arXiv subjects

Robert Richter

Publications and source records attributed to Robert Richter.

At least 19 recordsLinked to original sources

arXiv2026

More is not always better: Dissociative photoionization limits the EUV absorbing photacid generator pentafluorophenyl triflate in photolithography

Pentafluorophenyl triflate has been explored as a highly absorbing neutral photoacid generator (PAG) candidate for next generation chemically amplified resists used in extreme ultraviolet (EUV) lithography. Although increased fluorination enhances EUV absorption, this study demonstrates that such an approach does not necessarily improve photoacid generation efficiency. Using photoelectron-photoion coincidence (PEPICO) spectroscopy at the 92 eV photon energy of the EUV scanners in combination with quantum chemical calculations, the dissociative photoionization of pentafluorophenyl triflate was systematically investigated. The photoionization mass spectrum reveals extensive fragmentation, with the parent ion contributing only 3.1 % of the total signal and CF$_3^+$ representing the dominant product ion. Computed appearance energies align well with experimental trends and support a sequential fragmentation pathway involving loss of SO$_2$, CF$_3$, and CO. Crucially, none of the major dissociation channels yield precursors capable of forming triflic acid, the strong photoacid required for efficient deprotection reactions in chemically amplified resists. Combined with previous dissociative electron attachment studies indicating similarly unfavorable fragmentation, the results demonstrate that despite its high EUV absorption cross section, pentafluorophenyl triflate is unsuitable as a PAG for EUV lithography. The findings highlight the importance of understanding fundamental photoionization and electron interaction mechanisms to guide the rational design of next generation high performance EUV photoresists.

physics.chem-ph↗

arXiv2026

Valence Ionization Of Water Clusters Formed Inside Helium Nanodroplets

The ionization mechanisms of small H$_2$O/D$_2$O clusters embedded in helium nanodroplets (HNDs) irradiated with extreme ultraviolet photons of energy $h\nu$ = 21.6 eV are investigated using Penning ionization electron-ion coincidence spectroscopy. Both protonated (H$_2$O)$_{n-1}$H$^{+}$/(D$_2$O)$_{n-1}$D$^{+}$ ($n$ = 3-6) and unprotonated (H$_2$O)$_{n}$$^{+}$/(D$_2$O)$_{n}$$^{+}$ ($n$ = 2-5) cluster ions were observed. Penning ionization electron spectra measured in coincidence with water cluster ions emitted from water clusters doped into both large and small HNDs are analyzed and compared with photoelectron-photoion coincidence spectra measured for free water clusters at $h\nu$ =20.6 eV. The results reveal suppression of fragmentation inside HNDs and stabilization of intact cluster ions. Quantum chemical calculations support the coexistence of proton-transferred and hemibonded conformers under the cryogenic conditions of helium nanodroplets.

physics.chem-ph↗

arXiv2025

Tracking Microhydration of the NaCl Rocksalt Molecule in Helium Nanodroplets by Penning Ionization Electron Spectroscopy

The microhydration of rock salt (NaCl) molecules was investigated using high-resolution Penning ionization electron spectroscopy (PIES) in helium nanodroplets. Although model calculations predict that NaCl molecules are fully submerged inside the droplets, PIES of NaCl are highly resolved, in stark contrast to other molecular species. Co-doping the droplets with a controlled number of $n=5$--10 water molecules leads to efficient quenching of the NaCl Penning ionization signal and to its full suppression for $n\gtrsim 30$. Accompanying density-functional theory (DFT) and force field calculations reveal a transition from contact ion pair structures to solvent-separated ion pairs at $n=12$--15. However, it takes $n\approx 17$ water molecules to form a complete solvation shell around the Cl$^-$ anion and as many as $n\approx 34$ to fully hydrate the Na$^+$ cation, thus the entire NaCl molecule, which rationalizes the experimental findings.

physics.chem-ph↗

arXiv2025

Unraveling the relaxation dynamics of Uracil: insights from time-resolved X-ray photoelectron spectroscopy

We report a study of the electronic and nuclear relaxation dynamics of the photoexcited RNA base uracil in the gas phase, using time-resolved core level photoelectron spectroscopy together with high level calculations. The dynamics was investigated by trajectory surface-hopping calculations, and the core ionization energies were calculated for geometries sampled from these. The molecule was excited by a UV laser and dynamics was probed on the oxygen, nitrogen and carbon site by core electron spectroscopy. Assuming a particular model, we find that the initially excited $S_2(\pi\pi^*)$ state of uracil decays with a time constant of 17 $\pm$ 4 fs to the ground state directly, or to the $S_1(n\pi^*)$ state via internal conversion. We find no evidence that the $S_1(n\pi^*)$ state decays to the ground state by internal conversion; instead it decays to triplet states with a time constant of 1.6 $\pm$ 0.4 ps. Oscillations of the $S_1(n\pi^*)$ state O 1s intensity as a function of time correlate with those of calculated C4=O8 and C5=C6 bond lengths, which undergo a sudden expansion following the initial $\pi \to \pi^*$ excitation. We also observe oscillations in the mean energy of the main line (core ionized ionic state), which we tentatively assign to dynamics of the hot ground state. Our calculations support our interpretation of the data, and provide detailed insight into the relaxation processes of uracil.

physics.chem-ph↗

arXiv2024

Dissociative photoionization of EUV lithography photoresist models

The dissociative photoionization of \textit{tert}-butyl methyl methacrylate, a monomer unit found in many ESCAP resists, was investigated in a gas phase photoelectron photoion coincidence experiment employing extreme ultraviolet (EUV) synchrotron radiation at 13.5 nm. It was found that the interaction of EUV photons with the molecules leads almost exclusively to dissociation. However, the ionization can also directly deprotect the ester function, thus inducing the solubility switch wanted in a resist film. These results serve as a building block to reconstruct the full picture of the mechanism in widely used chemically amplified resist thin films, provide a knob to tailor more performant resist materials, and will aid interpreting advanced ultrafast time-resolved experiments.

cond-mat.mtrl-sci↗

arXiv2024

Performance of small-diameter muon drift tube chambers with new fast readout ASIC at high background rates

Experiments like ATLAS at the HL-LHC or detectors at future hadron colliders need muon detectors with excellent momentum resolution up to the TeV scale both at the trigger and offline reconstruction levels. This requires muon tracking chambers with high spatial resolution even at the highest background fluxes. Drift-tube chambers are the most cost-effective technology for large-area muon systems, providing the required high rate capability and three-dimensional spatial resolution. Thanks to advances in electronics, the new generation small-diameter Muon Drift Tube (sMDT) detectors with 15 mm tube diameter can be used in stand-alone mode up to the background rates expected at future hadron collider experiments, providing event times and second coordinates without additional trigger chambers. New developments in integrated front-end electronics include fast baseline restoration of the shaped signal and picosecond time-to-digital converters for second coordinate measurement with double-sided read-out. Self-triggered operation is now possible using modern high-performance FPGAs for real-time pattern recognition and track reconstruction. A new amplifier shaper discriminator chip in 65 nm TSMC CMOS technology with increased sensitivity and faster baseline recovery has been developed to cope with high background fluxes. Extensive test beam campaigns using sMDT chambers with new readout electronics have been performed at the CERN Gamma Irradiation Facility (GIF++). Results show that the shorter peaking time of the new chip enhances the spatial resolution of the drift tubes by up to 100 $μ$m at a background rate of 1 MHz, the maximum rate expected at the 100 TeV collider experiment.

hep-ex↗

arXiv2023

Electron energy loss and angular asymmetry induced by elastic scattering in helium droplets

Helium nanodroplets are ideal model systems to unravel the complex interaction of condensed matter with ionizing radiation. Here we study the effect of purely elastic electron scattering on angular and energy distributions of photoelectrons emitted from He nanodroplets of variable size ($10$-$10^9$ atoms per droplets). For large droplets, photoelectrons develop a pronounced anisotropy along the incident light beam due to a shadowing effect within the droplets. In contrast, the detected photoelectron spectra are only weakly perturbed. This opens up possibilities for photoelectron spectroscopy of dopants embedded in droplets provided they are smaller than the penetration depth of the light and the trapping range of emitted electrons.

physics.atm-clus↗

arXiv2022

Single-photon hot electron ionization of C$_{70}$

Gas phase C$_{70}$ molecules have been ionized with single photons of energies between 16 eV and 70 eV and the electron spectra measured with velocity map imaging in coincidence with the ions. The doubly ionized and unfragmented species was present at photon energies of 22 eV and up, and triply charged ions from 55 eV. The low kinetic energy parts of the spectra are explained with thermal emission of transient hot electrons. Deviations at high photon energies are used to determine a value for the initial electron equilibration time. We propose a generally applicable mechanism, named Resonance Ionization Shadowing, for the creation of hot electrons by absorption of above-threshold energy photons.

physics.atom-ph↗

arXiv2021

A systematic study of the valence electronic structure of cyclo(Gly-Phe), cyclo(Trp-Tyr) and cyclo(Trp-Trp) dipeptides in gas phase

The electronic energy levels of cyclo(Glycine-Phenylalanine), cyclo(Tryptophan-Tyrosine) and cyclo(Tryptophan-Tryptophan) dipeptides are investigated with a joint experimental and theoretical approach. Experimentally, valence photoelectron spectra in the gas phase are measured using VUV radiation. Theoretically, we first obtain low-energy conformers through an automated conformer-rotamer ensemble sampling scheme based on tight-binding simulations. Then, different first principles computational schemes are considered to simulate the spectra: Hartree-Fock (HF), density functional theory (DFT) within the B3LYP approximation, the quasi--particle GW correction, and the quantum-chemistry CCSD method. Theory allows to assign the main features of the spectra. A discussion on the role of electronic correlation is provided, by comparing computationally cheaper DFT scheme (and GW) results with the accurate CCSD method.

physics.chem-ph↗

arXiv2021

Coincident angle-resolved state-selective photoelectron spectroscopy of acetylene molecules: a candidate system for time-resolved dynamics

The acetylene-vinylidene system serves as a benchmark for investigations of ultrafast dynamical processes where the coupling of the electronic and nuclear degrees of freedom provides a fertile playground to explore the femto- and sub-femto-second physics with coherent extreme-ultraviolet (EUV) photon sources both on the table-top as well as free-electron lasers. We focus on detailed investigations of this molecular system in the photon energy range $19...40$ eV where EUV pulses can probe the dynamics effectively. We employ photoelectron-photoion coincidence (PEPICO) spectroscopy to uncover hitherto unrevealed aspects of this system. In this work, the role of excited states of the $C_{2}H_{2}^{+}$ cation, the primary photoion, is specifically addressed. From photoelectron energy spectra and angular distributions, the nature of the dissociation and isomerization channels is discerned. Exploiting the $4π$-collection geometry of velocity map imaging spectrometer, we not only probe pathways where the efficiency of photoionization is inherently high but also perform PEPICO spectroscopy on relatively weak channels.

physics.atom-ph↗

arXiv2020

A new method for measuring angle-resolved phases in photoemission

Quantum mechanically, photoionization can be fully described by the complex photoionization amplitudes that describe the transition between the ground state and the continuum state. Knowledge of the value of the phase of these amplitudes has been a central interest in photoionization studies and newly developing attosecond science, since the phase can reveal important information about phenomena such as electron correlation. We present a new attosecond-precision interferometric method of angle-resolved measurement for the phase of the photoionization amplitudes, using two phase-locked Extreme Ultraviolet pulses of frequency $ω$ and $2ω$, from a Free-Electron Laser. Phase differences $Δ\tilde η$ between one- and two-photon ionization channels, averaged over multiple wave packets, are extracted for neon $2p$ electrons as a function of emission angle at photoelectron energies 7.9, 10.2, and 16.6 eV. $Δ\tilde η$ is nearly constant for emission parallel to the electric vector but increases at 10.2 eV for emission perpendicular to the electric vector. We model our observations with both perturbation and \textit{ab initio} theory, and find excellent agreement. In the existing method for attosecond measurement, Reconstruction of Attosecond Beating By Interference of Two-photon Transitions (RABBITT), a phase difference between two-photon pathways involving absorption and emission of an infrared photon is extracted. Our method can be used for extraction of a phase difference between single-photon and two-photon pathways and provides a new tool for attosecond science, which is complementary to RABBITT.

physics.atom-ph↗

arXiv2020

Direct inner-shell photoionization of Xe atoms embedded in helium nanodroplets

We present the first measurements of photoelectron spectra of atomic clusters embedded in superfluid helium (He) nanodroplets. Owing to the large absorption cross section of xenon (Xe) around 100 eV photon energy (4d inner-shell ionization), direct dopant photoionization exceeds charge transfer ionization via the ionized He droplets. Despite the predominant creation of Xe^2+ and Xe^3+ by subsequent Auger decay of free Xe atoms, for Xe embedded in He droplets only singly charged Xe_k^+, k=1,2,3 fragments are observed. Broad Xe^+ ion kinetic-energy distributions indicate Coulomb explosion of the ions due to electron transfer to the primary Auger ions from surrounding neutral atoms. The electron spectra correlated with Xe ions emitted from the He nanodroplets contain a low-energy feature and nearly unshifted Xe photolines. These results pave the way to extreme ultraviolet (XUV) and x-ray photoelectron spectroscopy of clusters and molecular complexes embedded in He nanodroplets.

physics.atm-clus↗

arXiv2020

Double photoionization of propylene oxide: a coincidence study of the ejection of a pair of valence-shell electrons

Propylene oxide, a favorite target of experimental and theoretical studies of circular dichroism, was recently discovered in interstellar space, further amplifying the attention to its role in the current debate on protobiological homochirality. In the present work, a photoelectron-photoion-photoion coincidence technique, using an ion-imaging detector and tunable synchrotron radiation in the 18.0-37.0 eV energy range, permits: (i)-to observe six double-ionization fragmentation channels, their relative yields being accounted for about two-thirds by the couple (C2H4+, CH2O+), one-fifth by (C2H3+, CH3O+); (ii)-to measure thresholds for their openings as a function of photon energy; (iii)-to unravel a pronounced bimodality for a kinetic-energy-released distribution, fingerprint of competitive non-adiabatic mechanisms.

physics.atom-ph↗

arXiv2020

Electron transfer mediated decay of alkali dimers attached to He nanodroplets

Alkali metal dimers attached to the surface of helium nanodroplets are found to be efficiently doubly ionized by electron transfer-mediated decay (ETMD) when photoionizing the helium droplets. This process is evidenced by detecting in coincidence two energetic ions created by Coulomb explosion and one low-kinetic energy electron. The kinetic energy spectra of ions and electrons are reproduced by simple model calculations based on diatomic potential energy curves, and are in agreement with ab initio calculations for the He-Na_2 and He-KRb systems. This work demonstrates that ETMD is an important decay channel in heterogeneous nanosystems exposed to ionizing radiation.

physics.chem-ph↗

arXiv2019

Design and performance of a TDC ASIC for the upgrade of the ATLAS Monitored Drift Tube detector

We present the prototype of a time-to-digital (TDC) ASIC for the upgrade of the ATLAS Monitored Drift Tube (MDT) detector for high-luminosity LHC operation. This ASIC is based on a previously submitted demonstrator ASIC designed for timing performance evaluation, and includes all features necessary for the various operation modes, as well as the migration to the TSMC 130 nm CMOS technology. We present the TDC design with the emphasis on added features and performance optimization. Tests of the timing performance demonstrate that this ASIC meets the design specifications. The TDC has a bin size of about 780 ps, and a timing bin variations within 40 ps for all 24 channels with leading and trailing edge digitization, while the power consumption has been limited to 250 mW, corresponding to a consumption of about 5.2 mW per edge measurement.

physics.ins-det↗

arXiv2018

Impulsive laser-induced alignment of OCS molecules at FERMI

We demonstrate the experimental realization of impulsive alignment of carbonyl sulfide (OCS) molecules at the Low Density Matter Beamline (LDM) at the free-electron laser FERMI. OCS molecules in a molecular beam were impulsively aligned using 200 fs pulses from a near-infrared laser. The alignment was probed through time-delayed ionization above the sulphur 2p edge, resulting in multiple ionization via Auger decay and subsequent Coulomb explosion of the molecules. The ionic fragments were collected using a time-of-flight mass spectrometer and the analysis of ion-ion covariance maps confirmed the correlation between fragments after Coulomb explosion. The analysis of the CO+ and S+ channels allowed us to extract the rotational dynamics, which is in agreement with our theoretical description as well as with previous experiments. This result opens the way for a new class of experiments at LDM within the field of coherent control of molecules with the possibilities that a precisely synchronized optical-pump XUV-probe laser setup like FERMI can offer.

physics.chem-ph↗

arXiv2018

Hardware Implementation of a Fast Algorithm for the Reconstruction of Muon Tracks in ATLAS Muon Drift-Tube Chambers for the First-Level Muon Trigger at the HL-LHC

The High-Luminosity 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 the HL-LHC where the instantaneous luminosity will exceed the LHC Run 1 instantaneous luminosity by almost an order of magnitude. The ATLAS first level muon trigger rate is dominated by low momentum muons, selected due to the moderate momentum resolution of the resistive plate and thin gap trigger chambers. This limitation can be overcome by including the data of the precision muon drift tube (MDT) chambers in the first level trigger decision. This requires the fast continuous transfer of the MDT hits to the off-detector trigger logic and a fast track reconstruction algorithm performed in the trigger logic. In order to demonstrate the feasibility of reconstructing tracks in MDT chambers within the short available first-level trigger latency of about 3~$μ$s we implemented a seeded Hough transform on the ARM Cortex A9 microprocessor of a Xilinx Zynq FPGA and studied its performance with test-beam data recorded in CERN's Gamma Irradiation Facility. We could show that by using the ARM processor's Neon Single Instruction Multiple Data Engine to carry out 4 floating point operations in parallel the challenging latency requirement can be matched.

physics.ins-det↗

arXiv2018

Penning ionization of acene molecules by He nanodroplets

Acene molecules (anthracene, tetracene, pentacene) and fullerene (C$_{60}$) are embedded in He nanodroplets (He$_N$) and probed by EUV synchrotron radiation. When resonantly exciting the He nanodroplets, the embedded molecules M are efficiently ionized by the Penning reaction $\mathrm{He}_N^*+\mathrm{M}\rightarrow\mathrm{He}_N + \mathrm{M}^+ + e^-$. However, the Penning electron spectra are broad and structureless -- showing no resemblance neither with those measured by binary Penning collisions, nor with those measured for dopants bound to the He droplet surface. The similarity of all four spectra indicates that electron spectra of embedded species are substantially altered by electron-He scattering. Simulations based on elastic binary electron-He collisions qualitatively reproduce the measured spectra, but require the assumption of unexpectedly large He droplets.

physics.atm-clus↗