Helium superfluid form

Helium II Superfluid form of helium-4 liquid existing below 2.17 K. 

The 1996 Nobel Prize in physics went to three researchers who studied liquid helium at a temperature of 0.002 K, discovering superfluid helium. A superfluid behaves completely unlike conventional liquids. Liquids normally are viscous because their molecules interact with one another to reduce fluid motion. Superfluid helium has zero viscosity, because all of its atoms move together like a single superatom. This collective behavior also causes superfluid liquid helium to conduct heat perfectly, so heating a sample at one particular spot results in an immediate and equal increase in temperature throughout the entire volume. A superfluid also flows extremely easily, so it can form a fountain, shown in the photo, in apparent defiance of gravity. 

There are two characteristics that make helium attractive for space applications the first is weight (about 0.125 kg/1) the second is its superfluidity. Helium becomes superfluid at T< 2.17K (p < 37.8 torr). Thanks to superfluiduty, helium forms a film that completely covers the walls of the container and guarantees a homogeneous cooling even if most of the liquid does not have a fixed position inside the container (no gravity). 

As helium gas is cooled below -268.95°C, it forms a liquid. At -270.97°C, helium still looks like a liquid, but a liquid with unusual properties. Suddenly, liquid density drops and this "liquid" gains the ability to move through very small holes that helium gas cannot pass through. It flows up the walls of its container defying gravity, and has zero viscosity. Below -270.97°C, helium becomes a superfluid, the only one discovered so far. Helium never changes to a solid. —... 

First of all, the term supercritical fluid does not refer to superfluid helium, a state of matter for He and " He near absolute zero.ril As shown in the phase diagram in Ch. 1, a supercritical fluid refers to the state of matter above its critical point. The critical point is a temperature and pressure, Tg and Pc, at which two phases of a substance in equilibrium with each other become identical, forming one phase. Above the critical temperature, Tc, a substance can not... 

The ability to dope impurity atoms or molecules into large helium clusters by a pick-up method, pioneered by the groups of Toennies and Scoles, has helped make studies in superfluid helium clusters more accessible. In this method, an expansion through a nozzle produces a beam of helium clusters. Under appropriate conditions, helium droplets comprising up to 10 helium atoms can be formed. These droplets then traverse a collision cell containing a foreign gas at a pressure of 10 -10 Pa. Atoms or... 

At a temperature of about —456°F (—271°C), helium undergoes an unusual change. It remains a liquid, but a liquid with strange properties. Superfluidity is one of these properties. The forms of helium are so different that they are given different names. Above —456°F (—271°C), liquid helium is called helium I below that temperature, it is called helium II. 

Note Liquid helium has unique thermodynamic properties too complex to be adequately described here. Liquid He I has refr index 1.026,dO.l 25, and is called a quantum fluid because it exhibits atomic properties on a macroscopic scale. Its bp is near absolute zero and viscosity is 25 micropoises (water = 10,000). He II, formed on cooling He I below its transition point, has the unusual property of superfluidity, extremely high thermal conductivity, and viscosity approaching zero. 

The second reason helium and possibly hydrogen clusters are special is their capability to transform to a superfluid state. This is a state in which some or all of the component particles are in their lowest quantum state, a state common to all the particles in that state. This is only possible for particles with integral values of their total spin, particles known as bosons. The common isotope He is one such particle. At a sufficiently low temperature, bulk He goes into its superfluid state, in which a finite fraction of the atoms are in their lowest quantum state. This gives the very cold liquid helium special and sometimes dramatic properties, such as the capability to climb walls of a container, and to spout up through a capillary to form a fountain. 

Liquid helium exists in two forms— one with the properties of an ordinary liquid, and another superfluid one, which shows no cohesion at all. 

The liquefied helium is subdivided into two states He I and He II with a sharp transition point of 2.18 K at 5.04 kPa, the so-called A,-point. He I behaves like a normal liquid, whereas He II exhibits interesting properties of a superfluid or quantum fluid. During expansion of liquid He I below this pressure, the previously even surface forms a sharp meniscus at the wall of the container since at the 7.-point the viscosity decreases by the factor 10 and the thermal conductivity rises by the same factor. The thermal conductivity of He II is about 200 times higher than that of copper at 20 °C. Close to the absolute zero point, the viscosity turns zero and He II becomes an inviscid superfluid. He II flows over obstacles, which lie higher than the surface of the liquid, to reach the lowest level. If two containers of different temperatures are filled with He 11 and connected to each other by a capillary or another He Il-film, He II flows from the cold container into the warmer one. 

Photoelectron spectroscopy is a powerful technique to study ionic and electronically excited levels of atoms and molecules. In the case of single photon excitation of cold molecules the photoelectron spectrum reflects the internal energy levels of the ionic system. Many experiments are performed via two photon ionization enhanced by a one-photon resonance (R2PE spectroscopy) in which transitions to intermediate electronic levels are accessed which strongly enhance the ion yield. Photoelectron spectroscopy of molecules inside superfluid helium droplets is of particular interest since the interaction of free electrons with liquid helium is known to be highly repulsive, so much so that the electrons form bubbles of about 34 A diameter. In this section, three recent photoelectron spectra will be discussed those of bare helium droplets, of Ags clusters and of single aniline molecules in helium droplets. 

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