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A new method for determining these coefficients has been developed. The method employs pulse radiolysis for triplet excitation and, by means of triplet-triplet ( T–T ) energy transfer, leads to a comparison between the extinction coefficients of the benzophenone ketyl radical (ф 2 ĊOH) and the triplet states of several aromatic compounds whose triplet levels are lower than that of benzophenone. Since the ketyl extinction coefficient can be determined independently, the actual T–T extinction coefficients can be obtained. Values of the extinction coefficient at the absorption maximum were obtained for anthracene (57200 m -1 cm -1 ), naphthalene (22600 m -1 cm -1 ), phenanthrene (21000 m -1 cm -1 ) and 1,2-benzanthracene (25100 m -1 cm -1 ), all relative to an extinction coefficient for the benzophenone ketyl maxi­mum around 540 nm determined to be 3220 m -1 cm -1 . For several other compounds the above method for various reasons could not be applied. In such cases estimates were obtained by energy transfer experiments, leading to comparisons between their triplet absorptions and those of the compounds already determined. Estimates were thus obtained for the maximum T–T extinction coefficients of benzophenone (10300 m -1 cm -1 ), biacetyl (6400 m -1 cm -1 ), diphenyl (35400 m -1 cm -1 ), and acridine (28800 m -1 cm -1 ). During the course of these experiments estimates were also obtained for the rates of the following reactions : e - aq. + benzophenone ( k ii ═ 3·0 × 10 10 m -1 s -1 ), OH˙ + benzophenone ( k ii ═ 8·7 × 10 9 m -1 s -1 ) and hydrogen abstraction by triplet benzophenone from cyclo­hexane, to give ф 2 ĊOH ( k ii ~ 8 × 10 5 m -1 s -1 ).

Pulse radiolysis of solutions of iodine in cyclo hexane resulted in the formation of a transient absorption with wavelength maximum at 3300 Å. This transient spectrum was assigned to the C 6 H 12 ... I' charge-transfer complex. The spectrum and extinction coefficients of the complex over the wavelength range 2500 to 6500 Å were obtained, the coefficient at 3300 Å being 2025 1. mole -1 cm -1 . Hydrogen iodide in cyclo hexane gave rise to the same transient absorption. Pulse radiolysis of cyclo hexane and cyclo hexyl iodide alone gave rise to different transient spectra attributed to the cyclo hexyl radical, absorbing in the ultraviolet, extinction coefficient 3401. mole -1 cm -1 at 2550 Å and the C 6 H 11 .. .I* charge-transfer complex, absorption maximum 3900 A, extinction coefficient ~ 10 3 to 10 4 1. mole -1 cm -1 , respectively. During the course of the investigations rate constants were obtained for the following reactions: C 6 H 11 + I 2 C 6 H 11 I + I', k II = 7 x 10 9 1. mole -1 s -1 , 2C 6 H 12 ...I' -> 2C 6 H 12 + I 2 , k II = 1.06 x 10 10 1. mole -1 s -1 , or 21' -> I 2 , a C 6 H 10 + C 6 H 12 2C 6 H 11 k 11a + k 11b =2.5 x 10 9 1. mole -1 s -1 bC 12 H 22 Dilute solutions of iodine (10 -4 M) in cyclo hexane yielded experimental evidence for neutral radical intermediates only. There were, however, indications that ionic radical intermediates occur in more concentrated solutions ( > 10 -3 M).