arXiv subjects

Gert von Helden

Publications and source records attributed to Gert von Helden.

7 recordsLinked to original sources

arXiv2026

A two-color dual-oscillator infrared free-electron laser

We report on the design and performance of a two-color dual-oscillator infrared free-electron laser (FEL). The mid-infrared (MIR) FEL at the Fritz Haber Institute (FHI FEL) has been upgraded to include a second oscillator FEL beamline that permits lasing in the far-infrared (FIR) regime from 4.5 μm to 175 μm. In addition, a 500 MHz kicker cavity has been installed downstream of the electron accelerator. It allows to deflect electron bunches of up to 50 MeV energy alternately left and right by an angle of {\pm}2°. It can, thus, split the high-repetition-rate (1 GHz) electron bunch train from the accelerator into two bunch trains of 500 MHz repetition rate each; one is steered to the MIR FEL and the other one to the new FIR FEL. In this two-color mode of simultaneous, synchronized operation the wavelengths in both FELs can be tuned independently over wide ranges of up to a factor of four each by undulator-gap variation. In addition, two-color operation is also available at reduced repetition rates (e.g. 55.6 MHz of both MIR and FIR pulses), as needed for some applications. This unique two-color mode opens up a wealth of novel user applications such as, MIR-FIR pump-probe experiments.

physics.acc-ph↗

arXiv2026

Controlling isomer population using a dual-oscillator infrared free-electron laser

We report on the control and characterization of the isomer population of ions inside superfluid helium nanodroplets, using two-color operation of a dual-oscillator infrared free-electron laser. The timing of both lasers is highly synchronized, and their frequencies (or "colors") can be tuned independently over a wide range. Interaction of the singly deuterated proton-bound dimer of dihydrogen phosphate and formate inside helium nanodroplets with both colors enables the control over its isomer population and the recording of - one-color hidden - infrared spectra of individual isomers.

physics.chem-ph↗

arXiv2024

Observation of a core-excited dipole-bound state $\sim$1 eV above the electron detachment threshold in cryogenically cooled acetylacetonate

Dipole-bound states in anions exist when a polar neutral core binds an electron in a diffuse orbital through charge--dipole interaction. Electronically excited polar neutral cores can also bind an electron in a diffuse orbital to form Core-Excited Dipole-Bound States (CE-DBSs), which are difficult to observe because they usually lie above the electron detachment threshold, leading to very short lifetimes and, thus, unstructured transitions. We report here the photodetachment spectroscopy of cryogenically cooled acetylacetonate anion (C$_5$H$_7$O$_2^-$) recorded by detecting the neutral radical produced upon photodetachment and the infrared spectroscopy in He-nanodroplets. Two DBSs were identified in this anion. One of them lies close to the electron detachment threshold ($\sim$2.74 eV) and is associated with the ground state of the radical (D0-DBS). Surprisingly, the other DBS appears as resonant transitions at 3.69 eV and is assigned to the CE-DBS associated with the first excited state of the radical (D1-DBS). It is proposed that the resonant transitions of the D1-DBS are observed $\sim$1 eV above the detachment threshold because its lifetime is determined by the internal conversion to the D0-DBS, after which the fast electron detachment takes place.

physics.chem-ph↗

arXiv2024

Metal monoxide abundances as a function of the C/O ratio

The diatomic metal monoxides whose optical spectra define the classification of stars on the asymptotic giant branch (AGB), that is, TiO, YO, ZrO, and LaO, have the unusual property that their ionization energy is below their dissociation limit. The cations of these metal monoxides can be efficiently produced via associative ionization of their constituent ground state atoms and are long-lived. We present a simple model that can explain the observed relative abundance of these metal oxides as a function of the C/O ratio.

astro-ph.SR↗

arXiv2013

How Cations Change Peptide Structure

Specific interactions between cations and proteins have a strong impact on peptide and protein structure. We here shed light on the nature of the underlying interactions, especially regarding the effects on the polyamide backbone structure. To do so, we compare the conformational ensembles of model peptides in isolation and in the presence of either Li+ or Na+ cations by state-of-the-art density-functional theory (including van der Waals effects) and gas-phase infrared spectroscopy. These monovalent cations have a drastic effect on the local backbone conformation of turn-forming peptides, by disruption of the H bonding networks and the resulting severe distortion of the backbone conformations. In fact, Li+ and Na+ can even have different conformational effects on the same peptide. We also assess the predictive power of current approximate density functionals for peptide-cation systems and compare to results from established protein force fields as well as to high-level quantum chemistry (CCSD(T)).

q-bio.BM↗

arXiv2010

Catching proteins in liquid helium droplets

An experimental approach is presented that allows for the incorporation of large mass/charge selected ions in liquid helium droplets. It is demonstrated that droplets can be efficiently doped with a mass/charge selected amino acid as well as with the much bigger m$\approx$12 000 amu protein Cytochrome C in selected charge states. The sizes of the ion-doped droplets are determined via electrostatic deflection. Under the experimental conditions employed, the observed droplet sizes are very large and range, dependent on the incorporated ion, from 10$^{10}$ helium atoms for protonated Phenylalanine to 10$^{12}$ helium atoms for Cytochrome C. As a possible explanation, a simple model based on the size- and internal energy-dependence of the pickup efficiency is given.

physics.atm-clus↗

arXiv2008

A selector for structural isomers of neutral molecules

We have selected and spatially separated the two conformers of 3-aminophenol (C$_6$H$_7$NO) present in a molecular beam. Analogous to the separation of ions based on their mass-to-charge ratios in a quadrupole mass filter, the neutral conformers are separated based on their different mass-to-dipole-moment ratios in an ac electric quadrupole selector. For a given ac frequency, the individual conformers experience different focusing forces, resulting in different transmissions through the selector. These experiments demonstrate that conformer-selected samples of large molecules can be prepared, offering new possibilities for the study of gas-phase biomolecules.

physics.chem-ph↗