1. Department of Applied Physics, University of Eastern Finland, POB 1627, FI-70211 Kuopio,
Finland
2. Department of Chemistry, University of Eastern Finland, POB 111, FI-80101 Joensuu,
Finland
3. School of Medicine,
Institute
of Biomedicine, University of Eastern Finland, POB 1627, FI-70211 Kuopio, Finland
4. State Key Laboratory of Magnetic
Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics
and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China