Formation times of RbHe exciplexes on the surface of superfluid versus normal fluid helium nanodroplets

Nanodroplets of either superfluid 4He or normal fluid 3He are doped with Rb atoms that are bound to the surface of the droplets. The formation of RbHe exciplexes upon 5P(3/2) excitation is monitored in real time by femtosecond pump-probe techniques. We find formation times of 8.5 and 11.6 ps for Rb 4He and Rb 3He, respectively. A comparison to calculations based on a tunneling model introduced for these systems by Reho et al. [J. Chem. Phys. 113, 9694 (2000)]] shows that the proposed mechanism cannot account for our findings. Apparently, a different relaxation dynamics of the superfluid opposed to the normal fluid surface is responsible for the observed formation times.     

Quantum interference spectroscopy of rubidium-helium exciplexes formed on helium nanodroplets

Femtosecond multiphoton pump-probe photoionization is applied to helium nanodroplets doped with rubidium (Rb). The yield of Rb+ ions features pronounced quantum interference (QI) fringes demonstrating the coherence of a superposition of electronic states on a time scale of tens of picoseconds. Furthermore, we observe QI in the yield of formed RbHe exciplex molecules. The quantum interferogram allows us to determine the vibrational structure of these unstable molecules. From a sliced Fourier analysis one cannot only extract the population dynamics of vibrational states but also follow their energetic evolution during the RbHe formation.     

Formation of cold bialkali dimers on helium nanodroplets

Cold alkali diatomic molecules (LiCs, NaCs) in the lowest vibrational state of the electronic triplet ground state are formed on superfluid helium nanodroplets. Using photoionization detection the excitation spectra of the transitions are recorded. The splitting of the vibrational structure in the LiCs spectrum, not observed in the NaCs spectrum, is interpreted in terms of molecular fine structure. The spectra are well reproduced by a model based on quantum chemistry potential curves including spin-orbit coupling, in combination with an asymmetric line shape function to account for cluster-induced broadening. Our refined potential curves provide important input data for the photoassociation of ultracold dipolar alkali molecules from atomic quantum gases.Received: 1 July 2004, Published online: 26 October 2004PACS: 36.40.Mr Spectroscopy and geometrical structure of clusters - 34.50.Gb Electronic excitation and ionization of molecules; intermediate molecular states (including lifetimes, state mixing, etc.) - 33.20.-t Molecular spectra     

Magnetic dichroism of potassium atoms on the surface of helium nanodroplets

The population ratio of Zeeman sublevels of atoms on the surface of superfluid helium droplets (T=0.37 K) has been measured. Laser induced fluorescence spectra of K atoms are measured in the presence of a moderately strong magnetic field (2.9 kG). The relative difference between the two states of circular polarization of the exciting laser is used to determine the electron spin polarization of the ensemble. Equal fluorescence levels indicate that the two spin sublevels of the ground-state K atom are equipopulated, within 1%. Thermalization to 0.37 K would give a population ratio of 0.35. We deduce that the rate of spin relaxation induced by the droplet must be <520/s. For the K2 triplet dimer we find instead full thermalization of the spin.     

Discovery of dumbbell-shaped Cs*Hen exciplexes in solid 4He

We have observed several new spectral features in the fluorescence of cesium atoms implanted in the hcp phase of solid helium following laser excitation to the 62P states. Based on calculations of the emission spectra using semiempirical Cs-He pair potentials the newly discovered lines can be assigned to the decay of specific Cs*Hen exciplexes: an apple-shaped Cs(APi3/2)He2 and a dumbbell-shaped Cs(APi1/2)Hen exciplex with a well-defined number n of bound helium atoms. While the former has been observed in other environments, it was commonly believed that exciplexes with n>2 might not exist. The calculations suggest Cs(APi1/2)He7 to be the most probable candidate for that exciplex, in which the helium atoms are arranged on a ring around the waist of the dumbbell-shaped electronic density distribution of the cesium atom.     

Two-step excitation of Rb atoms on He nanodroplets

We present the first sequential laser excitation of atom-doped helium nanodroplets. Rubidium atoms on the surface of helium nanodroplets are selectively excited with a continuous wave laser to the 5²P_1/2 state so as not to desorb from the nanodroplets. From there they are excited by a laser pulse to the 5²D state; a laser-induced fluorescence (LIF) spectrum is recorded by monitoring the 6²P ?\rightarrow 5²S_1/2 emission. The LIF spectrum differs from that of the two-photon one-color direct excitation spectrum 5²D ?\leftarrow 5²S_1/2, indicating that the system does relax vibrationally during the lifetime of the 5²P_1/2 state. To model the LIF spectra we use calculated energy levels of the Rb atom as a function of its distance R from the center of the helium nanodroplet. The Franck-Condon factors of the resulting potential energy curves agree with the experimental spectra. In the future the 5²P_1/2 state can be used as a springboard to reach high-lying ²S and ²D states, and possibly create an artificial super-atom.     

Dipole moments of molecules solvated in helium nanodroplets

Stark spectra are reported for hydrogen cyanide and cyanoacetylene solvated in helium nanodroplets. The goal of this study is to understand the influence of the helium solvent on measurements of the permanent electric dipole moment of a molecule. We find that the dipole moments of the helium solvated molecules, calculated assuming the electric field is the same as in vacuum, are slightly smaller than the well-known gas-phase dipole moments of HCN and HCCCN. A simple elliptical cavity model quantitatively accounts for this difference, which arises from the dipole-induced polarization of the helium.     

Stability of two-component alkali clusters formed on helium nanodroplets

The stability of two-component clusters consisting of light (Na or K) and heavy (Rb or Cs) alkali atoms formed on helium nanodroplets is studied by femtosecond laser ionization in combination with mass spectrometry. Characteristic stability patterns reflecting electron shell-closures are observed in dependence of the total number of atoms contained in the mixed clusters. Faster decay of the stability of mixed clusters compared to the pure light ones as a function of size indicates a destabilizing effect of heavy alkali atoms on light alkali clusters, presumably due to second order spin-orbit interaction.     

Dopant-induced ignition of helium nanodroplets in intense few-cycle laser pulses

We demonstrate ultrafast resonant energy absorption of rare-gas doped He nanodroplets from intense few-cycle (~10 fs) laser pulses. We find that less than 10 dopant atoms "ignite" the droplet to generate a nonspherical electronic nanoplasma resulting ultimately in complete ionization and disintegration of all atoms, although the pristine He droplet is transparent for the laser intensities applied. Our calculations at those intensities reveal that the minimal pulse length required for ignition is about 9 fs.     

Infrared laser spectroscopy of diacetylene and cyclopropane in helium nanodroplets

High resolution infrared laser spectroscopy has been used to obtain rotationally resolved vibrational spectra of diactyelene and cyclopropane in helium nanodroplets. In part, this work was motivated by the need for a large database of rotational constants for molecules solvated in liquid helium. Data of this type provides benchmarks with which to test the theoretical methods that are currently being developed for calculating the effect of the helium on the rotational dynamics of these solvated molecules. In general, the correlated motion of the helium and the molecule results in an effective moment of inertia that is considerably larger than that of the gas phase molecule.     

Changing the fragmentation pattern of molecules in helium nanodroplets by co-embedding with water

Individual amino acid molecules embedded in helium nanodroplets fragment extensively when the beam is ionized by electron bombardment. However, we find that when glycine and tryptophan are picked up right after, or right before, a small amount of water, the mass spectra become significantly altered. For glycine, the detected ions consist almost entirely of intact protonated amino acids, with or without a few water molecules attached. In other words, the presence of water exerts a striking “buffering” effect on the ionization-induced fragmentation. For tryptophan the effect is weaker but also present. In both cases, the hydroxyl group lost upon ionization overwhelmingly comes from the water partner (in strong contrast to the situation observed when amino acids are picked up by neat water clusters). A complementary experiment involving DCl molecules co-embedded with water shows that in this case Cl and/or DCl invariably leave the droplet upon ionization. The observed patterns may be steered by the analytes' dipole moments or by solvation effects.     

Selecting molecules embedded in nanodroplets (clusters) of superfluid helium

A method of selecting molecules embedded in nanodroplets (clusters) of superfluid helium is proposed, which is based on the selective vibrational excitation of embedded molecules by intense IR laser radiation. This action leads to a significant decrease in size of the excited clusters, after which these clusters are separated with respect to size via scattering of the cluster beam on a crossing atomic beam. The method is described in detail and the possibility of selecting SF₆ molecules in liquid helium nanodroplets using the excitation by CO₂ laser radiation and the angular separation via scattering on a xenon atomic beam is demonstrated. The results show that, by using this technique, it is possible to separate molecules with respect to isotope (element) composition. Advantages and drawbacks of the method are analyzed.     

Resonant charging of Xe clusters in helium nanodroplets under intense laser fields

We theoretically investigate the impact of multiple plasmon resonances on the charging of Xe clusters embedded in He nanodroplets under intense pump-probe laser excitation (τ = 25 fs, I ₀ = 2.5 × 10¹⁴   W/cm², λ = 800 nm). Our molecular dynamics simulations on Xe₃₀₉He_10 000 and comparison to results for free Xe₃₀₉ give clear evidence for selective resonance heating in the He shell and the Xe cluster, but no corresponding double hump feature in the final Xe charge spectra is found. Though the presence of the He shell substantially increases the maximum charge states, the pump-probe dynamics of the Xe spectra from the embedded system is similar to that of the free species. In strong contrast to that, the predicted electron spectra do show well-separated and pronounced features from highly efficient plasmon assisted electron acceleration for both resonances in the embedded clusters. A detailed analysis of the underlying ionization and recombination dynamics is presented and explains the apparent disaccord between the resonance features in the ion and electron spectra.     

Electron spin pumping of Rb atoms on He nanodroplets via nondestructive optical excitation

We measured laser-induced-fluorescence (LIF) and beam-depletion (BD) spectra of rubidium atoms (5S-5P transition) on the surface of superfluid helium nanodroplets (M-He_{N} with M=Rb). It is known that when M is a lighter alkali atom electronic excitation always leads to detachment of the excited atom (M;{*}). The dissociation energy, few tens cm;{-1}, comes either as photon excess energy or from the barrierless formation of a M;{*}-He exciplex. We observe that this picture does not hold when M=Rb and the photon excess energy is small: we are able to excite atoms without detaching them from the droplet, thanks to a barrier preventing formation of the exciplex. This system is ideally suited for optical spin pumping in a He nanodroplet, whose achievement we explicitly demonstrate in a pump-probe magnetic circular dichroism experiment.     

Thermodynamic surface characteristics of liquid-metal nanodroplets at the vapor boundary

New relationships for the size-dependent surface tension σ(r), surface energy ū(r), the temperature T(r) of equilibrium between a liquid droplet and the surrounding vapor, and derivatives dσ/dT and dT/dr, which define spherical nanodroplets of liquid metals at the vapor boundary, are derived under the assumption that the two-phase (liquid nanodroplet-vapor) and three-phase (liquid nanodroplet-vapor-solid phase) equilibrium conditions are fulfilled. Consistent calculations of the aforementioned quantities are performed. The revealed dependences between the surface characteristics of nanodroplets of several metals and the calculation results are discussed.     

Modeling the formation of alkali clusters attached to helium nanodroplets and the abundance of high-spin states

The doping process of helium nanodroplets with alkali atoms has been modeled in order to study deviations from the Poissonian statistics of measured pick-up statistics which are important for assigning cluster or complex sizes in many experimental studies. Several, formally unexplained findings are reproduced and their origin has been analyzed: derivations from the expected functional form of the initial incline, the suppression of the formation of lithium clusters, the influence of the functional form and width of droplet size distributions. Furthermore, the controversially discussed formation of high-spin alkali clusters on helium droplets has been calculated within the model. The selection of high-spin states comes out to depend strongly on the experimental conditions, and is in general not pronounced for cluster sizes ≥ 3. The enhancement factor of 50 of high-spin states reported in earlier experiments is reproduced when choosing the conditions of these experiments.     

Structure and dynamics of the He 2 * (a 3Σ u + molecular complex in condensed phases of helium

An investigation is made of the absorption spectra of triplet metastable helium molecules in the a ³Σ _u ⁺ state in liquid ⁴He and ³He at various pressures and in dense ³He gas. An analysis of the spectrum corresponding to the a ³Σ _u ⁺ → c ³Σ _g ⁺ transition confirms the conclusion that there is a microscopic bubble surrounding the molecule in liquid helium. A simple approximation is proposed for the wave function of the valence electron of the molecule and the parameters of the bubble are determined for various experimental conditions. The coefficient of molecular recombination in liquid ³He and ⁴He was determined experimentally at various pressures and in dense cold ³He gas. The results show good agreement with the theory of mutual recombination limited by molecular diffusion under conditions of strong van der Waals interaction. It is shown that in the condensed phases of helium the polarization of the molecules under the action of the magnetic field does not lead to suppression of their mutual recombination, and this is confirmed experimentally.