At 100,000 times atmospheric pressure, solid oxygen turns red. Researchers have been aware of the phenomenon since 1979; now a team in Florence, Italy, has an explanation. Using Fourier transform IR spectroscopy, Federico A. Gorelli and Roberto Bini of the University of Florence and their colleagues determined that the optical effect accompanies the formation of O4 molecules. The physicists measured the absorption spectra of red oxygen from 100 to 1800 cm-1 at pressures of 10 to 63 GPa. A newly observed peak at about 300 cm-1 led them to the conclusion that the charge transfer mechanism forces two diatomic molecules of oxygen to bond in a sort of polymerization at between 10 and 20 GPa.