1. Kemira Pigments Oy, FIN-28840 Pori, Finland, Department of Medical Physics and Chemistry, University of Turku, FIN-20500 Turku, Finland, Departments of Applied Physics, Physics, and Chemistry, University of Turku, FIN-20014 Turku, Finland, and Espoo-Vantaa Institute of Technology, Leiritie 1, FIN-01600 Vantaa, Finland