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time span simulated ( fig. S6 and table S8). These results, combined with the lack of resolution within superclades of the metazoan tree, argue against models of metazoan radiation in which the temporal window of diversification is much larger (48 Molecules with three or more nuclei of nonzero spin exist as discrete spin isomers whose interconversion in the gas phase is generally considered improbable. We have studied the interconversion process in ethylene by creating a sample depleted in the B 2u nuclear spin isomer. The separation was achieved through spatial drift of this isomer induced by resonant absorption of narrowband infrared light. Evolution of the depleted sample revealed conversion between B 2u and B 3u isomers at a rate linearly proportional to pressure, with a rate constant of 5.5 (T0.8) Â 10 j4 s j1 torr j1 . However, almost no change was observed in the A g isomer populations. The results suggest a spin conversion mechanism in C 2 H 4 via quantum relaxation within the same inversion symmetry.

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