Wave packet approach to periodically driven scattering

For autonomous systems it is well known how to extract tunneling probabilities from wavepacket calculations. Here we present a corresponding approach for periodically time-dependent Hamiltonians, valid at all frequencies, field strengths, and transition orders. After mapping the periodically driven system onto a time-independent one with an additional degree of freedom, use is made of the correlation function formulation of scattering [J. Chem. Phys. {\bf 98}, 3884 (1993)]. The formalism is then applied to study the transmission properties of a resonant tunneling double barrier structure under the influence of a sinusoidal laser field, revealing an unexpected antiresonance in the zero photon transition for large field strengths.Comment: 4 pages, 2 figure

Dissociation and ionization of small molecules steered by external noise

We show that ionization and dissociation can be influenced separately in a molecule with appropriate external noise. Specifically we investigate the hydrogen molecular ion under a stochastic force quantum mechanically beyond the Born-Oppenheimer approximation. We find that up to 30% of dissociation without ionization can be achieved by suitably tuning the forcing parameters.Comment: 13 pages, 6 figure

Active Brownian particles with velocity-alignment and active fluctuations

We consider a model of active Brownian particles with velocity-alignment in two spatial dimensions with passive and active fluctuations. Hereby, active fluctuations refers to purely non-equilibrium stochastic forces correlated with the heading of an individual active particle. In the simplest case studied here, they are assumed as independent stochastic forces parallel (speed noise) and perpendicular (angular noise) to the velocity of the particle. On the other hand, passive fluctuations are defined by a noise vector independent of the direction of motion of a particle, and may account for example for thermal fluctuations. We derive a macroscopic description of the active Brownian particle gas with velocity-alignment interaction. Hereby, we start from the individual based description in terms of stochastic differential equations (Langevin equations) and derive equations of motion for the coarse grained kinetic variables (density, velocity and temperature) via a moment expansion of the corresponding probability density function. We focus here in particular on the different impact of active and passive fluctuations on the onset of collective motion and show how active fluctuations in the active Brownian dynamics can change the phase-transition behaviour of the system. In particular, we show that active angular fluctuation lead to an earlier breakdown of collective motion and to emergence of a new bistable regime in the mean-field case.Comment: 5 figures, 22 pages, submitted to New Journal of Physic

Naïve orangutans (Pongo abeliiand Pongo pygmaeus) individually acquire nut‐cracking using hammer tools

Nut-cracking with hammer tools (henceforth: nut-cracking) has been argued to be one of the most complex tool-use behaviors observed in nonhuman animals. So far, only chimpanzees, capuchins, and macaques have been observed using tools to crack nuts in the wild (Boesch and Boesch, 1990; Gumert et al., 2009; Mannu and Ottoni, 2009). However, the learning mechanisms behind this behavior, and the extent of nut-cracking in other primate species are still unknown. The aim of this study was two-fold. First, we investigated whether another great ape species would develop nut-cracking when provided with all the tools and appropriate conditions to do so. Second, we examined the mechanisms behind the emergence of nut-cracking by testing a naïve sample. Orangutans (Pongo abelii and Pongo pygmaeus) have the second most extensive tool-use repertoire among the great apes (after chimpanzees) and show flexible problem-solving capacities. Orangutans have not been observed cracking nuts in the wild, however, perhaps because their arboreal habits provide limited opportunities for nut-cracking. Therefore, orangutans are a valid candidate species for the investigation of the development of this behavior. Four nut-cracking-naïve orangutans at Leipzig zoo (P. abelii; Mage = 16; age range = 10–19; 4F; at the time of testing) were provided with nuts and hammers but were not demonstrated the nut-cracking behavioral form. Additionally, we report data from a previously unpublished study by one of the authors (Martina Funk) with eight orangutans housed at Zürich zoo (six P. abelii and two P. pygmaeus; Mage = 14; age range = 2–30; 5F; at the time of testing) that followed a similar testing paradigm. Out of the twelve orangutans tested, at least four individuals, one from Leipzig (P. abelii) and three from Zürich (P. abelii and P. pygmaeus), spontaneously expressed nut-cracking using wooden hammers. These results demonstrate that nut-cracking can emerge in orangutans through individual learning and certain types of non-copying social learning

Coulomb blockade effects in driven electron transport

We study numerically the influence of strong Coulomb repulsion on the current through molecular wires that are driven by external electromagnetic fields. The molecule is described by a tight-binding model whose first and last site is coupled to a respective lead. The leads are eliminated within a perturbation theory yielding a master equation for the wire. The decomposition into a Floquet basis enables an efficient treatment of the driving field. For the electronic excitations in bridged molecular wires, we find that strong Coulomb repulsion significantly sharpens resonance peaks which broaden again with increasing temperature. By contrast, Coulomb blockade has only a small influence on effects like non-adiabatic electron pumping and coherent current suppression.Comment: 9 pages, 7 figures. Added a plot for temperature dependence of resonance peaks. Published versio

Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem [Discussion paper]

We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O3) concentrations, except in polar regions where the model now underestimates O3 concentrations. Halogen chemistry reduces the global tropospheric O3 burden by 18.6 %, with the O3 lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28  ×  106 molecules cm−3 are 8.2 % lower than in a simulation without halogens, leading to an increase in the CH4 lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH4 by Cl is small (∼  2 %) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼  15–27 %). Oxidation of VOCs by Br is smaller, representing 3.9 % of the loss of acetaldehyde and 0.9 % of the loss of formaldehyde

Iodine monoxide in the Western Pacific marine boundary layer

A latitudinal cross-section and vertical profiles of iodine monoxide (IO) are reported from the marine boundary layer of the Western Pacific. The measurements were taken using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) during the TransBrom cruise of the German research vessel Sonne, which led from Tomakomai, Japan (42° N, 141° E) through the Western Pacific to Townsville, Australia (19° S, 146° E) in October 2009. In the marine boundary layer within the tropics (between 20° N and 5° S), IO mixing ratios ranged between 1 and 2.2 ppt, whereas in the subtropics and at mid-latitudes typical IO mixing ratios were around 1 ppt in the daytime. The profile retrieval reveals that the bulk of the IO was located in the lower part of the marine boundary layer. Photochemical simulations indicate that the organic iodine precursors observed during the cruise (CH3I, CH2I2, CH2ClI, CH2BrI) are not sufficient to explain the measured IO mixing ratios. Reasonable agreement between measured and modelled IO can only be achieved, if an additional sea-air flux of inorganic iodine (e.g. I2) is assumed in the model. Our observations add further evidence to previous studies that reactive iodine is an important oxidant in the marine boundary layer

Semiclassical description of multiphoton processes

We analyze strong field atomic dynamics semiclassically, based on a full time-dependent description with the Hermann-Kluk propagator. From the properties of the exact classical trajectories, in particular the accumulation of action in time, the prominent features of above threshold ionization (ATI) and higher harmonic generation (HHG) are proven to be interference phenomena. They are reproduced quantitatively in the semiclassical approximation. Moreover, the behavior of the action of the classical trajectories supports the so called strong field approximation which has been devised and postulated for strong field dynamics.Comment: 10 pages, 11 figure