TY - JOUR

T1 - Test of the Validity of Bragg´s Rule for Mean Excitation Energies of small molecules and ions

AU - Sauer, Stephan P. A.

AU - Sabin, John R.

AU - Oddershede, Jens

PY - 2019

Y1 - 2019

N2 - The purpose of this paper is to identify the fragmentation patterns of molecules and ions that give the best fulfilment of a Bragg’s rule estimation of the mean excitation energy of the molecules and ions. We investigate the effect of chemical binding on the validity of Bragg’s rule for the calculation of stopping cross sections and mean excitation energies. As test cases we use a series of small molecules and molecular ions, primarily carbon and nitrogen halides. We are using several nuclear fragments of the same molecule or ion to test the dependence of Bragg’s rule on the binding energy of the molecules/ions. The mean excitation energies of the molecules/ions and their fragments are computed with the same method. We find that neglect of chemical binding nearly always results in an underestimation of the mean excitation energies. We also find that the fully atomic decomposition of any molecule but a diatomic molecule never gives the best fulfilment of Bragg’s rule. The best fulfilment of Bragg’s rule is obtained when using a fragmentation pattern with as few bonds as possible broken and when the choice of fragments is guided by chemical knowledge and intuition. By investigating several alternative fragmentation patterns for a molecule/ion, guidelines for choice of optimal Bragg rule fragmentation are suggested. Using the best fragmentation pattern for the tested molecules and ions, the error in the mean excitation energy is of the order of 5 % or less, implying an error of not more than 1 % in the pure Bethe stopping power as a result of applying Bragg’s rule.

AB - The purpose of this paper is to identify the fragmentation patterns of molecules and ions that give the best fulfilment of a Bragg’s rule estimation of the mean excitation energy of the molecules and ions. We investigate the effect of chemical binding on the validity of Bragg’s rule for the calculation of stopping cross sections and mean excitation energies. As test cases we use a series of small molecules and molecular ions, primarily carbon and nitrogen halides. We are using several nuclear fragments of the same molecule or ion to test the dependence of Bragg’s rule on the binding energy of the molecules/ions. The mean excitation energies of the molecules/ions and their fragments are computed with the same method. We find that neglect of chemical binding nearly always results in an underestimation of the mean excitation energies. We also find that the fully atomic decomposition of any molecule but a diatomic molecule never gives the best fulfilment of Bragg’s rule. The best fulfilment of Bragg’s rule is obtained when using a fragmentation pattern with as few bonds as possible broken and when the choice of fragments is guided by chemical knowledge and intuition. By investigating several alternative fragmentation patterns for a molecule/ion, guidelines for choice of optimal Bragg rule fragmentation are suggested. Using the best fragmentation pattern for the tested molecules and ions, the error in the mean excitation energy is of the order of 5 % or less, implying an error of not more than 1 % in the pure Bethe stopping power as a result of applying Bragg’s rule.

KW - Faculty of Science

KW - Stopping Power

KW - Mean excitation energy

KW - Bragg's rule

KW - Quantum Chemistry

KW - random phase approximation

U2 - 10.1016/j.nimb.2019.02.019

DO - 10.1016/j.nimb.2019.02.019

M3 - Journal article

VL - 444

SP - 112

EP - 116

JO - Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms

JF - Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms

SN - 0168-583X

ER -