<i>An Introduction to Liquid Helium</i>

Wilks, J.; Betts, David S.; Hallock, R. B.

doi:10.1063/1.2810983

Cited by 75 publications

(50 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…After the two dipoles have been aligned, the molecules are drawn together by their mutual attraction; in contrast to a gas phase coagulation, however, their potential energy is not completely transformed into kinetic energy because of the steady cooling action of the "solvent". Consider that a potential energy of 200 K (i.e., 2.8 × 10 −21 J) would accelerate a molecule of mass 27 u to a velocity of 350 m/s, which is well above the critical velocity of 60 m/s [133] above which a foreign body will rapidly dissipate energy in superfluid helium. This dissipative cooling during complexation ensures that, at the contrary of what happens in the gas phase, the freshly formed complex is born cold, without the energy needed to surmount any barrier to isomerization toward the global energy minimum.…”

Section: Special Topics a Synthesis Of Nonequilibrium Structuresmentioning

confidence: 99%

Helium nanodroplet isolation rovibrational spectroscopy: Methods and recent results

Callegari¹,

Lehmann²,

Schmied³

et al. 2001

The Journal of Chemical Physics

350

459

View full text Add to dashboard Cite

In this article, recent developments in HElium NanoDroplet Isolation (HENDI) spectroscopy are reviewed, with an emphasis on the infrared region of the spectrum. We discuss how molecular beam spectroscopy and matrix isolation spectroscopy can be usefully combined into a method that provides a unique tool to tackle physical and chemical problems which had been outside our experimental possibilities. Next, in reviewing the experimental methodology, we present design criteria for droplet beam formation and its seeding with the chromophore(s) of interest, followed by a discussion of the merits and shortcomings of radiation sources currently used in this type of spectroscopy. In a second, more conceptual part of the review, we discuss several HENDI issues which are understood by the community to a varied level of depth and precision. In this context, we show first how a superfluid helium cluster adopts the symmetry of the molecule or complex seeded in it and discuss the nature of the potential well (and its isotropy) that acts on a solute inside a droplet, and of the energy levels that arise because of this confinement. Second, we treat the question of the homogeneous versus inhomogeneous broadening of the spectral profiles, moving after this to a discussion of the rotational dynamics of the molecules and of the surrounding superfluid medium. The change in rotational constants from their gas phase values, and their dependence on the angular velocity and vibrational quantum number are discussed. Finally, the spectral shifts generated by this very gentle matrix are analyzed and shown to be small because of a cancellation between the opposing action of the attractive and repulsive parts of the potential of interaction between molecules and their solvent. The review concludes with a discussion of three recent applications to (a) the synthesis of far-fromequilibrium molecular aggregates that could hardly be prepared in any other way, (b) the study of the influence of a simple and rather homogeneous solvent on large amplitude molecular motions, and (c) the study of mixed 3 He/ 4 He and other highly quantum clusters (e.g., H2 clusters) prepared inside helium droplets and interrogated by measuring the IR spectra of molecules embedded in them. In spite of the many open questions, we hope to convince the reader that HENDI has a great potential for the solution of several problems in modern chemistry and condensed matter physics, and that, even more interestingly, this unusual environment has the potential to generate new sets of issues which were not in our minds before its introduction.

show abstract

Section: Special Topics a Synthesis Of Nonequilibrium Structuresmentioning

confidence: 99%

Helium nanodroplet isolation rovibrational spectroscopy: Methods and recent results

Callegari¹,

Lehmann²,

Schmied³

et al. 2001

The Journal of Chemical Physics

350

459

View full text Add to dashboard Cite

show abstract

mentioning

confidence: 99%

“…Jackson et al [15] performed 3D simulations for a geometry similar to our experiment and obtained a critical velocity as low as 0.13c s , in good agreement with our results. The relevant critical velocity in our experiment is most likely related to vortex nucleation [3,[5][6][7]15,16,19,20], which is usually smaller than the Landau and Feynman critical velocities [8] at which phonons [18] or vortices [17] become energetically favorable.…”

mentioning

confidence: 99%

“…The onset of dissipation has been treated in the framework of the nonlinear Schrödinger equation [3][4][5][6][7] and shows an intriguing richness. Experiments on liquid helium could not test these theories, because superfluidity and dissipation are even more complex in this system due to the presence of strong interactions and surface effects [8].The creation of Bose-Einstein condensates in dilute gases has dramatically changed this situation, allowing for quantitative tests of microscopic theories using the tools and precision of atomic physics experiments [9]. A number of recent experiments have examined phenomenological features of superfluidity in gaseous BoseEinstein condensates.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Observation of Superfluid Flow in a Bose-Einstein Condensed Gas

et al. 2000

View full text Add to dashboard Cite

We have studied the hydrodynamic flow in a Bose-Einstein condensate stirred by a macroscopic object, a blue detuned laser beam, using nondestructive in situ phase contrast imaging. A critical velocity for the onset of a pressure gradient has been observed, and shown to be density dependent. The technique has been compared to a calorimetric method used previously to measure the heating induced by the motion of the laser beam.PACS 03.75.Fi, 67.40.Vs, 67.57.De Beginning with the London conjecture [1], BoseEinstein condensation has been considered crucial for the understanding of superfluidity. Since then, the weakly interacting Bose gas has served as an idealized model for a superfluid [2]. It has a phonon-like energy-momentum dispersion relation that does not allow for the generation of elementary excitations below a critical velocity, thus implying dissipationless flow at lower velocities. The onset of dissipation has been treated in the framework of the nonlinear Schrödinger equation [3][4][5][6][7] and shows an intriguing richness. Experiments on liquid helium could not test these theories, because superfluidity and dissipation are even more complex in this system due to the presence of strong interactions and surface effects [8].The creation of Bose-Einstein condensates in dilute gases has dramatically changed this situation, allowing for quantitative tests of microscopic theories using the tools and precision of atomic physics experiments [9]. A number of recent experiments have examined phenomenological features of superfluidity in gaseous BoseEinstein condensates. These include the observation of vortices [10,11], a non-classical moment of inertia [12], and suppression of collisions from microscopic impurities [13]. In previous work we found evidence for a critical velocity in a stirred condensate [14]. In this Letter we study the onset of dissipation with higher sensitivity using repeated in situ non-destructive imaging of the condensate. These images show the distortion of the density distribution around the moving object, thus directly probing the dynamics of the flow field that has been recently treated with different models [15][16][17][18][19][20]. The experimental setup was similar to the one used in our previous work [14]. Improvements in the evaporation strategy and decompression techniques allowed us to produce pure condensates with up to 5×10 7 sodium atoms, with densities ranging from 8.4×1013 to 3.5×10 14 cm −3 , corresponding to chemical potentials from 60 to 250 nK. We determined the Thomas-Fermi radius R z along the axial direction through in situ phase contrast imaging. The sound velocity at the center of the condensate was then evaluated through the relationship c s = 2πν z R z / √ 2, with ν z =20.1Hz being the axial trapping frequency. The macroscopic moving object was a 514 nm laser beam blue-detuned with respect to the sodium transitions, thereby creating an effective repulsive potential for the atoms. The beam is focused on the center of the condensate to a Gaussian 1/e 2 diameter o...

show abstract

“…21 The quantum liquid of 4 He undergoes a Bose-Einstein condensation (BEC) into the superfluid phase of 4 He II. In the case of the quantum spin-dimer system Sr 3 Cr 2 O 8 , an extremely sharp anomaly has also been found where the applied magnetic field H is well above a critical value H c of the quantum critical point.…”

mentioning

confidence: 99%