High-efficiency generation of attosecond pulses during atomic ionization from excited electronic statesM. Yu. Emelin, M. Yu. Ryabikin, A. M. Sergeev, M. D. Chernobrovtseva, T. Pfeifer, D. Walter and G. GerberEurophys. Lett., 69 6 (2005) 913-919DOI: https://doi.org/10.1209/epl/i2004-10449-1