1. Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, U.K., School of Biological and Chemical Sciences, Birkbeck University of London, Malet Street, London WC1E 7HX, U.K, and Central Laser Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, U.K.