The action spectrum of tetracene photooxygenation was measured in air-saturated carbon tetrachloride in the wavelength range of 1220-1290 nm using a wavelength-tunable forsterite laser. The data show that the photoreaction occurs due to laser excitation of the (1)Delta(g)(nu - 0) <-- (3)Sigma(g)(-)(nu = 0) transition in oxygen molecules. The molar absorption coefficient (epsilon(1273)) and the cross section of light absorption (sigma(1273)) corresponding to the spectral maximum of this transition were calculated from the observed photoreaction rates. The obtained values epsilon(1273) = 0.003 M-1 cm(-1) and sigma(1273) = 10(-23) cm(-2) (+/-20%) reasonably correlate with those extrapolated from the high-pressure oxygen absorption spectra. (C) 2004 Elsevier B.V. All rights reserved.