arXiv subjects

Tom Kirchner

Publications and source records attributed to Tom Kirchner.

23 records · Page 2Linked to original sources

arXiv2019

Electron capture and ionization cross-section calculations for proton collisions from methane and the DNA and RNA nucleobases

Net ionization and net capture cross-section calculations are presented for proton collisions from methane molecules and the DNA/RNA nucleobases adenine, cytosine, guanine, thymine, and uracil. We use the recently introduced independent-atom-model pixel counting method to calculate these cross sections in the 10 keV to 10 MeV impact energy range and compare them with results obtained from the simpler additivity rule, a previously used complete-neglect-of-differential-overlap method, and with experimental data and previous calculations where available. It is found that all theoretical results agree reasonably well at high energies, but deviate significantly in the low-to-intermediate energy range. In particular, the pixel counting method which takes the geometrical overlap of atomic cross sections into account is the only calculation that is able to describe the measurements for capture in proton-methane collisions down to 10 keV impact energy. For the nucleobases it also yields a significantly smaller cross section in this region than the other models. New measurements are urgently required to test this prediction.

physics.atm-clus↗

arXiv2019

Proton-impact-induced electron emission from biologically relevant molecules studied with a screened independent atom model

We use the recently introduced independent-atom-model pixel counting method to calculate proton-impact net ionization cross sections for a large class of biologically relevant systems including pyrimidines, purines, amino acids, and nucleotides from 10 keV to 10 MeV impact energy. Overall good agreement with experimental data, where available, is found. A scaling prescription that involves coefficients derived from the independent atom model is shown to represent the cross section results better than scalings based on the number of (bonding) valence electrons of the target molecules. It is shown that the scaled net ionization cross sections of the proton-nucleotide collision systems can be represented in terms of a simple analytical formula with four parameters to within 3% accuracy.

physics.atm-clus↗

arXiv2018

Doppler and collisional frequency shifts in trapped-atom clocks

Collisions with background gas particles can shift the resonance frequencies of atoms in atomic clocks. The internal quantum states of atoms can also become entangled with their motional states due to the recoil imparted by a collision, which leads to a further shift of the clock frequency through the relativistic Doppler shift. It can be complicated to evaluate the Doppler and collisional frequency shifts for clock atoms in such entangled states, but estimates of these shifts are essential in order to improve the accuracy of optical atomic clocks. We present a formalism that describes collisions and relativistic Doppler shifts in a unified manner, and can therefore be used to accurately estimate collisional frequency shifts in trapped-atom clocks.

physics.atom-ph↗

arXiv2018

A screened independent atom model for the description of ion collisions from atomic and molecular clusters

We apply a recently introduced model for an independent-atom-like calculation of ion-impact electron transfer and ionization cross sections to proton collisions from water, neon, and carbon clusters. The model is based on a geometrical interpretation of the cluster cross section as an effective area composed of overlapping circular disks that are representative of the atomic contributions. The latter are calculated using a time-dependent density-functional-theory-based single-particle description with accurate exchange-only ground-state potentials. We find that the net capture and ionization cross sections in p-X$_n$ collisions are proportional to $n^α$ with $2/3 \le α\le 1$. For capture from water clusters at 100 keV impact energy $α$ is close to one, which is substantially different from the value $α=2/3$ predicted by a previous theoretical work based on the simplest-level electron nuclear dynamics method. For ionization at 100 keV and for capture at lower energies we find smaller $α$ values than for capture at 100 keV. This can be understood by considering the magnitude of the atomic cross sections and the resulting overlaps of the circular disks that make up the cluster cross section in our model. Results for neon and carbon clusters confirm these trends. Simple parametrizations are found which fit the cross sections remarkably well and suggest that they depend on the relevant bond lengths.

physics.atm-clus↗

arXiv2017

The collisional frequency shift of a trapped-ion optical clock

Collisions with background gas can perturb the transition frequency of trapped ions in an optical atomic clock. We develop a non-perturbative framework based on a quantum channel description of the scattering process, and use it to derive a master equation which leads to a simple analytic expression for the collisional frequency shift. As a demonstration of our method, we calculate the frequency shift of the Sr$^+$ optical atomic clock transition due to elastic collisions with helium.

physics.atom-ph↗