1. Department of Chemistry; University of Bath; Claverton Down Bath BA2 7AY UK
2. National Physical Laboratory; Teddington TW11 0LW UK
3. Laser Processing Research Group; Lappeenranta University of Technology; Tuotantokatu 2 53850 LPR Lappeenranta Finland
4. Lappeenranta University of Technology; Skinnarilankatu 34 53850 Lappeenranta Finland
5. Department of Physics; The Oliver Lodge Laboratory; University of Liverpool; Oxford Street Liverpool L69 7ZE UK