Vacuum-UV negative photoion spectroscopy of CH3F, CH3Cl and CH3Br

Using tunable vacuum-UV radiation from a synchrotron, negative ions are detected by quadrupolar mass spectrometry following photoexcitation of three gaseous halogenated methanes CH$_3$X (X = F,Cl,Br). The anions X$^-$, H$^-$, CX$^-$, CHX$^-$ and CH$_2$X$^-$ are observed, and their ion yields recorded in the range 8-35 eV. The anions show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically described as AB + h$v$ $\rightarrow$ A$^-$ + B$^+$ (+ neutrals). Absolute cross sections for ion-pair formation are obtained by calibrating the signal intensities with those of F$^-$ from both SF$_6$ and CF$_4$. The cross sections for formation of X$^-$ + CH$_3$$^+$ are much greater than for formation of CH$_2$X$^-$ + H$^+$. In common with many quadrupoles, the spectra of $m$/$z$ 1 (H$^-$) anions show contributions from all anions, and only for CH$_3$Br is it possible to perform the necessary subtraction to obtain the true H$^-$ spectrum. The anion cross sections are normalised to vacuum-UV absorption cross sections to obtain quantum yields for their production. The appearance energies of X$^-$ and CH$_2$X$^-$ are used to calculate upper limits to 298 K bond dissociation energies for D$^o$ (H$_3$C-X) and D$^o$ (XH$_2$C-H) which are consistent with literature values. The spectra suggest that most of the anions are formed indirectly by crossing of Rydberg states of the parent molecule onto an ion-pair continuum. The one exception is the lowest-energy peak of F$^-$ from CH$_3$F at 13.4 eV, where its width and lack of structure suggest it may correspond to a direct ion-pair transition

Investigating Heating and Cooling in the BCS & B55 Cluster Samples

We study clusters in the BCS cluster sample which are observed by Chandra and are more distant than redshift, z>0.1. We select from this subsample the clusters which have both a short central cooling time and a central temperature drop, and also those with a central radio source. Six of the clusters have clear bubbles near the centre. We calculate the heating by these bubbles and express it as the ratio r_heat/r_cool=1.34+/-0.20. This result is used to calculate the average size of bubbles expected in all clusters with central radio sources. In three cases the predicted bubble sizes approximately match the observed radio lobe dimensions. We combine this cluster sample with the B55 sample studied in earlier work to increase the total sample size and redshift range. This extended sample contains 71 clusters in the redshift range 0<z<0.4. The average distance out to which the bubbles offset the X-ray cooling in the combined sample is at least r_heat/r_cool=0.92+/-0.11. The distribution of central cooling times for the combined sample shows no clusters with clear bubbles and t_cool>1.2Gyr. An investigation of the evolution of cluster parameters within the redshift range of the combined samples does not show any clear variation with redshift.Comment: 12 pages, 9 figures, accepted for publication in MNRA

Vacuum-UV negative photoion spectroscopy of CF3Cl, CF3Br and CF3I

Using synchrotron radiation negative ions have been detected by mass spectrometry following vacuum-UV photoexcitation of trifluorochloromethane (CF$_3$Cl), trifluorobromomethane (CF$_3$Br) and trifluoroiodomethane (CF$_3$I). The anions F$^-$, X$^-$, F$_2^-$, FX$^-$, CF$^-$, CF$_2^-$ and CF$_3^-$ were observed from all three molecules, where X = Cl, Br or I, and their ion yields recorded in the range 8-35 eV. With the exception of Br$^-$ and I$^-$, the anions observed show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Dissociative electron attachment, following photoionization of CF$_3$Br and CF$_3$I as the source of low-energy electrons, is shown to dominate the observed Br$^-$ and I$^-$ signals, respectively. Cross sections for ion-pair formation are put on to an absolute scale by calibrating the signal strengths with those of F$^-$ from both SF$_6$ and CF$_4$. These anion cross sections are normalized to vacuum-UV absorption cross sections, where available, and the resulting quantum yields are reported. Anion appearance energies are used to calculate upper limits to 298 K bond dissociation energies for $D^0$(CF$_3$-X) which are consistent with literature values. We report new data for $D^0$(CF$_2$I$^-$-F) ≤ 2.7 ± 0.2 eV and $\Delta_fH^0_{298}$ (CF$_2$I$^+$) ≤ (598 ± 22) kJ mol$^{-1}$. No ion-pair formation is observed below the ionization energy of the parent molecule for CF$_3$Cl and CF$_3$Br, and only weak signals (in both I$^-$ and F$^-$) are detected for CF$_3$I. These observations suggest neutral photodissociation is the dominant exit channel to Rydberg state photoexcitation at these lower energies

Feasibility study of a synthesis procedure for array feeds to improve radiation performance of large distorted reflector antennas

The topics covered include the following: (1) performance analysis of the Gregorian tri-reflector; (2) design and performance of the type 6 reflector antenna; (3) a new spherical main reflector system design; (4) optimization of reflector configurations using physical optics; (5) radiometric array design; and (7) beam efficiency studies

Hierarchy of stratigraphic forcing: Example from Middle Pennsylvanian shelf carbonates of the Paradox basin

Middle Pennsylvanian (Desmoinesian) shelf carbonates in the southwestern Paradox basin display three superimposed orders of stratigraphic cyclicity with a systematic vertical succession of facies, cycle, and sequence stacking patterns. Fifth-order cycles [34 cycles in a 645-ft (197-m) section; average 20 ft (6.1 m) thick; mean period 29,000 years] are grouped into fourth-order sequences [average 100 ft (30 m) thick; mean period 257,000 years], which in turn stack vertically to define a third-order sequence [650+ ft (200+ m) thick; 2-3 m.y. duration]. Fifth-order cycles are composed of shallow ing-upward packages of predominantly subtidal shelf carbonates with sharp cycle boundaries (either exposure or flooding surfaces). Fifth-order cycles are packaged into fourth-order sequences bounded by regionally correlative subaerial exposure surfaces. These type 1 sequences contain a downdip, restricted lowstand wedge of evaporites and quartz clastics in topographic lows on the Paradox shelf (intrashelf depressions). The lowstand systems tract is overlain by a regionally correlative transgressive shaly mudstone (condensed section) and a highstand systems tract composed of thinning-upward, aggradational fifth-order cycles. Systematic variation in the thickness of fourth-order sequences (thinning upward followed by thickening upward) and systematic variations in the number of fifth-order cycles and fourth-order sequences (decreasing followed by increasing number) defines a third-order accommodation trend that is also regionally correlative. High-frequency cycles and sequences are interpreted as predominantly aggradational allocycles generated in response to composite fourth- and fifth-order glacio-eustatic sea-level fluctuations. Two different orbital forcing (Milankovitch) scenarios are evaluated to explain the composite stratigraphic cyclicity of the Paradox sequences, each of which is plausible given Desmoinesian age estimates. The cycle, sequence, and facies stacking patterns have been replicated by means of computer modeling by superimposing composite high-frequency glacio-eustasy atop regional subsidence using depth-dependent, sedimentation

Hierarchy of stratigraphic forcing: Example from Middle Pennsylvanian shelf carbonates of the Paradox basin

Middle Pennsylvanian (Desmoinesian) shelf carbonates in the southwestern Paradox basin display three superimposed orders of stratigraphic cyclicity with a systematic vertical succession of facies, cycle, and sequence stacking patterns. Fifth-order cycles [34 cycles in a 645-ft (197-m) section; average 20 ft (6.1 m) thick; mean period 29,000 years] are grouped into fourth-order sequences [average 100 ft (30 m) thick; mean period 257,000 years], which in turn stack vertically to define a third-order sequence [650+ ft (200+ m) thick; 2-3 m.y. duration]. Fifth-order cycles are composed of shallow ing-upward packages of predominantly subtidal shelf carbonates with sharp cycle boundaries (either exposure or flooding surfaces). Fifth-order cycles are packaged into fourth-order sequences bounded by regionally correlative subaerial exposure surfaces. These type 1 sequences contain a downdip, restricted lowstand wedge of evaporites and quartz clastics in topographic lows on the Paradox shelf (intrashelf depressions). The lowstand systems tract is overlain by a regionally correlative transgressive shaly mudstone (condensed section) and a highstand systems tract composed of thinning-upward, aggradational fifth-order cycles. Systematic variation in the thickness of fourth-order sequences (thinning upward followed by thickening upward) and systematic variations in the number of fifth-order cycles and fourth-order sequences (decreasing followed by increasing number) defines a third-order accommodation trend that is also regionally correlative. High-frequency cycles and sequences are interpreted as predominantly aggradational allocycles generated in response to composite fourth- and fifth-order glacio-eustatic sea-level fluctuations. Two different orbital forcing (Milankovitch) scenarios are evaluated to explain the composite stratigraphic cyclicity of the Paradox sequences, each of which is plausible given Desmoinesian age estimates. The cycle, sequence, and facies stacking patterns have been replicated by means of computer modeling by superimposing composite high-frequency glacio-eustasy atop regional subsidence using depth-dependent, sedimentation

Colour Relations in Form

The orthodox monadic determination thesis holds that we represent colour relations by virtue of representing colours. Against this orthodoxy, I argue that it is possible to represent colour relations without representing any colours. I present a model of iconic perceptual content that allows for such primitive relational colour representation, and provide four empirical arguments in its support. I close by surveying alternative views of the relationship between monadic and relational colour representation

The strong influence of substrate conductivity on droplet evaporation

We report the results of physical experiments that demonstrate the strong influence of the thermal conductivity of the substrate on the evaporation of a pinned droplet. We show that this behaviour can be captured by a mathematical model including the variation of the saturation concentration with temperature, and hence coupling the problems for the vapour concentration in the atmosphere and the temperature in the liquid and the substrate. Furthermore, we show that including two ad hoc improvements to the model, namely a Newton's law of cooling on the unwetted surface of the substrate and the buoyancy of water vapour in the atmosphere, give excellent quantitative agreement for all of the combinations of liquid and substrate considered