arXiv subjects

Katrin Dulitz

Publications and source records attributed to Katrin Dulitz.

At least 19 recordsLinked to original sources

arXiv2025

H2-roaming dynamics in the formation of H3+ following two-photon double ionization of ethanol and aminoethanol

Roaming reactions involving a neutral fragment of a molecule that transiently wanders around another fragment before forming a new bond are intriguing and peculiar pathways for molecular rearrangement. Such reactions can occur for example upon double ionization of small organic molecules, and have recently sparked much scientific interest. We have studied the dynamics of the H$_2$-roaming reaction leading to the formation of H$_3^+$ after two-photon double ionization of ethanol and 2-aminoethanol, using an XUV-UV pump-probe scheme. For ethanol, we find dynamics similar to previous studies employing different pump-probe schemes, indicating the independence of the observed dynamics from the method of ionization and the photon energy of the disruptive probe pulse. Surprisingly, we do not observe a kinetic isotope effect in ethanol-D$_6$, in contrast to previous experiments on methanol where such an effect was observed. This distinction indicates fundamental differences in the energetics of the reaction pathways as compared to the methanol molecule. The larger number of possible roaming pathways compared to methanol complicates the analysis considerably. In contrast to previous studies, we additionally analyze a broad range of dissociative ionization products, which feature distinct dynamics from that of H$_{3}^{+}$ and allow initial insight into the action of the disruptive UV-probe pulse.

physics.chem-ph↗

arXiv2024

Method of Kinetic Energy Reconstruction from Time-of-Flight Mass Spectra

We present a method for the reconstruction of ion kinetic energy distributions from ion time-of-flight mass spectra through ion trajectory simulations. In particular, this method is applicable to complicated spectrometer geometries with largely anisotropic ion collection efficiencies. A calibration procedure using a single ion mass peak allows the accurate determination of parameters related to the spectrometer calibration, experimental alignment and instrument response function, which improves the agreement between simulations and experiment. The calibrated simulation is used to generate a set of basis functions for the time-of-flight spectra, which are then used to transform from time-of-flight to kinetic-energy spectra. We demonstrate this reconstruction method on a recent pump-probe experiment by Asmussen et al. (J. D. Asmussen et al., Phys. Chem. Chem. Phys., 23, 15138, (2021)) on helium nanodroplets and retrieve time-resolved kinetic-energy-release spectra for the ions from ion time-of-flight spectra.

physics.atm-clus↗

arXiv2024

Strong-field quantum control in the extreme ultraviolet using pulse shaping

Tailored light-matter interactions in the strong coupling regime enable the manipulation and control of quantum systems with up to unit efficiency, with applications ranging from quantum information to photochemistry. While strong light-matter interactions are readily induced at the valence electron level using long-wavelength radiation, comparable phenomena have been only recently observed with short wavelengths, accessing highly-excited multi-electron and inner-shell electron states. However, the quantum control of strong-field processes at short wavelengths has not been possible, so far, due to the lack of pulse shaping technologies in the extreme ultraviolet (XUV) and X-ray domain. Here, exploiting pulse shaping of the seeded free-electron laser (FEL) FERMI, we demonstrate the strong-field quantum control of ultrafast Rabi dynamics in helium atoms with high fidelity. Our approach unravels a strong dressing of the ionization continuum, otherwise elusive to experimental observables. The latter is exploited to achieve control of the total ionization rate, with prospective applications in many XUV and soft X-ray experiments. Leveraging recent advances in intense few-femtosecond to attosecond XUV to soft X-ray light sources, our results open an avenue to the efficient manipulation and selective control of core electron processes and electron correlation phenomena in real time.

physics.atom-ph↗

arXiv2022

Two-dimensional electronic spectroscopy of an ultracold gas

Femtosecond coherent multidimensional spectroscopy is demonstrated for an ultracold gas. For this, a setup for phase modulation spectroscopy is used to probe the $3^2\mathrm{S}_{1/2} - 2^2\mathrm{P}_{1/2, 3/2}$ transition in an 800 $\mu$K-cold sample of $^7$Li atoms confined in a magneto-optical trap. The observation of a double quantum coherence response, a signature of interparticle interactions, paves the way for detailed investigations of few- and many-body effects in ultracold atomic and molecular gases using this technique. The experiment combines a frequency resolution of 3 GHz with a potential time resolution of 200 fs, which allows for high-resolution studies of ultracold atoms and molecules both in the frequency and in the time domain.

physics.atom-ph↗

arXiv2022

Extreme ultraviolet wave packet interferometry of the autoionizing HeNe dimer

Femtosecond extreme ultraviolet wave packet interferometry (XUV-WPI) was applied to study resonant inter-atomic Coulombic decay (ICD) in the HeNe dimer. The high demands on phase stability and sensitivity for vibronic XUV-WPI of molecular-beam targets are met using an XUV phase-cycling scheme. The detected quantum interferences exhibit vibronic dephasing and rephasing signatures along with an ultrafast decoherence assigned to the ICD process. A Fourier analysis reveals the molecular absorption spectrum with high resolution. The demonstrated experiment shows a promising route for the real-time analysis of ultrafast ICD processes with both high temporal and spectral resolution.

physics.atm-clus↗

arXiv2022

Spin-state-controlled chemi-ionization reactions between metastable helium atoms and ground-state lithium atoms

We demonstrate the control of $^4$He(2$^3$S$_1$)-$^7$Li(2$^2$S$_{1/2}$) chemi-ionization reactions by all-optical electron-spin-state preparation of both atomic species prior to the collision process. Our results demonstrate that chemi-ionization is strongly suppressed (enhanced) for non-spin-conserving (spin-conserving) collisions at thermal energies. These findings are in good agreement with a model based on spin angular momentum coupling of the prepared atomic states to the quasi-molecular states. Small deviations from the model indicate the contribution of the $^4\Sigma^+$ channel to the reaction rate which is in violation of spin conservation.

physics.atom-ph↗

arXiv2021

Development and characterization of high-frequency sources for supersonic beams of fluorine radicals

We present and compare two high-pressure, high-frequency electric-discharge sources for the generation of supersonic beams of fluorine radicals. The sources are based on dielectric-barrier-discharge (DBD) and plate-discharge units attached to a pulsed solenoid valve. The corrosion-resistant discharge sources were operated with fluorine gas seeded in helium up to backing pressures as high as 30 bar. We employed a (3+1) resonance-enhanced multiphoton ionization combined with velocity-map imaging for the optimization, characterization and comparison of the fluorine beams. Additionally, universal femtosecond-laser-ionization detection was used for the characterization of the discharge sources at experimental repetition rates up to 200 Hz. Our results show that the plate discharge is more efficient in F$_{2}$ dissociation than the DBD by a factor of 8-9, whereas the DBD produces internally colder fluorine radicals.

physics.chem-ph↗

arXiv2021

Single-Source, Collinear Merged-Beam Experiment for the Study of Reactive Neutral-Neutral Collisions

We present two methods for studying reactive collisions between two atomic or molecular species: a collinear merged-beam method, in which two gas pulses from a single supersonic beam source are coalesced, and an intrabeam-scattering technique, in which a single gas pulse is used. Both approaches, which rely on the laser cooling and deceleration of a laser-coolable species inside a Zeeman slower, can be used for a wide range of scattering studies. Possible experimental implementations of the proposed methods are outlined for autoionizing collisions between helium atoms in the metastable $2^3\mathrm{S}_1$ state and a second, atomic or molecular species. Using numerical trajectory calculations, we provide estimates of the expected on-axis detection efficiency, collision-energy range and collision-energy resolution of the approach. We have experimentally tested the feasibility of such an experiment by producing two gas pulses at very short time intervals, and the results of these measurements are detailed as well.

physics.atom-ph↗

arXiv2021

Sensitive detection of metastable NO and \ce{N2} by reactive collisions with laser-excited Li

In a proof-of-principle experiment, we demonstrate that metastable nitric oxide molecules, NO(a$^4\Pi_i$), generated inside a pulsed, supersonic beam, can be detected by reactive gas-phase collisions with electronically excited Li atoms in the $2^2$P$_{3/2}$ state. Since the internal energy of NO(a$^4\Pi_i$, $v \leq 4$) is lower than the ionization potential of Li in the $2^2$S$_{1/2}$ electronic ground state, we observe that the product ion yield arising from autoionizing NO(a$^4\Pi_i$)+Li($2^2$S$_{1/2}$) collisions is a factor of 21 lower than the ion yield from NO(a$^4\Pi_i$)+Li($2^2$P$_{3/2}$) collisions. We also compare our findings with measurements of relative rates for collisions of metastable \ce{N2}+Li($2^2$S$_{1/2}$) and metastable \ce{N2}+Li($2^2$P$_{3/2}$) reactive collisions. Using this detection method, we infer densities of $\approx 600$ NO(a$^4\Pi_i$) molecules/cm$^3$ and $\approx 6 \cdot 10^{4}$ metastable \ce{N2} molecules/cm$^3$ in the interaction region. Our results also allow for an estimate of the fractional population of NO(a$^4\Pi_i$, $v \geq 5$) prior to the collision process. The production of NO(a$^4\Pi_i$) in selected vibrational states using laser excitation from the X$^2\Pi_r$ ground state will open possibilities for the detailed study of vibrational-state-selected NO(a$^4\Pi_i$)-Li($2^2$P$_{3/2}$) collisions.

physics.atom-ph↗

arXiv2021

Preparation of individual magnetic sub-levels of $^4$He($2^3$S$_1$) in a supersonic beam using laser optical pumping and magnetic hexapole focusing

We compare two different experimental techniques for the magnetic-sub-level preparation of metastable $^4$He in the $2^3$S$_1$ level in a supersonic beam, namely magnetic hexapole focusing and optical pumping by laser radiation. At a beam velocity of $v = 830\,$m/s, we deduce from a comparison with a particle trajectory simulation that up to $99\,$\% of the metastable atoms are in the $M_{J^{''}} = +1$ sub-level after magnetic hexapole focusing. Using laser optical pumping via the $2^3$P$_2-2^3$S$_1$ transition, we achieve a maximum efficiency of $94\pm3\,$\% for the population of the $M_{J^{''}} = +1$ sub-level. For the first time, we show that laser optical pumping via the $2^3$P$_1-2^3$S$_1$ transition can be used to selectively populate each of the three $M_{J^{''}}$ sub-levels ($M_{J^{''}} = $ -1, 0, +1). We also find that laser optical pumping leads to higher absolute atom numbers in specific $M_{J^{''}}$ sub-levels than magnetic hexapole focusing.

physics.atom-ph↗

arXiv2021

Dynamics of photo-excited Cs atoms attached to helium nanodroplets

We present an experimental study of the dynamics following the photo-excitation and subsequent photo-ionization of single Cs atoms on the surface of helium nanodroplets. The dynamics of excited-Cs-atom desorption and re-adsorption as well as CsHe exciplex formation are measured using femtosecond pump-probe velocity-map-imaging spectroscopy and ion-time-of-flight spectrometry. The timescales for the desorption of excited Cs atoms off helium nanodroplets as well as the timescales for CsHe exciplex formation are experimentally determined for the 6p states of Cs. For the 6p $^2\Pi_ {1/2}$ state, our results confirm that the excited Cs atoms only desorb from the nanodroplet when the excitation wavenumber is blue-shifted from the $6p\,^2\Pi_ {1/2} \leftarrow 6s\,^2\Sigma_ {1/2}$ resonance. Our results suggest that the dynamics following excitation to the 6p $^2\Pi_ {3/2}$ state can be described by an evaporation-like desorption mechanism, whereas the dynamics arising from excitation to the 6p $^2\Sigma_ {1/2}$ state is indicative for a more impulsive desorption process. Furthermore, our results suggest a helium-induced spin-orbit relaxation from the the 6p $^2\Sigma_ {1/2}$ state to the 6p $^2\Pi_ {1/2}$ state. Our findings largely agree with the results of time-dependent $^4$He-density-functional theory (DFT) simulations published earlier [Coppens et al., Eur. Phys. J. D 73, 94 (2019)].

physics.atm-clus↗

arXiv2021

Unravelling the Full Relaxation Dynamics of Superexcited Helium Nanodroplets

The relaxation dynamics of superexcited superfluid He nanodroplets is thoroughly investigated by means of extreme-ultraviolet (XUV) femtosecond electron and ion spectroscopy complemented by time-dependent density functional theory (TDDFT). Three main paths leading to the emission of electrons and ions are identified: Droplet autoionization, pump-probe photoionization, and autoionization induced by re-excitation of droplets relaxing into levels below the droplet ionization threshold. The most abundant product of both droplet autoionization and photoionization is He$_2^+$, whereas the delayed appearance of He$^+$ is indicative of the ejection of excited He atoms from the droplets. The state-resolved time-dependent photoelectron spectra reveal that intermediate excited states of the droplets are populated in the course of the relaxation, terminating in the lowest-lying metastable singlet and triplet He atomic states. The slightly faster relaxation of the triplet state compared to the singlet state is in agreement with the simulation showing faster formation of a bubble around a He atom in the triplet state.

physics.atm-clus↗

arXiv2020

Suppression of Penning ionization by orbital angular momentum conservation

The efficient suppression of Penning-ionizing collisions is a stringent requirement to achieve quantum degeneracy in metastable rare gases. Thus far, such loss processes have been avoided by electron-spin polarizing the collision partners. Here, we report on the efficient suppression of Penning ionization in collisions between metastable He and laser-excited Li atoms. The results illustrate that not only the electron spin, but also $Λ$ - the projection of the total molecular orbital angular momentum along the internuclear axis - is conserved during the ionization process. Our findings suggest that $Λ$ conservation can be used as a more general means of reaction control, for example, to improve schemes for the simultaneous laser cooling and trapping of metastable He and alkali atoms.

physics.chem-ph↗

arXiv2019

Optical Quenching of Metastable Helium Atoms using Excitation to the $4P$ State

Discharge and electron-impact excitation lead to the production of metastable helium atoms in two metastable states, 2$^1$S$_0$ and 2$^3$S$_1$. However, many applications require pure beams of one of these species or at least a detailed knowledge of the relative state populations. In this paper, we present the characterization of an original experimental scheme for the optical depletion of He(2$^1$S$_0$) in a supersonic beam which is based on the optical excitation of the 4$^1$P$_1 \leftarrow 2^1$S$_0$ transition at 397 nm using a diode laser. From our experimental results and from a comparison with numerical calculations, we infer a near unit depletion efficiency at all beam velocities under study (1070 m/s $\leq v \leq$ 1750 m/s). Since the technique provides a direct means to determine the singlet-to-triplet ratio in a pulsed supersonic helium beam, our results show that the intrabeam singlet-to-triplet ratio is different at the trailing edges of the gas pulse.

physics.atom-ph↗

arXiv2019

Production of rotationally cold methyl radicals in pulsed supersonic beams

We present a comparison of two technically distinct methods for the generation of rotationally cold, pulsed supersonic beams of methyl radicals (CH3): a plate discharge source operating in the glow regime, and a dielectric barrier discharge source (DBD). The results imply that the efficiency of both sources is comparable, and that molecular beams with similar translational and rotational temperatures are formed. Methane (CH4) proved to be the most suitable radical precursor species.

physics.chem-ph↗

arXiv2019

Penning collisions between supersonically expanded metastable helium atoms and laser-cooled lithium atoms

We describe an experimental setup comprised of a discharge source for supersonic beams of metastable helium atoms and a magneto-optical trap (MOT) for ultracold lithium atoms that makes it possible to study Penning ionization and associative ionization processes at high ion count rates. The cationic reaction products are analyzed using a novel ion detection scheme which allows for mass selection, a high ion extraction efficiency and a good collision-energy resolution. The influence of elastic He-Li collisions on the steady-state Li atom number in the MOT is described, and the collision data are used to estimate the excitation efficiency of the discharge source. We also show that Penning collisions can be directly used to probe the temperature of the Li cloud without the need for an additional time-resolved absorption or fluorescence detection system.

physics.chem-ph↗

arXiv2017

Spin-orbit coupling and rovibrational structure in the iododiacetylene radical cation by PFI-ZEKE photoelectron spectroscopy

The photoelectron spectrum of the $\textrm{X}^{+}\,{}^{2}Π\leftarrow \textrm{X}\,{}^{1}Σ^{+}$ photoionising transition in iododiacetylene, HC$_4$I, has been recorded using pulsed-field-ionisation zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy with partial resolution of the rotational structure. The first adiabatic ionisation energy of HC$_4$I and the spin-orbit splitting of the X$^{+}\,{}^{2}Π$ state of HC$_4$I$^+$ are determined as $E^{\textrm{ad}}_{\textrm{I}}/(hc) = 74470.7(2)$ cm$^{-1}$ and $Δ\tildeν_{\textrm{so}} = 1916.7(4)$ cm$^{-1}$, respectively. Several vibrational levels of the X$^{+}\,{}^{2}Π$ electronic ground state of the HC$_4$I$^+$ cation have been observed. The experimental data are discussed in the realm of a simple three-state charge-transfer model without adjustable parameters which allows for a qualitative description of the electronic structure and spin-orbit coupling in HC$_4$I$^+$ and of the change in bond lengths upon ionisation of HC$_4$I.

physics.chem-ph↗

arXiv2015

A compact design for a magnetic synchrotron to store beams of hydrogen atoms

We present a design for an atomic synchrotron consisting of 40 hybrid magnetic hexapole lenses arranged in a circle. We show that for realistic parameters, hydrogen atoms with a velocity up to 600 m/s can be stored in a 1-meter diameter ring, which implies that the atoms can be injected in the ring directly from a pulsed supersonic beam source. This ring can be used to study collisions between stored hydrogen atoms and molecular beams of many different atoms and molecules. The advantage of using a synchrotron is two-fold: (i) the collision partners move in the same direction as the stored atoms, resulting in a small relative velocity and thus a low collision energy, and (ii) by storing atoms for many round-trips, the sensitivity to collisions is enhanced by a factor of 100-1000. In the proposed ring, the cross-sections for collisions between hydrogen, the most abundant atom in the universe, with any atom or molecule that can be put in a beam, including He, H$_2$, CO, ammonia and OH can be measured at energies below 100 K. We discuss the possibility to use optical transitions to load hydrogen atoms into the ring without influencing the atoms that are already stored. In this way it will be possible to reach high densities of stored hydrogen atoms.

physics.atom-ph↗