1. Exxon Research and Engineering Company, Corporate Research Science Laboratories, Route 22 East, Annandale, NJ 08801
2. Exxon Research and Engineering Company, Corporate Research Science Laboratories, Route 22 East Annandale, NJ 08801, and Department of Physics, University of Colorado, Boulder, CO 80309
3. Department of Physics, University of Colorado, Boulder, CO 80309