The word "femtosecond" is a unit of time, specifically one quadrillionth of a second. It is a very short timeframe used to measure extremely fast processes, such as chemical reactions or the movement of light particles. Full definition
Electron motions induced by a strong electric field are mapped in space and time with the help of femtosecond x-ray pulses. (sciencedaily.com)
While some theories point to polarizability and ion size, Saykally and his colleagues used femtosecond laser spectroscopy to examine water molecules around simple ions. (pnnl.gov)
Kim et al. used femtosecond x-ray laser pulses to probe micrometer - sized water droplets cooled to 227 K (see the Perspective by Gallo and Stanley). (science.sciencemag.org)