1. V.A. Fock Institute of Physics, St. Petersburg State University, 198904 St. Petersburg, Russia
2. A.F. Ioffe Physico-Technical Institute RAS, 194021 St. Petersburg, Russia
3. Laboratoire de Spectrométrie Physique, Université Joseph Fourier de Grenoble, Grenoble, France
4. Department of Physics, University of Bath, BA2 7AY Bath, UK