Threshold photoelectron photoion coincidence spectroscopy and selected ion flow tube reactions of CHF3: comparison of product branching ratios

The threshold photoelectron and threshold photoelectron photoion coincidence spectra of CHF$_3$ in the range 13.5 – 24.5 eV have been recorded. Ion yields and branching ratios have been determined for the three fragments CF$_3^+$, CHF2$^+$ and CF$^+$. The mean kinetic energy releases into fragment ions involving either C-H or C-F bond cleavage have been measured, and compared with statistical and impulsive models. CHF$_3^+$ behaves in a non-statistical manner characteristic of the small-molecule limit, with the ground electronic state and low-lying excited states of CHF$_3^+$ being largely repulsive along the C-H and C-F coordinates, respectively. The rate coefficients and product ion branching ratios have been measured at 298 K in a selected ion flow tube for the reactions of CHF$_3$ with a large number of gas-phase cations whose recombination energies span the range 6.3 through 21.6 eV. A comparison between the branching ratios from the two experiments, together with an analysis of the threshold photoelectron spectrum of CHF$_3$, shows that long-range charge transfer probably occurs for the Ar$^+$ and F$^+$ atomic ions whose recombination energies lie above ca. 15 eV. Below this energy, the mechanism involves a combination of short-range charge transfer and chemical reactions involving a transition state intermediate

A study on the compressive strength of thick carbon fibre-epoxy laminates

This paper describes an experimental study that examines the effect of specimen size on the axial compressive strength of IM7/8552 carbon fibre/epoxy unidirectional laminates (UD). Laminate gauge length, width and thickness were increased by a scaling factor of 2 and 4 from the baseline specimen size of 10 mm x 10 mm x 2 mm. In all cases, strength decreased as specimen size increased, with a maximum reduction of 45%; no significant changes were observed for the axial modulus. Optical micrographs show that the failure mechanism is fibre microbuckling accompanied by matrix cracking and splitting. The location of failure in most specimens, especially the thicker ones, is where the tabs terminate and the gauge section begins suggesting that the high local stresses developed due to geometric discontinuity contribute to premature failure and hence reduced compressive strength. Two generic quasi-isotropic multi-directional (MD) lay-ups were also tested in compression, one with blocked plies [45n/90n/-45n/0n]s and the other with distributed plies [45/90/-45/0]ns with n=2, 4 and 8. The material used and test fixture was identical to that of the unidirectional specimens with three different gauge sections (30 mm x 30 mm, 60 mm x 60 mm and 120 mm x 120 mm) to establish any size effects. Strength results showed no evidence of a size effect when the specimens are scaled up using distributed plies and compared to the 2 mm thick specimens. All blocked specimens had similar compressive strengths to the sub-laminate ones apart of the 8 mm specimens that showed a 30% reduction due to extensive matrix cracking introduced during the specimen's cutting process. The calculated unidirectional failure stress (of the 0° ply within the multidirectional laminate) of about 1710 MPa is slightly higher than the average measured value of 1570 MPa of the 2 mm thick baseline unidirectional specimen, suggesting that the reduced unidirectional strength observed for the thicker specimens is a testing artefact. It appears that the unidirectional compressive strength in thicker specimens (>2 mm) is found to be limited by the stress concentration developed at the end tabs and manufacturing induced defects

Efficient HMAC-based secure communication for VANETs

Vehicular Ad Hoc Network (VANET) is an emerging type of network which facilitates vehicles on roads to communicate for driving safety. It requires a mechanism to help authenticate messages, identify valid vehicles, and remove malevolent vehicles which do not obey the rules. Most existing solutions either do not have an effective message verification scheme, or use the public key infrastructure (PKI). In this network, vehicles are able to broadcast messages to other vehicles and a group of known vehicles can also communicate securely among themselves. So group communication is necessary for the network. However, most existing solutions either do not consider this or use pairing operation to realize this. They are either not secure or not effective. In this paper, we provide a more comprehensive set of secure schemes with Hash-based Message Authentication Code (HMAC) in VANETs to overcome their shortcomings. Of course, we still need to use Pairing operation in some place. Our scheme is composed of three schemes: (1) Communications between Vehicles and Road-Side Units (RSUs), (2) One to One Communications within a Group, (3) One to One Communications without a Group. Based on our simulation study, we show that our schemes are effective and the delay caused is much lower. The average delay caused by our first scheme is nearly thousands of times lower than prior schemes. The average delay caused by our second scheme is 0.312 ms, while the delay caused by prior scheme is 12.3 ms. Meanwhile the average delay caused by our third scheme is 0.312 ms, and the delay caused by prior scheme is about 9 s. © 2012 Elsevier B.V. All rights reserved.postprin

On the Integrable Structure of the Ising Model

Starting from the lattice $A_3$ realization of the Ising model defined on a strip with integrable boundary conditions, the exact spectrum (including excited states) of all the local integrals of motion is derived in the continuum limit by means of TBA techniques. It is also possible to follow the massive flow of this spectrum between the UV $c=1/2$ conformal fixed point and the massive IR theory. The UV expression of the eigenstates of such integrals of motion in terms of Virasoro modes is found to have only rational coefficients and their fermionic representation turns out to be simply related to the quantum numbers describing the spectrum.Comment: 18 pages, no figure

The Influence of Analyser Geometry Effects in Scanning Electron Microscope Voltage Contrast Measurements

A computer simulation model used to study specimen dependent and analyser geometry dependent effects is described in this paper. With this model, the influence of the specimen dependent effect on quantitative voltage contrast measurements can be isolated from the analyser geometry dependent effect. Linearization error voltages in quantitative voltage contrast measurements arising from the individual influences of the specimen dependent and analyser geometry dependent effects are presented. The results show that the error component due to very narrow analysers dominate the total linearization error. The same situation arises when the voltage measurement point on the specimen is very near to the edge of the analyser