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 (±0.8) x 10~(-4) s~(-1) torr~(-1). However, almost no change was observed in the A_g isomer populations. The results suggest a spin conversion mechanism in C_2H_4 via quantum relaxation within the same inversion symmetry.
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