Abstract
All symmetrical molecules with non-zero nuclear spin exist in nature as nuclear-spin isomers (NSIs). However, owing to the lack of experimental information, knowledge is rare about interconversions of NSIs of gaseous molecules with torsional symmetry. Here we report our separation and conversion observations on NSI-torsion-specific transition systems of gaseous methanol from a light-induced drift experiment involving partially spatial separation of the ortho and para isomers. We find that vibrationally excited molecules of the methanol spin isomer have a smaller collision cross-section than their ground-state counterparts. Interconversion of the enriched ortho isomer with the para isomer, which is generally considered improbable, has been quantitatively studied by sensitive detections of the spectral intensities. Rather counterintuitively, this reveals that the interconversion is inhibited with increasing pressure. Our results suggest that the spin conversion mechanism in methanol is via a quantum relaxation process with the quantum Zeno effect induced by molecular collisions.
Highlights
All symmetrical molecules with non-zero nuclear spin exist in nature as nuclear-spin isomers (NSIs)
The LID effects and the decay curves arising from subsequent population transfer between the ortho and para isomers after closing the valve-T were observed by employing a three-period signal recording method
We have quantitatively accounted for our nuclear-spin conversion observations by the model of quantum relaxation and experimentally determined the spin-conversion rate g; the latter for the ortho isomer in Case 1, owing to larger induced spin-conversion rate by the molecule–surface interactions is bigger than that for the para isomer in Case 2
Summary
All symmetrical molecules with non-zero nuclear spin exist in nature as nuclear-spin isomers (NSIs). All the NSIs of these molecules are identified by rotational quantum numbers Among these molecules, only the NSIs of CH3F, 13CH3F, 13CCH4 and C2H4 have been separated at ambient temperature in the gas phase by the method of light-induced drift[13] (LID) and the nuclear-spin conversion mechanisms of them have been explained by the model of quantum relaxation[14]. To seek the possibility of separation of the ortho and para isomers by the LID method and to provide experimental evidence and explanation of mechanisms to the interconversion between these two spin isomers, we performed separation and conversion observations on CH3OH in the gas phase. In contrast to the molecular NSIs studied before, the mixing between ortho and para states involves torsional wavefunctions and the intramolecular torsion is involved in the nuclear spin conversion in CH3OH. In this work we provide experimental evidence for the existence of the weak nuclear-spin-dependent intramolecular hyperfine interactions in gaseous CH3OH
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