1. Current address: Physical Chemistry Department, General Motors Research Laboratories, Warren, MI 48090
2. Current address: 3M, 3M Center Road, Central Research, Building 208-01-01, Process Technology, St. Paul, MN 55144
3. Current address: Max Planck Institut für Strahlenchemie, D-4330, Mülheim a.d. Rühr, Federal Republic of Germany