Markus Koch

Imaging with neutral atoms—a new matter-wave microscope

Matter‐wave microscopy can be dated back to 1932 when Max Knoll and Ernst Ruska published the first image obtained with a beam of focussed electrons. In this paper a new step in the development of matter‐wave microscopy is presented. We have created an instrument where a focussed beam of neutral, ground‐state atoms (helium) is used to image a sample. We present the first 2D images obtained using this new technique. The imaged sample is a free‐standing hexagonal copper grating (with a period of about 36 μm and rod thickness of about 8 μm). The images were obtained in transmission mode by scanning the focussed beam, which had a minimum spot size of about 2.0 μm in diameter (full width at half maximum) across the sample. The smallest focus achieved was 1.9 ± 0.1 μm. The resolution for this experiment was limited by the speed ratio of the atomic beam through the chromatic aberrations of the zone plate that was used to focus. Ultimately the theoretical resolution limit is set by the wavelength of the probing particle. In praxis, the resolution is limited by the source and the focussing optics.

High resolution electron microscopy of Ag-clusters in crystalline and non-crystalline morphologies grown inside superfluid helium nanodroplets

We present a first investigation of structural properties of Ag clusters with a diameter of up to 5.5 nm grown inside superfluid helium nanodroplets (He(N)) and deposited on an amorphous C surface. With high resolution transmission electron microscope images we are able to show that in addition to the crystalline face centered cubic (fcc) structure, noncrystalline icosahedral (Ih), and decahedral (Dh) morphologies are grown. Relative abundances (56% fcc, 31% Dh, and 13% Ih) as well as the size distribution of each morphology (mean diameters d(fcc)=2.62(5) nm, d(Dh)=3.34(7) nm, and d(Ih)=3.93(2) nm) do not reflect the situation expected from pure energetic considerations, where small Ihs should be followed by medium sized Dhs and large fccs. Instead, kinetic factors seem to play an important role in the formation of these structures, as it appears to be the case for clusters formed by inert gas aggregation. Considering the low temperatures (0.37 K) and extremely high cooling rates, we discuss basic ideas that might lead to a qualitative picture of the cluster formation process inside He(N).

Doping helium nanodroplets with high temperature metals: Formation of chromium clusters

A new method for stable and continuous doping of superfluid helium nanodroplets (He(N)) with high-melting elements such as refractory metals is presented. The method exploits the advantages of electron bombardment heating and avoids stray fields induced by high currents or high frequency fields. It is thus especially suitable for magnetic studies of atoms and clusters in He(N). The source is characterized by means of mass spectroscopic investigations of He(N) doped with chromium atoms and clusters. Source temperatures of up to (1650 ± 50) °C were reached and Cr clusters up to Cr(9) could be formed in He(N).

Molecular dynamics simulation of the deposition process of cold Ag-clusters under different landing conditions.

We present a series of molecular dynamics simulations on the surface deposition process of initially free silver clusters (Agn) with different sizes (n = 100-2000) and morphologies. During the whole deposition process the morphology of the clusters was studied as a function of the landing conditions. These conditions include variations of the depth and range of the substrate potential as well as the thermal coupling to the surface and a variation of the impact velocity of the free clusters. Depending on the applied conditions the clusters final form ranges from spread out fragments via deformed and restructured heaps to quasi unchanged spherical clusters sitting at the surface. Under certain landing conditions larger clusters retain their initial multiply twinned morphology upon deposition, while smaller ones undergo structural transitions to form single domain particles. Furthermore, the occurrence of a structural transition depends on the initial structure-initially decahedral clusters tend to conserve their morphology better than icosahedral ones. The same behavior can also be observed in our experiments, where silver clusters were grown in helium nanodroplets and subsequently deposited on amorphous carbon substrates.

Rubidium on Helium Droplets: Analysis of an Exotic Rydberg Complex for n* < 20 and 0 ≤ l ≤ 3

Rubidium atom Rydberg states perturbed by helium droplets of different sizes provide insight into the role of a nanosized dielectric on the Coulomb potential. The observation of droplet size-dependent shifts of excited states with respect to bare atom states is explained by a decreased quantum defect and a lowered ionization threshold. Within the scope of a Rydberg model, we demonstrate that quantum defects and ionization potentials are constant for each specific Rydberg series, which confirms the Rydberg character of excited Rubidium states on helium droplets. A set of six Rydberg series could be identified. Individual Rydberg states are observed with effective principal quantum numbers up to n* ≈ 19 and l ≤ 3, for which the expectation value of the electron orbital radius is about 10 times larger than the droplet radius.

Molecular Beam Magnetic Resonance in Doped Helium Nanodroplets. A Setup for Optically Detected ESR/NMR in the Presence of Unresolved Zeeman Splittings†

An apparatus is presented to perform magnetic resonance measurements in a beam of doped helium nanodroplets. This type of experiment faces the same difficulties as traditional molecular beam electric/magnetic resonance experiments, namely, an optically thin sample. Like many of these traditional experiments, it uses lasers to prepare the states of interest and to detect them after manipulation with a microwave field. Unlike these traditional experiments, Zeeman substates cannot be resolved by the laser transition because of the droplet-induced line broadening. A magnetic dichroism scheme is used instead, exploiting the favorable selection rules for the absorption of circularly polarized light. ESR spectra are shown for K atoms captured on the surface of He nanodroplets. The extension of the method to other atoms and molecules, and to NMR spectroscopy, is discussed.

Electronic Relaxation after Resonant Laser Excitation of Cr in Superfluid Helium Nanodroplets

Chromium (Cr) atoms embedded into helium nanodroplets (HeN) are ejected from the droplets upon photoexcitation. During ejection they undergo electronic relaxation resulting in bare Cr atoms in various excited states. In a study of the relaxation process we present absorption spectra observed via laser induced fluorescence and beam depletion as well as dispersed fluorescence spectra and time-resolved fluorescence measurements. Broad and shifted absorption structures were found for the strong z7P° ← a7S3 and y7P° ← a7S3 excitations from the ground state. Emission lines are, in contrast, very narrow, which indicates that fluorescence is obtained from bare excited Cr atoms after ejection. Upon excitation into the y7P2,3,4° states we observed fluorescence from y7P2°, z5P1,2,3°, and z7P2,3,4°, indicating that these states are populated by electronic relaxation during the ejection processes. Relative population ratios are obtained from the intensities of individual spectral lines. Excitation into the z7P2,3,4° states resulted in fluorescence only from z7P2°. Estimates of the time duration of the ejection process are obtained from time-resolved measurements.

Alkali-metal electron spin density shift induced by a helium nanodroplet

Helium (He) nanodroplets provide a cold and virtually unperturbing environment for the study of weakly bound molecules and van der Waals aggregates. High resolution microwave spectroscopy and the detection of electron spin transitions in doped He droplets have recently become possible. Measurements of hyperfine-resolved electron spin resonance in potassium (39K) and rubidium (85Rb) atoms on the surface of He droplets show small line shifts relative to the bare atoms. These shifts were recorded for all 2I + 1 components (I is the nuclear spin) of a transition at high accuracy for He droplets ranging in size from 1000 to 15,000 He atoms. Evaluation of the spectra yields the influence of the He environment on the electron spin density at the alkali-metal nucleus. A semi-empirical model is presented that shows good qualitative agreement with the measured droplet size dependent increase of Fermi contact interaction at the nuclei of dopant K and Rb.

A Beamline for Time-Resolved Extreme Ultraviolet and Soft X-Ray Spectroscopy

High harmonic generation is a convenient way to obtain extreme ultraviolet light from table-top laser systems and the experimental tools to exploit this simple and powerful light source for time-resolved spectroscopy are being developed by several groups. For these applications, brightness and stability of the high harmonic generation is a key feature. This article focuses on practical aspects in the generation of extreme ultraviolet pulses with ultrafast commercial lasers, namely generation parameters and online monitoring as well as analysis of generation yield and stability.

One- and Two-Color Resonant Photoionization Spectroscopy of Chromium-Doped Helium Nanodroplets

We investigate the photoinduced relaxation dynamics of Cr atoms embedded into superfluid helium nanodroplets. One- and two-color resonant two-photon ionization (1CR2PI and 2CR2PI, respectively) are applied to study the two strong ground state transitions z7P2,3,4° ← a7S3 and y7P2,3,4° ← a7S3. Upon photoexcitation, Cr* atoms are ejected from the droplet in various excited states, as well as paired with helium atoms as Cr*–Hen exciplexes. For the y7P2,3,4° intermediate state, comparison of the two methods reveals that energetically lower states than previously identified are also populated. With 1CR2PI we find that the population of ejected z5P3° states is reduced for increasing droplet size, indicating that population is transferred preferentially to lower states during longer interaction with the droplet. In the 2CR2PI spectra we find evidence for generation of bare Cr atoms in their septet ground state (a7S3) and metastable quintet state (a5S2), which we attribute to a photoinduced fast excitation–relaxation cycle mediated by the droplet. A fraction of Cr atoms in these ground and metastable states is attached to helium atoms, as indicated by blue wings next to bare atom spectral lines. These relaxation channels provide new insight into the interaction of excited transition metal atoms with helium nanodroplets.