The influence of nuclear spins on the transport properties of molecular gases and application to HD

Abstract

Coupling between the angular momentum J and the nuclear spins of a molecule modifies those contributions to the transport properties of a molecular gas which arise from the macroscopic orientational polarization. This (low pressure) effect is studied in detailfor molecular HD at low temperature (J = 1 states). Because the rotational quantum numbers are low (J = 1), a quantum treatment is essential and moreover be classical motion of J differs significantly from simple precession: the “nutation” of J being also important. This nutation gives rise to significant phase randomization of the angular-momentum polarizations at zero field, and to a field effect on the axial polarizations which are in addition to the usual field effects on nonaxial polarizations.The complicated dependence of the microscopic transport coefficients on the magnetic field and pressure reflects not only the various intramolecular couplings involved, but also the discreteness of the energy-level structure. In fact, it is possible to resolve effects of individual energy off-diagonalities in the density matrix, which persist at energy level crossings. These contribute adsorption and dispersion peaks respectively to the field dependence of the even and odd transport coefficients. The collisionally uncoupled model used in the calculation leads to good agreement both with the experimental transverse thermal-conductivity coefficient, and with the experimental thermomagnetic torque.