Original: He always used new observation techniques like low-energy electron diffraction (LEED) at the beginning of his career, later  ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscope (STM) yielding ground breaking results.
Simplified: One was called low-energy electron diffraction (LEED) used at the beginning of his career and later he worked on ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscope (STM).