Sentence:  In addition to the basic programs in wavelength standards , spectroscopy , solid state physics , interactions of the <WSD>free</WSD> electron and atomic constants which are necessary to provide the foundation for technological progress , the Bureau has strengthened its activities in laboratory astrophysics . 
Meaning of the word marked with <WSD> tags:  unconstrained or not chemically bound in a molecule or not fixed and capable of relatively unrestricted motion