or thereabouts. Because of this concentration effect, presumably related to the dielectric constant of the medium, they assigned that band to an ionic species, not to oxonium ion, however, but erroneously to the hypothetical counterion HF2". On the contrary Huong and Couzi,33 who extended their measurements to many other solvents, concluded against proton transfer mainly on the basis of electrical conductivity34 and Raman spectra.35 Instead, they assigned the bands at about 1850 cm"1 to the overtone of the O-H-F bending in molecular complexes. But this also is untenable on the grounds both of intensity (overtones very much stronger than fundamentals) and frequency (large positive anharmonicities). Another objection they invoked had to do with the almost constant frequency of the 1850-cm"1 band in widely different organic bases. These objections can be dismissed in terms of proton-transfer complexes, ROH+-P. The near invariance of the 1850-cm"1 frequency (Table I) is a consequence of the strong electrostatic character of the H bond in these complexes. There may also be some partial compensation of the mass effect of the substituent and the strength of the organic base. In fact, the weaker bases, such as acetonitrile and the mercaptans, may be only slightly protonated by HF, hence the lower bending frequency.Besides the OH (or XH) bending there is also the stretching frequency which, generally speaking, varies in the opposite direction.

No abstract

A quantitative study of the differential beam hardening effect of the flattening filter on the 6-MV beam of Clinac 2100C has been conducted with Monte Carlo simulations using EGS4 code. The fluence-weighted photon energy of the unfiltered beam decreases from 1.35 MeV at central axis (CAX) to 1.22 MeV at an off-axis distance (OAD) of 20.0 cm. Compared to the unfiltered beam, the fluence-weighted photon energy of the filtered beam increases to 1.93 MeV at CAX and to 1.36 MeV at an OAD f 20.0 cm, respectively. The beam hardening effect was found to be 2.1 times higher at CAX than at an OAD of 20.0 cm. With the differential filtration of the flattening filter, the photon energy fluence reduced to 44% and 78% at CAX and an OAD of 20.0 cm respectively, resulting in the energy fluence of the filtered beam being flat from CAX to an OAD of 20.0 cm. The differential transmission ratios between the high energy and low energy photons decrease as the OAD increases. The percentage depth doses (PDDs) at field size of 10.0 cm x 10.0 cm for both the filtered and unfiltered 6-MV beams at CAX and at an OAD of 15.0 cm were calculated with a Monte Carlo technique based on the simulated spectra and fluence. The calculated PDDs were found to be consistent with the measured data for the filtered beam at CAX and an OAD of 15.0 cm. The beam quality (BQ) of the filtered beam at CAX is also higher than that of the same beam at an OAD of 15.0 cm. All the above results quantitatively demonstrate the differential beam hardening effects of a flattening filter on a therapeutic x-ray beam.

No abstract

Abstract— High concentrations of organic chromophores, in particular pyrene, were found to incorporate into the cluster network of water swollen Nation. The vibrational structure of the fluorescent molecule and its fluorescence lifetime were used to probe the location of pyrene in this structure. Strong heavy‐atom induced room temperature phosphorescence was observed from the chromophore in the water swollen membrane. Excimer formation in tm‐butanol swollen Nation was observed and was utilized to estimate the micro viscosity in this system.