Resolving Length Scale Dependent Transient Disorder Through an Ultrafast Phase Transition

Material functionality can be strongly determined by structure extending only over nanoscale distances. The pair distribution function presents an opportunity to shift structural studies beyond idealized crystal models and investigate structure over varying length scales. Applying this method with ultrafast time resolution has the potential to similarly disrupt the study of structural dynamics and phase transitions. Here, we demonstrate such a measurement of CuIr$_{2}$S$_{4}$ optically pumped from its low temperature Ir-dimerized phase. Dimers are optically removed without spatial correlation, generating a structure whose level of disorder depends strongly on length scale. The re-development of structural ordering over tens of picoseconds is directly tracked over both space and time as a non-equilibrium state is approached. This measurement demonstrates both the crucial role of local structure and disorder in non-equilibrium processes and the feasibility of accessing this information with state-of-the-art XFEL facilities.Comment: 14 page manuscript with 5 figures. 6 page Supplementary with 8 figures. 20 pages and 11 figures in tota

Emergence of liquid following laser melting of gold thin films

X-ray structural science is undergoing a revolution driven by the emergence of X-ray Free-electron Laser (XFEL) facilities. The structures of crystalline solids can now be studied on the picosecond time scale relevant to phonons, atomic vibrations which travel at acoustic velocities. In the work presented here, X-ray diffuse scattering is employed to characterize the time dependence of the liquid phase emerging from femtosecond laser-induced melting of polycrystalline gold thin films using an XFEL. In a previous analysis of Bragg peak profiles, we showed the supersonic disappearance of the solid phase and presented a model of pumped hot electrons carrying energy from the gold surface to scatter at internal grain boundaries. This generates melt fronts propagating relatively slowly into the crystal grains. By conversion of diffuse scattering to a partial X-ray pair distribution function, we demonstrate that it has the characteristic shape obtained by Fourier transformation of the measured F(Q). The diffuse signal fraction increases with a characteristic rise-time of 13 ps, roughly independent of the incident pump fluence and consequent final liquid fraction. This suggests the role of further melt-front nucleation processes beyond grain boundaries

BIOGEOCHEMICAL AND MICROBIOLOGICAL STUDY OF ESTUARINE SURFACE MICROLAYERS: COMPARISON OF THE TAMAR AND PLYM ESTUARIES

The Sea surface microlayer (SML) is enriched in transparent exopolymer particles (TEP) which are the most ubiquitous gel particles in the ocean and are implicated in global carbon cycles as well as providing a substrate for microbial settlement and growth. Although there have been a number of papers examining oceanic TEP dynamics, there have been few estuarine studies into TEP. In this study we investigated the TEP concentrations in the SML and subsurface water from April to mid-July of two estuaries in South-West England, the Plym and the Tamar. Additionally, the microbial community structure of the SML and subsurface water in the two estuaries was analysed. TEP varied between the SML and subsurface water, as well as between estuaries. The Plym showed significant enrichment in the microlayer over the sample period whereas the Tamar did not. The enrichment factors in both estuaries were low, which is indicative of eutrophic environments. In the Plym, no relationships were found between the estuarine environmental parameters and TEP concentration but in the Tamar SML TEP concentration was correlated with salinity. This indicates that TEP concentration is affected by a suite of environmental parameters relationships are not simple. A distinct microbial community is present in the SML of both estuaries. There was a high amount of variation in the microbial community structure between the estuaries at low tide and over time. The findings of this paper support the idea that estuarine microlayers are a distinct and dynamic environment

Investigation into the dynamics of dissolved organic phosphorus concentrations over an annual period in a river and estuary system

Dissolved organic phosphorus (DOP) may play an important role in the control of primary production in aquatic and coastal systems. However, whereas dissolved inorganic phosphorus is routinely measured in most UK rivers and estuaries by the Environment Agency, DOP is not determined and there are few high frequency temporal records of how DOP concentrations vary seasonally and in relation to changes in river flow. The aim of research presented in this thesis was to investigate the DOP dynamics in the lower reaches of two major UK south coast rivers (the Hampshire Avon and Dorset Stour) and the receiving waters of the Christchurch Harbour estuary over an annual period at a high (i.e. weekly) sampling frequency. Water samples were collected from 1 site on the Hampshire Avon river (Knapp Mill), two sites on Dorset Stour river (Throop and Iford Bridge), and one site at Christchurch Harbour estuary (Mudeford Quay) on a weekly basis from April 2013 to April 2014.In order to investigate the concentrations and fluxes of DOP in the two rivers and the estuary, a new method of analysis of total dissolved phosphorus (TDP) using inductively coupled mass spectrometry (ICP-MS) was implemented, in addition to a well described spectrophotometric technique to measure soluble reactive phosphorus (SRP). The DOP concentration was then assumed to be the difference between the TDP concentration and the SRP concentration. ICP-MS analysis of certified reference materials containing known concentrations of TDP indicated that the technique was detecting TDP to within 5% of the expected concentration.DOP concentrations were highest in the River Stour, with the highest concentrations detected at Throop (up to 12.1μM) and Iford Bridge (up to 21.0μM) on the River Stour. The River Avon had the lowest DOP concentration of the four sites (up to 1.45μM). The estuarine site at Mudeford Quay was high in DOP concentration in the summer (ranging between 5.7μM and 18.8μM), with lower concentrations in the winter (ranging from undetectable concentrations to 2.8μM). The DOP concentration at all of the sites decreased during periods of high river flow, most likely due to dilution of DOP by rainfall.During periods of high river flow increased fluxes of TDP, SRP and DOP occurred in both rivers flowing into the Christchurch Harbour estuary, with 45.2% of the annual SRP flux at Knapp Mill and 49.5% at Throop occurring during an extended period of high river flow from 16/12/13 to 27/3/14. Riverine fluxes of DOP at Knapp Mill and Throop were highest over periods of high river flow during January and February 2014 when river flows were sustained at peak values over 69m3/s and 50m3/s respectively for several weeks. At Knapp Mill, 50% of the annual flux of DOP occurred over a 9-week period of high river flow in early 2014.During summer 2014 several transect surveys of Christchurch Harbour were conducted to investigate the relationship between TDP, SRP and DOP with salinity. On most dates TDP and SRP showed non-conservative removal of phosphate between the riverine and seawater end member concentrations. DOP concentrations on some dates showed a non-conservative removal of phosphate at low salinities whereas on other dates DOP showed a small increasing linear relationship with salinity.In addition to TDP, SRP and derived DOP concentrations, weekly measurements of nitrate and silicate concentrations were analysed on samples collected from each site as well as measurements of water quality parameters including temperature, conductivity/salinity, chlorophyll a concentration and turbidity. Mean dissolved inorganic nitrate concentrations over the sampling period were 391.6μM at Knapp Mill, 508.9μM at Throop, 545.6μM at Iford Bridge and 319.3μM at Mudeford Quay. Mean silicate concentrations over the sampling period were 135.4μM at Knapp Mill, 104.3μM at Throop, 112.8μM at Iford Bridge and 84.1μM at Mudeford Quay. Highest concentrations of inorganic nutrients were seen over summer, with the lowest seen in winter.The results from this study have provided new data on the seasonal changes in DOP in comparison to SRP measured at weekly intervals in two contrasting south coast UK rivers that feed into the small shallow eutrophic estuary of Christchurch Harbour. These high frequency measurements have provided new insight into the processes influencing the flux of dissolved phosphate from temperate rivers into estuaries highlighting the importance of including analysis of organic forms of nutrients when assessing the nutrient status of natural waters

Seasonal microbial community dynamics correlate with phytoplankton-derived polysaccharides in surface coastal waters

Phytoplankton produce large amounts of polysaccharide gel material known as transparent exopolymer particles (TEP). We investigated the potential links between phytoplankton-derived TEP and microbial community structure in the sea surface microlayer and underlying water at the English Channel time-series station L4 during a spring diatom bloom, and in two adjacent estuaries. Major changes in bacterioneuston and bacterioplankton community structure occurred after the peak of the spring bloom at L4, and coincided with the significant decline of microlayer and water column TEP. Increased abundance of Flavobacteriales and Rhodobacterales in bacterioneuston and bacterioplankton communities at L4 was significantly related to the TEP decline, indicating that both taxa could be responsible. The results suggest that TEP is an important factor in determining microbial diversity in coastal waters, and that TEP utilisation could be a niche occupied by Flavobacteriales and Rhodobacterales