1. Laboratory of Chemical Physics, National institute of diabetes and Digestive and Kidney diseases, The National institutes of health, Bethesda, Maryland 20892 (P.C., L.H.K., I.W.L., E.N.L.); Vanguard Research, Inc., Fairfax, Virginia 22030 (J.C.F.); and Space Sciences Laboratory, university of California, Berkeley, California 94720 (J.F.A.)