Abstract: The nucleus is composed of protons and neutrons. The total angular momentum of the protons and neutrons in an atomic nucleus is usually referred to as nuclear spin. Nuclear spin isomers and their stability are fundamental concepts in quantum mechanics, and all molecules possessing identical nuclei with nonzero spin have two or more distinct nuclear spin isomers with different arrangements of total nuclear spin quantum numbers. Thus, researchers have tried to separate the nuclear spin isomers of gaseous polyatomic molecules and study the conversion mechanisms among them for many years. However, the studies in this field did not make progress until the early 90s. In 2005, we made a study on the separation and conversion of nuclear spin isomers of ethylene for the first time, to which we gave a special introduction in this paper. Firstly, the scientific history about the discovery of the protons and neutrons that form nucleus is briefly described chronologically; the concept of nuclear spin isomers is presented, and the nuclear spin isomers of hydrogen and ethylene molecules aref described in detail. Secondly, we give a full description of the progresses made in the experimental study on the separating the nuclear spin isomers of gaseous ethylene with light-induced drift technique, and make a quantitative explanation on the interconversion dynamics among the nuclear spin isomers of ethylene based on parity conservation and quantum relaxation theory. Finally, we briefly summarize the latest research finding. The enrichment of the nuclear spin isomers of ethylene can be achieved through chemical synthesis using the acetylene catalytically hydrogenated with para-hydrogen, which would sufficiently and successfully enhance nuclear magnetic resonance signal.

Key words: the proton; the neutron; nuclear constitution of atoms; nuclear spin isomers; separation and conversion rate of nuclear spin isomers; light-induced drift; quantum relaxation; parity conservation; ethylene; precision laser spectrum; nuclear magnetic resonance