Ortho-para separations of ${}^{7}{\mathrm{Li}}_{2}$ and ${}^{6}{\mathrm{Li}}_{2}$ molecules were achieved by using a monochromatic laser beam to align atoms into a single nuclear spin state. This approach is unlike the cryogenic cooling technique, which has been successful only on very light molecules (such as ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ ), which have large energy gaps between the lowest rotational states. The atoms were ``pumped into'' a single hyperfine Zeeman state with defined electron and nuclear spin orientations. The well-resolved rotational structures of the lithium molecules, as shown by laser-induced fluorescence spectra, revealed the enrichment of ortho-components over para-components. Using laser radiation with a power density of $\ensuremath{\sim}10{\mathrm{m}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ can increase the amount of ortho-components by a factor of 2 for ${}^{7}{\mathrm{Li}}_{2}$ vapor and by a factor of 3 for ${}^{6}{\mathrm{Li}}_{2}$ vapor on a time scale of milliseconds. This paper proposes a model to account for the experimental observations. These increases in the concentrations of ortho-components to para-components are explained by a combined effect of (i) a decrease in molecular density due to the electron spin orientation and (ii) a transfer of nuclear spin momenta from oriented atoms to molecules. The exchange reaction, ${\mathrm{Li}}^{\ensuremath{'}}{+\mathrm{L}\mathrm{i}}_{2}{\ensuremath{\rightarrow}\mathrm{Li}}_{3}{\ensuremath{\rightarrow}\mathrm{Li}}^{\ensuremath{'}}\mathrm{L}\mathrm{i}+\mathrm{L}\mathrm{i},$ appears to transfer the spin orientation from atoms to molecules. A substantial number of ortho-para separations can be achieved through multiple exchange collisions. Applications of this technique to other homonuclear diatomic molecules and polyatomic molecules with nuclear exchange symmetry are predicted.Ortho-para separations of ${}^{7}{\mathrm{Li}}_{2}$ and ${}^{6}{\mathrm{Li}}_{2}$ molecules were achieved by using a monochromatic laser beam to align atoms into a single nuclear spin state. This approach is unlike the cryogenic cooling technique, which has been successful only on very light molecules (such as ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$), which have large energy gaps between the lowest rotational states. The atoms were ``pumped into'' a single hyperfine Zeeman state with defined electron and nuclear spin orientations. The well-resolved rotational structures of the lithium molecules, as shown by laser-induced fluorescence spectra, revealed the enrichment of ortho-components over para-components. Using laser radiation with a power density of $\ensuremath{\sim}10{\mathrm{m}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ can increase the amount of ortho-components by a factor of 2 for ${}^{7}{\mathrm{Li}}_{2}$ vapor and by a factor of 3 for ${}^{6}{\mathrm{Li}}_{2}$ vapor on a time scale of milliseconds. This paper proposes a model to account for the experimental observations. These increases in the concentrations of ortho-components to para-components are explained by a combined effect of (i) a decrease in molecular density due to the electron spin orientation and (ii) a transfer of nuclear spin momenta from oriented atoms to molecules. The exchange reaction, ${\mathrm{Li}}^{\ensuremath{'}}{+\mathrm{L}\mathrm{i}}_{2}{\ensuremath{\rightarrow}\mathrm{Li}}_{3}{\ensuremath{\rightarrow}\mathrm{Li}}^{\ensuremath{'}}\mathrm{L}\mathrm{i}+\mathrm{L}\mathrm{i},$ appears to transfer the spin orientation from atoms to molecules. A substantial number of ortho-para separations can be achieved through multiple exchange collisions. Applications of this technique to other homonuclear diatomic molecules and polyatomic molecules with nuclear exchange symmetry are predicted.

Physical Review AMay 1, 19982 citations

Binding energies for clusters of atomic-triplet hydrogen

Richard David Etters

Physics Letters AJan 1, 197316 citations

The Stabilization of Atomic Hydrogen

Isaac F. Silvera, Jook T. M. Walraven

Scientific AmericanJan 1, 198214 citations

Theory of Small Para-Hydrogen Clusters: Magic Numbers and Superfluid Sizes

Saad A. Khairallah, Mikhail Borisovich Sevryuk, David M. Ceperley +1 more

The interplay between magic number stabilities and superfluidity of small para-hydrogen clusters with sizes $N = 5$ to 40 and temperatures $0.5 K \leq T \leq 4.5 $K is explored with classical and quantum Path Integral Monte Carlo calculations. Clusters with $N < 26$ and T $\leq 1.5 K$ have large superfluid fractions even at the stable magic numbers 13, 19, and 23. In larger clusters, superfluidity is quenched especially at the magic numbers 23, 26, 29, 32, and 37 while below 1 K, superfluidity is recovered for the pairs $(27,28)$ , $(30,31)$ , and $(35,36)$ . For all clusters superfluidity is localized at the surface and correlates with long exchange cycles involving loosely bound surface molecules.The interplay between magic number stabilities and superfluidity of small para-hydrogen clusters with sizes $N = 5$ to 40 and temperatures $0.5 K \leq T \leq 4.5 $K is explored with classical and quantum Path Integral Monte Carlo calculations. Clusters with $N < 26$ and T $\leq 1.5 K$ have large superfluid fractions even at the stable magic numbers 13, 19, and 23. In larger clusters, superfluidity is quenched especially at the magic numbers 23, 26, 29, 32, and 37 while below 1 K, superfluidity is recovered for the pairs $(27,28)$, $(30,31)$, and $(35,36)$. For all clusters superfluidity is localized at the surface and correlates with long exchange cycles involving loosely bound surface molecules.

arXiv:physics/0612161Bulletin of the American Physical SocietyDec 16, 20060 citations

Ortho-para separations of

{}^{7}{\mathrm{Li}}_{2}

and

{}^{6}{\mathrm{Li}}_{2}

molecules were achieved by using a monochromatic laser beam to align atoms into a single nuclear spin state. This approach is unlike the cryogenic cooling technique, which has been successful only on very light molecules (such as

{\mathrm{H}}_{2}

and

{\mathrm{D}}_{2}

), which have large energy gaps between the lowest rotational states. The atoms were ``pumped into'' a single hyperfine Zeeman state with defined electron and nuclear spin orientations. The well-resolved rotational structures of the lithium molecules, as shown by laser-induced fluorescence spectra, revealed the enrichment of ortho-components over para-components. Using laser radiation with a power density of $\ensuremath{\sim}10{\mathrm{m}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ can increase the amount of ortho-components by a factor of 2 for

{}^{7}{\mathrm{Li}}_{2}

vapor and by a factor of 3 for

{}^{6}{\mathrm{Li}}_{2}

vapor on a time scale of milliseconds. This paper proposes a model to account for the experimental observations. These increases in the concentrations of ortho-components to para-components are explained by a combined effect of (i) a decrease in molecular density due to the electron spin orientation and (ii) a transfer of nuclear spin momenta from oriented atoms to molecules. The exchange reaction, ${\mathrm{Li}}^{\ensuremath{'}}{+\mathrm{L}\mathrm{i}}_{2}{\ensuremath{\rightarrow}\mathrm{Li}}_{3}{\ensuremath{\rightarrow}\mathrm{Li}}^{\ensuremath{'}}\mathrm{L}\mathrm{i}+\mathrm{L}\mathrm{i},$ appears to transfer the spin orientation from atoms to molecules. A substantial number of ortho-para separations can be achieved through multiple exchange collisions. Applications of this technique to other homonuclear diatomic molecules and polyatomic molecules with nuclear exchange symmetry are predicted.

The interplay between magic number stabilities and superfluidity of small para-hydrogen clusters with sizes

N = 5

to 40 and temperatures $0.5 K \leq T \leq 4.5 $K is explored with classical and quantum Path Integral Monte Carlo calculations. Clusters with

N < 26

and T

\leq 1.5 K

have large superfluid fractions even at the stable magic numbers 13, 19, and 23. In larger clusters, superfluidity is quenched especially at the magic numbers 23, 26, 29, 32, and 37 while below 1 K, superfluidity is recovered for the pairs

(27,28)

(30,31)

, and

(35,36)

. For all clusters superfluidity is localized at the surface and correlates with long exchange cycles involving loosely bound surface molecules.