Doping helium nanodroplets with high temperature metals: formation of chromium clusters

A new method for stable and continuous doping of superfluid helium nanodroplets (He(N)) with high-melting elements such as refractory metals is presented. The method exploits the advantages of electron bombardment heating and avoids stray fields induced by high currents or high frequency fields. It is thus especially suitable for magnetic studies of atoms and clusters in He(N). The source is characterized by means of mass spectroscopic investigations of He(N) doped with chromium atoms and clusters. Source temperatures of up to (1650 ± 50) °C were reached and Cr clusters up to Cr(9) could be formed in He(N).     

Density functional theory for triplet superconductors

The density functional theory of superconductivity is extended to triplet superconductors and superfluid helium 3. We prove a Hohenberg-Kohn-type theorem for these systems and derive effective single-particle equations. The latter include exchange and correlations in a formally exact way and allow the treatment of both electronic and phonon-induced superconductivity. The relation of this approach to the Bogolubov-de Gennes mean-field theory and to phenomenological theories based on Ginzburg-Landau functionals is discussed. © 1997 John Wiley & Sons, Inc.     

Rotational excitations of N2O in small helium clusters and the role of Bose permutation symmetry

We present a detailed study of the energetics, structures, and Bose properties of small clusters of (4)He containing a single nitrous oxide (N(2)O) molecule, from N=1 (4)He up to sizes corresponding to completion of the first solvation shell around N(2)O (N=16 (4)He). Ground state properties are calculated using the importance-sampled rigid-body diffusion Monte Carlo method, rotational excited state calculations are made with the projection operator imaginary time spectral evolution method, and Bose permutation exchange and associated superfluid properties are calculated with the finite temperature path integral method. For N< or =5 the helium atoms are seen to form an equatorial ring around the molecular axis, at N=6 helium density starts to occupy the second (local) minimum of the N(2)O-He interaction at the oxygen side of the molecule, and N=9 is the critical size at which there is onset of helium solvation all along the molecular axis. For N> or =8 six (4)He atoms are distributed in a symmetric, quasirigid ring around N(2)O. Path integral calculations show essentially complete superfluid response to rotation about the molecular axis for N> or =5, and a rise of the perpendicular superfluid response from zero to appreciable values for N> or =8. Rotational excited states are computed for three values of the total angular momentum, J=1-3, and the energy levels fitted to obtain effective spectroscopic constants that show excellent agreement with the experimentally observed N dependence of the effective rotational constant B(eff). The non-monotonic behavior of the rotational constant is seen to be due to the onset of long (4)He permutation exchanges and associated perpendicular superfluid response of the clusters for N> or =8. We provide a detailed analysis of the role of the helium solvation structure and superfluid properties in determining the effective rotational constants.     

Rovibrational spectra for the HCCCN*HCN and HCN*HCCCN binary complexes in 4He droplets

Rovibrational spectra are measured for the HCCCN*HCN and HCN*HCCCN binary complexes in helium droplets at low temperature. Though no Q-branch is observed in the infrared spectrum of the linear HCN*HCCCN dimer, which is consistent with previous experimental results obtained for other linear molecules, a prominent Q-branch is found in the corresponding infrared spectrum of the HCCCN*HCN complex. This Q-branch, which is reminiscent of the spectrum of a parallel band of a prolate symmetric top, implies that some component of the total angular momentum is parallel to the molecular axis. The appearance of this particular spectroscopic feature is analyzed here in terms of a nonsuperfluid helium density induced by the molecular interactions. Finite temperature path integral Monte Carlo simulations are performed using potential energy surfaces calculated with second-order M?ller-Plesset perturbation theory, to investigate the structural and superfluid properties of both HCCCN*HCN(4He)N and HCN*HCCCN(4He)N clusters with N < or = 200. Explicit calculation of local and global nonsuperfluid densities demonstrates that this difference in the rovibrational spectra of the HCCCN*HCN and HCN*HCCCN binary complexes in helium can be accounted for by local differences in the superfluid response to rotations about the molecular axis, i.e., different parallel nonsuperfluid densities. The parallel and perpendicular nonsuperfluid densities are found to be correlated with the locations and strengths of extrema in the dimer interaction potentials with helium, differences between which derive from the variable extent of polarization of the CN bond in cyanoacetylene and the hydrogen-bonded CH unit in the two isomers. Calculation of the corresponding helium moments of inertia and effective rotational constants of the binary complexes yields overall good agreement with the experimental values.     

Multiply Charged Helium Droplet Anions

The detection of multiply charged helium droplet anions is reported for the first time. By ionizing droplets of superfluid helium with low energy electrons (up to 25 eV), it was possible to produce droplets containing up to five negative charges, which remain intact on the timescale of the experiment. The appearance sizes for different charge states are determined and are found to be orders of magnitude larger than for the equivalent cationic droplets, starting at 4 million He atoms for dianions. Droplets with He*⁻ as charge carriers show signs of being metastable, but this effect is quenched by the pickup of water molecules.     

3He NMR studies on helium–pyrrole,helium–indole,and helium–carbazole systems: a new tool for following chemistry of heterocyclic compounds

The ³He nuclear magnetic shieldings were calculated for free helium atom and He–pyrrole, He–indole, and He–carbazole complexes. Several levels of theory, including Hartree–Fock (HF), Second‐order Møller‐Plesset Perturbation Theory (MP2), and Density Functional Theory (DFT) (VSXC, M062X, APFD, BHandHLYP, and mPW1PW91), combined with polarization‐consistent pcS‐2 and aug‐pcS‐2 basis sets were employed. Gauge‐including atomic orbital (GIAO) calculated ³He nuclear magnetic shieldings reproduced accurately previously reported theoretical values for helium gas. ³He nuclear magnetic shieldings and energy changes as result of single helium atom approaching to the five‐membered ring of pyrrole, indole, and carbazole were tested. It was observed that ³He NMR parameters of single helium atom, calculated at various levels of theory (HF, MP2, and DFT) are sensitive to the presence of heteroatomic rings. The helium atom was insensitive to the studied molecules at distances above 5 Å. Our results, obtained with BHandHLYP method, predicted fairly accurately the He–pyrrole plane separation of 3.15 Å (close to 3.24 Å, calculated by MP2) and yielded a sizable ³He NMR chemical shift (about ?1.5 ppm). The changes of calculated nucleus‐independent chemical shifts (NICS) with the distance above the rings showed a very similar pattern to helium‐3 NMR chemical shift. The ring currents above the five‐membered rings were seen by helium magnetic probe to about 5 Å above the ring planes verified by the calculated NICS index. Copyright © 2014 John Wiley & Sons, Ltd.     

3He NMR: from free gas to its encapsulation in fullerene

The ³He nuclear magnetic shieldings were calculated for single helium atom, its dimer, simple models of fullerene cages (He@C_n), and single wall carbon nanotubes. The performances of several levels of theory (HF, MP2, DFT‐VSXC, CCSD, CCSD(T), and CCSDT) were tested. Two sets of polarization‐consistent basis sets were used (pcS‐n and aug‐pcS‐n), and an estimate of ³He nuclear magnetic shieldings in the complete basis set limit using a two‐parameter fit was established. Theoretical ³He results reproduced accurately previously reported theoretical values for helium gas, dimer, and helium probe inside several fullerene cages. Excellent agreement with experimental values was achieved. ³He nuclear magnetic shieldings of single helium atom approaching various points of benzene ring were tested, and an impact of ³He confinement within fullerene cages of different size on the ³He chemical shift was determined. Copyright © 2013 John Wiley & Sons, Ltd.     

Commensurate-incommensurate transition of 4He adsorbed on a single C60 molecule

Path-integral Monte Carlo calculations have been performed to study (4)He adsorption on a single C(60) molecule. Helium corrugations on the fullerene molecular surface are incorporated with the (4)He-C(60) interaction described by the sum of all (4)He-C interatomic pair potentials. Radial density distributions show a layer-by-layer growth of (4)He with the first adlayer being located at a distance of ~6.3 ? from the center of the C(60) molecule. The monolayer shows different quantum states as the number of (4)He adatoms N varies. For N = 32, we find a commensurate solid, with each of the 32 adsorption sites on the molecular surface being occupied by a single (4)He atom. Various domain-wall structures are observed as more (4)He atoms are added and the first layer crystallizes into an incommensurate solid when it is completely filled. This commensurate-incommensurate transition of the helium monolayer is found to be accompanied by re-entrant superfluid response at a low temperature of 0.31 K with the superfluidity being totally quenched at N = 32, 44, and 48. Finally, the different quantum states observed in the helium monolayer around C(60) are compared with phase diagrams proposed for the corresponding layer on a graphite surface.     

Superfluid helium droplets: a uniquely cold nanomatrix for molecules and molecular complexes

Herein, recent experiments on the spectroscopy and chemical reactions of molecules and complexes embedded in helium droplets are reviewed. In the droplets, a high spectroscopic resolution, which is comparable to the gas phase is achieved, while an isothermal low-temperature environment is maintained by evaporative cooling at T =0.37 K (4He droplets) or 0.15 K (3He droplets), lower than possible in most solid matrices. Thus the helium-droplet technique combines the benefits of both the gas phase and the classical matrix-isolation techniques. Most important, the superfluid helium facilitates binary encounters, and absorbs the released binding energy upon recombination. Thus the droplet can be viewed as an isothermal nanoscopic reactor, which isolates single molecules, clusters, or even a single reactive encounter at ultralow temperatures.     

Method and basis set dependence of the NICS indexes of aromaticity for benzene

The role of theory level in prediction of benzene magnetic indexes of aromaticity is analysed and compared with calculated nuclear magnetic shieldings of ³He used as NMR probe. Three closely related nucleus‐independent chemical shift (NICS) based indexes were calculated for benzene at SCF‐HF, MP2, and DFT levels of theory and the impact of basis set on these quantities was studied. The changes of benzene NICS(0), NICS(1), and NICS(1)zz parameters calculated using SCF‐HF, MP2 and several density functionals were within 1 to 3 ppm. Similar deviations between magnetic indexes of aromaticity were observed for values calculated with selected basis sets. Only very small effect of polar solvent on benzene aromaticity was predicted. The ³He nuclear magnetic isotropic shielding (σ) and its zz‐components (σ_zz) of helium atom approaching the centre of benzene ring from above produced similar curves versus benzene‐He distance to NICS parameters calculated for similarly moving Bq ghost atom. We also propose an experimental verification of NICS calculations by designing the ³He NMR measurement for benzene saturated with helium gas or in low temperature matrices.     

Quantum statistical effects in helium clusters

We discuss ground state and statistical properties of helium clusters with special emphasis to quantum statistical effects. We investigate the density of states of⁴He and³He clusters, their cooling due to evaporation as well as the role played by surface excitations in the superfluid motion.     

Rotovibrational spectroscopy of hydrogen peroxide embedded in superfluid helium nanodroplets

We report the infrared depletion spectrum of para- and ortho-hydrogen peroxide embedded in superfluid helium nanodroplets in the OH stretching region. Six transitions were observed in the antisymmetric stretching band (v(5)) of H(2)O(2), and three in the weaker symmetric stretching band (v(1)). While rotations about the b- and c-axes are slowed by a factor of ～0.4 relative to the gas phase, rotations about the a-axis are not significantly affected; this relates to the rotational speed about the a-axis being too fast for helium density to adiabatically follow. The trans tunneling splitting does not appear to be considerably affected by the helium droplet environment, and is reduced by only 6% relative to the gas phase, under the assumption that the vibrational shifts of the v(5) and v(1) torsional subbands are the same. The linewidths increase with increasing rotorsional energies, and are significantly narrower for energies which fall within the "phonon gap" of superfluid helium. These narrower lines are asymmetrically broadened, indicative of a dynamical coupling between the H(2)O(2) rotor and surrounding helium density.     

Spectroscopic investigation of the solvation of organic molecules in superfluid helium droplets

The spectroscopy of molecules doped into superfluid helium nanodroplets provides valuable information on the process of solvation in superfluid helium. In continuation of an earlier report on emission spectra of various phthalocyanines showing a splitting of all molecular transitions in the range of about 5-12 cm(-1), the emission spectra of tetracene, pentacene, and perylene in superfluid helium droplets are presented. The new spectra and the results obtained for the phthalocyanines are explained by an empirical model which accounts for the existence of different metastable configurations of a nonsuperfluid solvation layer around the guest molecule.     

Electronic polarization spectroscopy of metal phthalocyanine chloride compounds in superfluid helium droplets

We report the electronic polarization spectroscopy of two metal phthalocyanine chloride compounds (MPcCl, M=Al,Ga) embedded in superfluid helium droplets and oriented in a dc electric field. For both compounds, the laser induced fluorescence spectra show preference for perpendicular excitation relative to the orientation field. This result indicates that the permanent dipoles of both compounds are predominantly perpendicular to the transition dipole. Since the permanent dipole derives from the metal chloride, while the transition dipole derives from the phthalocyanine chromophore, in the plane of phthalocyanine, this qualitative result is not surprising. However, quantitative modeling reveals that this intuitive model is inadequate and that the transition dipole might have tilted away from the molecular plane of phthalocyanine. The out of plane component of the transition dipole amounts to approximately 10% if the permanent dipole is assumed to be approximately 4 debye. The origin for this tilt is puzzling, and we tentatively attribute it to the transition of nonbonding orbitals, either from the chlorine atom or from the bridge nitrogen atom, to the pi* orbitals of the phthalocyanine chromophore. On the other hand, although unlikely, we cannot completely exclude the possibility that both our high level density functional theory calculation and ab initio results severely deviate from reality. The droplet matrix induces redshifts in the origin of the electronic transition and produces discrete phonon wings. Nevertheless, in dc electric fields, all phonon wings and the zero phonon line demonstrate the same dependence on the polarization direction of the excitation laser. Although electronic excitation does couple to the superfluid helium matrix and the resulting phonon wings add complications to the electronic spectrum, this coupling does not affect the direction of the electronic transition dipole. Electronic polarization spectroscopy in superfluid helium droplets is thus still informative in revealing the permanent dipole and its relation relative to the transition dipole.     

Effects of dissolved electrolytes on the solubility and partial molar volume of helium in water from 50 to 400 atmospheres at 25°C

The solubility of helium in water and aqueous CsCl, NaCl and MgCl₂ solutions at concentrations up to 3.380 molal has been measured at 50 atm intervals from 50 to 400 atm at 25°C. Setschenow coefficients for helium are practically invariant with pressure in each solution, decrease significantly with increasing electrolyte concentration and vary with the type of electrolyte in a fashion identical to that observed for the low pressure solubilities of other gases. The pressure dependence of the helium solubility in each solution follows a form of Henry's law in which the helium partial molar volume at infinite dilution \(\bar V^{_{He}^o } \) is independent of pressure. Values of \(\bar V^{_{He}^o } \) , computed from Henry's law, are smaller for the electrolyte solutions than for water and vary systematically with the type and concentration of dissolved electrolyte. This result is explained qualitatively in terms of ion hydration and its influence on the ability of the intrinsic configurational volume in each liquid to accommodate the relatively small helium molecules. It is concluded that intrinsic solvent structure is an important factor governing the partial molar volume of helium and the pressure dependence of helium solubility in water and aqueous electrolytes.     

Radiochemical study on the recombination reactions of H+T→HT and T+T→T2 in superfluid and normalfluid 3He-4He media at 1.4-2.5 K

The recombination reactions of thermal H and T atoms, H+THT and T+TT₂, have been investigated using radiochemical methods at 1.4-2.5 K in a mixture of ³He-⁴He. The reactive species were produced via ³He(n,p)T by neutron irradiation at the upgraded Japan Research Reactor No. 3 (JRR-3M). Nonstatistical formation of ortho-T₂ was observed over the entire range of ³He atomic fraction at 1.4-2.5 K. The relative yield of T₂ decreased with an increase in superfluid component, which is a second liquid phase observed only in liquid helium, at 1.6 K.     

Electronic spectroscopy of molecules in superfluid helium nanodroplets: an excellent sensor for intramolecular charge redistribution

Electronic spectra of molecules doped into superfluid (4)He nanodroplets reveal important details of the microsolvation in superfluid helium. The vibrational fine structure in the electronic spectra of phthalocyanine derivatives and pyrromethene dye molecules doped into superfluid helium droplets have been investigated. Together with previous studies on anthracene derivatives [J. Chem. Phys.2010, 133, 114505] and 3-hydroxyflavone [J. Chem. Phys.2009, 131, 194307], the line shapes vary between two limiting cases, namely, sharp Lorentzians and nonresolved vibrational fine structure. All different spectral signatures are initiated by the same effect, namely, the change of the electron density distribution initiated by the electronic excitation. This change can be quantified by the difference of the electrostatic moments of the molecule in the electronic ground state and the corresponding Franck-Condon point in the excited state. According to the experimental data, electronic spectroscopy suffers from drastic line broadening when accompanied by significant changes of the charge distribution, in particular, changes of the dipole moment. Vice versa, the vibrational fine structure in electronic spectra of molecules doped into helium droplets is highly sensitive to changes of the electron density distribution.     

On superconductivity and superfluidity (what I have and have not managed to do), as well as on the 'physical minimum' at the beginning of the 21 st century.

The question of thermeolectric effects in superconductors is still a particular problem, which evidently emerges only in the presence of a temperature gradient. The well-known Londons theory yielded much, and is widley employed under certain conditions even nowadays, but is absolutely insufficient and has to be generalized. This problem was solved in the psi-theory of superconductivity by V.L. Ginzburg and L.D. Landau. Together they developed a phenomenological theory of superconductivity in the late 1940s. This theory proposes that those electrons that contribute to superconduction form a superfluid. The superconductor is described by a complex function psi called the order parameter, and /psi/ indicates the fraction of electrons that has condensed into a superfluid. In his Nobel lecture V. L. Ginzburg also gives a 'list' of top problems in contemporary physics. Acquaintance with all subjects included in this 'list' is what he calls the 'physical minimum'.     

Multiple solvation configurations around phthalocyanine in helium droplets

Recent measurements of the emission spectrum of phthalocyanine solvated in superfluid helium nanodroplets exhibit a constant 10.3 cm(-1) splitting of each emission line relative to the absorption spectrum. This splitting has been attributed to two distinct helium environments near the surface of the phthalocyanine molecule. Rigid-body path-integral Monte Carlo provides a means of investigating the origin of the splitting on a detailed microscopic level. Path-integral Monte Carlo simulations of 4He(N)-phthalocyanine at 0.625 K with N ranging from 24 to 150 show two distinct helium configurations. One configuration is commensurate with the molecular substrate and the other is a triangular lattice. We investigate the energetics of these two configurations and use a method for calculating electronic spectral shifts for aromatic molecule-rare-gas clusters due to dispersive interactions to estimate the spectral splitting that would arise from the two helium configurations seen for N=150. The results are in reasonable agreement with the experimentally measured splitting, supporting the existence of two distinct local helium environments near the surface of the molecule in the nanodroplets.     

Effet Josephson et expérience d'Andronikashvili

A way of making a weak link in superfluid helium 4 is proposed. Putting this weak link in a torsion pendulum of the Andronikashvili type adds high frequency vibrations to the regular oscillations. We also propose a schema of SQUIDac type for a gyroscope.