4He Droplets Research Articles

Optical transitions in the centres with soft dynamics in the final state

Anomalous optical spectra of the centres with strongly-reduced local elastic springs in the final electronic states are studied. In this case the transitions take place to the vicinity of a flat potential curve in the configurational coordinate space which corresponds to a very strong mode mixing at the electronic transition. To take this mixing into account we apply the operator transform method. We have found that the mixing results in a strong enhancement of the effect of low-frequency vibrations, causing an intensive low-energy phonon sideband. In this case the spectrum (or its envelope) has the shape of an asymmetrical lambda-letter. Suchtype spectra have been observed experimentally. The spectra may also reveal the Airy-type oscillations if the potential of the final state has a finite slope. This-type oscillations have been observed in the hot luminescence; the oscillating structure of zero-point lines in the absorption spectra of super-fluid 4He droplets probably has an analogous origin.

Time-domain analysis of electronic spectra in superfluid 4He

Electronic absorption spectra of impurities in superfluid helium is developed in time domain, using time-dependent density functional theory to describe liquid 4He and time-dependent perturbation theory to describe the electronic degrees of freedom of the impurity. Angularly isotropic potentials are used to describe the molecule–helium interactions in the ground and excited electronic states. The calculations rationalize experimentally observed phonon side-bands in 4He droplets and in bulk helium, and allow assignments of spectral features to specific motions of the liquid.

Charged cores in ionized $\mathsf{{^{4}He}}$ clusters III: A quantum modeling for the collisional relaxation dynamics

When a pure 4He droplet is ionized by electron impact, the most abundant fragment detected in mass spectra after ionization is He2 + . All the models that have been proposed thus far to explain the experimental evidence therefore involve the formation of the He2 + molecular ion. The understanding of the interactions between this ion and the surrounding He atoms in the cluster and of their dynamical behavior during cluster break-up is an important element for the modeling of the cluster evolution after the ionization event. In previous works [1,2] we have computed and described the Potential Energy Surface (PES) of the electronic ground state for the He3 + system that provides the required forces between He2 + and He. After ionization He2 + is presumably formed by association of an He + and any of the neutral atoms in the cluster via a 3-body collision process. The ensuing vibrational quenching of the “hot” molecular ion may release the energy necessary to evaporate the entire droplet, or most of it, and give the fragmentation patterns detected by experiments. We present here a model quantum dynamics that generates vibrational deexcitation cross-sections and the corresponding rate coefficients for the collision of He2 + with He. A timescale of the cluster evaporation due to vibrational relaxation is estimated and the present findings are compared with earlier studies on the same system.

Probing Vortices inHe4Nanodroplets

We present static and dynamical properties of linear vortices in 4He droplets obtained from density functional calculations. By comparing the adsorption properties of different atomic impurities embedded in pure droplets and in droplets where a quantized vortex has been created, we suggest that Ca atoms should be the dopant of choice to detect vortices by means of spectroscopic experiments.

Microscopic two-fluid theory of rotational constants of the OCS–H2 complex in He4 droplets

We present a microscopic quantum analysis for rotational constants of the OCS–H2 complex in helium droplets using the local two-fluid theory in conjunction with path integral Monte Carlo simulations. Rotational constants are derived from effective moments of inertia calculated assuming that motion of the H2 molecule and the local nonsuperfluid helium density is rigidly coupled to the molecular rotation of OCS and employing path integral methods to sample the corresponding H2 and helium densities. The rigid coupling assumption for H2–OCS is calibrated by comparison with exact calculations of the free OCS–H2 complex. The presence of the H2 molecule is found to induce a small local nonsuperfluid helium density in the second solvation shell which makes a non-negligible contribution to the moment of inertia of the complex in helium. The resulting moments of inertia for the OCS–H2 complex embedded in a cluster of 63 helium atoms are found to be in good agreement with experimentally measured values in large helium droplets. Implications for analysis of rotational constants of larger complexes of OCS with multiple H2 molecules in helium are discussed.

Emission spectra of free base phthalocyanine in superfluid helium droplets

We report on the electronic emission spectra of free-base phthalocyanine doped into superfluid He4 droplets. The emission spectra of phthalocyanine obtained upon vibronic excitation show a doubling of all lines, absent in the gas phase [Fitch, Wharton, and Levy, J. Chem. Phys. 70, 2018 (1979)]. Similar features found for Mg-phthalocyanine in He4 droplets suggest that this doubling is due to the helium environment. Quantized relaxation states of the first helium layer surrounding the embedded molecule are discussed as likely causes of the doubling.

The influence of embedded atoms, molecules, and clusters on the lifetimes of electron bubbles in large He4 droplets

The lifetimes of electrons injected into large (≈106 atoms) superfluid He4 droplets have been measured in the presence of different prior embedded rare gas atoms, their clusters, and the molecules O2, H2O, and SF6. For the light rare gas atoms Ne and Ar the lifetimes of about 6⋅10−2 s, found previously for pure droplets, are reduced to between 1–2⋅10−2 s. Single molecules with large electron affinities, such as SF6 and O2 and small clusters of H2O lead to a significant increase in the lifetimes. In the case of Kr and Xe, larger clusters with n̄>14 are needed before the lifetimes increase. A simple model, which takes account of the energy levels occupied by the electrons in the corresponding molecules or the clusters, assumed to be solid, can qualitatively explain the observations.

Superfluidity in a doped helium droplet.

Path-integral Monte Carlo calculations of the superfluid density throughout 4He droplets doped with linear impurities are presented. After deriving a local estimator for the superfluid density distribution, we find a decreased superfluid response in the cylindrically symmetric region of the first solvation layer. The helium in this region has a superfluid transition temperature similar to that of a two-dimensional helium system and may be responsible for previously unexplained experimental Q-branch measurements.

Spontaneous alignment of tetracene molecules in 4He droplets

The polarization dependence of the laser induced fluorescence and absorption was investigated for single tetracene molecules embedded in 4He droplets having N ̄ 4=3×10 3–3×10 5 atoms. The measurements revealed an increase in the intensity up to 20% in the spectra, when the laser polarization vector was aligned parallel to the direction of the droplet beam as compared to the perpendicular alignment. This observation suggests a spontaneous alignment of the molecules in the laboratory frame. A mechanism of the molecular alignment in the pick-up process is proposed.

Effect of the Symmetry ofH2Molecules on their Rotations around an OCS Molecule in SuperfluidH4eDroplets

The infrared spectra of OCS-(H2)(n) clusters in cold (0.15 K) superfluid 4He droplets coated with 3He exhibit resolved rotational bands for each n up to n=8 para-H2 (pH(2)) or ortho-D2 (oD(2)) molecules. An analysis of the different Q-branch intensities based on the different spin symmetries of pH(2) and oD(2) indicates the formation of symmetric 5- or 6-membered rings around the linear carbonyl sulfide (OCS) chromophore. The rings of distinguishable oD(2) are found to undergo axial rotations, whereas for 6 pH(2) molecules the symmetry-allowed rotational levels lie too high to be excited at the 0.15 K droplet temperatures.

The observation of large changes in the rotational constants of glyoxal in superfluid helium droplets upon electronic excitation

The rotational fine structure of the vibrationless 000 band of the electronic S1←S0 transition of glyoxal in large superfluid He4 droplets (N̄4=2⋅103–2⋅104) has been interrogated with high resolution laser depletion spectroscopy. In the electronic ground singlet state S0 the rotational constants A and B̄=(B+C)/2 are less than for the free molecule by a factor of 2.87 and 2.16, respectively. In the electronic excited state S1 the rotational constant A is found to be 17% larger and the average rotational constants B̄ to be 25% smaller than in the S0 state. The unexpected large magnitudes and different signs in the observed changes are attributed to modifications in the shape of the outer electron density distribution of the molecule upon excitation. None of the previous models introduced to explain the changes in rotational constants is entirely satisfactory, presumably because of the much weaker interactions with the helium environment.

The phonon wings in the (S1 ← S0) spectra of tetracene, pentacene, porphin and phthalocyanine in liquid helium droplets

The vibronic spectra of the large organic molecules tetracene (C18H12), pentacene (C24H14), base-free porphin (C20H14N4) and base-free phthalocyanine (C32H18N8) have been investigated in large (N ≈ 103–104) 4He droplets using laser induced fluorescence. The spectra show sharp zero phonon lines (Δν ⩽ 0.2 cm−1), accompanied by weaker phonon wings on the blue side. The very small inhomogeneous broadening of the zero phonon lines supports the unique properties of liquid He as an extremely gentle ultra-cold matrix for high resolution spectroscopy of larger molecules. Pronounced structures in the phonon wings, which are characteristic for each molecule, are attributed to the excitation of vibrational modes of the helium atoms located in shells surrounding the molecule.

Spectroscopy of OCS–hydrogen clusters in He droplets

Clusters of para-hydrogen (pH2) and ortho-deuterium (oD2) have been assembled around an OCS chromophore molecule inside He droplets in a molecular beam and studied via IR diode laser depletion spectroscopy (nu approximately 2060 cm-1). The superfluid 4He droplets provide a gentle host ensuring a constant low temperature of either T = 0.38 K for 4He droplets or T = 0.15 K for both the pure 3He and mixed 4He-3He droplets. The spectra show well resolved rotational structure of the vibrational bands for each attached hydrogen molecule in the range n = 1-8. With only one (n = 1) attached pH2, HD or an oD2 molecule the best fit rotational constants were used to determine the structure of the complex, which was found to be in surprisingly good agreement with quantum chemical calculations for the free complex. With n = 5 and 6 the Q-branch disappears for the pH2 clusters but not for the oD2 clusters which is consistent with a donut model. The moments of inertia of the pH2 and the oD2 complexes are explained by a new model in which each of the 18 attached helium atoms in a shell surrounding the OCS molecule are assigned a mass of 0.55, while each attached H2 and D2 molecule has an effective mass of about 10 and 12 u, respectively.

Microwave-infrared double resonance spectroscopy of an OCS molecule inside a He4 droplet

The dynamical behavior of single OCS molecules embedded in large (N4=6000) superfluid 4He droplets has been investigated with high resolution double resonance infrared-microwave spectroscopy. The individual lines for the rotational transitions 2↔1, 3↔2, 4↔3, and 5↔4 are clearly resolved in the difference double resonance spectra and reveal a fine structure. This is especially well resolved for the 3↔2 transition which exhibits five separate lines having widths of about 50 MHz and separated by 200 MHz. The possible origin of the splitting is discussed. © 2000 American Institute of Physics.

Evidence for Superfluidity in Para-Hydrogen Clusters Inside Helium-4 Droplets at 0.15 Kelvin.

A linear carbonyl sulfide (OCS) molecule surrounded by 14 to 16 para-hydrogen (pH(2)) molecules, or similar numbers of ortho-deuterium (oD(2)) molecules, within large helium-4 ((4)He) droplets and inside mixed (4)He/(3)He droplets was investigated by infrared spectroscopy. In the pure (4)He droplets (0.38 kelvin), both systems exhibited spectral features that indicate the excitation of angular momentum around the OCS axis. In the colder (4)He/(3)He droplets (0.15 kelvin), these features remained in the oD(2) cluster spectra but disappeared in the pH(2) spectra, indicating that the angular momentum is no longer excited. These results are consistent with the onset of superfluidity, thereby providing the first evidence for superfluidity in a liquid other than helium.

Pinning of quantized vortices in helium drops by dopant atoms and molecules

Using a density functional method, we investigate the properties of liquid 4He droplets doped with atoms (Ne and Xe) and molecules ( SF6 and hydrogen cyanide). We consider the case of droplets having a quantized vortex pinned to the dopant. A liquid-drop formula is proposed that accurately describes the total energy of the complex and allows one to extrapolate the density functional results to large N. For a given impurity, we find that the formation of a dopant+vortex+(4)He(N) complex is energetically favored below a critical size N(cr). Our results support the possibility to observe quantized vortices in helium droplets by means of spectroscopic techniques.

Elastic and inelastic scattering off 4He droplets

The properties of pure 4He droplets and of 3He atoms in droplets are studied within the manifestly microscopic Jastrow–Feenberg theory. Collective excitations of the droplet and the dynamic structure function are calculated, as well as the elastic, inelastic and absorption scattering cross sections for the scattering of 4He and 3He atoms off the droplet.

Spectroscopy of molecules in helium droplets

The work reviewed in the talk explores the possibilities of using molecules immersed in liquid helium as probes to study superfluidity on the microscopic level. For this purpose liquid 4He, 3He and mixed 4He/ 3He droplets consisting of 10 3–10 5 atoms have been doped with single molecules or a small defined number of molecules which form clusters in the droplet interior. The dopants were investigated with high-resolution optical spectroscopy. The electronic and rovibrational spectra show unusually sharp spectral lines and unexpected features such as a distinct gap between the zero phonon line and the phonon wing as well as the free rotations of single molecules. Both are not observed in colder nonsuperfluid 3He droplets and therefore are considered to be new microscopic manifestations of superfluidity. Spectroscopic studies of small p-H 2 and o-D 2 clusters surrounding a single chromophore molecule formed in the interior of the He droplets are also reported.

The rotational spectrum of single OCS molecules in liquid 4He droplets

Single OCS molecules have been embedded in large 4He droplets (N̄=1×103–8×103 atoms) and their infrared spectra in the vicinity of the ν3-fundamental at 2062 cm−1 have been studied using coaxial laser depletion spectroscopy. Sharp lines corresponding to the P- and R-branches with a linewidth of 160 MHz or greater are observed. From the line intensities a droplet temperature of 0.37±0.02 K is obtained and from the line positions the rotational constants B for the ground and excited states and an average centrifugal distortion constant D are determined. The former are about a factor of 2.8 smaller and the latter four orders-of-magnitude larger than for the free molecule. The decrease in B is attributed to an attached nonsuperfluid component which has the same effect as a ring of about six 4He atoms in an equatorial plane around the waist of the OCS molecule, which is carried along in the end-over-end rotation. The widths of the individual lines show an interesting asymmetry which is oppositely sloped for the P- and R-branches and increases with the rotational quantum number. Several possible explanations for the line shapes and widths are presented.

Kinetics of Liquid 3He Droplets in Solid 4He Matrix

Precise measurements of pressure in the crystal at constant volume were used to obtain the data on growth and dissolution kinetics of liquid 3 He droplets formed as a result of isotopic phase separation of solid 3 He- 4 He Mixtures. We studied several crystals with an initial 3 He concentration of 2.05% in the pressure range of 26–27 bar. It is shown that the growth of the liquid droplets during the stepwise cooling of the two-phase crystal is correctly described by the superposition of two exponential processes: diffusion decomposition with a small time constant and strain relaxation with a big time constant. The strain layer near the droplet boundaries is due to a great difference in molar volume between the droplets and the matrix, and leads to a plastic deformation of the matrix and to a non-equilibrium 3 He concentration in the matrix. Under such conditions quantum diffusion is significantly suppressed and 3 He atom transport occurs only as the strain is relaxed.