Pulse overlap ambiguities in multiple quantum coherence spectroscopy.

Coherent two-dimensional electronic spectroscopy probes ultrafast dynamics using femtosecond pulses. In the case where the time scale of the studied dynamics become comparable to the pulse duration, pulse overlap effects may compromise the experimental data. Here, we perform one-dimensional coherence scans and study pulse overlap effects in clean two-level systems. We find parasitic multiple-quantum coherences as a consequence of the arbitrary time ordering during the temporal pulse overlap. Surprisingly, the coherence lifetimes exceed the pulse coherence time by a factor of 1.85. These findings have important implications for the interpretation of higher-order coherent two-dimensional and related spectroscopy experiments.

Two-dimensional electronic spectroscopy of an ultracold gas.

Femtosecond coherent multidimensional spectroscopy is demonstrated for an ultracold gas.  A setup for phase modulation spectroscopy is used to probe the 32S1/2-22P1/2,3/2 transition in an 800µK-cold sample of 7Li 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 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.

Symmetry Dependence of the Continuum Coupling in the Chemi-ionization of Li(22S1/2) by He(23S1, 23PJ).

In the literature, the chemi-ionization of Li in the 22S1/2 ground level by He in a metastable state is typically described as an electron transfer process in which an electron from the 2s orbital of Li is transferred to the 1s orbital of He while an electron from the 2s orbital of He is ejected. Therefore, one would not assume that the orbital of the valence electron of He strongly influences the coupling strength of the collision complex to the ionization continuum. However, we observe that the chemi-ionization rate is decreased when He is laser-excited from the metastable 23S1 level to the 23PJ level (with J = 0, 1, 2). A semiclassical treatment of the reaction dynamics reveals a strong dependence of the ionization rate on the reaction-channel-specific ionization width functions to which the observed decrease of the rate coefficients can be related to. The results are relevant for the improved understanding and control of chemi-ionization processes in merged beams and in traps.

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

We demonstrate the control of 4He(23S1)-7Li(22S1/2) 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 4Σ+ channel to the reaction rate, which is in violation of spin conservation.

Excitation dynamics in polyacene molecules on rare-gas clusters.

Laser-induced fluorescence spectra and excitation lifetimes of anthracene, tetracene, and pentacene molecules attached to the surface of solid argon clusters have been measured with respect to cluster size, density of molecules, and excitation density. Results are compared to previous studies on the same sample molecules attached to neon clusters. A contrasting lifetime behavior of anthracene on neon and argon clusters is discussed, and mechanisms are suggested to interpret the results. Although both neon and argon clusters are considered to be weakly interacting environments, we find that the excitation decay dynamics of the studied acenes depends significantly on the cluster material. Moreover, we find even qualitative differences regarding the dependence on the dopant density. Based on these observations, previous assignments of collective radiative and non-radiative decay mechanisms are discussed in the context of the new experimental findings.

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 ions are He2+, generated by droplet autoionization and by photoionization of droplet-bound excited He atoms. He+ appear with some pump-probe delay as a result of the ejection He atoms in their lowest excited states 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.

Dynamics of Photoexcited Cs Atoms Attached to Helium Nanodroplets.

We present an experimental study of the dynamics following the photoexcitation and subsequent photoionization of single Cs atoms on the surface of helium nanodroplets. The dynamics of excited Cs atom desorption and readsorption as well as CsHe exciplex formation are measured by using femtosecond pump-probe velocity map imaging spectroscopy and ion time-of-flight spectrometry. The time scales for the desorption of excited Cs atoms off helium nanodroplets as well as the time scales for CsHe exciplex formation are experimentally determined for the 6p states of Cs. For the 6p 2Π1/2 state, our results confirm that the excited Cs atoms only desorb from the nanodroplet when the excitation wavenumber is blue-shifted from the 6p 2Π1/2 ← 6s 2Σ1/2 resonance. Our results suggest that the dynamics following excitation to the 6p 2Π3/2 state can be described by an evaporation-like desorption mechanism, whereas the dynamics arising from excitation to the 6p 2Σ1/2 state is indicative for a more impulsive desorption process. Furthermore, our results suggest a helium-induced spin-orbit relaxation from the 6p 2Σ1/2 state to the 6p 2Π1/2 state. Our findings largely agree with the results of time-dependent 4He density functional theory (DFT) simulations published earlier [Eur. Phys. J. D 2019, 73, 94].

Effects of high pulse intensity and chirp in two-dimensional electronic spectroscopy of an atomic vapor.

The effects of high pulse intensity and chirp on two-dimensional electronic spectroscopy signals are experimentally investigated in the highly non-perturbative regime using atomic rubidium vapor as clean model system. Data analysis is performed based on higher-order Feynman diagrams and non-perturbative numerical simulations of the system response. It is shown that higher-order contributions may lead to a fundamental change of the static appearance and beating-maps of the 2D spectra and that chirped pulses enhance or suppress distinct higher-order pathways. We further give an estimate of the threshold intensity beyond which the high-intensity effects become visible for the system under consideration.

High-gain harmonic generation with temporally overlapping seed pulses and application to ultrafast spectroscopy.

Collinear double-pulse seeding of the High-Gain Harmonic Generation (HGHG) process in a free-electron laser (FEL) is a promising approach to facilitate various coherent nonlinear spectroscopy schemes in the extreme ultraviolet (XUV) spectral range. However, in collinear arrangements using a single nonlinear medium, temporally overlapping seed pulses may introduce nonlinear mixing signals that compromise the experiment at short time delays. Here, we investigate these effects in detail by extending the analysis described in a recent publication (Wituschek et al., Nat. Commun., 11, 883, 2020). High-order fringe-resolved autocorrelation and wave packet interferometry experiments at photon energies > 23 eV are performed, accompanied by numerical simulations. It turns out that both the autocorrelation and the wave-packet interferometry data are very sensitive to saturation effects and can thus be used to characterize saturation in the HGHG process. Our results further imply that time-resolved spectroscopy experiments are feasible even for time delays smaller than the seed pulse duration.

Time-resolved formation of excited atomic and molecular states in XUV-induced nanoplasmas in ammonia clusters.

High intensity XUV radiation from a free-electron laser (FEL) was used to create a nanoplasma inside ammonia clusters with the intent of studying the resulting electron-ion interactions and their interplay with plasma evolution. In a plasma-like state, electrons with kinetic energy lower than the local collective Coulomb potential of the positive ionic core are trapped in the cluster and take part in secondary processes (e.g. electron-impact excitation/ionization and electron-ion recombination) which lead to subsequent excited and neutral molecular fragmentation. Using a time-delayed UV laser, the dynamics of the excited atomic and molecular states are probed from -0.1 ps to 18 ps. We identify three different phases of molecular fragmentation that are clearly distinguished by the effect of the probe laser on the ionic and electronic yield. We propose a simple model to rationalize our data and further identify two separate channels leading to the formation of excited hydrogen.

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 23S1 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 approaches. 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 also detailed.

Alkali atoms attached to vortex-hosting helium nanodroplets.

Light absorption or fluorescence excitation spectroscopy of alkali atoms attached to 4He droplets is investigated as a possible way for detecting the presence of vortices. To this end, we have calculated the equilibrium configuration and energetics of alkali atoms attached to a 4He1000 droplet hosting a vortex line using 4He density functional theory. We use them to study how the dipole absorption spectrum of the alkali atom is modified when the impurity is attached to a vortex line. Spectra are found to be blue-shifted (higher frequencies) and broadened compared to vortex-free droplets because the dimple in which the alkali atom sits at the intersection of the vortex line and the droplet surface is deeper. This effect is smaller for lighter alkali atoms and all the more so when using a quantum description since, in this case, they sit further away from the droplet surface on average due to their zero-point motion. Spectral modifications due to the presence of a vortex line are minor for np ← ns excitation and therefore insufficient for vortex detection. In the case of higher n'p ← ns or n's ← ns (n' > n) excitations, the shifts are larger as the excited state orbital is more extended and therefore more sensitive to changes in the surrounding helium density.

Stable interferometric platform for phase modulation of seeded free-electron lasers.

We present a compact phase modulation setup designed for high laser intensities sufficient to drive highly nonlinear processes, such as high-gain harmonic generation in seeded free-electron lasers. This paves the way for all-extreme-ultravioloet coherent nonlinear spectroscopy. The high linearity, phase stability, and sensitivity of the setup are demonstrated by probing the quantum interference of electronic wave packets in the deep ultraviolet region (268 nm) combined with photoion time-of-flight mass spectrometry.

Delocalized excitons and interaction effects in extremely dilute thermal ensembles.

Long-range interparticle interactions are revealed in extremely dilute thermal atomic ensembles using highly sensitive nonlinear femtosecond spectroscopy. Delocalized excitons are detected in the atomic systems at particle densities where the mean interatomic distance (>10 µm) is much greater than the laser wavelength and multi-particle coherences should destructively interfere over the ensemble average. With a combined experimental and theoretical analysis, we identify an effective interaction mechanism, presumably of dipolar nature, as the origin of the excitonic signals. Our study implies that even in highly-dilute thermal atom ensembles, significant transition dipole-dipole interaction networks may form that require advanced modeling beyond the nearest neighbor approximation to quantitatively capture the details of their many-body properties.

Optical signatures of pentacene in soft rare-gas environments.

Acenes and pentacene (Pc), in particular, are promising candidates for organic dyes with interesting properties important for solar light to energy conversion. We present a combined experimental and computational study of Pc in an ultracold environment that allows for high resolution optical spectroscopy. The spectra and their vibrational substructure are interpreted with the help of density functional theory calculations. While there are only slight changes within superfluid helium as compared to vacuum, the neon surface shows more prominent effects. Additional vibrational coupling by neon modes leads to broadening as well as the emergence of new features, like the otherwise symmetry forbidden out-of-plane butterfly mode.

Penning collisions between supersonically expanded metastable He atoms and laser-cooled Li atoms.

We describe an experimental setup comprised of a discharge source for supersonic beams of metastable He atoms and a magneto-optical trap (MOT) for ultracold Li 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.

Phase-synchronous undersampling in nonlinear spectroscopy.

We introduce the concept of phase-synchronous undersampling in nonlinear spectroscopy. The respective theory is presented and validated experimentally in a phase-modulated quantum beat experiment by sampling high phase modulation frequencies with low laser repetition rates. The advantage of undersampling in terms of signal quality and reduced acquisition time is demonstrated, and breakdown conditions are identified. The presented method is particularly beneficial for experimental setups with limited signal/detection rates.

Three-Dimensional Shapes of Spinning Helium Nanodroplets.

A significant fraction of superfluid helium nanodroplets produced in a free-jet expansion has been observed to gain high angular momentum resulting in large centrifugal deformation. We measured single-shot diffraction patterns of individual rotating helium nanodroplets up to large scattering angles using intense extreme ultraviolet light pulses from the FERMI free-electron laser. Distinct asymmetric features in the wide-angle diffraction patterns enable the unique and systematic identification of the three-dimensional droplet shapes. The analysis of a large data set allows us to follow the evolution from axisymmetric oblate to triaxial prolate and two-lobed droplets. We find that the shapes of spinning superfluid helium droplets exhibit the same stages as classical rotating droplets while the previously reported metastable, oblate shapes of quantum droplets are not observed. Our three-dimensional analysis represents a valuable landmark for clarifying the interrelation between morphology and superfluidity on the nanometer scale.

Desorption dynamics of RbHe exciplexes off He nanodroplets induced by spin-relaxation.

Doped He nanodroplets are ideal model systems to study the dynamics of elementary photophysical processes in heterogeneous nanosystems. Here we present a combined experimental and theoretical investigation of the formation of free RbHe exciplex molecules from laser-excited Rb-doped He nanodroplets. Upon excitation of a droplet-bound Rb atom to the 5p3/22Π3/2-state, a stable RbHe exciplex forms within about 20 ps. Only due to 2Π3/2 → 2Π1/2 spin-relaxation does the RbHe exciplex detach from the He droplet surface with a half life of about 700 ps, given by the spin-relaxation time and the coupling of spin and translational degrees of freedom.

Phase-modulated harmonic light spectroscopy.

By combining phase-modulated nonlinear spectroscopy with second harmonic generation, the concept of phase-modulated harmonic light spectroscopy is introduced. Simultaneous spectroscopy with different harmonics of the light is demonstrated and linear and nonlinear excitation of the spectroscopic sample is investigated. Sum frequency generation and stray light effects during temporal pulse overlap have been evaluated in detail, accompanied by simulations. The presented work provides a promising concept to facilitate coherent nonlinear time-domain spectroscopy in the extreme ultraviolet wavelength regime and contributes valuable insights for future studies in this direction.