Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThree-Dimensional Shapes of Spinning Helium Nanodroplets
189
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
A significant fraction of superfluid helium nanodroplets produced in a free-jet expansion have 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 dataset 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.
rate research
Read More
Helium tagging in action spectroscopy is an efficient method for measuring the absorption spectrum of complex molecular ions with minimal perturbations to the gas phase spectrum. We have used superfluid helium nanodroplets doped with corannulene to prepare cations of these molecules complexed with different numbers of He atoms. In total we identify 13 different absorption bands from corannulene cations between 5500 {AA} and 6000 {AA}. The He atoms cause a small, chemically induced redshift to the band positions of the corannulene ion. By studying this effect as a function of the number of solvating atoms we are able to identify the formation of solvation structures that are not visible in the mass spectrum. The solvation features detected with the action spectroscopy agree very well with the results of atomistic modeling based on path-integral molecular dynamics simulations. By additionally doping our He droplets with D$_2$, we produce protonated corannulene ions. The absorption spectrum of these ions differs significantly from the case of the radical cations as the numerous narrow bands are replaced by a broad absorption feature that spans nearly 2000 {AA} in width.
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.
A new setup for doping helium nanodroplets by means of laser ablation at kilohertz repetition rate is presented. The doping process is characterized and two distinct regimes of laser ablation are identified. The setup is shown to be efficient and stable enough to be used for spectroscopy, as demonstrated on beam-depletion spectra of lithium atoms attached to helium nanodroplets. For the first time, helium droplets are doped with high temperature refractory materials such as titanium and tantalum. Doping with the non-volatile DNA basis Guanine is found to be efficient and a number of oligomers are detected.
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.
We present the first measurement of a one-photon extreme-ultraviolet photoelectron spectrum (PES) of molecules embedded in superfluid helium nanodroplets. The PES of coronene is compared to gas phase and the solid phase PES, and to electron spectra of embedded coronene generated by charge transfer and Penning ionization through ionized or excited helium. The resemblence of the He-droplet PES to the one of the solid phase indicates that mostly Cor clusters are photoionized. In contrast, the He-droplet Penning-ionization electron spectrum is nearly structureless, indicating strong perturbation of the ionization process by the He droplet. These results pave the way to extreme ultraviolet photoelectron spectroscopy (UPS) of clusters and molecular complexes embedded in helium nanodroplets.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
