Total cross section measurements for positron and electron scattering on benzene molecules

Abstract

The total cross sections (TCS) for 0.7-400 eV positrons and 1.0-400 eV electrons scattered by benzene (C,H,) vapour molecules obtained by a relative method are presented. The values of TCS for positrons at low energies are very high, and the positron scattering at low energies is found to be sharply forward peaked. On the other hand, the TCS values for electrons decrease with decreasing energy at low energies. The positronium (Ps) formation cross sections are rather low in comparison with the TCS.Benzene, being one of the fundamental molecules, has been studied experimentally and theoretically in various problems. However, there has been no study of positron collisions with the benzene vapour molecule, and also no total cross section (TCS) study for electron collisions. In this letter, TCS measurements for et and e-colliding with C6H6 molecules have been presented using an absorption-type TOF apparatus. Part of this work has been already reported (Sueoka 1987).The apparatus and experimental procedure are almost the same as used in previous work ). In the present TOF technique, the retarding method, in which the retarding potential is equal to the acceleration potential, is applied to eliminate the contribution from projectiles which have suffered a large energy loss by inelastic scattering and to minimise contributions from projectiles which have scattered forward elastically but, due to angular deflection, have reduced axial velocity. For the transportation of projectiles, a magnetic field provided by a solenoid is applied on the flight path including the collision cell.As C6H6 vapour reacts with organic substances, the rotary pump oil was frequently exchanged. No problems with the vacuum system were encountered, although oil diffusion pumps were used. A glass reservoir was used. The sample vapour is controlled with an error of 1 '/o using the same system as described in our previous paper (Katayama et al 1987). The error in the gas density Ap is estimated to be almost identical to the accuracy of the pressure gauge, because the density p and temperature of the gas are controlled to sufficiently good accuracy. On the error estimation, Ap is determined by checking the sensitivity of a Baratron pressure gauge. The check was carried out by comparing with the sensitivity of an ionisation gauge in the range of Torr. It was found that the sensitivity of the Baratron in the low-pressure range was four times better than the value stated in the catalogue. The TCS measurements for et were performed at pressures lower than 1 x Torr at low energies where the TCS values are extremely high. The full scale pressure range of the Baratron used (220 BHS) was

Elastic scattering of low-energy electrons by benzene

“…Sueoka 1 and Mozejko et al 2 have measured total cross sections for a very broad energy range. Gianturco and Lucchese 3 have computed integral, differential, and partial ͑by symmetry͒ elastic cross sections using a parameter-free exact-static-exchange-plus-correlation-polarization potential.…”

“…Benzene is an intrinsically interesting target molecule due to its geometry and symmetry and is of additional interest in light of recent experimental and theoretical studies. [1][2][3][10][11][12] Our calculations employed the Schwinger multichannel ͑SMC͒ method 17,18 as implemented for parallel computers 19 and were carried out both in the static-exchange and in the static-exchange-plus-polarization ͑SEP͒ approximations. This study may be considered a first step toward studies of more complex aromatic molecules such as the fluoro-, chloro-, and fluorochlorobenzenes 20 or the DNA bases.…”

Positron Scattering on Atoms and Molecules in the Limit of Low Energy

“…1 we compare our crosssections in benzene with experiments by Sueoka [5][6][7][8][9] and collaborators, obtained by them in different time periods. Depending on experimental details, the total cross-sections by Sueoka show a rise [5] or a fall in the zero-energy limit [8]. We guess that these discrepancies depend on the magnitude of the guiding magnetic field used (varying between 3 G and 27 G [5]).…”

“…Depending on experimental details, the total cross-sections by Sueoka show a rise [5] or a fall in the zero-energy limit [8]. We guess that these discrepancies depend on the magnitude of the guiding magnetic field used (varying between 3 G and 27 G [5]). Our data show a rise, down to the lowest energy measured, in qualitative agreement with the ab-initio theory by Occhigrossi and Gianturco [10].…”

“…It is caused by the fact that due to the finite dimensions of the scattering cell exit, some of the scattered positrons can still reach the detector. This error is particularly big when guiding magnetic [5][6][7][8][9] and theory [9]. Fig.…”