^0C$ temperature of the Rb vapour cell. For a better understanding of the experimental results, we have derived an analytical solution for the delay characteristics considering the thermal averaging. The analytical solution was derived for a three level V-type system. The theoretical plots of the delay and the group velocity dispersion show the same characteristics as we observed in the experiment. This analytical approach can be further generalized for the higher level schemes to calculate different quantities such as susceptibility, group velocity delay or group velocity dispersion characteristics.

The dispersive properties of 87Rb atomic medium in electromagnetically induced transparency (EIT) have been studied experimentally as well as theoretically. To measure the dispersion signal, a balanced homodyne detection technique was used by forming a Mach–Zehnder interferometer. We studied the system in a Λ-type configuration. From the slope of the dispersive EIT signal, the group index (n g ) of the atomic medium, the group velocity () and the time delay () of the light wave through the atomic medium were calculated. We measured the group index of the atomic medium from the dispersion signal and observed that it non-linearly depends on the pump power. This non-linearity signifies that to achieve the maximum reduced group velocity, optimization of the pump power is needed. We got the maximum reduction in the group velocity of the probe beam as m s−1. The corresponding time delay was ns. To support our experimental observation, we have performed a theoretical simulation by taking the semi-classical approach of density matrix formalism of a Λ-type three level system. To get the probe response we solved the optical Bloch equation analytically assuming the Lorentzian velocity distribution of the atom. We have observed that our analytical solution matched exactly with the numerical solution. Our experimental observation of non-linear variation of the group index with the pump Rabi frequency matched quite well with both of our numerical and analytical simulations.

Polarization rotation in electromagnetically induced transparency (PREIT) in rubidium vapour at room temperature has been studied experimentally as well as theoretically in the absence of any magnetic field. PREIT is observed for both 85Rb and 87Rb combining the D1 and the D2 transitions with a V-type configuration. For 85Rb, two EIT peaks and for 87Rb, one EIT peak were observed. Nonlinear characteristics of the variations of the angle of rotation corresponding to the EIT position were observed for both the isotopes of Rb. We have modelled a four level system to explain the occurrence of the double EIT in case of the 85Rb and the occurrence of one EIT peak for 87Rb. The rotation of plane of polarization of the probe beam can be further explained with this model. The characteristic behaviour of the variation of rotation corresponding to the EIT position from theoretical analysis has matched well with the experimental observations. A sharp rotation with a high signal to noise ratio in the vicinity of the EIT can be used as a modulation free and noise insensitive signal for the frequency stabilization.

The dissociation dynamics of multiply charged methanol molecules formed in collision with 1.2 MeV Ar 8+ projectiles is studied. Using coincidence mapping techniques, we can separate out the different dissociation pathways between carbon, oxygen and hydrogen ionic fragments as well as two- and three-body breakup events. Reactions involving intramolecular bond rearrangements within the CH 3 group of the dissociative molecule are discussed in detail. A signature of hydrogen migration in doubly charged methanol is observed. Kinetic energy releases of different breakup channels are reported here and compared with values calculated from a Coulomb explosion model. The shape and orientation of the islands in the coincidence map give further information about the momentum balance in the fragmentation process of two- or many-body dissociation pathways.

Having a bank account is widely regarded as the first step towards financial inclusion of the poor, as funds deposited in a bank account is expected to lead to higher savings. However, the existing literature on the savings potential of new bank accounts for the poor usually stops short of investigating whether the savings are productively used. In this paper we attempt to fill in this gap. Moreover, having a bank account to the poor is not synonymous to using it. Therefore, we measure the impact of being paid at bank on the investment in human capital of the households. Our empirical approach differs from most existing studies which have used field experiments with a limited sample size and a one-time payment to subjects of the experiments. In order to ensure generalizability of our findings, we use a large nationally representative sample and repeated wage payments to the poor. Our empirical tests exploiting special features of the National Rural Employment Guarantee Scheme (NREGS) of India indicate that the beneficiary households (recipients of NREGS wage payments through bank accounts) spend significantly less than their counterparts (cash payment recipients) on education, arguably the most important human capital development investment for the poor. The results are consistent between standard OLS and instrumental variable estimates designed to correct for omitted variable bias in OLS tests. Our tests for other discretionary and non-discretionary expenses provide corroborating evidence.

A very simple and inexpensive tunable semiconductor diode laser controller is designed for stable operation of the diode laser. The diode laser controller is stable within +/−8 μA and +/−10 mK, respectively. The noise spectrum of the current controller is studied by FFT analysis. We have used our home-made diode laser system in a tunable diode laser absorption spectrometer (TDLAS) to probe weak overtone transitions of water vapour molecule. The diode laser wavelength is coarsely tuned by changing the operating temperature to probe (2, 1, 1)←(0, 0, 0) band overtone transitions of water vapour within 818–835 nm. To demonstrate line shape study, seven transitions are scanned by ramping the drive current of the diode laser (at constant operating temperature) under different perturber (laboratory air) pressures within the sample cell. A balanced detector and a lock-in amplifier are used for phase sensitive detection purpose. Small current modulation amplitude, balanced detection and proper adjustment of the lock-in amplifier help to obtain a S/N ratio ranging from ∼100 to ∼7 using a small sample path length of 1.5 m. Experimentally obtained derivative spectrum is numerically integrated to reveal the original line shape and fitted with a nonlinear least squares fitting program to extract air broadening coefficients and line strength parameters. The spectroscopic line parameters are compared with the results from HITRAN database.

We describe a setup for studies of dissociation dynamics of multiply charged molecules formed due to highly charged ion impact. The setup consists of a multihit, position-sensitive, time of flight measurement system along with an electrostatic analyzer with offset detector. Brief description of different components of the setup and electronics for multihit data acquisition is presented in this paper. Coulomb explosion of multiply ionized N 2 and charge exchange studies on Ar with highly charged projectile ion is carried out to test the performance of the spectrometer and its multifold coincidence capabilities.

A typical conflict is a complex phenomenon. It can have multiple roots; social (ethnic and religious differences), political (civil wars), economic (control of natural resources). In this paper, we investigate whether a specific economic intervention can mitigate conflicts, regardless of their origins and complexity. The intervention that we consider is financial development, measured either as an increase in bank credit supply or an increase in the number of bank accounts, in a conflict-affected area. Using a model as well as extensive empirical tests with district-level data from India over a long sample period (1983-2010), we find consistent and strong evidence that supports our model’s prediction that financial development mitigates conflicts. Multiple identification checks establish causality of our findings. Further tests indicate that employment growth due to financial development serves as a beneficial channel from financial development to conflicts. Our findings offer important policy implications for conflict mitigation.

In quality-differentiated audit markets with client-firms of unknown types, insider-managers of client firms strategically select auditors who respond to legal liabilities to decide their care level. In this signaling game, uninformed-investors use the audit report and the auditors' identity for firm valuation. The analysis shows that increased legal liability increases the auditor's effort and audit accuracy but reduces the demand for high quality auditing because, apart from the increased audit costs, the adverse selection benefit of the worse type reduces with increased accuracy. Furthermore, alternative legal regimes and damage allocation rules alter informational efficiency of the financial market.

Electromagnetically induced absorption (EIA) and velocity-selective transparency peaks are observed in the probe absorption signal of V-type five-level atomic systems interacting with copropagating control and probe fields. At high control beam intensity, a splitting of EIA occurs. The experiment shows similar results for both Rb87 and Rb85 isotopes at room temperature. A density matrix-based theoretical model of this five-level system has been developed and solved numerically under steady-state condition. The simulated spectra agree quite well with the experimental findings. An analytic expression for the probe response is derived and used to find out the causes for the splitting of EIA.

Surface stress can have profound effects on nanoscale materials and can lead to a contraction of the lattice in nanoparticles to compensate for the under-coordination of the surface atoms. The effect of elastic properties like Poisson's ratio can be accentuated in lower dimensional systems. The current study focuses on hollow metal nanoshells (MNSs), wherein there is interplay between the surface stresses existing in the inner and outer surfaces. Using a two scale computational method and transmission electron microscopy, we not only show a lattice expansion (in the radial direction) due to purely surface stress effects in a metallic system but also discover anomalous lattice expansion in the case of very thin walled MNSs. We argue that this effect, wherein the stress in the outer surface causes expansion in the radial lattice parameter (instead of compression), is a Poisson effect driven phenomenon. Although Ni nanoshells are used as an illustrative system for the studies, we generalize this effect for a...

We report techniques developed to utilize three-dimensional momentum information as feedback in adaptive femtosecond control of molecular dynamics. Velocity map imaging is used to obtain the three-dimensional momentum map of the dissociating ions following interaction with a shaped intense ultrafast laser pulse. In order to recover robust feedback information, however, the two-dimensional momentum projection from the detector must be inverted to reconstruct the full three-dimensional momentum of the photofragments. These methods are typically slow or require manual inputs and are therefore accomplished offline after the images have been obtained. Using an algorithm based upon an “onion-peeling” (also known as “back projection”) method, we are able to invert 1040 × 1054 pixel images in under 1 s. This rapid inversion allows the full photofragment momentum to be used as feedback in a closed-loop adaptive control scheme, in which a genetic algorithm tailors an ultrafast laser pulse to optimize a specific outcome. Examples of three-dimensional velocity map image based control applied to strong-field dissociation of CO and O2 are presented.

This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray Free-Electron Laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C8H5F) and dissociating, laseraligned 1,4-dibromobenzene (C6H4Br2) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments., 24 pages, 10 figures, Faraday Discussions 171