1. Department of Physics, Laser Research Institute, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa
2. BIOPEP Peptide Group, Department of Biochemistry, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa
3. Department of Chemistry and York Biomedical Research Institute, University of York, Heslington, York YO10 5DD, U.K.
4. Research Complex at Harwell, Rutherford Appleton Laboratory, STFC Central Laser Facility, Harwell Science and Innovation Campus, Didcot, Oxon OX11 0QX, U.K.
5. Department of Physics, University of Strathclyde, SUPA, 107 Rottenrow East, Glasgow G4 0NG, U.K.