1. Contribution from the Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611-0011, Japan, Centre for Science at Extreme Conditions and School of Chemistry, University of Edinburgh, Mayfield Road, Edinburgh, EH9 3JZ, United Kingdom, and ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom