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Europhysics Letters (EPL)

Nanosecond control and high-density production of spin-polarized hydrogen atoms

Dimitris Sofikitis, L. Rubio Lago, Andrew J. Alexander, Theodore Peter Rakitzis

Published Feb 25, 2008• 16 citations
Publisher

Abstract

We demonstrate directly the nanosecond-timescale production of spin-polarized hydrogen (SPH) atoms from photodissociation of thermal HBr molecules, and the spin-state and Doppler-resolved detection using polarized fluorescence at 121.6 nm, without requiring hyperfine resolution. These techniques allow a variety of spin-dependent, nanosecond pump-probe experiments with SPH, which were not previously realizable. The potential of surpassing the SPH densities and production rates of current techniques is discussed.

DOI
10.1209/0295-5075/81/68002

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We demonstrate directly the nanosecond-timescale production of spin-polarized hydrogen (SPH) atoms from photodissociation of thermal HBr molecules, and the spin-state and Doppler-resolved detection using polarized fluorescence at 121.6 nm, without requiring hyperfine resolution. These techniques allow a variety of spin-dependent, nanosecond pump-probe experiments with SPH, which were not previously realizable. The potential of surpassing the SPH densities and production rates of current techniques is discussed.
We describe methods for the production of spin-polarized H and D atoms from the IR-excitation and photodissociation of molecular beams of HBr, HI, and NH3{\rm NH_{3}}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 1021 photons s−1{\rm 10^{21}\, photons\, s^{-1}}1021photonss−1, and how the production rates of conventional methods of ∼1017 s−1{\sim}{\rm 10^{17} \, s^{-1}}∼1017s−1 may be surpassed significantly.
We discuss how nuclear spin can be highly polarized from the transfer of molecular rotational polarization via the hyperfine interaction, and how large atomic electronic polarization can be produced via molecular photodissociation with polarized light. In combination, these two pulsed-laser techniques can be used to produce highly polarized atoms at densities close to the density of the parent molecules. We compare these methods to existing methods for producing polarized atoms and present specific examples.
We have produced highly spin-polarized atomic hydrogen by spin-exchange optical pumping. A tunable ring dye laser is used to polarize rubidium atoms by optical pumping. The cell containing the rubidium vapor is coated with paraffin in order to reduce spin relaxation due to wall collisions. Hydrogen gas is dissociated in an inductive discharge and flows continuously through the cell, in which the hydrogen atoms are polarized by spin-exchange collisions with the polarized rubidium atoms. Atomic-hydrogen polarization as high as 2〈Jz{\mathit{J}}_{\mathit{z}}Jz​〉H{\mathrm{〉}}_{\mathrm{H}}〉H​=0.72(6) has been observed, which is the highest polarization yet produced by this method. However, the rubidium polarization may be limited to this value due to radiation trapping at higher rubidium densities. The spin-relaxation rate of atomic hydrogen on a paraffin-coated cell is also measured and corresponds to about 7600 wall bounces between wall relaxation.
We investigate the excited-state dynamics of hydrogen (H) atoms in flames by using a femtosecond (fs) pump-probe scheme and measuring directional emission signals. An approximately 100-fs pump pulse at 205.1 nm excites H atoms through a two-photon process (n = 3 ←← n = 1), which is followed by detection of the forward emission signals induced by a broadband fs probe pulse near 656 nm. Above a certain threshold, we observe a quadratic dependence of the emission signal on the pump laser energy. Moreover, the linewidth of the forward emission signal varies with the probe delay and the probe laser energy. This behavior can be explained in terms of superradiance. We perform a theoretical analysis and compare the experimental results with the theory, and conclude that, within the conditions of our experiment, the behavior of the atomic system involves atomic coherence, which is produced non-adiabatically and corresponds to a superradiant process. Variations in the duration of the gain time window and lifetime of the excited-state H atoms in flames are explored at different flame conditions (i.e., equivalence ratio and heights above the burner). The fs pump-probe technique demonstrated here can also be extended to characterize the time-resolved population dynamics and the corresponding collisional energy transfer rates for energy levels involved in laser-induced fluorescence detection of H atoms in flames relevant to practical combustion applications.
Polarized atomic beam sources have been in operation for many years to produce either nuclear polarized atomic hydrogen or deuterium beams. In recent experiments, such a source was used to polarize both isotopes independently at the same time. By recombination of the atoms, hydrogen-deuterium molecules with all possible nuclear spin combinations can be created. Those spin isomers are useful for further applications, like precision spectroscopy, as polarized targets for laser-particle acceleration, polarized fuel for fusion reactors, or as an option for future measurements of electric dipole moments.