Starting from 1960, loan-lovitz Popescu was a scientific researcher of the Institute of Physics and Technology of Radiation Devices, IFTAR, principal scientific researcher (1962-1972) of the institute, and during 1977-1981, director of the institution. Between the years 1971-1981 he worked with Prof. Carl B. Collins, from the University of Texas, Dallas, at the "U.S.-Romanian Project in Atomic and Plasma Physics" program, funded by the U.S. National Science Foundation. In 1981, he was proposed as a candidate for the "Nobel" Prize in physics, together with Denisa Popeseu and Carl B. Collins, for the experimental discovery of atomic multiphoton spectra and the pioneering research in the field of laser spectroscopy, canied out in the framework of Romanian-American cooperation. He made original research contributions with international impact: - the cathodic fall theory of glow discharges and resonant charge transfer (1960); - the introduction of optogalvanic methods in laser spectroscopy: thermionic detection (1964); - spectroscopy of Rydberg states (1965); - discovery of the first multiphoton spectra of atoms and molecules (1973); - discovery of the multiphoton hybrid molecular-atomic resonance process (1974); - introduction of photolytic spectroscopy (1980); - optical impedance spectroscopy through radio-frequency detection (1980); - detection of the frequency shift of ion waves in plasma (1990); - the creation of a new type of compact plasma source of intense pulsed beams of electrons, ions and electromagnetic radiation (1994); - the first gamma laser project with optical pumping (1982); - the first experimental evidence of accelerated gamma emission (1999).