Decay technique  In chemistry, the decay technique is a method to generate chemical species such as radicals, carbocations, and other potentially unstable covalent structures by radioactive decay of other compounds. For example, decay of a tritium-labeled molecule yields an ionized helium atom, which might then break off to leave a cationic molecular fragment. The technique was developed in 1963 by the Italian chemist Fulvio Cacace at the University of Rome. It has allowed the study of a vast number of otherwise inaccessible compounds and reactions. It has also provided much of our current knowledge about the chemistry of the helium hydride ion . In the basic method, a molecule is prepared where the vacant bond of the desired radical or ion is satisfied by an atom of tritium , the radioactive isotope of hydrogen with mass number 3. As the tritium undergoes beta decay (with a half-life of 12.32 years), it is transformed into an ion of helium-3, creating the cation . In the decay, an electron and an antineutrino are ejected at great speed from the tritium nucleus, changing one of the neutrons into a proton with the release of 18,600 electronvolts (eV) of energy. The neutrino escapes the system; the electron is generally captured within a short distance, but far enough away from the site of the decay that it can be considered lost from the molecule. Those two particles carry away most of the released energy, but their departure causes the nucleus to recoil, with about 1.6 eV of energy. This recoil energy is larger than the bond strength of the carbon–helium bond (about 1 eV), so this bond breaks. The helium atom almost always leaves as a neutral , leaving behind the carbocation . These events happen very quickly compared to typical molecular relaxation times, so the carbocation is usually created in the same conformation and electronic configuration as the original neutral molecule. For example, decay of tritiated methane, (R = R′ = R″ = H) produces the carbenium ion in a tetrahedral conformation, with one of the orbitals having a single unpaired electron and the other three forming a trigonal pyramid. The ion then relaxes to its more favorable trigonal planar form, with release of about 30 kcal/mol of energy—that goes into vibrations and rotation of the ion. The carbocation then can interact with surrounding molecules in many reactions that cannot be achieved by other means.