Hyperfine coupling constants of muonium in sub and supercritical water

Muonium, like the hydrogen atom, is a hydrophobic solute in water under sta ndard conditions. Molecular dynamics simulations suggest that the free atom exists in a transient clathrate-like cage of hydrogen-bonded water molecul es. The hyperfine constants of Mu and H are very close to their vacuum valu es, supporting the picture of an atom "rattling" around in a hole in the li quid. Muonium has now been studied in water over a wide range of temperatur es and pressures, from standard conditions to over 400 degrees C and 400 ba r (the critical point is at 374 degrees C, 221 bar). Drastic changes occur in the properties of water over this range of conditions, so large changes in the muonium hyperfine constant might well be expected. Surprisingly, the changes are small. The hyperfine coupling constant goes through a minimum in the subcritical region, and then increases toward the vacuum value under supercritical conditions. (C) 2000 Elsevier Science B.V. All rights reserv ed.