arXiv subjects

Sebastian George

Publications and source records attributed to Sebastian George.

9 recordsLinked to original sources

arXiv2024

Unimolecular processes in diatomic carbon anions at high rotational excitation

On the millisecond to second time scale, stored beams of diatomic carbon anions C$_2{}^-$ from a sputter ion source feature unimolecular decay of yet unexplained origin by electron emission and fragmentation. To account for the magnitude and time dependence of the experimental rates, levels with high rotational and vibrational excitation are modeled for the lowest electronic states of C$_2{}^-$, also including the lowest quartet potential. Energies, spontaneous radiative decay rates (including spin-forbidden quartet-level decay), and tunneling dissociation rates are determined for a large number of highly excited C$_2{}^-$ levels and their population in sputter-type ion sources is considered. For the quartet levels, the stability against autodetachment is addressed and recently calculated rates of rotationally assisted autodetachment are applied. Non-adiabatic vibrational autodetachment rates of high vibrational levels in the doublet C$_2{}^-$ ground potential are also calculated. The results are combined to model the experimental unimolecular decay signals. Comparison of the modeled to the experimental rates measured at the Croygenic Storage Ring (CSR) gives strong evidence that C$_2{}^-$ ions in quasi-stable levels of the quartet electronic states are the so far unidentified source of unimolecular decay.

physics.chem-ph↗

arXiv2024

Autodetachment of diatomic carbon anions from long-lived high-rotation quartet states

Highly excited C$_2{}^{-}$ ions prominently feature electron detachment at a mean decay time near 3 milliseconds with hitherto unexplained origin. Considering various sources of unimolecular decay, we attribute the signal to the electronic C$^4\Sigma^+_u$ state. Quartet C$_2{}^{-}$ levels are found to be stabilized against autodetachment by high rotation. Time constants of their rotationally assisted autodetachment into levels opening energetically at lower rotation are calculated by a theory based on the non-local resonance model. For some final levels of significantly less rotation the results conclusively explain the puzzling observations.

physics.chem-ph↗

arXiv2020

Layering of magnetic nanoparticles at amorphous magnetic templates with perpendicular anisotropy

We reveal the assembly of monodisperse magnetite nanoparticles of sizes 5 nm, 15 nm and 25 nm from dilute water-based ferrofluids onto an amorphous magnetic template with out-of-plane anisotropy. From neutron reflectometry experiments we extract density profiles and show that the particles self-assemble into layers at the magnetic surface. The layers are extremely stable against cleaning and rinsing of the substrate. The density of the layers is determined by and increases with the remanent magnetic moment of the particles.

cond-mat.soft↗

arXiv2020

Examining the $N$ = 28 shell closure through high-precision mass measurements of $^{46-48}$Ar

The strength of the $N$ = 28 magic number in neutron-rich argon isotopes is examined through high-precision mass measurements of $^{46-48}$Ar, performed with the ISOLTRAP mass spectrometer at ISOLDE/CERN. The new mass values are up to 90 times more precise than previous measurements. While they suggest the persistence of the $N$ = 28 shell closure for argon, we show that this conclusion has to be nuanced in light of the wealth of spectroscopic data and theoretical investigations performed with the \emph{SDPF-U} phenomenological shell model interaction. Our results are also compared with \emph{ab initio} calculations using the Valence Space In-Medium Similarity Renormalization Group and the Self-Consistent Green's Function approaches. Both calculations provide a very good account of mass systematics at and around $Z$ = 18 and, generally, a consistent description of the physics in this region. This combined analysis indicates that $^{46}$Ar is the transition between the closed-shell $^{48}$Ca and collective $^{44}$S.

nucl-ex↗

arXiv2019

$Q_{\textrm{EC}}$-value determination for $^{21}$Na$\rightarrow^{21}$Ne and $^{23}$Mg$\rightarrow^{23}$Na mirror-nuclei decays using high-precision mass spectrometry with ISOLTRAP at ISOLDE/CERN

We report on high-precision $Q_{\textrm{EC}}$ values of the $^{21}$Na$\rightarrow^{21}$Ne and $^{23}$Mg$\rightarrow^{23}$Na mirror $\beta$-transitions from mass measurements with ISOLTRAP at ISOLDE/CERN. A precision of $\delta m/m = 9 \cdot 10^{-10}$ and $\delta m/m = 1.5 \cdot 10^{-9}$ was reached for the masses of $^{21}$Na and $^{23}$Mg, respectively. We reduce the uncertainty of the $Q_{\textrm{EC}}$ values by a factor five, making them the most precise experimental input data for the calculation of the corrected $\mathcal{F} t$-value of these mixed Fermi/Gamow-Teller transitions. For the $^{21}$Na$\rightarrow^{21}$Ne $Q_{\textrm{EC}}$ value, a $2.3 \sigma$ deviation from the literature $Q_{\textrm{EC}}$-value was found.

nucl-ex↗

arXiv2018

Precision Mass Measurement of $^{58-63}$Cr: Nuclear Collectivity towards the \emph{N}=40 Island of Inversion

The neutron-rich isotopes $^{58-63}$Cr were produced for the first time at the ISOLDE facility and their masses were measured with the ISOLTRAP spectrometer. The new values are up to 300 times more precise than those in the literature and indicate significantly different nuclear structure from the new mass-surface trend. A gradual onset of deformation is found in this proton and neutron mid-shell region, which is a gateway to the second island of inversion around \emph{N}=40. In addition to comparisons with density-functional theory and large-scale shell-model calculations, we present predictions from the valence-space formulation of the \emph{ab initio} in-medium similarity renormalization group, the first such results for open-shell chromium isotopes.

nucl-ex↗

arXiv2018

Long-term monitoring of the internal energy distribution of isolated cluster systems

A method is presented to monitor the internal energy distribution of cluster anions via delayed electron detachment by pulsed photoexcitation and demonstrated on Co$_4{}^-$ in an electrostatic ion beam trap. In cryogenic operation, we calibrate the detachment delay to internal energy. By laser frequency scans, at room temperature, we reconstruct the time-dependent internal energy distribution of the clusters. The mean energies of ensembles from a cold and a hot ion source both approach thermal equilibrium. Our data yield a radiative emission law and the absorptivity of the cluster for thermal radiation.

physics.atm-clus↗

arXiv2016

The Cryogenic Storage Ring CSR

An electrostatic cryogenic storage ring, CSR, for beams of anions and cations with up to 300 keV kinetic energy per unit charge has been designed, constructed and put into operation. With a circumference of 35 m, the ion-beam vacuum chambers and all beam optics are in a cryostat and cooled by a closed-cycle liquid helium system. At temperatures as low as (5.5 $\pm$ 1) K inside the ring, storage time constants of several minutes up to almost an hour were observed for atomic and molecular, anion and cation beams at an energy of 60 keV. The ion-beam intensity, energy-dependent closed-orbit shifts (dispersion) and the focusing properties of the machine were studied by a system of capacitive pickups. The Schottky-noise spectrum of the stored ions revealed a broadening of the momentum distribution on a time scale of 1000 s. Photodetachment of stored anions was used in the beam lifetime measurements. The detachment rate by anion collisions with residual-gas molecules was found to be extremely low. A residual-gas density below 140 cm$^{-3}$ is derived, equivalent to a room-temperature pressure below 10$^{-14}$ mbar. Fast atomic, molecular and cluster ion beams stored for long periods of time in a cryogenic environment will allow experiments on collision- and radiation-induced fragmentation processes of ions in known internal quantum states with merged and crossed photon and particle beams.

physics.atom-ph↗

arXiv2006

Some properties of complex matrix-variate generalized Dirichlet integrals

Dirichlet integrals and the associated Dirichlet statistical densities are widely used in various areas. Generalizations of Dirichlet integrals and Dirichlet models to matrix-variate cases, when the matrices are real symmetric positive definite or hermitian positive definite, are available \cite{4}. Real scalar variables case of the Dirichlet models are generalized in various directions. One such generalization of the type-2 or inverted Dirichlet is looked into in this article. Matrix-variate analogue, when the matrices are hermitian positive definite, are worked out along with some properties which are mathematically and statistically interesting.

math.LO↗