Positron interactions with water–total elastic, total inelastic, and elastic differential cross section measurements

Abstract

Utilising a high-resolution, trap-based positron beam, we have measured both elastic and inelastic scattering of positrons from water vapour. The measurements comprise differential elastic, total elastic, and total inelastic (not including positronium formation) absolute cross sections. The energy range investigated is from 1 eV to 60 eV. Comparison with theory is made with both R-Matrix and distorted wave calculations, and with our own application of the Independent Atom Model for positron interactions.

Cross sections for positron and electron collisions with an analog of the purine nucleobases: Indole

“…Specifically, the IAM SCAR results are a factor of 3.4-4.4 larger in magnitude than the corrected experimental data at the common impact energies. This is somewhat surprising, given the quite fair accord we have found between the IAM SCAR calculations and cross sections measured at Trento or the Australian Positron Beamline Facility [26,59,73] for some other large polyatomic molecules. We note, however, that for yet further targets the level of accord was not so good or not uniformly good at all energies [55,58,72].…”

High-Resolution Measurements of e++H2O Total Cross Section

“…Since then, only a few new results have been added, experimentally [13][14][15] and theoretically [16,17], without a satisfactory agreement emerging among them. The integral (σ el ) and differential (dσ el =dΩ) elastic (el) scattering cross sections for e þ þ H 2 O have also been measured recently [17], complementing theoretical determinations [18,19].Because of the long-range forces involved in the scattering of charged projectiles from a polar molecule such as H 2 O, one of the major difficulties in measuring σ T (even in the case of electrons, e.g., [20][21][22][23]) lies in discriminating against the considerable flux of small forward-angle scattered particles (FSPs) (e.g., [16,19]). The largest error associated with FSPs arises from elastic scattering and rovibrational inelastic processes which cannot be easily distinguished from the incident flux via energy loss discrimination since this is smaller than (or comparable to) typical beam energy resolutions (e.g., the first vibrational excitation from the ground state J ¼ 0 is ≃1595 cm −1 [24]).…”

“…Since then, only a few new results have been added, experimentally [13][14][15] and theoretically [16,17], without a satisfactory agreement emerging among them. The integral (σ el ) and differential (dσ el =dΩ) elastic (el) scattering cross sections for e þ þ H 2 O have also been measured recently [17], complementing theoretical determinations [18,19].…”

“…However, in the case of H 2 O (α ≃ 9.8 a:u: and permanent dipole moment, d ≃ 1.85 D), they are expected to result in large errors [16,19], e.g., ≃100% of the measured σ T at 10 eV [28]. In order to remedy this, theoretical dσ el =dΩ, in conjunction with the experimental angular resolutions, are routinely employed to compute the FSPs contribution and to correct the measured σ T [15,17,28].In this Letter, we present σ T for e þ þ H 2 O measured for the first time with a fully electrostatic positron beam characterized by an angular discrimination against FSPs of ≃1°. The total cross sections have been measured in the energy range (10-300) eV.…”

“…The spherical complex optical potential (SCOP) [16] employed an isotropic complex potential to describe the interaction between positron and water molecule, the imaginary component of the potential chosen of the same form as for electrons and empirically scaled using e þ data. The independent atomic model (IAM) [17] calculated the scattering amplitudes for each of the atoms independently using the optical potential theory, then adding them in order to obtain σ T for the molecule. The internal structure of the molecule was considered by applying the screen corrected additive rule (SCAR), previously used to calculate electron-molecule scattering with good accuracy in the energy range (10-1000) eV (e.g., [34,35]) and, more recently, adapted to describe positronmolecule scattering [17,36,37].…”