The isolation of atoms, molecules, clusters or nano-sized complexes in helium nanodroplets allows detailed spectroscopic studies at temperatures in the millikelvin range. Moreover, femtosecond real-time spectroscopy has been introduced to study dynamical processes in the ultracold helium environment. On the one hand, wave packet propagation opens a window to dynamical processes, allowing even a view to superfluid properties at the nanoscale. This is exemplified at decoherence effects in the wave packet propargation of small molecules attached to the droplets. On the other hand, high-resolution mass spectra using both femtosecond photoionisation (PI) as well as electron impact ionization enable us to characterize reactive processes at temperatures in the millikelvin range. As an example, alkali cluster -- water complexes are formed in helium droplets. By recording multi-photon PI spectra we can distinguish between reactive processes of the neutral clusters and ionic reactions occurring after ionisation of the alkali cluster component. These studies pave the way to time-resolved reaction dynamics at very low temperatures.    [Preview Abstract]

Doping helium droplets with alkaline earth atoms is an interesting tool to investigate the interaction with the superfluid helium. Magnesium is a corner case regarding the degree of solvation in helium [1,2] which may enable the detection of quantized vortices in helium droplets. In this contribution we add another facet to the discussion. The absorption of helium droplets doped with magnesium atoms is measured with resonant two-photon ionization at different combinations of droplet size and the number of doped Mg atoms. This enables the unambiguous identification of the absorption of an isolated atom inside the droplet centered around 279\,nm. When increasing the Mg content of the droplet we find evidence for the formation of metastable, weakly bound Mg complexes. After excitation of such a complex it collapses to a Mg cluster on a timescale of 20\,ps. \newline [1] J. Reho \emph{et al}., J. Chem. Phys. {\bf 112}, 8409 (2000) \newline [2] Y. Ren and V.V. Kresin, Phys. Rev. A {\bf 76}, 043204 (2007)    [Preview Abstract]

Bosonic density functional theory describing superfluid $^4$He is formulated in 2-D cylindrical coordinates and a numerical implementation of the model using a regular spatial grid basis is presented. The 2-D formulation has many important applications as the 1-D treatment cannot, for example, describe translational motion of atoms and molecules solvated in the liquid and the 3-D theory is usually computationally too expensive, especially when describing dynamics in bulk superfluid $^4$He. The theory is implemented in both real and imaginary time forms for allowing solution of both time-dependent and time-independent problems. Two test cases for the developed method are presented and the results are compared against the previously published results. Finally, the method is applied to describe solvation of single wall carbon nanotubes in superfluid $^4$He at 0 K and the implications of the results to dynamic liquid response are discussed.    [Preview Abstract]

Near threshold photo-induced isomerization and photo-induced chemical reactions have long been sough after as sensitive probes of the underlying potential energy surface. One of the most important questions asked is how the initially bright quantum state couples to the reaction coordinate, and thus relates to energy transfer in general. Helium droplets have now allowed us to stabilize entrance channel clusters behind very small reaction barriers such that vibrational excitation may result in reaction. Through two examples, namely the isomerization of the 2 binary complexes of HF-HCN {\{}Douberly et al. PCCP 2005, 7,463{\}}, and the induced reaction of the gallium-HCN complex {\{}Merritt et al. JPCA 2007, DOI:10.1021/jp074981e{\}} we will show how the branching ratios for reaction and predissociation can determined and the influence of the superfluid He solvent.    [Preview Abstract]

Photoelectron images of helium nanodroplets doped with Kr and Ne atoms are reported. The images and resulting photoelectron spectra were obtained using tunable synchrotron radiation to ionize the droplets. Droplets were excited at 21.6 eV, corresponding to a strong droplet electronic excitation. The rare gas dopant is then ionized via a Penning excitation transfer process. The electron kinetic energy distributions reflect complex ionization and electron escape dynamics.    [Preview Abstract]

Superfluid helium droplets serve as a very gentle cryogenic matrix for molecular spectroscopy. The low temperature and high thermal conductivity of superfluid helium droplets are of great advantage for the investigation of dispersed emission spectra of molecules. As a complement to the fluorescence excitation spectrum the emission spectra provide important details on dynamic processes of intramolecular as well as intermolecular nature. This will be demonstrated for various examples such as intramolecular proton tunnelling, isomeric van der Waals complexes, tautomerization and microsolvation.    [Preview Abstract]

Planar aromatic molecules provide strongly localizing potentials for helium that considerably modify the local superfluid properties of a solvating helium environment. I shall describe some of the effects of these interactions on the solvation structure and spectroscopy of tetracene and phthalocyanine in helium droplets, comparing results of zero and finite temperature quantum Monte Carlo simulations with experimental data. The helium atoms closest to the molecule are seen to show similarities to trapped cold atoms in multi-well potentials. Studies of cold bosons with attractive and repulsive interactions in double well potentials will also be presented, showing formation of squeezed and quantum superposition states of cold atoms.    [Preview Abstract]

We present a theoretical study on the effect of a helium nanodroplet environment on the fragmentation dynamics of embedded rare gas cluster ions. The helium atoms are treated explicitly, with zero-point effects taken into account through an effective helium-helium interaction potential. All the nonadiabatic effects between electronic states of the ionized rare gas cluster are taken into account. Our results reveal new mechanisms for the cooling by helium, and show that the dopant can be ejected from the helium droplet. These results will be presented and discussed.    [Preview Abstract]

Molecular scattering behaviour has generally proven difficult to study at low collision energies. We formed a molecular beam of OH radicals with a narrow velocity distribution and a tunable velocity by passing the beam through a Stark decelerator [1]. The transition probabilities for inelastic scattering of the OH radicals with Xe atoms were measured as a function of the collision energy in the range of 50 to 400 wavenumbers. The behaviour of the cross-sections for inelastic scattering near the energetic thresholds was accurately measured, and excellent agreement was obtained with cross-sections derived from coupled- channel calculations on ab initio computed potential energy surfaces [2]. For collision studies at lower energies, the decelerated beams of molecules can be loaded into a variety of traps. In these traps, electric fields are used to keep the molecules confined in a region of space where they can be studied in complete isolation from the (hot) environment. Typically, 10$^5$ state- selected molecules can be trapped for times up to several seconds at a density of 10$^7$ mol/cm$^3$ and at a temperature of several tens of mK [3]. The long interaction time afforded by the trap has been exploited to measure the infrared radiative lifetime of vibrationally excited OH radicals, for instance, as well as to study the far-infrared optical pumping of these polar molecules due to blackbody radiation [4]. As an alternative to these traps, we have demonstrated an electrostatic storage ring for neutral molecules. In its simplest form, a storage ring is a trap in which the molecules - rather than having a minimum potential energy at a single location in space - have a minimum potential energy on a circle. To fully exploit the possibilities offered by a ring structure, it is imperative that the molecules remain in a bunch as they revolve around the ring. This ensures a high density of stored molecules, moreover, this makes it possible to inject multiple - either co-linear or counter propagating - packets into the ring without affecting the packet(s) already stored. We have recently demonstrated a prototype molecular synchrotron, which will be used as a low-energy collider for neutral molecules in the future [5].\newline [1] H.L. Bethlem, G. Berden, and G. Meijer, Phys. Rev. Lett. 83, (1999) 1558-1561.\newline [2] J.J. Gilijamse, S. Hoekstra, S.Y.T. van de Meerakker, G.C. Groenenboom, and G. Meijer, Science 313, (2006) 1617-1620.\newline [3] S.Y.T. van de Meerakker, P.H.M. Smeets, N. Vanhaecke, R.T. Jongma, and G. Meijer, Phys. Rev. Lett. 94, (2005) Artn. 023004.\newline [4] S. Hoekstra, J.J. Gilijamse, B. Sartakov, N. Vanhaecke, L. Scharfenberg, S.Y.T. van de Meerakker, and G. Meijer, Phys. Rev. Lett. 98, (2007) Artn. 133001.\newline [5] C.E. Heiner, D. Carty, G. Meijer, and H.L. Bethlem, Nature Physics 3, (2007) 115-118.    [Preview Abstract]

Advances in cold molecule production promise to profoundly impact research on precision measurement, quantum information, and controlled chemistry. To this end, we employ a Stark decelerator to remove 99.5{\%} of the center-of-mass kinetic energy of a supersonic beam of ground-state OH molecules. We subsequently trap a 70 mK sample of the decelerated molecules at a density of $>$10$^{5}$ cm$^{-3}$ within a magnetic quadrupole whose center lies $\sim $1cm from the decelerator exit. Our magnetoelectrostatic trap (MET) design allows for the addition of an electric field of variable magnitude to the trapped sample to facilitate polar-molecule collision studies. We report progress toward observation of cold collisions between samples of polar molecules.    [Preview Abstract]

We discuss the possibility to use the photodissociation of cold SO$_2$ molecules to produce internally and translationally cold photofragments SO and O. It is expected from our measurements of the molecular Stark effect~[1] that the dissociation pathways and excess energies of the fragments are tunable by electric fields~[2]. Cold SO$_2$ molecules are produced by Stark deceleration. We have realized a Stark decelerator that is able to slow down packages SO$_2$ in weak-field seeking levels to a few 10~m/s center of mass velocity. A Stark decelerator with 326~stages is required for this purpose, since the ratio of Stark shift to initial kinetic energy is small for SO$_2$. The photofragments SO and O have triplet ground states, while the ground state of SO$_2$ is diamagnetic. In combination with the photodissociation at the threshold we want to employ this constellation to accumulate fragments in a magnetic trap by dissociating SO$_2$ as it is stopped by electric fields in the center of the trap. \newline [1] J. Phys. B \textbf{39}, S1085 (2006). \newline [2] Phys. Rev.~A \textbf{74}, 040701(R) (2006).    [Preview Abstract]

Our group has recently demonstrated the ability to assemble ultracold, polar molecules from laser-cooled atoms. We use photoassociation followed by stimulated emission pumping to produce RbCs molecules in their absolute ground state, at temperatures $T\sim 100\mu \mbox{K}$. In recent work, we have moved towards the goal of accumulated large, high-density samples of ultracold RbCs. Here we present new results on the trapping and collisional properties of RbCs in levels of high vibrational excitation.    [Preview Abstract]

``Kinematic'' cooling is a general technique by which a vast array of molecules can be translationally cooled using crossed atomic and molecular beams. The success of the technique relies primarily on the existence of an approximate mass degeneracy between the molecule to be cooled and its atomic (or molecular) collision partner. Here, we discuss factors that affect the efficiency of cold molecule production by this method, as well as schemes that may allow tunability of the velocity and temperature of the cold molecules on a fine scale.    [Preview Abstract]

Clusters of molecular hydrogen (H$_{2}$) at low temperatures have been attracted much attention because of the possible superfluid phase of molecular hydrogen. Parahydrogen has been predicted to undergo Bose-Einstein condensate (BEC) and to exhibit a superfluid phase below 6 K. However, since the freezing point of H$_{2}$ (14 K) is much higher than the predicted superfluid transltion temperature, the supercooling of bulk H$_{2}$ system has not been achieved despite many attempts. Clusters are known to exhibit lower freezing and melting temperatures than their bulk system due to the size effect. In addition, the melting temperature may become significantly lower than the freezing temperature in such clusters, and coexistence of liquid and solid phases between the melting and freezing temperatures has been predicted theoretically. Thus, clusters of molecular hydrogen are very appealing system for the observation of possible superfluid phase of molecular hydrogen. Since superfluid is a macroscopic property, we have studied properties of hydrogen clusters with fairly large size ($N=100 - 10^{6}$) by using He droplet spectroscopy. Some advantages of using droplet spectroscopy for this study include (1) cluster size can be precisely controlled by its pickup process, and (2) the temperature of clusters is well defined. Laser induced fluorescence of several molecules doped in H$_{2}$ clusters showed clear evidence of non-rigidity of hydrogen clusters at 0.4 K or 4 K. We have also observed a clear difference in the LIF spectra between {\it parahydrogen} and {\it orthohydrogen} clusters. We will discuss the properties of large parahydrogen clusters from the dependence on the cluster size and concentration of orthohydrogen.    [Preview Abstract]

In the cryogenic low-density liquid and solid phases of H$_2$ and D$_2$, the H$_2$ and D$_2$ molecules retain good rotational and vibrational quantum numbers that characterize their internal degrees of freedom. High-resolution infrared and Raman spectroscopic experiments provide extremely sensitive probes of these degrees of freedom. We present here fully-first-principles calculations of the infrared and Raman spectra of liquid and solid H$_2$ and D$_2$, calculations that employ a high-quality six-dimensional coupled-cluster H$_2$-H$_2$ potential energy surface and quantum Monte Carlo treatments of the single-molecule translational degrees of freedom. The computed spectra agree very well with experimental results once we include three-body interactions among the molecules, interactions which we also compute using coupled-cluster quantum chemical methods. We predict the vibrational spectra of liquid and solid H$_2$ at several temperatures and densities to provide a framework for interpreting recent experiments designed to search for superfluid behavior in small H$_2$ droplets. We also present preliminary calculations of the spectra of mixed H$_2$/D$_2$ solids that show how positional disorder affects the spectral line shapes in these systems.    [Preview Abstract]

Clusters of parahydroge comprising between 10 and 50 molecules have been extensively studied by computer simulations based on the continuous-space Worm Algorithm, which allows one to go down to temperatures as low as a few hundredths of a K. These clusters display an intriguing interplay of liquid- and solid-like behavior as a function of both temperature and cluster size. In this sense, their physics is far richer than that of helium clusters. An intriguing phenomenon predicted by our simulations is {\it quantum melting}, whereby clusters in some size range (roughly between 22 and 30 molecules) are observed to go from rigid, solid-like, to essentially structureless and liquid-like as the temperature is lowered, due to the onset of quantum exchange cycles involving all the molecules in the cluster. At low temperature these clusters turn superfluid; their local superfluid response has been analyzed, and found to be essentially uniform throughout the system in the $T\to 0$ limit, even in clusters with a pronounced shell structure. In particular, exchanges involving molecules in the inner and outer shells are shown to be underlying the superfluid response. This system can also allow one to gain insight into the relationship of the superfluid properties with Bose condensation, and aspect that has been thoroughly investigated.    [Preview Abstract]

Infrared laser spectroscopy is used to probe the rotational dynamics of the binary HCN-M (M=Ca, Sr) complexes, either solvated within or bound to helium droplets. The ``surface bound'' spectral signatures reported previously for the HCN-alkali atom complexes are observed for both species, while a second band is observed for HCN-Ca that corresponds to a solvated species. IR-IR double resonance spectroscopy is used to probe the interconversion of the two distinct HCN-Ca populations. Above a threshold droplet size, vibrational excitation results in the solvation of the surface bound HCN-Sr complex.    [Preview Abstract]

Photoionization of He droplets doped with Xe and Kr atoms have been investigated by photoelectron imaging utilizing VUV synchrotron radiation. Photoelectron images were recorded over a wide range of He droplet sizes, photon energies, and dopant pick-up conditions. Significant ionization of dopants was observed at 21.6 eV, the absorption maximum of 2${ }^1P$electronic excited state of He droplets, suggesting an indirect ionization via excitation transfer. Photoelectron images and spectra indicate multiple pathways for photoelectrons generated by this process to escape the droplet. Special attention is paid to the excitation transfer dynamics and the electron relaxation in He droplets. It is found that excitation transfer from 2${ }^1P$state to dopants competes with relaxation to the lower 2${ }^1S$ state. The excitation is likely a localized exciton that transfers the energy to the dopant via a dipole-dipole hopping mechanism. The conduction band of He droplets as a function of droplet size is also observed. The conduction band edge reaches the bulk limit for the largest He droplets. The electron under the conduction band becomes trapped and forms an electron bubble that escapes the droplet by transcending a barrier near the liquid/vapor interface.    [Preview Abstract]

Infrared spectra of HCl dimers have been obtained in helium nanodroplets. The splitting in the vibrationally excited state of the bonded H-Cl stretching band ($v_{2})$ in (H$^{35}$Cl - H$^{37}$Cl) dimers was obtained to be 2.7 cm$^{-1}$ as compared to 3.7 cm$^{-1}$ in free dimer. From the splitting, the strength of the interchange-tunneling interaction in liquid helium was obtained to be 0.85 cm$^{-1}$, which is about a factor of two smaller than in the free dimer. The results are compared with the previous spectroscopic study of (HF)$_{2}$ in He droplets as well as to the theoretical study of (HF)$_{2}$ and (HCl)$_{2}$ dimers in small He clusters.    [Preview Abstract]

Path integral simulations have been carried out to study the rotations of a methane inside a single shell of $^4$He atoms at 0.3~K to address the question of whether dopant molecule rotations can be used to probe the quantum statistics and superfluidity of the shell. We examined the effects of the probe molecule on the $^4$He exchanges and their counter effects on the renormalized rotation constant of the probe systematically by varying the intrinsic moment of inertia of the methane. The observed effects show strong dependence on the intrinsic moment of inertia of the rotating probe, with a heavy probe favoring stronger templating of the $^4$He density and a corresponding suppression of exchanges in the shell, as well as a large renormalization in the probe's effective rotation constant, while a light probe shows almost no effect on the shell density or the effective rotation constant. These results can be rationalized in terms of a rotational smearing effect and suggest that there is no clearly quantifiable relationship between the superfluid fraction of the shell and the renormalized rotation constant of the probe for cases where the probe molecule has weak anisotropic interactions with the $^4$He atoms.    [Preview Abstract]

We have recorded and analyzed the \mbox{\textit{X}$\rightarrow$\textit{B}} spectra for three Rg--Br-Br linear isomers [Rg = He, Ne, Ar] using pump-probe spectroscopy. This work is an interesting test case for the transition from quantum to quasi-classical dynamics, and how the dynamics are interconnected with changes in the potential energy surface. Helium is not only much lighter than argon, but the He-Br$_2$ potential well is much shallower than that of Ar-Br$_2$. Excitation spectra to individual Rg-Br$_2$ (\textit{B}, $\nu$') intermolecular potentials were recorded by probing the Br$_2$ (\textit{B}, $\nu$') asymptotic limit of the potential while scanning the pump laser. The continuum spectra of the three species are very different, with the He-Br$_2$ spectrum peaking at threshold while the Ar-Br$_2$ spectrum is negligible at threshold and strongly blue shifted. The linear Ne-Br$_2$ bond energy was measured to be \mbox{71 $\pm$ 3 cm$^{-1}$} by the threshold energy for the onset of the continuum. Since excitation tends to move electron density to the $\sigma^*$ orbital of the Br-Br bond near the rare gas atom, the intramolecular stretching vibration (Br-Br) and the intermolecular stretching vibration (Rg-Br) are strongly coupled. The experiments will be compared to a two dimensional model using the best available potential energy functions.    [Preview Abstract]