Sydney Leach has made many contributions concerning transient molecules and molecular ions. Using novel spectroscopic sources, he discovered spectra of SH+, H2S+, OCS+, and other molecular ions, and has made pioneering studies of dications such as SO22+, CH42+, CO22+ and C10H82+.
Major contributions have been made to the analysis of many electronic spectra, such as those of CO2+, C6F6+ and sym–C6H3F3+, and on the following dynamical effects: spin-orbit calculations and the Renner–Teller effect in linear species; Jahn–Teller effects in aromatic ions; matrix-induced effects on the photophysics of benzenoid species; and fluorescence and dissociation kinetics, leading to an understanding of radiationless transitions. He has applied this knowledge to astrophysical problems, including ionospheric data for Mars and laboratory tests of the role of polycyclic aromatic ions in the interstellar medium.
Since 1990, Sydney has studied the spectroscopy and photophysics of fullerenes. His recent work on survival in space includes the VUV spectroscopy and photophysics of amino acids and nucleic acids, and two 18-month duration experiments on plant seeds on the International Space Station.