Abstract
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 B2u nuclear spin isomer. The separation was achieved through spatial drift of this isomer induced by resonant absorption of narrow-band infrared light. Evolution of the depleted sample revealed conversion between B2u and B3u 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 Ag isomer populations. The results suggest a spin conversion mechanism in C2H4 via quantum relaxation within the same inversion symmetry.
Highlights
We have studied the interconversion process in ethylene by creating a sample depleted in the B2u nuclear spin isomer
It is widely assumed that the conversion probabilities among nuclear spin isomers for the various molecules in the interstellar medium (ISM) are zero, even over time spans of millions of years
Separation and conversion of nuclear spin isomers have been successfully studied for only a small number of polyatomic molecules: CH3F (6, 7), 13C12CH4 (8), H2CO (9), and H2O (11)
Summary
Molecules with three or more nuclei of nonzero spin exist as discrete spin isomers whose interconversion in the gas phase is generally considered improbable. It is widely assumed that the conversion probabilities among nuclear spin isomers for the various molecules in the ISM are zero, even over time spans of millions of years. This is not necessarily the case (6–10). Because the excited molecules usually have a larger cross section than the ground-state molecules, their mean free path will be smaller than that of the groundstate molecules This produces a drift of the absorbing species moving in the direction of the laser beam with respect to the buffer gas and results in a concentration difference between the two ends of the closed cell. Frequency offset Df denotes the C2H4 transition frequency minus the CO2 laser frequency
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