Intense radiation from lasers has opened up many new areas of research in physics and chemistry, and has revolutionized optical technology. So far, most work in the field of nonlinear processes has been restricted to infrared, visible and ultraviolet light, although progress in the development of X-ray lasers has been made recently. With the advent of a free-electron laser in the soft-X-ray regime below 100 nm wavelength, a new light source is now available for experiments with intense, short-wavelength radiation that could be used to obtain deeper insights into the structure of matter. Other free-electron sources with even shorter wavelengths are planned for the future. Here we present initial results from a study of the interaction of soft X-ray radiation, generated by a free-electron laser, with Xe atoms and clusters. We find that, whereas Xe atoms become only singly ionized by the absorption of single photons, absorption in clusters is strongly enhanced. On average, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons. We suggest that the clusters are heated up and electrons are emitted after acquiring sufficient energy. The clusters finally disintegrate completely by Coulomb explosion.

Discrete visible and near-infrared luminescence of a beam of photoexcited helium clusters is reported. The emission lines are attributed to free helium atoms and molecules desorbing from clusters in electronically excited states. Depending on the excitation energy, various atomic and molecular singlet and triplet states are involved in the relaxation process. With increasing cluster size the intensity of molecular transitions becomes dominant. The temperature of ejected molecules could be estimated to T vib ϳ 2500 K and T rot ϳ 450 K and is much higher than that of the cluster itself.[ S0031-9007(97) PACS numbers: 36.40. Mr, 33.50.Dq, 36.40.Qv Visible and infrared (ir) luminescence spectroscopy is one of the oldest and most established methods for the exploration of electronic level structure and excited states dynamics of atoms and molecules. The derivation of the well known Rydberg formula which was based on the observation of luminescence spectra of metal atoms is one of the most prominent examples of its usefulness. Condensed matter physics has also benefited substantially from luminescence spectroscopy, but detailed information on the excited state dynamics of pure materials is often difficult to obtain for two reasons: (i) In most cases only rather broad luminescence bands due to transitions between the lowest electronically excited state and the ground state are observed. (ii) Transitions between electronically excited states of pure solids or liquids similar to that in atoms and molecules are usually not observed because nonradiative relaxation processes following electronic excitation depopulate the highly excited states extremely efficiently.Liquid helium is presumably the only exception to this rule. In contrast to all other pure condensed materials liquid helium emits a very rich discrete spectrum in the ir which is due to transitions between electronically excited states of helium atoms or molecules enclosed in bubbles [1,2]. Thus, in the context of cluster science helium is an ideal material to study the excited state dynamics and radiative and nonradiative processes as a function of the cluster size.In this Letter we report on the observation of visible and ir luminescence of helium cluster beams. Similar to He 2 [3] and the bulk liquid [2], helium clusters emit a large number of discrete lines in the visible and infrared range. The excited states dynamics is investigated as a function of the cluster size and in addition the excitation energy is varied. This permits detailed insight into the relaxation pathway. Further, this is the first luminescence study of helium with photoexcitation at all, since all work done in the past on the excited states of atomic, molecular, or liquid helium has been performed with unspecific excitation using charged particles, e.g., electrons, protons, a particles. In general, nonradiative processes dominating the relaxation pathway depend on the particle density. In helium clusters the particle density depends strongly on the cluster size [4] which makes them even...

We have recorded the R(0)nu(CO) = 1 <- 0 IR spectrum of CO and its isotopomers in superfluid helium nanodroplets. For droplets with average size N greater than or similar to 2000 helium atoms, the transition exhibits a Lorentzian shaped linewidth of 0.034 cm(-1), indicating a homogeneous broadening mechanism. The rotational constants could be deduced and were found to be reduced to about 60% of the corresponding gas-phase values (63% for the reference C-12 O-16 species). Accompanying calculations of the pure rotational spectra were carried out using the method of correlated basis functions in combination with diffusion Monte Carlo (CBF/DMC). These calculations show that both the reduction of the rotational B constant and the line broadening can be attributed to phonon-rotation coupling. The reduction in B is confirmed by path integral correlation function calculations for a cluster of 64 He-4 atoms, which also reveal a non-negligible effect of finite size on the collective modes. The phonon-rotation coupling strength is seen to depend strongly on the strength and anisotropy of the molecule-helium interaction potential. Comparison with other light rotors shows that this coupling is particularly high for CO. The CBF/DMC analysis shows that the J = 1 rotational state couples effectively to phonon states, which are only present in large helium droplets or bulk. In particular, they are not present in small clusters with n <= 20, thereby accounting for the much narrower linewidths and larger B constant measured for these sizes

A high sensitivity, high resolution tandem mass spectrometer to research low-energy, reactive ion-surface interactions Review of Scientific Instruments 91, 065101 (2020);

The nature of the electronically excited states of He clusters and their relaxation mechanisms are investigated by spectroscopy using monochromatized synchrotron radiation. Time correlated fluorescence excitation and energy resolved luminescence spectra of the clusters are recorded in separate wavelength ranges. The size of the clusters and the isotopic constitution is also varied. The spectral features are analysed and discussed particularly with regard to the high lying states and their possible Rydberg nature. While Rydberg states seem not to exist in the interior region of large clusters there is experimental evidence that sharp lines in the spectrum are either due to He Rydberg atoms or excimer molecules in high vibrational states bound at the surface of large clusters or due to very small positively charged clusters with the Rydberg electron outside. The spectra of large 3 He clusters exhibit a larger contribution of Rydberg lines than 4 He clusters. He clusters also emit fluorescence at energies above the ionization energy of He atoms. This is attributed to the barrier for the injection of electrons into the conduction band which was found to be 1.35 eV for 4 He and 0.95 eV for 3 He clusters, respectively.

The energy transfer in 3He and 4He clusters electronically excited by monochromatic synchrotron radiation is investigated by luminescence spectroscopy. Depending on the cluster size and the isotopic constitution, either sharp, broadened, or shifted emission bands of single He molecules are observed. The spectral features show that He molecules emit light either within a bubble inside the cluster or in the vacuum after desorption from the cluster. From the luminescence intensity, the cluster diameter, and the radiative lifetime, an average velocity of approximately 7 m/s of bubbles in 4He clusters could be deduced. In the nonsuperfluid 3He clusters this velocity was observed to be significantly lower.

The helium atom is the simplest example of a many-electron atom. In light of the simplicity and model character of helium atoms one would naturally be interested to learn how the electronically excited states become modified when an excited helium atom is placed near one, two, or more helium atoms. A straightforward experiment to investigate this problem is to produce a beam of helium clusters of variable size and to photoexcite these clusters in the vacuum ultraviolet (VUV) spectral range using synchrotron radiation. Such an experiment probes a disordered arrangement of atoms because helium clusters and droplets in a free beam are always liquid. A molecular beam of clusters is optically thin and absorbs very little light, but helium clusters have a large fluorescence yield. Therefore, photofluorescence yield detection is the method of choice. The first experiments of 4 He clusters of variable size using a VUV fluorescence light detector identified a number of bands and it was found that the energies of these excitations, unlike the electronic excitations of heavy rare gas clusters, 1,2 did not fit to a Wannier-type excitonic series. 3 Subsequent experiments investigated fluorescence decay channels in different wavelength regions, 4,5 the effect of helium particle density, 6 fluorescence in cavities within large helium droplets, 7 and the possible Rydberg nature of the excited states of small helium clusters. 8 First quantum chemical calculations of an octahedral model cluster reported the energies of the n = 2, 3, and 4 states of the central atom as a function of the internuclear separation in the octahedron. 9 Electronic spectra simulations of a perturbed octahedron as well as an N = 25 atom-large cluster 10 were able to reproduce previously reported experimental data of small clusters. 6 Despite these efforts, we still have an incomplete picture of the electronically excited states of large helium clusters. The number of states increases dramatically with size making the computation and interpretation of the results laborious. The availability of experimental data that cover the entire size range of helium clusters and droplets is prerequisite as a benchmark for testing theoretical interpretation and likewise prerequisite in providing evidence for empirical interpretation. Furthermore, the comprehensive data set presented in this paper shows that each helium cluster size has a specific spectral fingerprint. A unique feature of helium clusters and droplets is the 6À7 Å thick surface region where the density drops smoothly from the bulk value to zero. 11,11,12 With regard to electronically excited states, such a surface layer represents a ABSTRACT: We report a comprehensive investigation of the electronically excited states of helium clusters and droplets of sizes ranging from a few to several 10 7 atoms using time-resolved fluorescence excitation spectroscopy and quantum chemical ab initio calculations. We employ various approaches for our analysis considering the lifetime-dependence of the fluorescence intens...

We have measured the high-resolution infrared spectrum of the radical NO in the 2 1=2 state in superfluid helium nanodroplets. The features are attributed to the -doubling splitting and the hyperfine structure. The hyperfine interaction is found to be unaffected by the He solvation. For the -doubling splitting, we find a considerable increase by 55% compared to the gas phase. This is explained by a confinement of the electronically excited NO states by the surrounding He. The rotational level spacing is decreased to 76% of the gas phase value. The IR transition to the J 1:5 state is found to be homogeneously broadened. We attribute both observations to the coupling between the molecular rotation and phonon/roton excitations in superfluid 4 He droplets. DOI: 10.1103/PhysRevLett.95.215301 PACS numbers: 67.40.Fd, 33.15.Pw, 33.20.Ea, 33.20.Wr Spectroscopy of molecules in He nanodroplets is a very promising technique for investigating chemical reactions at ultralow temperatures. In such reactions, radicals play an important role [1]. It is, therefore, of interest to probe the influence of the He environment on a radical. While at present quite a number of closed shell molecules have been studied in He droplets [2], only little is known about the influence of the helium droplets on the spectra [3] and the electronic structure of radicals. The infrared spectra of many radicals exhibit hyperfine splitting and -type doubling. These effects are well known in the gas phase but have never been studied in solvents.In this Letter, we report the measurement of the IR spectrum of NO in He droplets. NO is an open shell molecule and has a 2 1=2 electronic ground state. The rotational levels of NO show hyperfine structure and -type doubling. While the hyperfine splitting reflects the magnetic interaction, -type doubling arises from rotational interaction of the ground state with higher electronic states. The coupling to the higher electronic or ÿ states splits the degenerate energy levels of the 2 1=2ground state. We will show that the droplets increase the magnitude of the -type doubling and discuss the implications for the electronically excited states of NO within the He solvent. The lower rotational energy levels of NO lie in the interesting region of the collective excitations of superfluid He. In particular, they are close to the roton excitations. NO is therefore a good candidate to investigate the role of these excitations in the rotational energy relaxation or in the shift of the rotational levels, as recently pointed out by theory [4,5]. For this purpose, NO as a diatomic molecule is ideal because it has only a single vibrational mode. The vibrational energy is much larger than the collective excitations of liquid He (1875 cm ÿ1 10 cm ÿ1 ). Therefore, vibrational relaxation is extremely slow.The experiments have been carried out using a new apparatus at Bochum. The setup is similar to that used by Hartmann et al. and explained in detail elsewhere [5,6]. Here we give only a brief description. Helium is expanded at 40 ...