1. A.F. Ioffe Physico-Technical Institute, RAS, 194021 St. Petersburg, Russia
2. Laboratoire de Spectrométrie Physique, Université Joseph Fourier de Grenoble, Grenoble, France
3. Institute of Physics, St. Petersburg State University, St. Petersburg, Russia
4. Department of Physics, University of Bath, Bath BA2 7AY, UK