ALBERT

Notes: Abstract. Deterministic leaching models are used to estimate regional losses of nitrate from agricultural land to the environment. The estimated leaching losses are associated with uncertainty arising from uncertainty in the input data used. In the present case study we have assessed this uncertainty by use of Monte Carlo analysis, using the Latin hypercube sampling technique. Input data have preferably been adopted from publicly available data. Data which could not be retrieved from the databases was assessed by guided estimates or based on local data. The estimated annual leaching loss from the study region was around 106 kg N ha−1, which is in agreement with previous findings. The uncertainty in the leaching expressed in terms of coefficients of variation (CV) depended on the agricultural practices. CV's for arable farm rotations, cattle farm rotations, and pig farm rotations were around 20, 30 and 40%, respectively. Breakdown of the total uncertainty into contributions of different error sources did not isolate one single all important source.

Notes: The temporal population profiles of F*, CF, and CF2 in a sharp-edged, pulsed (500 μs), fluorocarbon discharge are examined. F* population rises and falls with the discharge current suggesting that electron impact of the parent fluorocarbon is the primary source of emitting fluorine atoms. Ground-state CF and CF2, monitored by laser-induced fluorescence, show noticeably slower formation and decay, but a simple kinetic model assuming that each arises from direct electron impact of the parent gas fits the data. It is shown that CF can be conveniently monitored by exciting the B˜(v'=2)−X˜(v‘=0) transition with a 193 nm ArF excimer laser.

Notes: Films were produced by pulsed laser evaporation of various solid polymer targets in vacuum. Smooth films and relatively low deposition power thresholds (〈107 W/cm2 peak) were observed for strongly absorbed ultraviolet wavelengths. Poorly absorbed wavelengths gave powdery deposits. For many polymers the evaporation process did not significantly alter the chemical structure, but the molecular weight was reduced.

Notes: By measuring the rapid change in reflectivity of a substrate during film growth induced by pulsed laser evaporation, the time-of-arrival profiles of material emanating from polycarbonate and selenium targets were determined. Results for both targets are reasonably well described by Maxwell–Boltzmann velocity distributions. Selenium evaporation appears to be atomic while for polycarbonate a range of masses are involved. The high velocity of the material leaving polycarbonate strongly suggests that small polymers are not transported directly. The mechanism for polymer film formation must involve repolymerization on the substrate of species not weighing more than a few hundred amu. For both polycarbonate and selenium the time-of-arrival profiles were affected very little by changing the excitation wavelength from 248 to 1064 nm.

Notes: The laser ablation properties of a (50%)-isopropyl methyl–(50%)-n-propyl methyl silane copolymer are examined. Both 193- and 248-nm-pulsed excimer laser radiation cleanly and completely remove this material in vacuum above certain energy thresholds (30 and 50 mJ/cm2, respectively). Under these conditions the ablation properties are quite similar to those reported for typical organic polymers. Below threshold, ablation is less efficient and becomes increasingly inefficient as irradiation continues due to spectral bleaching. In the presence of air, material removal is incomplete even for high-energy densities and long exposures. The ablation rate is shown to be independent of substrate material both above and below threshold.

Notes: Aggregation of casein micelles after addition of the proteolytic enzyme chymosin has been studied by static and dynamic light scattering at three different concentrations of casein corresponding to dilutions 1:100, 1:500, and 1:1000 of native milk. The static light scattering data have been analyzed by an indirect Fourier transformation method which gives the distance distributions as a function of time. From these curves radius of gyration and an average number of casein micelles in the aggregates have been derived as a function of time. The dynamic light scattering experiments give the hydrodynamic radius as a function of time after the addition of rennet. The initial radius of gyration for the intact casein micelles is 140 nm. The corresponding hydrodynamic radius is also 140 nm. This shows that the casein micelles are not solid spheres. Inspection of a plot of relative mass versus radius of gyration for the aggregates appearing after the addition of chymosin shows that two processes take place. First extended linear aggregates are built up to a relative mass of the aggregates of about 10 and then restructuring of aggregates occurs such that increasingly compact objects are formed. Whereas the first process exhibits a relatively fast growth in size, the aggregates grow slowly in size during the second process. Further evidence of the formation of linear aggregates followed by more dense aggregates was obtained by forming the ratio between the radius of gyration and the hydrodynamic radius.This ratio increases to values of about 2.5 (indicating that linearly extended molecules are present followed) by a decrease to about 1. The log–log plot of mass versus radius of gyration is linear up to relative masses of about 10 with a slope of about 2. This extends up to sizes of 1 μm in diameter. The slope then increases to values indicating branching and thereby the formation of more compact aggregates. For relative masses below 10 and sizes below 1 μm sedimentation is unlikely to occur and information about the mechanism of aggregation can be obtained. The aggregation number as a function of time has been analyzed in terms of Smoluchowski's equations with a rate constant including both functionality and a changing barrier height as a function of the extent of proteolysis. The functionality obtained from Smoluchowski's equations is about 2.1. © 1995 American Institute of Physics.

Notes: Velocity distributions of molecular species ejected by ∼80 mJ/cm2, 266-nm laser ablation of polycarbonate, polyimide, poly(ethylene terephthalate), and poly(α-methylstyrene) are presented and discussed. Time-of-flight mass spectroscopy in conjunction with both 248- and 193-nm laser ionization was used to probe the escaping vapor. Up to three distinct waves of material pass through the ionization zone. The fastest wave (6–8×105 cm/s) appears to consist of highly degraded species such as C3; the arrival profiles are well fit by a velocity offset Maxwell–Boltzmann distribution with offsets typically 3–6×105 cm/s and transverse temperatures above 10 000 K. The second wave has a characteristic velocity of 1–2×105 cm/s, and, except with the poly(α-methylstyrene) target, the associated material is not cleanly ionized to parent ions under our typical conditions. It is hypothesized that this wave consists of hot, fairly heavy (up to a few hundred amu) radicals. The slow wave (2–5×104 cm/s) is composed of stable molecules which do not readily condense on the chamber walls. Its arrival profile is too broad to be described by a simple Maxwell–Boltzmann velocity distribution. A mechanism involving a thermal velocity distribution combined with laser-associated background vapor might explain the broad profiles. Problems related to the largely unknown and highly variable ionization cross sections of diverse organic molecules with 193- and 248-nm light are briefly discussed.

Notes: The material ejected into vacuum by 266-nm pulsed laser ablation of poly(α-methylstyrene) (PαMS), polycarbonate, poly(ethylene terephthalate), polybenzimidazole, and polyimide is examined using time-of-flight mass spectroscopy with both 193- and 248-nm ionization. PαMS is well behaved in that the primary ejected species are based on the monomer, and intact units ranging up to trimer are observed. The other four polymers show two distinct waves of material passing through the ionization zone: a fast wave (105–106 cm/s) consisting of small bare or nearly bare carbon clusters and a much slower one composed of mainly aromatic fragments in the 128±50 amu range. These species are all smaller than the corresponding monomers and tend to be fairly similar regardless of target material although the spectrum arising from each polymer is unique. It is speculated that the difference in behavior between PαMS and the others relates to the known favorable, thermally induced "unzipping'' which occurs in PαMS; when this low energy decomposition channel is not open, the laser-induced temperature rise is greater, and more severe bond-breaking processes occur. This work supports our previous conclusion that polymer film formation by laser ablation proceeds by a fragmentation/repolymerization mechanism but does not generally identify the film-forming species. As part of establishing the range of molecules our ionization scheme is sensitive to, mass spectra of a number of different permanent organic vapors were taken using 193-, 248-, and 266-nm ionization. These results are also discussed.