Production of Spin-Polarized Molecular Beams via Microwave or Infrared Rotational Excitation

Abstract

We propose schemes to produce highly nuclear-spin polarized small molecules in an intense and cold molecular beam via microwave or infrared rotational excitation, followed by hyperfine-induced quantum beats. Repumping schemes can be used to achieve polarization above $90\%$ in cases where single-pumping schemes are insufficient. We discuss the possibility of high production rates which allow applications including nuclear-magnetic-resonance signal enhancement, and spin-polarized nuclear fusion, where polarized nuclei are known to enhance D-T and D-$^3$He fusion cross sections by $50\%$.

Figures (8)

• Figure 1: Sketch of the experimental setup. A cool molecular beam is excited to the state $\left\vert \varv \, J \, m_{J} \right\rangle$---i.e., rotationally polarized---via microwave or IR radiation. This polarization is subsequently transferred to the nuclear spin. Additional excitation and polarization-transfer steps may be required. Finally, the polarized beam is collected on a cold surface.
• Figure 2: Excitation-polarization scheme for HD or DT via a $J=2$ state. (a) Initial excitation from the lowest rovibrational state to $\left\vert \varv^{\prime}\, 2\, 2 \right\rangle$ at $t=0$. (b) Redistribution of population among all $m_{J}$ states until $t=t_{1}$, when the $m_{J}=-1$ population reaches a local minimum. (c) At $t=t_{1}$, the $m_{J}=0,1,2$ states are selectively repumped to the ground state, preserving the developed nuclear polarization.
• Figure 3: Time evolution of expectation values of the rotational angular momentum projection $\langle m_{J}\rangle$ and nuclear spin projections $\langle m_{d}\rangle$ and $\langle m_{p/t}\rangle$ in panels (a) and (c), and of population loss in panels (b) and (d), over six polarization cycles in HD and DT. The plots correspond to the $\left\vert \varv^{\prime} =2 \, J=2\right\rangle$ excited state, and therefore the slight reduction in nuclear polarization due to the repumping process at the end of each cycle is not depicted.
• Figure 4: Excitation-polarization scheme for the $^{2}\Pi_{1/2}$ state of NO: (a) Transitions with $\Delta m_{J} = 1$: $\left\vert J = 1/2 \, m_{J} = - 1/2 \right\rangle \rightarrow\left\vert 3/2 \, 1/2 \right\rangle$ and $\left\vert 1/2 \, 1/2 \right\rangle\rightarrow\left\vert 3/2 \, 3/2 \right\rangle$. (b) Transition with $\Delta m_{J} = 0$: $\left\vert 3/2 \, 1/2 \right\rangle\rightarrow\left\vert 1/2 \, 1/2 \right\rangle$.
• Figure 5: Time evolution of the average projections $\langle m_{J}\rangle$ (green), $\langle m_{S}\rangle$ (red), and $\langle m_{I}\rangle$ (blue) for the molecules: (a) 14NO and (b) 15NO.