Low-energy positron scattering from gas-phase pyrimidine: A quantum treatment of the dynamics and a comparison with experiments

Abstract

In the present work we are reporting detailed quantum scattering calculations that describe the diffusion of a beam of low-energy positrons interacting with the pyrimidine target as a gas-phase partner. The calculations have employed an essentially ab initio model for the short-range correlation interaction and for the electrostatic interaction of an impinging positron and the electron+nuclear structure of the target molecule at its equilibrium geometry. The available experiments were also performed in the low-energy region below about 30 eV and have been reported by two different experimental groups cited in the main text. Those data include integral elastic plus rotationally and vibrationally summed cross sections, together with angular distributions over the same range of energies. The effects on the scattering observables which stem from the permanent dipole moment of the title molecule are carefully analyzed and computational corrections which ensure numerical convergence are introduced and discussed. The additional uncertainties introduced by the angular discrimination error present in the experiments are also discussed and analyzed, thereby providing a numerical procedure for correcting all available data. The final comparison between experimental angular distributions and the computed counterparts produced in the present work turns out to be very good. The same applies to the comparison in size and energy dependence of the integral cross sections, where we show that our calculated quantities and the corrected experiments are in very good agreement over the whole range of available energies.

Positron-electron correlation-polarization potentials for the calculation of positron collisions with atoms and molecules*

“…Calculations for polar targets are difficult due to the slow convergence of the partial wave expansion. We reached good agreement between experiments and theory for targets such as water [14], pyrimidine [15] and tetrahydrofurane [16], after correcting the measurements for the angular discrimination error and the calculations by employing the Born approximation [17]. For non-polar molecules the angular discrimination is much smaller [3,18].…”

Low-energy positron scattering from DNA nucleobases: the effects from permanent dipoles

“…In this study all molecules are initially in the lowest rotational state (J = 0, τ = 0), because the effects on the cross section of populating higher rotationally excited states are less than a few percent for temperatures up to 500 K, as we have shown earlier for positron scattering from pyrimidine [9].…”

“…In order to compare experimental and computational integrated cross sections, one therefore needs to add the estimated partially integrated differential cross section, which includes all scattering events into forward direction with scattering angles smaller than the angular discrimination angle θ min . In our recent work on positron scattering off pyrimidine [9] and tetrahydrofuran [21] we have compared computed cross sections with experimental data in this way. This can be done by an integration over the computed differential cross sections dσ calc dΩ carried out between the forward direction and the angular discrimination angle θ min…”

The POCOBIO Database for Computed Scattering Cross-Sections for Positron Collisions with Biomolecular Systems

“…Currently we have data available for the positron collisions with the following biomolecules: pyrimidine [9], tetrahydrofuran (THF) [10], uracil [11] and the DNA nucleobases adenine, guanine, thymine and cytosine [12]. The last manuscript includes cross section data for 6 tautomers of adenine, for 9 tautomers of guanine and for 5 tautomers of cytosine.…”

“…The errors are usually below 10 per cent. In Franz and Gianturco [9,10] we are correcting the experimental integral cross sections for the angular discrimination error. We follow the procedure outlined by Kauppila et al [19] and Kwan et al [20] in order to take into account the effect of electro-magnetic fields in the scattering cell.…”