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arXiv:2606.04242

Spin dynamics and ortho-para conversion in H$_{2}$O during the phase transition from gas to solid in external magnetic fields

Chrysovalantis S. Kannis, Ralf Engels, Nicolas Faatz, Simon J. Pütz, Markus Büscher

physics.chem-phquant-phSubmitted Jun 2, 2026• Announced Jun 4, 2026• Revised Aug 1, 2026 (v2)
v1 · Jun 2, 2026v2 · Aug 1, 2026
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Abstract

The spin dynamics of water ice in the presence of external magnetic fields are investigated. The employed model is based on the approach introduced by Buntkowsky et al. [Z. Phys. Chem. 222, 1049 (2008)], which considers two nearest-neighbor water molecules and yields a four-spin system, as the abundant oxygen isotope has zero nuclear spin. The model is extended to include coupling to external magnetic fields, allowing us to analyze the interplay between magnetic dipole-dipole interactions and magnetic field coupling. Two types of configurations are examined: (i) static, homogeneous fields, corresponding to a time-independent interaction, and (ii) spatially varying sinusoidal fields in relative motion with the molecules, leading to a time-dependent interaction. All computations are performed within the density operator formalism. The ortho/para populations and the total spin projections are evaluated during the first tens of milliseconds following the gas-to-solid phase transition. For static homogeneous fields, we show that increasing field strength suppresses dipolar-induced depolarization. Assuming that all molecules are initially in the para state, we show that static homogeneous fields can drive the ortho population up to approximately $50\%$, whereas suitably chosen sinusoidal-field configurations can increase it beyond $90\%$. These results are relevant for schemes aiming to preserve or manipulate nuclear-spin polarization during deposition.

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Spin dynamics and ortho-para conversion in H2_{2}2​O during the phase transition from gas to solid in external magnetic fields
The spin dynamics of water ice in the presence of external magnetic fields are investigated. The employed model is based on the approach introduced by Buntkowsky et al. [Z. Phys. Chem. 222, 1049 (2008)], which considers two nearest-neighbor water molecules and yields a four-spin system, as the abundant oxygen isotope has zero nuclear spin. The model is extended to include coupling to external magnetic fields, allowing us to analyze the interplay between magnetic dipole-dipole interactions and magnetic field coupling. Two types of configurations are examined: (i) static, homogeneous fields, corresponding to a time-independent interaction, and (ii) spatially varying sinusoidal fields in relative motion with the molecules, leading to a time-dependent interaction. All computations are performed within the density operator formalism. The ortho/para populations and the total spin projections are evaluated during the first tens of milliseconds following the gas-to-solid phase transition. For static homogeneous fields, we show that increasing field strength suppresses dipolar-induced depolarization. Assuming that all molecules are initially in the para state, we show that static homogeneous fields can drive the ortho population up to approximately 50%50\%50%, whereas suitably chosen sinusoidal-field configurations can increase it beyond 90%90\%90%. These results are relevant for schemes aiming to preserve or manipulate nuclear-spin polarization during deposition.
Magnetic properties of two-dimensional (2D) spin-polarized Fermi gas with dipole-dipole interaction are studied in the presence of external magnetic field at zero temperature. Within perturbation theory and the second quantization formalism, the total energy is explicitly obtained as a function of three dimensionless parameters: the spin polarization, dipolar coupling, and Zeeman parameters. We examine the effects of these agents on the magnetic properties of 2D Fermi gas. The results show that an induced ferromagnetic phase transition is observed only for adequately large values of magnetic field. This paper offers two controllable factors to change the spin polarization of the system.
We evaluate a population of ortho nuclear spin states in H2 gas in the 22-pole linear radio-frequency trap using N + ions as a chemical probe. The rate coecient of the reaction N + + H2 ! NH + + H strongly depends on the nuclear spin states of the H2 molecules at the temperature 11 K. To achieve astrophysically relevant conditions in other our experiments, we convert the normal H2 gas (25% of the molecules in para-states) to H2 gas with a distribution of the nuclear spin states corresponding to the temperatures in prestellar cores (> 99 % in para-states at temperatures 20 K) by a catalytic converter (para-hydrogen generator). Tests by the aforementioned probing reaction show that the abundance of ortho-H2 in such H2 gas is (0:5 0:2)% in the trap. Para-to-ortho back-conversion of H2 gas in the generator's outlet tube was studied and its rate was estimated to lie in the interval (10 4 ; 10 2 )s 1 .
We study the polarization dynamics of the molecular rotation and the nuclear spin in the presence of magnetic fields, in the hydrogen isotope molecules, H2,D2{\mathrm{H}}_{2},\phantom{\rule{0.16em}{0ex}}{\mathrm{D}}_{2}H2​,D2​, and HD. If only the rotational or the nuclear angular momentum is polarized, polarization is transferred from one to the other and back in a oscillatory fashion, due to hyperfine quantum beats. Application of magnetic fields is shown to increase the frequencies of these oscillations and reduce their amplitude so that the polarization exchange can be strongly suppressed. We derive the time-averaged polarization as a function of the (static) magnetic field. Finally, we show, using continuous lasers, molecular beams, and static magnetic fields, that the molecular rotation polarization can be transferred and maintained in the nuclear spin, providing highly nuclear-spin-polarized molecules.
We study the static and the dynamic response of coherently coupled two component Bose–Einstein condensates due to a spin-dipole perturbation. The static dipole susceptibility is determined and is shown to be a key quantity to identify the second order ferromagnetic transition occurring at large inter-species interaction. The dynamics, which is obtained by quenching the spin-dipole perturbation, is very much affected by the system being paramagnetic or ferromagnetic and by the correlation between the motional and the internal degrees of freedom. In the paramagnetic phase the gas exhibits well defined out-of-phase dipole oscillations, whose frequency can be related to the susceptibility of the system using a sum rule approach. In particular in the interaction SU (2) symmetric case, i.e., all the two-body interactions are the same, the external dipole oscillation coincides with the internal Rabi flipping frequency. In the ferromagnetic case, where linear response theory in not applicable, the system shows highly non-linear dynamics. In particular we observe phenomena related to ground state selection: the gas, initially trapped in a domain wall configuration, reaches a final state corresponding to the magnetic ground state plus small density ripples. Interestingly, the time during which the gas is unable to escape from its initial configuration is found to be proportional to the square root of the wall surface tension.
We investigate the fine and spin structure of ultralong-range Rydberg molecules exposed to a homogeneous magnetic field. Each molecule consists of a 87Rb^{87}\mathrm{Rb}87Rb Rydberg atom the outer electron of which interacts via spin-dependent sss- and ppp-wave scattering with a polarizable 87Rb^{87}\mathrm{Rb}87Rb ground-state atom. Our model includes also the hyperfine structure of the ground-state atom as well as spin-orbit couplings of the Rydberg and ground-state atom. We focus on ddd-Rydberg states and principal quantum numbers nnn in the vicinity of 40. The electronic structure and vibrational states are determined in the framework of the Born-Oppenheimer approximation for varying field strengths ranging from a few up to hundred Gauss. The results show that the interplay between the scattering interactions and the spin couplings gives rise to a large variety of molecular states in different spin configurations as well as in different spatial arrangements that can be tuned by the magnetic field. This includes relatively regularly shaped energy surfaces in a regime where the Zeeman splitting is large compared to the scattering interaction but small compared to the Rydberg fine structure, as well as more complex structures for both weaker and stronger fields. We quantify the impact of spin couplings by comparing the extended theory to a spin-independent model.