Transient absorption spectroscopy on spiropyran monolayers using nanosecond pump–probe Brewster angle reflectometry

Self-assembled monolayers of 11-(3’,3’-dimethyl-6,8-dinitrospiro[chromene-2,2’-indoline]-1’-yl) undecanoic acid (amphiphilic spiropyran) at the air–water interface are studied using Brewster angle reflectometry. Transient kinetics of the spiropyran to merocyanine conversion are recorded in a UV-pump, VIS-probe configuration. By varying the probe wavelength using an optical parametric oscillator, we are able to reconstruct absorption spectra of intermediate states with a time-resolution of 10 nanoseconds, limited by the temporal convolution of the two laser pulses. After UV irradiation, spiropyran converts to merocyanine in two stages. The first occurs within a timescale of several tens of nanoseconds and is heavily convoluted with the system response time, whereas the second stage occurs over a few hundred nanoseconds. During the rise time there is a small red shift in the transient absorption spectrum of ∼20 nm. We assign the red shift and the slower kinetics to the isomerization of a merocyanine isomer cis about the central methine bond to those that are trans about the same bond

Vocational pedagogies: The science of teaching or the teaching of science?

This paper draws on recent reports and articles concerning vocational education and training which in 2014 aim to offer a theoretical underpinning for vocational pedagogy and one that promotes a greater understanding of how the ‘practical knowledge’ within vocational education and training can be developed through a consideration of different pedagogic practices. What these reports fail to do however is to address a consideration of the ‘theoretical knowledge’ within vocational areas of study currently on offer. This article, therefore seeks to provide an alternative view of vocational pedagogy that arises from a practitioner perspective of teaching science to vocational ‘subjects’ for a number of years. Hence it is an attempt to reframe the arguments for vocational pedagogy into a consideration of the teaching of ‘science’ within vocational education rather than an analysis of the newly defined concept of vocational pedagogy which implies that teaching vocational subjects is an art in itself

Transient Brewster angle reflectometry of spiropyran monolayers

Brewster angle reflectometry has been developed as a tool for determining the absorbance and refractive index changes in molecular monolayers containing spiropyran. The method is sensitive to changes in both the real and imaginary parts of the refractive index in the monolayers. It was used to monitor the conversion of spiropyran to merocyanine and the reversal of this reaction when the molecules were immobilised on quartz using silane coupling. An analytical solution of Fresnel formula allowed the transient reflectometry data to be converted into transient absorption information. Absorbances of transients as low as ~10-6 were possible using the current apparatus with a single laser pulse transient measurement. It was found that spiropyran photoconverted to merocyanine with an efficiency of ~0.1. The photochemical reversion of converted merocyanine to spiropyran occurred with efficiencies of 0.03–0.2 and this was probably site dependent. It was found that the thermal conversion from merocyanine to spiropyran was slow and even after 10 min there was no significant thermal reversion. This measurement was possible because the real part of the refractive index of the monolayer could be monitored with time using an off-resonance probe at a wavelength where the merocyanine did not absorb light meaning that the probe did not photobleach the sample. Thus our method also provides a non-intrusive method for probing changes in molecules in thin films

A lattice grain model of hillslope evolution

Thispaperdescribesandexploresanewcontinuous-timestochasticcellularautomatonmodelofhill- slope evolution. The Grain Hill model provides a computational framework with which to study slope forms that arise from stochastic disturbance and rock weathering events. The model operates on a hexagonal lattice, with cell states representing fluid, rock, and grain aggregates that are either stationary or in a state of motion in one of the six cardinal lattice directions. Cells representing near-surface soil material undergo stochastic disturbance events, in which initially stationary material is put into motion. Net downslope transport emerges from the greater likelihood for disturbed material to move downhill than to move uphill. Cells representing rock undergo stochas- tic weathering events in which the rock is converted into regolith. The model can reproduce a range of common slope forms, from fully soil mantled to rocky or partially mantled, and from convex-upward to planar shapes. An optional additional state represents large blocks that cannot be displaced upward by disturbance events. With the addition of this state, the model captures the morphology of hogbacks, scarps, and similar features. In its simplest form, the model has only three process parameters, which represent disturbance frequency, characteris- tic disturbance depth, and base-level lowering rate, respectively. Incorporating physical weathering of rock adds one additional parameter, representing the characteristic rock weathering rate. These parameters are not arbitrary but rather have a direct link with corresponding parameters in continuum theory. Comparison between observed and modeled slope forms demonstrates that the model can reproduce both the shape and scale of real hillslope profiles. Model experiments highlight the importance of regolith cover fraction in governing both the downslope mass transport rate and the rate of physical weathering. Equilibrium rocky hillslope profiles are possible even when the rate of base-level lowering exceeds the nominal bare-rock weathering rate, because increases in both slope gradient and roughness can allow for rock weathering rates that are greater than the flat-surface maximum. Examples of transient relaxation of steep, rocky slopes predict the formation of a regolith-mantled pediment that migrates headward through time while maintaining a sharp slope break

Fresh shallow valleys in the Martian midlatitudes as features formed by meltwater flow beneath ice

Significant numbers of valleys have been identified in the Martian midlatitudes (30–60°N/S), spatially associated with extant or recent ice accumulations. Many of these valleys date to the Amazonian, but their formation during these cold, dry epochs is problematic. In this study, we look in detail at the form, distribution, and quantitative geomorphology of two suites of these valleys and their associated landforms in order to better constrain the processes of their formation. Since the valleys themselves are so young and thus well preserved, uniquely, we can constrain valley widths and courses and link these to the topography from the Mars Orbiter Laser Altimeter and High-Resolution Stereo Camera data. We show that the valleys are both qualitatively and quantitatively very similar, despite their being >5000 km apart in different hemispheres and around 7 km apart in elevation. Buffered crater counting indicates that the ages of these networks are statistically identical, probably forming during the Late Amazonian, ~100 Ma. In both localities, at least tens of valleys cross local drainage divides, apparently flowing uphill. We interpret these uphill reaches to be characteristic of flow occurring beneath a now absent, relatively thin (order 101–102 m), regionally extensive ice cover. Ridges and mounds occasionally found at the foot of these valley systems are analogous to eskers and aufeis-like refreezing features. On the basis of their interaction with these aufeis-like mounds, we suggest that this suite of landforms may have formed in a single, short episode (perhaps order of days), probably forced by global climate change

Dynamics of long term fluvial response in postglacial catchments of the Ladakh Batholith, Northwest Indian Himalaya

Upland rivers control the large-scale topographic form of mountain belts, allow coupling of climate and tectonics at the earth’s surface and are responsible for large scale redistribution of sediment from source areas to sinks. However, the details of how these rivers behave when perturbed by changes to their boundary conditions are not well understood. I have used a combination of fieldwork, remotely sensed data, mathematical analysis and computer modelling to investigate the response of channels to well constrained changes in the forcings upon them, focussing in particular on the effects of glacial remoulding of the catchments draining the south flank of the Ladakh batholith, northwest Indian Himalaya. The last glacial maximum for these catchments is atypically old (~100 ka), and this allows investigation of the response to glaciation on a timescale not usually available. The geomorphology of the catchments is divided into three distinct domains on the basis of the behaviour of the trunk stream – an upper domain where the channel neither aggrades above or incises into the valley form previously carved by glacial abrasion, a middle domain where the channel incises a gorge down into glacial sediments which mantle the valley floor, and a lower domain where the channel aggrades above this postglacial sediment surface. This landscape provides a framework in which to analyze the processes and timescales of fluvial response to glacial modification. The dimensions of the gorge and the known dates of glacial retreat record a time averaged peak river incision rate of approximately 0.5 mm/y; the timescale for the river long profile to recover to a smooth, concave up form must exceed 1 Ma. These values are comparable with those from similarly sized catchments that have been transiently perturbed by changing tectonics, but have never been quoted for a glacially forced basin-scale response. I have also demonstrated that lowering of the upper reaches of the Ladakh channel long profiles by glacial processes can systematically and nonlinearly perturb the slope-area (concavity) scaling of the channel downstream of the resulting profile convexities, or knickzones. The concavity values are elevated significantly above the expected equilibrium values of 0.3-0.6, with the magnitude controlled by the relative position of the knickzone within the catchment, and thus also by the degree of glacial modification of the fluvial system. This work also documents the existence of very similar trends in measured concavities downstream of long profile convexities in other transiently responding river systems in different tectonoclimatic settings, including those responding to changes in relative channel uplift. This previously unrecognised unity of response across a wide variety of different environments argues that such a trend is an intrinsic property of river response to perturbation. Importantly, it is consistent with the scaling expected from variation in incision efficiency driven by evolving sediment flux downstream of knickzones. The pervasive nature of this altered scaling, and its implications for fluvial erosion laws in perturbed settings, have significant consequences for efforts to interpret past changes in forcings acting on river systems from modern topography. I follow this by examining in detail the channel hydraulics of the Ladakh streams as they incise in response to the glacial perturbation. I present a new framework under which the style of erosion of a natural channel can be characterized as either detachment- or transport-limited based upon comparison of the downstream distribution of shear stress with the resulting magnitude of incision. This framework also allows assessment of the importance of sediment flux driven effects in studied channels. This approach is then used to demonstrate that fluvial erosion and deposition in the Ladakh catchments is best modelled as a sediment flux dependent, thresholded, detachment-limited system. The exceptional quality of the incision record in this landscape enables an unprecedented calibration of the sediment flux function within this incision law for three different trunk streams. The resulting curves are not compatible with the theoretically-derived parabolic form of this relation, instead showing nonzero erosion rates at zero sediment flux, a rapid rise and peak at relative sediment fluxes of less than 0.5 and a quasi exponential decrease in erosional efficiency beyond this. The position of the erosional efficiency peak in relative sediment flux space and the magnitude of the curve are shown to be both variable between the catchments explored and also correlated with absolute sediment flux in the streams

Pyrene emission from monolayers 'clicked' onto quartz

A series of quartz surfaces were modifed with a series of crosslinkers and functional groups in order to obtain an azide-terminated monolayer, which was then used to immobilize pyrene onto the surface via alkyne-azide \click" chemistry. During the course of the immobilization, different ratios of tert-butyl diphenyl chlorosilane were used to control the distribution and hence the photophysical properties of the pyrene on the surface. The preparative surface reactions and photophysical properties were investigated with contact angle, X-ray photoelectron spectroscopy, UV-visible absorption and emission spectroscopy. High surface coverage was achieved of just under 1molecule per nm2. At this coverage all emission from the pyrene was in the form of excimer emission. Excimer emission dominated at all surface coverages greater than 0.45 molecules per nm2. Below this coverage the monomer emission could also be observed. The conclusions that can be drawn are important for understanding the interactions of neighboring molecules in molecular monolayers. Our results suggest that at high surface coverage a substantial number of the pyrene molecules are already close enough to their neighbors that pairs of them can be directly excited to form excimer with no requirement for diffusion. This can be stated because the long wavelength end of the pyrene absorption and excitation spectra show a broad tail that is assigned to a charge transfer band resulting from an electron being directly transferred from a ground state pyrene to a neighboring pyrene molecule. Furthermore, absorption spectra shifts also indicate that the pyrene molecules undergo some interactions on the surface when they are closely packed

Modeling the shape and evolution of normal-fault facets

Facets formed along the footwalls of active normal-fault blocks display a variety of longitudinal profile forms, with variations in gradient, shape, degree of soil cover, and presence or absence of a slope break at the fault trace. We show that a two-dimensional, process-oriented cellular automaton model of facet profile evolution can account for the observed morphologic diversity. The model uses two dimensionless parameters to represent fault slip, progressive rock weathering, and downslope colluvial-soil transport driven by gravity and stochastic disturbance events. The parameters represent rock weathering and soil disturbance rates, respectively, scaled by fault slip rate; both can be derived from field-estimated rate coefficients. In the model's transport-limited regime, slope gradient depends on the ratio of disturbance to slip rate, with a maximum that represents the angle of repose for colluvium. In this regime, facet evolution is consistent with nonlinear diffusion models of soil-mantled hillslope evolution. Under the weathering-limited regime, bedrock becomes partly exposed but microtopography helps trap some colluvium even when facet gradient exceeds the threshold angle. Whereas the model predicts a continuous gradient from footwall to colluvial wedge under transport-limited behavior, fully weathering-limited facets tend to develop a slope break between footwall and basal colluvium as a result of reduced transport efficiency on the rocky footwall slope. To the extent that the model provides a reasonable analogy for natural facets, its behavior suggests that facet profile morphology can provide useful constraints on relative potential rates of rock weathering, soil disturbance, and fault slip

Norspermidine is not a self-produced trigger for biofilm disassembly

SummaryFormation of Bacillus subtilis biofilms, consisting of cells encapsulated within an extracellular matrix of exopolysaccharide and protein, requires the polyamine spermidine. A recent study reported that (1) related polyamine norspermidine is synthesized by B. subtilis using the equivalent of the Vibrio cholerae biosynthetic pathway, (2) exogenous norspermidine at 25 μM prevents B. subtilis biofilm formation, (3) endogenous norspermidine is present in biofilms at 50–80 μM, and (4) norspermidine prevents biofilm formation by condensing biofilm exopolysaccharide. In contrast, we find that, at concentrations up to 200 μM, exogenous norspermidine promotes biofilm formation. We find that norspermidine is absent in wild-type B. subtilis biofilms at all stages, and higher concentrations of exogenous norspermidine eventually inhibit planktonic growth and biofilm formation in an exopolysaccharide-independent manner. Moreover, orthologs of the V. cholerae norspermidine biosynthetic pathway are absent from B. subtilis, confirming that norspermidine is not physiologically relevant to biofilm function in this species

2-{(E)-[1-(2-Hydroxy­ethyl)-3,3-dimethyl-3H-indol-1-ium-2-yl]vin­yl}-6-hydroxy­meth­yl-4-nitro­phenolate dihydrate

The title merocyanine-type mol­ecule, C21H22N2O5·2H2O, crystallizes in a zwitterionic form and has an E configuration at the styryl C=C bond. The styryl part of the mol­ecule and the indolium ring are slightly twisted and form a dihedral angle of 13.4 (1)°. The 1.274 (3) Å C—O bond length in the phenolate fragment is the longest among similar mol­ecules. Hydrogen bonds between solvent water mol­ecules, two hydroxyl groups and the phenolate O atom dictate the packing arrangement of mol­ecules in the crystal and join the mol­ecules into a two-dimensional polymeric network which propagates parallel to (001). Four water mol­ecules and four hydr­oxy groups form a centrosymmetric homodromic cyclic motif of O—H⋯O hydrogen bonds. Another cyclic centrosymmetric motif is generated by four water mol­ecules and two phenolate O atoms