Unusual Rydberg system consisting of a positively charged helium nanodroplet with an orbiting electron

Relaxation dynamics of 3He and 4He clusters and droplets studied using near infrared and visible fluorescence excitation spectroscopy

The relaxation dynamics of electronically excited ³He and ⁴He clusters and droplets is investigated using time-correlated near-infrared and visible (NIR/VIS) fluorescence excitation spectroscopy. A rich data set spanning a wide range of cluster and droplet sizes is produced. The spectral features broadly follow the vacuum ultraviolet excitation (VUV) spectra. However, when the NIR/VIS spectra are normalised to the VUV fluorescence, regions with distinctly different cluster size and isotope dependence are identified, enabling deeper insight into the relaxation mechanism. Particle density, location of atomic-like states and their principal quantum number, n, are found to play an important role in the relaxation. For states with n = 3 and higher, only energy within the surface region is transferred to excited atoms which are subsequently ejected from the surface and fluoresce in vacuum. For states with n = 2, energy from the entire region within clusters and droplets is transferred to the surface, leading to the ejection of excited atoms and excimers. Here, the energy is transferred by excitation hopping, which competes with radiative and non-radiative decay, making ejection and NIR/VIS fluorescence inefficient in increasingly larger droplets.

Reaction dynamics within a cluster environment

This perspective article reviews experimental and theoretical works where rare gas clusters and helium nanodroplets are used as a nanoreactor to investigate chemical dynamics in a solvent environment. A historical perspective is presented first followed by specific considerations on the mobility of reactants within these reaction media. The dynamical response of pure clusters and nanodroplets to photoexcitation is shortly reviewed before examining the role of the cluster (or nanodroplet) degrees of freedom in the photodynamics of the guest atoms and molecules.

Suppression of Multiphoton Ionization of Aniline in Large Superfluid Helium Droplets

We report suppression of multiphoton ionization (MPI) of aniline doped large superfluid helium droplets containing over 5 × 10⁶ atoms. In contrast, surface-bound sodium atoms and dimers are readily desorbed and ionized. Adequacy of the experimental conditions is also confirmed from ejection of embedded aniline cations from smaller droplets containing multiple cations, and MPI of gaseous aniline. The photoelectrons have a mean-free-path of less than 1 nm and a thermalization distance of 10 nm. In a droplet with a diameter of over 70 nm, effective charge recombination within the droplet is expected.

Lithium atoms on helium nanodroplets: Rydberg series and ionization dynamics

The electronic excitation spectrum of lithium atoms residing on the surface of helium nanodroplets is presented and analyzed employing a Rydberg-Ritz approach. Utilizing resonant two-photon ionization spectroscopy, two different Rydberg series have been identified: one assigned to the nS(Σ) series and the other with predominantly nP(Π) character. For high Rydberg states, which have been resolved up to n = 13, the surrounding helium effectively screens the valence electron from the Li ion core, as indicated by the apparent red-shift of Li transitions and lowered quantum defects on the droplet with respect to their free atom counterparts. For low n states, the screening effect is weakened and the prevailing repulsive interaction gives rise to strongly broadened and blue-shifted transitions. The red-shifts originate from the polarization of nearby He atoms by the positive Li ion core. As a consequence of this effect, the ionization threshold is lowered by 116 ± 10 cm^-1 for Li on helium droplets with a radius of about 40 Å. Upon single-photon ionization, heavy complexes corresponding to Li ions attached to intact helium droplets are detected. We conclude that ionization close to the on-droplet ionization threshold triggers a dynamic process in which the Li ion core undergoes a transition from a surface site into the droplet.

Rydberg states of alkali atoms on superfluid helium nanodroplets: inside or outside?

Electronic excitations of an electron bound to an alkali metal ion inside a droplet of superfluid 4He are computed via a combination of helium density functional theory and the numerical integration of the Schrödinger equation for a single electron in a modified, He density dependent atomic pseudopotential. The application of a spectral method to the radial part of the valence electron wavefunction allows the computation of highly excited Rydberg states. For low principal quantum numbers, the energy required to push the electron outward is larger than the solvation energy of the ion. However, for higher principal quantum numbers the situation is reversed, which suggests the stability of a system where the ion sits inside the droplet while the valence electron orbits the nanodroplet.

Desorption Dynamics of Rb2 Molecules Off the Surface of Helium Nanodroplets

The desorption dynamics of rubidium dimers (Rb₂) off the surface of helium nanodroplets induced by laser excitation is studied by employing both nanosecond and femtosecond ion imaging spectroscopy. Similarly to alkali metal atoms, we find that the Rb₂ desorption process resembles the dissociation of a diatomic molecule. However, both angular and energy distributions of detected Rb₂⁺ ions appear to be most crucially determined by the Rb₂ intramolecular degrees of freedom rather than by those of the Rb₂He_N complex. The pump-probe dynamics of Rb₂⁺ is found to be slower than that of Rb⁺, pointing at a weaker effective guest-host repulsion for excited molecules than for single atoms.

Communication: nucleation of quantized vortex rings in 4He nanodroplets

Whereas most of the phenomena associated with superfluidity have been observed in finite-size helium systems, the nucleation of quantized vortices has proven elusive. Here we show using time-dependent density functional simulations that the solvation of a Ba(+) ion created by photoionization of neutral Ba at the surface of a (4)He nanodroplet leads to the nucleation of a quantized ring vortex. The vortex is nucleated on a 10 ps timescale at the equator of a solid-like solvation structure that forms around the Ba(+) ion. The process is expected to be quite general and very efficient under standard experimental conditions.

Coupled electronic and structural relaxation pathways in the postexcitation dynamics of Rydberg states of BaArN clusters

We investigate, theoretically, the joint relaxation of orbital and structure in postexcitation dynamics of Rydberg states of cluster BaArN (N=250). Mixed quantum-classical dynamics is used to account for the nonadiabatic transitions among more than 160 electronic states, represented via a diatomics-in-molecules Hamiltonian. The simulation illustrates the complex multistep relaxation processes and provides detailed insight in the mechanisms contributing to the final-time experimental photoelectron spectrum.

One- and two-color resonant photoionization spectroscopy of chromium-doped helium nanodroplets

We investigate the photoinduced relaxation dynamics of Cr atoms embedded into superfluid helium nanodroplets. One- and two-color resonant two-photon ionization (1CR2PI and 2CR2PI, respectively) are applied to study the two strong ground state transitions z⁷P_2,3,4^° ← a⁷S₃ and y⁷P_2,3,4^° ← a⁷S₃. Upon photoexcitation, Cr* atoms are ejected from the droplet in various excited states, as well as paired with helium atoms as Cr*–He_n exciplexes. For the y⁷P_2,3,4^° intermediate state, comparison of the two methods reveals that energetically lower states than previously identified are also populated. With 1CR2PI we find that the population of ejected z⁵P₃^° states is reduced for increasing droplet size, indicating that population is transferred preferentially to lower states during longer interaction with the droplet. In the 2CR2PI spectra we find evidence for generation of bare Cr atoms in their septet ground state (a⁷S₃) and metastable quintet state (a⁵S₂), which we attribute to a photoinduced fast excitation–relaxation cycle mediated by the droplet. A fraction of Cr atoms in these ground and metastable states is attached to helium atoms, as indicated by blue wings next to bare atom spectral lines. These relaxation channels provide new insight into the interaction of excited transition metal atoms with helium nanodroplets.

Critical Landau velocity in helium nanodroplets

The best-known property of superfluid helium is the vanishing viscosity that objects experience while moving through the liquid with speeds below the so-called critical Landau velocity. This critical velocity is generally considered a macroscopic property as it is related to the collective excitations of the helium atoms in the liquid. In the present work we determine to what extent this concept can still be applied to nanometer-scale, finite size helium systems. To this end, atoms and molecules embedded in helium nanodroplets of various sizes are accelerated out of the droplets by means of optical excitation, and the speed distributions of the ejected particles are determined. The measurements reveal the existence of a critical velocity in these systems, even for nanodroplets consisting of only a thousand helium atoms. Accompanying theoretical simulations based on a time-dependent density functional description of the helium confirm and further elucidate this experimental finding.

Spectroscopy of lithium atoms and molecules on helium nanodroplets

We report on the spectroscopic investigation of lithium atoms and lithium dimers in their triplet manifold on the surface of helium nanodroplets (He_N). We present the excitation spectrum of the 3p ← 2s and 3d ← 2s two-photon transitions for single Li atoms on He_N. The atoms are excited from the 2S(Σ) ground state into Δ, Π, and Σ pseudodiatomic molecular substates. Excitation spectra are recorded by resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) mass spectroscopy, which allows an investigation of the exciplex (Li*–He_m, m = 1–3) formation process in the Li–He_N system. Electronic states are shifted and broadened with respect to free atom states, which is explained within the pseudodiatomic model. The assignment is assisted by theoretical calculations, which are based on the Orsay–Trento density functional where the interaction between the helium droplet and the lithium atom is introduced by a pairwise additive approach. When a droplet is doped with more than one alkali atom, the fragility of the alkali–He_N systems leads preferably to the formation of high-spin molecules on the droplets. We use this property of helium nanodroplets for the preparation of Li dimers in their triplet ground state (1³Σ_u⁺). The excitation spectrum of the 2³Π_g(ν′ = 0–11) ← 1³Σ_u⁺(ν″ = 0) transition is presented. The interaction between the molecule and the droplet manifests in a broadening of the transitions with a characteristic asymmetric form. The broadening extends to the blue side of each vibronic level, which is caused by the simultaneous excitation of the molecule and vibrations of the droplet (phonons). The two isotopes of Li form ⁶Li₂ and ⁷Li₂ as well as isotope mixed ⁶Li⁷Li molecules on the droplet surface. By using REMPI-TOF mass spectroscopy, isotope-dependent effects could be studied.

Fluorescence emission of Ca-atom from photodissociated Ca2 in Ar doped helium droplets. II. Theoretical

The stability of the ground or excited state calcium atom in an argon-doped helium droplet has been investigated using an extension of the helium density functional method to treat clusters. This work was motivated by the experimental study presented in a companion paper, hereafter called Paper I [A. Masson, M. Briant, J. M. Mestdagh, M. A. Gaveau, A. Hernando, and N. Halberstadt, J. Chem. Phys. 137, 184310 (2012)], which investigated Ca(2) photodissociation in an argon-doped helium droplet and the nature of the fluorescent species. It is found that one single argon atom is sufficient to bring the calcium atom inside the droplet, for droplets of over 200 helium atoms. The absorption and emission spectra of CaAr(M) (M = 0-7) clusters have been simulated using the recently developed density sampling method to describe the influence of the helium environment. Absorption spectra exhibit broad, double bands that are significantly blueshifted with respect to the calcium atomic line. The emission spectra are less broad and redshifted with respect to the calcium resonance line. The shifts are found to be additive only for M ≤ 2, because only the first two argon atoms are located in equivalent positions around the calcium p orbital. This finding gives a justification for the fit presented in the companion paper, which uses the observed shifts in the emission spectra as a function of argon pressure to deduce the shifts as a function of the number of argon atoms present in the cluster. An analysis of this fit is presented here, based on the calculated shifts. It is concluded that the emitting species following Ca(2) photodissociation in an argon-doped droplet in Paper I could be Ca∗Ar(M) in a partly evaporated droplet where less than 200 helium atoms remain.

Rubidium on Helium Droplets: Analysis of an Exotic Rydberg Complex for n* < 20 and 0 ≤ l ≤ 3

Rubidium atom Rydberg states perturbed by helium droplets of different sizes provide insight into the role of a nanosized dielectric on the Coulomb potential. The observation of droplet size-dependent shifts of excited states with respect to bare atom states is explained by a decreased quantum defect and a lowered ionization threshold. Within the scope of a Rydberg model, we demonstrate that quantum defects and ionization potentials are constant for each specific Rydberg series, which confirms the Rydberg character of excited Rubidium states on helium droplets. A set of six Rydberg series could be identified. Individual Rydberg states are observed with effective principal quantum numbers up to n* ≈ 19 and l ≤ 3, for which the expectation value of the electron orbital radius is about 10 times larger than the droplet radius.