arXiv subjects

R. Richter

Publications and source records attributed to R. Richter.

At least 19 recordsLinked to original sources

arXiv2026

The ASD2 Chip for the Upgrade of the ATLAS MDT Chamber Readout at HL-LHC

Upgrading the ATLAS detector for operation at the High-Luminosity LHC requires a new, more se-lective trigger scheme to control the readout of MDT drift-tube chambers by incorporating RPC trigger information. This necessitates replacing the MDT readout electronics, including the front-end boards that house the ASD amplifiers and the custom-designed TDC. The latter buffers timing measurements for transmission to a chamber-mounted data concentrator (CSM), which, in turn, communicates with the MDT Data Processor, where MDT tracking data are combined with RPC timing information, to specifically identify high-energy muon tracks. In this article, we report on the new ASD2 preamplifier, discussing its architecture, design details, functionality and measured performance. We also present test results for chips from MPW runs, the engineering run and the volume production of 80,000 chips. Unlike the ASD1 preamplifier, currently used for MDT readout, which relies on 500 nm chip tech-nology, the ASD2 is manufactured using the more advanced 130 nm technology. This offers a range of technical improvements that enhance critical performance parameters such as signal rise time, noise levels, and the reproducibility of threshold settings across a chip's channels. Comparative measurements are presented to verify this improved performance. Finally, we discuss the robust-ness of ASD2 against environmental conditions like radiation exposure and potential high-voltage discharges within the MDT drift tubes.

physics.ins-det↗

arXiv2025

Intracluster ion-molecule reaction in quinoline and isoquinoline dimers under the influence of diverse ionizing radiations

This work demonstrates the tendency of two model PANH isomers to dimerize under pure ambient evaporative conditions and then undergo complex intracluster ion-molecule reactions to produce rich chemistry. Despite the population of such dimers at room temperature is found to be relatively low, they are found to produce observable effects in typical stellar radiation conditions. It is also demonstrated that various types of energetic radiation (UV radiation at 266 nm, synchrotron VUV radiation and high-energy protons) can induce intracluster ion-molecule reactions in the dimers. The existence of such dimers is confirmed via the analysis of the mass-selected photoelectron spectra of various species observed in the mass spectra. The signal from such processes is enhanced by UV multiphoton ionization/dissociation and is analysed using energy-correlated time-of-flight mass spectrometry. These measurements, together with the dependence on laser intensity, disclose the reaction energetics as well as the hierarchy of the decay of the reaction products. The findings of this work on dimer-driven ion-molecular reactions in quinoline and isoquinoline provide an alternative to the path for molecular growth in the astrochemical environment through cluster dynamics, which is otherwise attributed to dust and ice-driven processes.

physics.chem-ph↗

arXiv2024

Observation of sequential three-body dissociation of camphor molecule -- a native frame approach

The three-body dissociation dynamics of the dicationic camphor molecule (C$_{10}$H$_{16}$O$^{2+}$) resulting from Auger decay are investigated using soft X-ray synchrotron radiation. A photoelectron-photoion-photoion coincidence (PEPIPICO) method, a combination of a velocity map imaging (VMI) spectrometer and a time-of-flight (ToF) spectrometer is employed to measure the 3D momenta of ions detected in coincidence. The ion mass spectra and the ion-ion coincidence map at photon energies of 287.9 eV (below the C 1s ionization potential) and 292.4 eV (above the C 1s ionization potential for skeletal carbon) reveal that fragmentation depends on the final dicationic state rather than the initial excitation. Using the native frame method, three new fragmentation channels are discussed; (1) CH$_2$CO$^+$ + C$_7$H$_{11}^+$ + CH$_3$, (2) CH$_3^+$ + C$_7$H$_{11}^+$ + CH$_2$CO, and (3) C$_2$H$_5^+$ + C$_6$H$_9^+$ + CH$_2$CO. The dominating nature of sequential decay with deferred charge separation is clearly evidenced in all three channels. The results are discussed based on the experimental angular distributions and momenta distributions, corroborated by geometry optimization of the ground, monocationic, and dicationic camphor molecule.

physics.chem-ph↗

arXiv2023

Observation of interatomic Coulombic decay induced by double excitation of helium in nanodroplets

Interatomic Coulombic decay (ICD) plays a crucial role in weakly bound complexes exposed to intense or high-energy radiation. So far, neutral or ionic atoms or molecules have been prepared in singly excited electron or hole states which can transfer energy to neighboring centers and cause ionization and radiation damage. Here we demonstrate that a doubly excited atom, despite its extremely short lifetime, can decay by ICD; evidenced by high-resolution photoelectron spectra of He nanodroplets excited to the 2s2p+ state. We find that ICD proceeds by relaxation into excited He$^*$He$^+$ atom-pair states, in agreement with calculations. The ability of inducing ICD by resonant excitation far above the single-ionization threshold opens opportunities for controlling radiation damage to a high degree of element specificity and spectral selectivity.

physics.atom-ph↗

arXiv2023

Spectroscopically resolved resonant interatomic Coulombic decay in photoexcited large He nanodroplets

Interatomic Coulombic decay (ICD) processes play a crucial role in weakly bound complexes exposed to intense or high-energy radiation. Using large helium nanodroplets, we demonstrate that ICD is efficient even when the droplets are irradiated by weak synchrotron radiation at relatively low photon energies. Below the ionization threshold, resonant excitation of multiple centers efficiently induces resonant ICD as previously observed for intense pulses [A. C. LaForge et al., PRX 11, 021011 (2021)]. More surprisingly, we observe ICD even above the ionization threshold due to recombination of photoelectrons and ions into excited states which subsequently decay by ICD. This demonstrates the importance of secondary processes, in particular electron scattering and recombination, in inducing ICD in extended condensed phase systems. In addition, we show that ICD can serve as a diagnostic tool for monitoring the relaxation dynamics of highly-excited and ionized weakly-bound nanosystems.

physics.atom-ph↗

arXiv2023

Efficient Indirect Interatomic Coulombic Decay Induced by Photoelectron Impact Excitation in Large He Nanodroplets

Ionization of matter by energetic radiation generally causes complex secondary reactions which are hard to decipher. Using large helium nanodroplets irradiated by XUV photons, we show that the full chain of processes ensuing primary photoionization can be tracked in detail by means of high-resolution electron spectroscopy. We find that elastic and inelastic scattering of photoelectrons efficiently induces interatomic Coulombic decay (ICD) in the droplets. This type of indirect ICD even becomes the dominant process of electron emission in nearly the entire XUV range in large droplets with radius $\gtrsim40~$nm. Indirect ICD processes induced by electron scattering likely play an important role in other condensed phase systems exposed to ionizing radiation as well, including biological matter.

physics.atm-clus↗

arXiv2022

The photochemical ring-opening reaction of 1,3-cyclohexadiene: complex dynamical evolution of the reactive state

The photochemically induced ring-opening isomerization reaction of 1,3-cyclohexadiene (CHD) to 1,3,5-hexatriene (HT) is a textbook example of a pericyclic reaction, and has been amply investigated with advanced spectroscopic techniques. The generally accepted description of the isomerization pathway starts with a valence excitation to the lowest-lying bright state, followed by a passage through a conical intersection to a dark doubly excited state, and finally a branching between either the return to the ground state of the cyclic molecule or the actual ring-opening reaction leading to the open-chain isomer. It was traditionally assumed that the dark reactive state corresponds to the second excited state of CHD at the Franck-Condon geometry. Here in a joint experimental and computational effort we demonstrate that the evolution of the excitation-deexcitation process is much more complex than usually described. In particular, we show that an initially high-lying electronic state smoothly decreasing in energy along the reaction path plays a key role in the ring-opening reaction. The conceptual basis of our work is that the dynamics to consider here is determined by diabatic states, whose populations are the ones closely related to the observed photoelectron signal.

physics.chem-ph↗

arXiv2020

Photoelectron spectroscopy of coronene molecules embedded in helium nanodroplets

We present the first measurement of a one-photon extreme-ultraviolet photoelectron spectrum (PES) of molecules embedded in superfluid helium nanodroplets. The PES of coronene is compared to gas phase and the solid phase PES, and to electron spectra of embedded coronene generated by charge transfer and Penning ionization through ionized or excited helium. The resemblence of the He-droplet PES to the one of the solid phase indicates that mostly Cor clusters are photoionized. In contrast, the He-droplet Penning-ionization electron spectrum is nearly structureless, indicating strong perturbation of the ionization process by the He droplet. These results pave the way to extreme ultraviolet photoelectron spectroscopy (UPS) of clusters and molecular complexes embedded in helium nanodroplets.

physics.atm-clus↗

arXiv2020

Penning Spectroscopy and Structure of Acetylene Oligomers in He Nanodroplets

Embedded atoms or molecules in a photoexcited He nanodroplet are well-known to be ionized through inter-atomic relaxation in a Penning process. In this work, we investigate the Penning ionization of acetylene oligomers occurring from the photoexcitation bands of He nanodroplets. In close analogy to conventional Penning electron spectroscopy by thermal atomic collisions, the n = 2 photoexcitation band plays the role of the metastable atomic $1s2s$ $^{3,1}S$ He$^\ast$. This facilitates electron spectroscopy of acetylene aggregates in the sub-kelvin He environment, providing the following insight into their structure: The molecules in the dopant cluster are loosely bound van der Waals complexes rather than forming covalent compounds. In addition, this work reveals a Penning process stemming from the n = 4 band where charge-transfer from autoionized He in the droplets is known to be the dominant relaxation channel. This allows for excited states of the remnant dopant oligomer Penning-ions to be studied. Hence, we demonstrate Penning ionization electron spectroscopy of doped droplets as an effective technique for investigating dopant oligomers which are easily formed by attachment to the host cluster.

physics.atm-clus↗

arXiv2019

Charge-exchange dominates long-range interatomic Coulombic decay of excited metal-doped He nanodroplets

Atoms and molecules attached to rare gas clusters are ionized by an interatomic autoionization process traditionally termed 'Penning ionization' when the host cluster is resonantly excited. Here we analyze this process in the light of the interatomic Coulombic decay (ICD) mechanism, which usually contains a contribution from charge exchange at short interatomic distance, and one from virtual photon transfer at large interatomic distance. For helium (He) nanodroplets doped with alkali metal atoms (Li, Rb), we show that long-range and short-range contributions to the interatomic autoionization can be clearly distinguished by detecting electrons and ions in coincidence. Surprisingly, ab initio calculations show that even for alkali metal atoms floating in dimples at large distance from the nanodroplet surface, autoionization is largely dominated by charge exchange ICD. Furthermore, the measured electron spectra manifest ultrafast internal relaxation of the droplet into mainly the 1s2s 1^S state and partially into the metastable 1s2s 3^S state.

physics.atm-clus↗

arXiv2019

Ultrafast relaxation of photoexcited superfluid He nanodroplets

The relaxation of photoexcited nanosystems is a fundamental process of light-matter interaction. Depending on the couplings of the internal degrees of freedom, relaxation can be ultrafast, converting electronic energy in a few fs, or slow, if the energy is trapped in a metastable state that decouples from its environment. Here, helium nanodroplets are resonantly excited by femtosecond extreme-ultraviolet (XUV) pulses from a seeded free-electron laser. Despite their superfluid nature, we find that helium nanodroplets in the lowest electronically excited states undergo ultrafast relaxation. By comparing experimental photoelectron spectra with time-dependent density functional theory simulations, we unravel the full relaxation pathway: Following an ultrafast interband transition, a void nanometer-sized bubble forms around the localized excitation (He*) within 1 ps. Subsequently, the bubble collapses and releases metastable He* at the droplet surface. This study highlights the high level of detail achievable in probing the photodynamics of nanosystems using tunable XUV pulses.

physics.atm-clus↗

arXiv2019

Autoionization dynamics of He nanodroplets resonantly excited by intense XUV laser pulses

The ionization dynamics of helium droplets in a wide size range from 220 to 10^6 He atoms irradiated with intense femtosecond extreme ultraviolet (XUV) pulses of 10^9 ÷ 10^{12} W/cm2 power density is investigated in detail by photoelectron spectroscopy. Helium droplets are resonantly excited in the photon energy range from ~ 21 eV (corresponding to the atomic 1s2s state) up to the atomic ionization potential (IP) at ~ 25 eV. A complex evolution of the electron spectra as a function of droplet size and XUV intensity is observed, ranging from atomic-like narrow peaks due to binary autoionization, to an unstructured feature characteristic of electron emission from a nanoplasma. The experimental results are analyzed and interpreted with the help of numerical simulations based on rate equations taking into account various processes such as multi-step ionization, interatomic Coulombic decay (ICD), secondary inelastic collisions, desorption of electronically excited atoms, collective autoionization (CAI) and further relaxation processes.

physics.atm-clus↗

arXiv2018

Inelastic scattering of photoelectrons from He nanodroplets

We present a detailed study of inelastic energy-loss collisions of photoelectrons emitted from He nanodroplets by tunable extreme ultraviolet (XUV) radiation. Using coincidence imaging detection of electrons and ions, we probe the lowest He droplet excited states up to the electron impact ionization threshold. We find significant signal contributions from photoelectrons emitted from free He atoms accompanying the He nanodroplet beam. Furthermore, signal contributions from photoionization and electron impact excitation/ionization occurring in pairs of nearest-neighbor atoms in the He droplets are detected. This work highlights the importance of inelastic electron scattering in the interaction of nanoparticles with XUV radiation.

physics.atm-clus↗

arXiv2018

Highly efficient double ionization of mixed alkali dimers by intermolecular Coulombic decay

As opposed to purely molecular systems where electron dynamics proceed only through intramolecular processes, weakly bound complexes such as He droplets offer an environment where local excitations can interact with neighbouring embedded molecules leading to new intermolecular relaxation mechanisms. Here, we report on a new decay mechanism leading to the double ionization of alkali dimers attached to He droplets by intermolecular energy transfer. From the electron spectra, the process is similar to the well-known shake-off mechanism observed in double Auger decay and single-photon double ionization, however, in this case, the process is dominant, occurring with efficiencies equal to, or greater than, single ionization by energy transfer. Although an alkali dimer attached to a He droplet is a model case, the decay mechanism is relevant for any system where the excitation energy of one constituent exceeds the double ionization potential of another neighbouring molecule. The process is, in particular, relevant for biological systems, where radicals and slow electrons are known to cause radiation damage

physics.atm-clus↗

arXiv2017

A compact design for velocity-map imaging energetic electrons and ions

We present a compact design for a velocity-map imaging spectrometer for energetic electrons and ions. The standard geometry by Eppink and Parker [A. T. J. B. Eppink and D. H. Parker, Rev. Sci. Instrum. 68, 3477 (1997)] is augmented by just two extended electrodes so as to realize an additional einzel lens. In this way, for a maximum electrode voltage of 7 kV we experimentally demonstrate imaging of electrons with energies up to 65 eV. Simulations show that energy acceptances of <270 and <1,200 eV with an energy resolution of dE / E <5% are achievable for electrode voltages of <20 kV when using diameters of the position-sensitive detector of 42 and 78 mm, respectively.

physics.ins-det↗

arXiv2017

Interatomic Coulombic decay in helium nanodroplets

Interatomic Coulombic decay (ICD) is induced in helium (He) nanodroplets by photoexciting the n=2 excited state of He^+ using XUV synchrotron radiation. By recording multiple coincidence electron and ion images we find that ICD occurs in various locations at the droplet surface, inside the surface region, or in the droplet interior. ICD at the surface gives rise to energetic He^+ ions as previously observed for free He dimers. ICD deeper inside leads to the ejection of slow He^+ ions due to Coulomb explosion delayed by elastic collisions with neighboring He atoms, and to the formation of He_k^+ complexes.

physics.atm-clus↗

arXiv2017

Off-axis parabolic mirror optics for polarized Raman spectroscopy at low temperature

We report the development of a detection optics for the integration of Raman scattering and scanning probe microscopy at low temperature based on a parabolic mirror. In our set-up half of the paraboloid mirror covers a solid angle of $π$ corresponding to a numerical aperture of N.A.\,$\approx 0.85$. The optical system can be used for far- and near-field spectroscopy. In the far field the polarizations can be maintained to within 80-90\%. In combination with a scanning microscope (AFM/STM), tunneling or near-field experiments are possible with less than 10\% loss of aperture. Our set-up provides ideal conditions for the future development of tip-enhanced Raman spectroscopy (TERS) at low temperature.

physics.ins-det↗

arXiv2017

Development of a Highly Selective First-Level Muon Trigger for ATLAS at HL-LHC Exploiting Precision Muon Drift-Tube Data

The High-Luminosity LHC (HL-LHC) will provide the unique opportunity to explore the nature of physics beyond the Standard Model of strong and electroweak interactions. Highly selective first-level triggers are essential for the physics programme of the ATLAS experiment at HL-LHC, where the instantaneous luminosity will exceed the instantaneous LHC Run 1 luminosity by about an order of magnitude. The ATLAS first-level muon trigger rate is dominated by low momentum muons, which are accepted because of the moderate momentum resolution of the RPC and TGC trigger chambers. This limitation can be overcome by exploiting the data of the precision Muon Drift-Tube (MDT) chambers in the first-level trigger decision. This requires continuous fast transfer of the MDT hits to the off-detector trigger logic and fast track reconstruction algorithms. The reduction of the muon trigger rate achievable with the proposed new trigger concept, the performance of a novel fast track reconstruction algorithm, and the first hardware demonstration of the scheme with muon testbeam data taken at the CERN Gamma Irradiation Facility are discussed.

physics.ins-det↗