1. One may use Varian Vacsorb molecular sieve, catalog No. 944–0000, obtainable from Varian Vacuum Products, 121 Hartwell Avenue, Lexington, MA 02173.
2. T.P. Beebe, D.W. Goodman, B.D. Kay, and J.T. Yates, Jr., J. Chem. Phys. 87, 2305 (1987).
3. Dr. Jorg Wambach, Danish Technical University, Department of Physics, Building 307, DK-2800 Lyngby, Denmark (private communication).
4. L. Hanley, Z. Xu, and J.T. Yates, Jr., Surface Sci. 248, 265 (1991).
5. J.T. Yates, Jr., J.J. Zinck, and W.H. Weinberg, Rev. Sci. Inst. 50,132 (1979).