Interface Excitons in Krmnen Clusters : The Role of Electron Affinity in the Formation of Electronic Structure

The formation of the electronic structure of small Kr_m clusters (m<150) embedded inside Ne_N clusters (1200<N<7500) has been investigated with the help of fluorescence excitation spectroscopy using synchrotron radiation. Electronically excited states, assigned to excitons at the Ne/Kr interface, 1i and 1'i were observed. The absorption bands, which are related to the lowest spin-orbit split atomic Kr 3P1 and 1P1 states, initially appear and shift towards lower energy when the krypton cluster size m increases. The characteristic bulk 1t and 1't excitons appear in the spectra, when the cluster radius exceeds some critical value, R_cl>Delta_1i . Kr clusters comprising up to 70 atoms do not exhibit bulk absorption bands. We suggest that this is due to the penetration of the interface excitons into the Kr_m cluster volume, because of the negative electron affinity of surrounding Ne atoms. From the energy shift of the interface absorption bands with cluster size an unexpectedly large penetration depth of delta_1i =7.0+/-0.1 A is estimated, which can be explained by the interplay between the electron affinities of the guest and the host cluster

Investigating Mechanisms of State Localization in Highly-Ionized Dense Plasmas

We present the first experimental observation of K$_{\beta}$ emission from highly charged Mg ions at solid density, driven by intense x-rays from a free electron laser. The presence of K$_{\beta}$ emission indicates the $n=3$ atomic shell is relocalized for high charge states, providing an upper constraint on the depression of the ionization potential. We explore the process of state relocalization in dense plasmas from first principles using finite-temperature density functional theory alongside a wavefunction localization metric, and find excellent agreement with experimental results.Comment: 22 pages, 13 figure

Investigating mechanisms of state localization in highly ionized dense plasmas

Producción CientíficaWe present experimental observations of Kβ emission from highly charged Mg ions at solid density, driven by intense x rays from a free electron laser. The presence of Kβ emission indicates the n=3 atomic shell is relocalized for high charge states, providing an upper constraint on the depression of the ionization potential. We explore the process of state relocalization in dense plasmas from first principles using finite-temperature density functional theory alongside a wave-function localization metric, and find excellent agreement with experimental results.This work has been supported by the Spanish Ministry of Science and Innovation under Research Grant No. PID2019-108764RB-I0

Electronic excitation, decay and photochemical processes in rare gas clusters

An overview of processes of electronic excitation and the subsequent following decay in clusters is presented. Pure rare gas clusters (Xe-Ne) are chosen as model systems for non-metallic clusters. Selective photo-excitation with synchrotron radiation offers the possibility to study a variety of different decay and relaxation processes (exciton trapping, desorption of electronically excited species) depending both on the cluster size and of the excitation energy. Special emphasis is given to the role of energy transfer, and in particular, vibrational energy flow, radiative and non-radiative electronic relaxation. Some recent results on the decay processes in He clusters are given in the last part of the article. Decay processes in He clusters differ remarkably from that observed in the heavier rare gas clusters

Electronic excitations in liquid helium: The evolution from small clusters to large droplets

The absorption coefficient of liquid He droplets is reported in the energy range from the first electronic excitation up to the ionization limit. It is obtained from total fluorescence yield curves. The size of He droplets is varied between approximately 10$^6$ atoms/particle to small clusters containing on the order of 50 atoms. The character of the excited states is discussed in view of different theoretical approaches, i.e., molecular excitations, Frenkel and Wannier type excitons. It turned out that the Frenkel and Wannier model being very successful for the description of excited states in insulators has severe shortcomings in the case of liquid He. Furthermore, the implications for the structural peculiarities of liquid He are addressed

Electronic excitation, decay and photochemical processes in rare gas clusters

An overview of processes of electronic excitation and the subsequent following decay in clusters is presented. Pure rare gas clusters (Xe-Ne) are chosen as model systems for non-metallic clusters. Selective photo-excitation with synchrotron radiation offers the possibility to study a variety of different decay and relaxation processes (exciton trapping, desorption of electronically excited species) depending both on the cluster size and of the excitation energy. Special emphasis is given to the role of energy transfer, and in particular, vibrational energy flow, radiative and non-radiative electronic relaxation. Some recent results on the decay processes in He clusters are given in the last part of the article. Decay processes in He clusters differ remarkably from that observed in the heavier rare gas clusters

Evolution of excitonic energy levels in ArN\mathrm{Ar_N} clusters: Confinement of bulk, surface, and deep valence shell excitons

The evolution of excitonic energy levels (Wannier and Frenkel type) is investigated for ArN clusters in the range N=200–106 using fluorescence excitation spectroscopy. In the case of Wannier excitons, a pronounced blue shift of the absorption bands relative to the position in the infinite solid is observed. As a consequence of the lower dimensionality, the shift of the transition energy of surface excitons is considerably smaller than that of the bulk states of clusters. The evolution with size is discussed within several theoretical models for exciton confinement. In addition, model calculations are performed for bulk excitons which give good quantitative agreement with the experimental results. In the case of n=1 Frenkel or intermediate type excitons, there are blue and red shifts observed. The spectral shift of (3p→4s) and deep valence (3s→4p) excitations differs considerably. From the shift of the transition energies the exciton mass of the (3p→4s) exciton is derived

CLULU: An experimental setup for luminescence measurements on van der Waals clusters with synchrotron radiation

We describe a new experimental setup for photoluminescence spectroscopy on van der Waals clusters. It consists of a molecular beam apparatus with a cluster beam installed behind a high intensity vacuum ultraviolet synchrotron radiation beamline. Special emphasis was given to the design of a very intense cluster source which can also be used for the preparation of quantum clusters (He,H$_2$). For the determination of the cluster size a time‐of‐flight mass spectrometer can be attached to the setup. In addition, an atomic cross jet is installed in the experimental chamber which can be used for mass separation or for doping of the clusters. The luminescence light can be recorded with several different detectors or spectrally analyzed with a secondary monochromator which is equipped with a position sensitive detector. The pulsed nature of synchrotron radiation provides the basis for time‐resolved measurements in the regime 100 ps$^{–3}$ μs

Molecular dynamics simulations of inelastic X-Ray scattering from shocked copper

By taking the spatial and temporal Fourier transforms of the coordinates of the atoms in molecular dynamics simulations conducted using an embedded-atom-method potential, we calculate the inelastic scattering of x-rays from copper singlecrystals shocked along [001] to pressures of up to 70 GPa. Above the Hugoniot elastic limit (HEL), we find that the copious stacking faults generated at the shock front introduce strong quasi-elastic scattering (QES) that competes with the inelastic scattering signal, which remains discernible within the first Brillouin zone; for specific directions in reciprocal space outside the first zone, the QES dominates the inelastic signal overwhelmingly. The synthetic scattering spectra we generate from our Fourier transforms suggest that energy resolutions of order 10 meV would be required to distinguish inelastic from quasi-elastic scattering within the first Brillouin zone of shock-loaded copper. We further note that high-resolution inelastic scattering also affords the possibility of directly measuring particle velocities via the Doppler shift. These simulations are of relevance to future planned inelastic scattering experiments at x-ray Free Electron Laser (FEL) facilities