Spin-polarized Atomic Hydrogen Research Articles

Isolated spin polarized hydrogen atoms as targets for laser-induced polarized electron acceleration

High density Spin Polarized Hydrogen (SPH) atoms, which can be prepared using UV dissociation of hydro-halide molecules, can be attractive as potential targets for laser ionization/acceleration schemes aiming to create high energy and high current polarized electron beams. However, for these SPH targets to be of practical use, they have to be spatially isolated from the halide atoms which accompany hydrogen in the parent hydro-halide molecule. We show how the UV dissociation dynamics of hydro-halides and the dissociation geometry and timing can be combined to prepare a variety of isolated SPH targets aimed to accommodate laser acceleration schemes.

Thermodynamic Properties and Critical Behavior of Spin-Polarized Atomic Hydrogen (H↓) Using the Quantum Second-Virial Coefficient

The quantum–mechanical formulations for the second-virial, and the acoustic virial, coefficients are derived for spin-polarized atomic hydrogen (H↓). The dependence of these coefficients on the temperature T as well as the nuclear polarization $$\zeta \;$$ is analyzed. This is done for the first time. The main inputs in these computations are the scattering phase shifts, which are obtained using the Lippmann–Schwinger equation, with the Silvera triplet-state potential. Starting with these phase shifts, comprehensive calculations of the thermophysical properties for this system are performed for T ranging from 1 µK to 100 K, and $$\zeta \;$$ varying from 0 to 1. These properties include, in addition to the quantum second-virial coefficient and the acoustic virial coefficient: the pressure–polarization–temperature $$\left( {P {-} \zeta {-} T} \right)$$ behavior, the entropy (per atom), the speed of sound, the second-virial correction (to the total internal energy per atom and unit density), and the specific heat capacity (per atom and unit density). The Boyle temperature and the Joule inversion temperature are determined. The T-dependence of these thermophysical properties, and related quantities, is explored. As expected, this dependence is most evident in the low-T limit, where quantum effects predominate. The corresponding $$\zeta$$ -dependence becomes noticeable at 80 K and below. It is observed that the virial coefficients tend to decrease with increasing $$\zeta$$ . Comparison of the present results to previous results are included whenever possible. The overall agreement is very good. As T is lowered, the disruptive effects of thermal energy are weakened relative to the attractive interactions between the atoms. Consequently, the H↓ gas makes a transition to a state of higher order and lower entropy—Bose–Einstein condensation (BEC). This is explored carefully.

Macroscopic production of spin-polarised hydrogen atoms from the IR-excitation and photodissociation of molecular beams

We describe methods for the production of spin-polarised H and D atoms from the IR-excitation and photodissociation of molecular beams of HBr, HI, and NH3 isotopes, including optical excitation schemes with partial hyperfine resolution. We discuss the extent to which the production rates may approach the IR-laser production rates of , and how the production rates of conventional methods of may be surpassed significantly.

Searching for magnetostatic modes in spin-polarized atomic hydrogen

We consider a possibility of the magnetostatic type spin waves driven by a long-range magnetic dipole interactions, to account for the peaks in the ESR spectra observed in our previous work (Lehtonen et al 2018 New J. Phys. 20 055010). The Walker equation for magnetostatic modes is solved for a cylinder of atomic hydrogen, first in a uniform magnetic field and second in a linearly decreasing magnetic field. The frequency behavior of the solved modes with length of the cylinder and density of the gas is compared to experimental data. We conclude that magnetostatic modes are unlikely to account for the observed modulations of ESR spectra.

Transport Properties of Spin-Polarized Atomic Hydrogen Using Generalized Scattering Theory

Our results for the scattering and thermophysical properties of spin-polarized atomic hydrogen \((\hbox {H}{\downarrow })\) have been presented in the temperature range 0.01–10 K using the Galitskii–Migdal–Feynman formalism. These results include the quantum second virial coefficient, the average total and viscosity cross sections, the viscosity, the diffusion coefficient, and the thermal conductivity. The calculations have been undertaken using three triplet-state potentials: Morse-type, Silvera and Born–Oppenheimer potentials. The Morse potential is less attractive and very simple, but less accurate to describe spin-polarized atomic hydrogen. That explains the differences between it and the other two potentials, which are clearly better. From the results of the average total cross sections, it is concluded the \(\hbox {H}{\downarrow }\) remains a gas even at low temperature. The viscosity, the thermal conductivity, and the diffusion coefficients of \(\hbox {H}{\downarrow }\) increase in all cases with increasing temperature.

Rydberg Detection of Spin-Polarized Hydrogen Atoms in Chemical Reactions

An approach to detect the velocity dependence of spin-polarized H atoms has recently been developed. The technique, termed spin-polarized hydrogen Rydberg time-of-flight (SPH-RTOF), permits direct measurement of spin-polarized H atoms born from a photodissociation or scattering event. The resulting H-atom spin polarization can be fully described by three anisotropy parameters, all of which are captured in SPH-RTOF. This review describes the approach in detail, provides a theoretical description that allows the experimentally obtained values to relate to the underlying dissociation dynamics of a molecule, and summarizes the first applications of SPH-RTOF to the UV photodissociation of H(D)Br.

Imaging detection of spin-polarized hydrogen atoms

We demonstrate that a two-color high-n Rydberg excitation of hydrogen atoms using a circularly polarized tagging laser may be combined with pulsed-field ionization to allow for direct velocity map imaging of the H atom spin polarization. In this proof-of-principle study we only detect the incoherent spin polarization for a single velocity component in the 213nm dissociation of HBr, but anticipate that by combining this strategy with slice imaging, the full velocity dependent H atom spin polarization may be obtained in a single experimental geometry.

Spin-polarized hydrogen and its isotopes: A rich class of quantum phases (Review Article)

We review the recent activity in the theoretical description of spin-polarized atomic hydrogen and its isotopes at very low temperatures. Spin-polarized hydrogen is the only system in nature that remains stable in the gas phase even in the zero temperature limit due to its small mass and weak interatomic interaction. Hydrogen and its heavier isotope tritium are bosons, the heavier mass of tritium producing a self-bound (liquid) system at zero temperature. The other isotope, deuterium, is a fermion with nuclear spin one making possible the study of three different quantum systems depending on the population of the three degenerate spin states. From the theoretical point of view, spin-polarized hydrogen is specially appealing because its interatomic potential is very accurately known making possible its precise quantum many-body study. The experimental study of atomic hydrogen has been very difficult due to its high recombination rate, but it finally led to its Bose-Einstein condensate state in 1998. Degeneracy has also been observed in thin films of hydrogen adsorbed on the 4He surface allowing for the possibility of observing the Berezinskii–Kosterlitz–Thouless superfluid transition.

Scattering properties of ground-state spin-polarized atomic hydrogen

In this paper, the scattering properties of ground-state spin-polarized atomic hydrogen (H↓) are studied at 0K using the Lippmann–Schwinger formalism. The total, diffusion and viscosity cross sections, as well as the S-wave scattering length, are calculated. The S-wave scattering cross section is found to be the most significant partial wave contributing to the total cross section at low energy. The contribution of the higher angular momentum waves, especially the D-wave (ℓ=2), to the scattering increases with increasing relative momentum k. Our calculations are performed for three triplet-state potentials: Morse-type, Silvera and Born–Oppenheimer potentials. It is also noted that as k→0, the results of the Morse potential are larger than those of the Silvera and Born–Oppenheimer potentials. This is because of the exponential tail of the Morse potential which falls off more rapidly than the r−6 behavior of the Silvera and Born–Oppenheimer potentials. Also, the Morse potential is relatively shallower than the other two potentials. For high k, the Morse cross sections approach the corresponding Silvera cross sections. This is because these triplet-state potentials have almost the same short-range part. The total cross sections reflect the quantum oscillations arising from the diffraction caused by the repulsive short-range part of the potential. Our results are consistent with those obtained by other methods.

Bose-Einstein condensation and heat capacity of two-dimensional spin-polarized atomic hydrogen

The static fluctuation approximation (SFA) is used to study the condensate fraction and the specific heat capacity of finite two-dimensional spin-polarized atomic hydrogen. It is found that Bose-Einstein condensation occurs in this system. The transition temperature at different densities decreases as the number of particles of the system increases. At low density, a sharp peak in the specific heat capacity is observed at the transition temperature. On the other hand, as the density of the system increases, the transition temperature becomes no longer well-defined, and a hump is observed in the specific heat capacity around the transition temperature. A qualitative comparison of our results to published results for finite Bose systems shows good agreement.

Theoretical Analysis of a Spin-Polarized Atomic Hydrogen Beam Optics

Theoretical Analysis of a Spin-Polarized Atomic Hydrogen Beam Optics

( 2 + 1 ) laser-induced fluorescence of spin-polarized hydrogen atoms

We report the measurement of the spin polarization of hydrogen (SPH) atoms by (2+1) laser-induced fluorescence, produced via the photodissociation of thermal HBr molecules with circularly polarized 193 nm light. This scheme, which involves two-photon laser excitation at 205 nm and fluorescence at 656 nm, offers an experimentally simpler polarization-detection method than the previously reported vacuum ultraviolet detection scheme, allowing the detection of SPH atoms to be performed more straightforwardly, from the photodissociation of a wide range of molecules and from a variety of collision experiments.

Bose–Einstein condensation and heat capacity of spin-polarized atomic hydrogen

The condensation fraction, transition temperature, and heat capacity per particle have been calculated for the spin-polarized atomic hydrogen system using the static fluctuation approximation (SFA). The transition temperature has been found to be greater than that for the noninteracting gas, which is consistent with dilute systems results. The heat capacity has a cusp at the transition temperature similar to liquid helium-4. The results obtained indicate that superfluidity behavior occurs in spin-polarized atomic hydrogen system.

Nonadiabatic coupling in cold collisions of spin-polarized metastable hydrogen atoms

Previous calculations of low-temperature cross sections for collisions between spin-polarized metastable hydrogen atoms are improved to include nonadiabatic radial and angular coupling at large interatomic separations. The electrostatic dipole-quadrupole interaction produces nonadiabatic radial coupling between (2s,2p) and (2p,2p) states, while the Coriolis interaction produces nonadiabatic angular coupling. Both of these long-range contributions are handled in a space-fixed atomic gauge that is particularly convenient for a spin-polarized system. The improved theoretical results are compared with an existing experiment.

Thermal Compression of Atomic Hydrogen on Helium Surface

We describe experiments with spin-polarized atomic hydrogen gas adsorbed on liquid 4He surface. The surface gas density is increased locally by thermal compression up to 5.5 × 1012 cm−2 at 110 mK. This corresponds to the onset of quantum degeneracy with the thermal de-Broglie wavelength being 1.5 times larger than the mean interatomic spacing. The atoms were detected directly with a 129 GHz electron-spin resonance spectrometer probing both the surface and the bulk gas. This and the simultaneous measurement of the recombination power, allowed us to make accurate studies of the adsorption isotherm and the heat removal from the adsorbed hydrogen gas. From the data, we estimate the thermal contact between 2D hydrogen gas and phonons of the helium film. We analyze the limitations of the thermal compression method and the possibility to reach the superfluid transition in 2D hydrogen gas.

A study of spin-polarized atomic hydrogen in the Brueckner–Bethe–Goldstone theory

In this paper the Brueckner–Bethe–Goldstone (BBG) formalism is applied to spin-polarized atomic hydrogen (SPAH) for the first time. To this end, the ground-state energy per atom and the depletion are calculated. The ground-state energy per atom for SPAH is positive definite for all allowed densities. This means that SPAH never forms a many-body bound state. From the depletion results, it is deduced that this system behaves like a weakly interacting Bose gas for relatively low densities. Our calculations are performed for three triplet-state potentials: Morse-type, Silvera, and Born–Oppenheimer. As a bonus, the BBG formalism is also applied to spin-polarized tritium. Unlike SPAH, spin-polarized tritium forms a liquid at zero temperature and pressure, where the binding energy is −0.759 K.

Three-body recombination in two-dimensional atomic hydrogen gas

We study three-body recombination in the gas of spin-polarized atomic hydrogen adsorbed on the surface of superfluid helium at temperatures from $45\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}130\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. The two-dimensional gas is thermally compressed to densities up to $\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ using the ``cold spot'' method which makes the three-body process the dominant decay channel in the system. We measure the loss rate and surface density of atoms directly and independently and observe the former to be proportional to the third power of the latter. The result for the surface three-body recombination rate constant at $4.6\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, ${L}_{s}=2.0(7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$, significantly reduces the discrepancy between the theory and earlier measurements where the surface density was inferred from the adsorption isotherm. We also measured the three-body rate constant on a 0.1% $^{3}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$ film and found ${L}_{s}=1.3(4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$. This larger value is attributed to an increased delocalization of adsorbed hydrogen atoms in the direction normal to the surface.

SPIN-POLARIZED ATOMIC HYDROGEN IN THE STATIC FLUCTUATION APPROXIMATION

In this paper we use the so-called static fluctuation approximation (SFA) to calculate the thermodynamic properties of spin-polarized atomic hydrogen. This approximation is based on the replacement of the square of the local-field operator with its mean value. A closed set of nonlinear integral equations is derived for neutral many-bosonic systems. This set is solved numerically by an iteration method for two triplet-state potentials: a Morse- and Silvera-type potentials. It is found that the mean internal energy per unit volume, the pressure, the entropy per unit volume, and the specific heat per unit volume increase with temperature and decrease with spin polarization in the low-temperature region (<8 mK ); whereas the condensate fraction increases with an applied magnetic field. It is also found that these quantities are nearly independent of spin polarization for high temperatures (>0.1 K ), and that they are independent of the number density up to 10-3Å-3 in the low-temperature region.

Pulsed‐Laser Production and Detection of Spin‐Polarized Hydrogen Atoms

Spin-polarized hydrogen (SPH) atoms have traditionally been produced and detected using complex experimental methods with poor time resolution. Recently, SPH has been produced by pulsed-laser photodissociation of HCl using circularly polarized light. In combination with the proposed detection of SPH via polarized fluorescence, this approach should allow the production and spatially resolved detection of SPH with a higher sensitivity than that currently available, and with a time resolution in the nanosecond regime. This represents an improvement of several orders of magnitude over the existing methods.

Measurement of Br photofragment orientation and alignment from HBr photodissociation: production of highly spin-polarized hydrogen atoms.

The orientation and alignment of the (2)P(3/2) and (2)P(1/2) Br photofragments from the photodissociation of HBr is measured at 193 nm in terms of a(q) ((k))(p) parameters, using slice imaging. The A (1)Pi state is excited almost exclusively, and the measured a(q) ((k))(p) parameters and the spin-orbit branching ratio show that the dissociation proceeds predominantly via nonadiabatic transitions to the a (3)Pi and 1 (3)Sigma(+) states. Conservation of angular momentum shows that the electrons of the nascent H atom cofragments (recoiling parallel to the photolysis polarization) are highly spin polarized: about 100% for the Br((2)P(1/2)) channel, and 86% for the Br((2)P(3/2)) channel. A similar analysis is demonstrated for the photodissociation of HCl.