INTERACTION OF OXYGEN AND ACRIFLAVINE 3257tion that 1 0 is almost temperature independent if the phosphorescence is observed following a photostationary illumination with a tungsten lamp. For this condition steady-state kinetics leads to equation 5 for the triplet populationwhere Iabs is the quantal rate of light absorption during the photostationary state. The population of triplets is seen to be dependent on the rate both of the S' + T process and also the reverse T -+ S' process. The temperature dependence of the two just about cancel each other. The energy of activation for a-phosphorescence, Eg, was estimated in two ways. ks was estimated from kobsdj the reciprocal of the mean life as tabulated in Table 11, by use of equation 3. k~, ranges from 0 a t -100" and below, to 5.3 set.-' at +20°.The resulting values of k6 gave a linear semi-log vs. 1/T plot, with E6 = 6 kcal. Because of errors inherent in the several subtractions required for this procedure, the following method was used.The ratio of a-to ,&phosphorescence intensities a t any time is proportional to kzkc/k6ks'. The proportionality constant is independent of temperature because of constant geometrical and optical factors and invariant band shapes. Also, kz and k5 are independent of temperature. Thus, the plot in Fig. 2 represents the temperature dependence of kslks'. At the lower temperatures, where ks'is independent of temperature, the observed slope leads to a value of 8 kcal. for Eg. This method is believed to be more reliable than the use of kg values obtained from the decay rates. The hightemperature break in the Fig. 2 curve is attributed to the increase in ks' and to the slight temperature dependence of the green band shape.TO k41abd(ks'kT -kakd (5)The energy difference between radiation from the S' and T states, obtained from the peak wave lengths of the bands, is about 5 kcal. The difference in activation energies for the T + . S' and S' -+ T steps, E6 -Eq, is 1 to 3 kcal. short of the spectroscopic difference in the levels. This is not surprising if the T state radiates to a higher vibrational level of S than does the S' state. This would correspond to a greater nuclear displacement in the T state than in the S' state.Internal Conversion.-The assumption was made above that k3 = 0. Semiquantitative estimates of the ratio of steady-state fluorescence to a-phosphorescence at room temperature indicate that k3iks' does not exceed 0.1. We tried to estimate the effectiveness of the direct radiationless depopulation of the triplet state, reaction vii. From the experimental comparison of the low temperature ,!%phosphorescence yield with the room temperature a-phosphorescence, we estimate that k 6 / k T is a t least 0.25. We thus cannot exclude the participation of step (vi;) in depopulating the triplet state on the basis of our data. Reactions of type (vii) occur very rapidly in fluid media. Apparently the degrees of freedom responsible for this type of quenching are not completely frozen out in adsorbates.Triplet-Triplet Interactions.-The observed...
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