Investigation of impurities in superfluid Helium by optical spectroscopy

Atomic and molecular defects in solid 4He

“…In the range of temperatures and pressures relevant for the present review (T = 1 -2 K, p = 25 -100 bar) there exist two crystalline phases of 4 He (see Fig. 1): hexagonal close-packed (hcp) and the body-centered cubic (bcc) phase.…”

“…The de Broglie wavelength of a He atom λ B = h/ √ 2πm He kT is very large and at low temperatures it becomes comparable to the interatomic separation in condensed helium (R ≈ 3.7Å at the solidification pressure at 1.5 K). The He atoms are strongly delocalized (even in the crystalline phase) and form a so-called quantum liquid or quantum solid because of the large overlap of the wavefunctions which makes condensed, i.e., liquid or solid 4 He a macroscopic quantum object. The degree of delocalization is best characterized by the Lindemann ratio γ = u 2 /R or by the de Boer parameter Λ = λ B /(2πσ), in which u 2 is the rms value of the vibrational amplitude of an atom in the matrix and σ is the distance at which the interatomic potential passes through zero.…”

“…The elastic constants of bcc and hcp solid He were obtained from measurements of the orientation-dependent sound velocity [17,18]. Recently evidence for a supersolid phase of 4 He at temperatures below 0.2 K was obtained [19] and is under active discussion [20].…”

“…Rare gas solids are typically used in those studies because of their chemical inertness, and the low temperature required for the matrix formation is an additional asset for the studies. In the present paper we review experiments that have extended the matrix isolation technique to the use of solid 4 He as the matrix material. Those studies were initiated by one of the authors (AW) together with Sergei Kanorsky in 1994 at the Max Planck Institute for Quantum Optics in Garching (Germany).…”

“…The studies of impurities in solid helium were reviewed in [1,2] and more recently in [3]. This field of solid helium matrix isolation spectroscopy is closely connected with the studies of atomic and molecular impurities in bulk liquid He (reviewed, e.g., in [4]) and in He nano-droplets. The latter research domain was pioneered by the teams of J. P. Toennies in Göttingen and G. Scoles at Princeton in 1993 and has been rapidly expanding since with review articles of the topic being published on average every 2 to 3 years: [5][6][7][8].…”