arXiv subjects

M. Mudrich

Publications and source records attributed to M. Mudrich.

At least 37 records · Page 2Linked to original sources

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

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arXiv2018

Quantum dynamics of Rb atoms desorbing off the surface of He nanodroplets

The desorption of excited rubidium (Rb) atoms off the surface of helium (He) nanodroplets is studied in detail using femtosecond time-resolved photoion and photoelectron imaging spectroscopy in combination with quantum wave packet simulations. The good agreement of the measured time-dependent velocity distributions with the simulation when exciting the Rb dopant atoms into the 6p-state supports the pseudo-diatomic model (PDM) for the Rb-He droplet interaction, even on the level of quantum wave packet dynamics. Time-resolved photoelectron spectra reveal the partitioning of excitation energy into the dopant and the droplet degrees of freedom.

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

A simple photoionization scheme for characterizing electron and ion spectrometers

We present a simple diode laser-based photoionization scheme for generating electrons and ions with well-defined spatial and energetic (<2 eV) structures. This scheme can easily be implemented into ion or electron imaging spectrometers for the purpose of off-line characterization and calibration. The low laser power ~1$ mW needed from a passively stabilized diode laser and the low flux of potassium atoms in an effusive beam make our scheme a versatile source of ions and electrons for applications in research and education.

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↗

arXiv2016

Desorption Dynamics of Rb_2 Molecules off the Surface of Helium Nanodroplets

The desorption dynamics of rubidium dimers (Rb_2) off the surface of helium nanodroplets induced by laser excitation is studied employing both nanosecond and femtosecond ion imaging spectroscopy. Similarly to alkali metal atoms, we find that the Rb_2 desorption process resembles the dissociation of a diatomic molecule. However, both angular and energy distributions of detected Rb_2^+ ions appear to be most crucially determined by the Rb_2 intramolecular degrees of freedom rather than by those of the Rb_2He_N complex. The pump-probe dynamics of Rb_2^+ is found to be slower than that of Rb^+ pointing at a weaker effective guest-host repulsion for excited molecules than for single atoms.

physics.atm-clus↗

arXiv2016

Efficiency of Dopant-Induced Ignition of Helium Nanoplasmas

Helium nanodroplets irradiated by intense near-infrared laser pulses ignite and form highly ionized nanoplasmas even at laser intensities where helium is not directly ionized by the optical field, provided the droplets contain a few dopant atoms. We present a combined theoretical and experimental study of the He nanoplasma ignition dynamics for various dopant species. We find that the efficiency of dopants to ignite a nanoplasma in helium droplets strongly varies and mostly depends on (i) the pick-up process, (ii) the number of free electrons each dopant donates upon ionization, and remarkably, (iii) by the hitherto unexplored effect of the dopant location in or on the droplet.

physics.atm-clus↗

arXiv2015

Fano Resonances observed in Helium Nanodroplets

Doubly-excited Rydberg states of helium (He) nanodroplets have been studied using synchrotron radiation. We observed Fano resonances related to the atomic N = 2,0 series as a function of droplet size. Although similar qualitatively to their atomic counterparts, the resonance lines are broader and exhibit a shift in energy which increases for the higher excited states. Furthermore, additional resonances are observed which are not seen in atomic systems. We discuss these features in terms of delocalized atomic states perturbed by the surrounding He atoms and compare to singly excited droplets.

physics.chem-ph↗

arXiv2015

Enhanced ionization of embedded clusters by Electron Transfer Mediated Decay in helium nanodroplets

Here, we report the observation of electron transfer mediated decay (ETMD) involving Mg clusters embedded in helium nanodroplets which is initiated by the ionization of helium followed by removal of two electrons from the Mg clusters of which one is transferred to the He environment neutralizing it while the other electron is emitted into the continuum. The process is shown to be the dominant ionization mechanism for embedded clusters for photon energies above the ionization potential of He. The photoelectron spectrum reveals a low energy ETMD peak. For Mg clusters larger than 5 atoms we observe stable doubly-ionized clusters. We argue that ETMD provides a new pathway to the formation of doubly-ionized cold species.

physics.chem-ph↗

arXiv2015

Dynamics of solvation and desolvation of rubidium attached to He nanodroplets

The real-time dynamics of photoexcited and photoionized rubidium (Rb) atoms attached to helium (He) nanodroplets is studied by femtosecond pump-probe mass spectrometry. While excited Rb atoms in the perturbed 6p-state (Rb*) desorb off the He droplets, Rb^+ photoions tend to sink into the droplet interior when created near the droplet surface. The transition from Rb^+ solvation to full Rb* desorption is found to occur at a delay time t~600 fs for Rb* in the 6pSigma-state and t~1200 fs for the 6pPi-state. Rb^+He ions are found to be created by directly exciting bound Rb*He exciplex states as well as by populating bound Rb^+He-states in an photoassociative ionization process.

physics.atm-clus↗

arXiv2015

Predissociation dynamics of lithium iodide

The predissociation dynamics of lithium iodide (LiI) in the first excited A-state is investigated for molecules in the gas phase and embedded in helium nanodroplets, using femtosecond pump-probe photoionization spectroscopy. In the gas phase, the transient Li+ and LiI+ ion signals feature damped oscillations due to the excitation and decay of a vibrational wave packet. Based on high-level ab initio calculations of the electronic structure of LiI and simulations of the wave packet dynamics, the exponential signal decay is found to result from predissociation predominantly at the lowest avoided X-A potential curve crossing, for which we infer a coupling constant V=650(20) reciprocal cm. The lack of a pump-probe delay dependence for the case of LiI embedded in helium nanodroplets indicates fast droplet-induced relaxation of the vibrational excitation.

physics.atm-clus↗

arXiv2014

Desorption Dynamics of Heavy Alkali Metal Atoms (Rb, Cs) off the Surface of Helium Nanodroplets

We present a combined ion imaging and density functional theory study of the dynamics of the desorption process of rubidium and cesium atoms off the surface of helium nanodroplets upon excitation of the perturbed $6s$ and $7s$ states, respectively. Both experimental and theoretical results are well represented by the pseudodiatomic model for effective masses of the helium droplet in the desorption reaction of m_eff/m_He~10 (Rb) and 13 (Cs). Deviations from this model are found for Rb excited to the 6p state. Photoelectron spectra indicate that the dopant-droplet interaction induces relaxation into low-lying electronic states of the desorbed atoms in the course of the ejection process.

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arXiv2014

Photoionizaton of Pure and Doped Helium Nanodroplets

Helium nanodroplets, commonly regarded as the "nearly ideal spectroscopic matrix", are being actively studied for more than two decades now. While they mostly serve as cold, weakly perturbing and transparent medium for high-resolution spectroscopy of embedded molecules, their intrinsic quantum properties such as microscopic superfluidity still are subject-matter of current research. This article reviews recent work on pure and doped He nanodroplets using PI spectroscopy, an approach which has greatly advanced in the past years. While the notion of the ideal spectroscopic matrix mostly no longer holds in this context, photoionization techniques provide detailed insights into the photo-physical properties of pure and doped He nanodroplets and their relaxation dynamics following electronic excitation. Exploiting nowadays available high laser fields, even highly ionized states of matter on the nanoscale can be formed. Our particular focus lies on recent experimental progress including fs time-resolved spectroscopy, photoion and electron imaging, and novel sources of highly energetic radiation.

physics.atm-clus↗

arXiv2013

Collective Autoionization in Multiply-Excited Systems: A novel ionization process observed in Helium Nanodroplets

Free electron lasers (FELs) offer the unprecedented capability to study reaction dynamics and image the structure of complex systems. When multiple photons are absorbed in complex systems, a plasma-like state is formed where many atoms are ionized on a femtosecond timescale. If multiphoton absorption is resonantly-enhanced, the system becomes electronically-excited prior to plasma formation, with subsequent decay paths which have been scarcely investigated to date. Here, we show using helium nanodroplets as an example that these systems can decay by a new type of process, named collective autoionization. In addition, we show that this process is surprisingly efficient, leading to ion abundances much greater than that of direct single-photon ionization. This novel collective ionization process is expected to be important in many other complex systems, e.g. macromolecules and nanoparticles, exposed to high intensity radiation fields.

physics.atm-clus↗

arXiv2013

EUV ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization and electron energy-loss spectra

The ionization dynamics of pure He nanodroplets irradiated by EUV radiation is studied using Velocity-Map Imaging PhotoElectron-PhotoIon COincidence (VMI-PEPICO) spectroscopy. We present photoelectron energy spectra and angular distributions measured in coincidence with the most abundant ions He+, He2+, and He3+. Surprisingly, below the autoionization threshold of He droplets we find indications for multiple excitation and subsequent ionization of the droplets by a Penning-like process. At high photon energies we evidence inelastic collisions of photoelectrons with the surrounding He atoms in the droplets.

physics.atm-clus↗

arXiv2013

Penning ionization of doped helium nanodroplets following EUV excitation

Helium nanodroplets are widely used as a cold, weakly interacting matrix for spectroscopy of embedded species. In this work we excite or ionize doped He droplets using synchrotron radiation and study the effect onto the dopant atoms depending on their location inside the droplets (rare gases) or outside at the droplet surface (alkali metals). Using photoelectron-photoion coincidence imaging spectroscopy at variable photon energies (20-25 eV), we compare the rates of charge-transfer to Penning ionization of the dopants in the two cases. The surprising finding is that alkali metals, in contrast to the rare gases, are efficiently Penning ionized upon excitation of the (n=2)-bands of the host droplets. This indicates rapid migration of the excitation to the droplet surface, followed by relaxation, and eventually energy transfer to the alkali dopants.

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