NUCLEAR SPIN CONVERSION IN POLYATOMIC MOLECULES

Abstract

Except for ortho- and para-H2, little is known about nuclear spin isomers (or spin modifications) of molecules. The main reason is the lack of practical enrichment techniques. Recently, a few enrichment methods were developed, which opened up new possibilities in the field. These methods are briefly reviewed. Substantial progress in the field has been made by the introduction of light-induced drift as a gas-phase separation tool. This is illustrated by extensive data on CH3F, which reveal that the gas-phase ortho-para conversion is governed by intramolecular mixing of the nuclear spin states. The role of direct ortho-para transitions is small. Various aspects of the conversion were investigated in detail: pressure and collision partner dependence, isotope effect, and temperature dependence. The most decisive information on the spin conversion mechanism is derived from the observation of level-crossing resonances in an electric field and the quantum Zeno effect induced by collisions.

Nuclear spin conversion in formaldehyde

“…It can be illustrated by a few examples in which quantum relaxation was established to be a leading process. The most studied case is the spin conversion in 13 CH 3 F (for the review see [12]). …”

“…1. Simple physical picture of spin conversion by quantum relaxation is given elsewhere, e.g., [12]. Quantitative description of the process can be performed in the framework of the density matrix formalism.…”

Separating Para and Ortho Water

“…This ranges from the astronomical importance of the ortho-para ratio [1][2][3][4][5] , to studies of nuclear-spin conversion [6,7] , selection rules and reactive collisions [8][9][10] or symmetry breaking [11] . Spin-enriched samples furthermore would allow for hypersensitized NMR experiments via polarization transfer reactions [12][13][14] .…”

“…Spin-enriched samples furthermore would allow for hypersensitized NMR experiments via polarization transfer reactions [12][13][14] . However, unlike other small polyatomic molecules exhibiting spin isomerism, such as fluoromethane or ethylene [15,16] , which were spin-isomerically enriched using the light induced drift technique [7] , this has not been achieved for water. Separation through selective adsorption on surfaces was reported [17] , but these results remain controversial and could not be reproduced [18][19][20][21] .…”

“…Combined with the strong deflection experienced in the slower beam, this leads to molecules colliding with the skimmers or electrodes and not reaching the interaction region anymore. These densities are sufficient for precision spectroscopy or laboratory scattering experiments [6,7] .…”