Molecules made of identical nuclei of non-zero spin exist in nuclear-spin modifications, and the interconversion of these spin isomers is often forbidden for isolated states1, 2, 3. The interconversion between the nuclear-spin modifications, however, is promoted by inhomogeneous magnetic fields, such as those present on the surfaces of magnetic materials4. Nuclear-spin conversion on diamagnetic and insulating solid substances, on the other hand, is generally considered improbable. Here we present the observation of nuclear-spin flips of H2 and D2 occurring on amorphous solid water surfaces with time constants of 370−140+340 s and 1,220−580+2,980 s, respectively. To explain these unexpected conversion processes, we propose a model of electric-field-induced nuclear-spin flips. In this model, giant and inhomogeneous electric fields present on the ice surface5 mix the electronic states of opposite parities by the Stark effect6, and significantly enhance the spin–orbit couplings between the electronic singlet–triplet spin states of the molecules. By virtue of these effects, the intramolecular hyperfine contact interaction induces the nuclear-spin conversion. This concept should have implications for controlling nuclear magnetization using external electric fields7.
At a glance
- 1935). Orthohydrogen, Parahydrogen and Heavy Hydrogen (Cambridge Univ. Press,
- Ortho–para transition in molecular hydrogen. Phys. Rev. A 77, 030501 (2008). &
- Nuclear spin conversion in molecules. Science 310, 1913–1914 (2005). &
- Ortho–para conversion of hydrogen molecules physisorbed on surfaces. Prog. Surf. Sci. 41, 217–335 (1992).
- Molecular atmospheric pollutant adsorption on ice: A theoretical survey. Surf. Sci. Rep. 44, 159–238 (2001). &
- 1950). Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules (Krieger,
- Multiferroic and magnetoelectric materials. Nature 442, 759–765 (2006). , &
- 1961). Principles of Nuclear Magnetism (Oxford Science,
- Theory of ortho–para conversion in hydrogen adsorbed on metal and paramagnetic surfaces at low temperatures. Phys. Rev. B 39, 3104–3114 (1989).
- Fast ortho–para conversion of H2 adsorbed at copper surface step atoms. Phys. Rev. Lett. 98, 096105 (2007). &
- Photostimulated desorption and ortho–para conversion of H2 on Ag surfaces. Phys. Rev. Lett. 90, 096103 (2003). et al.
- Effects of coadsorbed O2 on hydrogen ortho–para conversion on Ag surfaces. Phys. Rev. B 79, 085408 (2009). , , , &
- Interstellar ice: The Infrared Space Observatory legacy. Astrophys. J. Suppl. Ser. 151, 35–73 (2004). , , &
- Molecular hydrogen. Space Sci. Rev. 119, 71–91 (2005). et al.
- Spitzer observations of HH 54 and HH 7–11: Mapping the H2 ortho-to-para ratio in shocked molecular gas. Astrophys. J. 649, 816–835 (2006). et al.
- Rotational and vibrational effects in the E1Σg+−X1Σg+ two-photon transitions of H2, HD and D2. J. Chem. Phys. 95, 205–213 (1991). , &
- Measurement of the adsorption energy difference between ortho- and para-D2 on an amorphous ice surface. Phys. Rev. Lett. 100, 056101 (2008). et al.
- Experimental and simulated vibrational spectra of H2 adsorbed in amorphous ice: Surface structures, energetics, and relaxations. J. Chem. Phys. 97, 753–767 (1992). , , , &
- Ab initio calculation of electronic transition moments for singlet excited states of the H2 molecule. J. Mol. Spectrosc. 217, 162–172 (2003).
- Transition moments among 3Σ and 3Π states of the H2 molecule. J. Mol. Spectrosc. 198, 416–420 (1999). &
- Laser tunnel ionization from multiple orbitals in HCl. Science 325, 1364–1367 (2009). et al.
- 2004). & The Spectra and Dynamics of Diatomic Molecules (Academic,
- Natural orbital of hydrogen-molecule excited states. J. Chem. Phys. 45, 2560–2576 (1966). &
- Adiabatic potentials of lowest 1Πg and 1,3Δg states of the hydrogen molecule. J. Mol. Spectrosc. 169, 329–340 (1995).
- Three lowest 3Πg and three lowest 3Πu states of the hydrogen molecule. Comput. Methods Sci. Tech. 5, 39–52 (1999). &
- Theory of the low-temperature chromatographic separation of the hydrogen isotopes. J. Chem. Phys. 44, 1007–1014 (1966). &
- Control of amorphous solid water morphology using molecular beams. 1. Experimental results. J. Chem. Phys. 114, 5284–5294 (2001). , , , &
- Control of amorphous solid water morphology using molecular beams. 2. Ballistic deposition simulation. J. Chem. Phys. 114, 5295–5303 (2001). , , , &
- The adsorption of H2O on clean and oxygen-dosed silver single crystal surfaces. Surf. Sci. 136, L42–L50 (1984). &
- Experimental evidence for water formation via ozone hydrogenation on dust grains at 10 K. Astrophys. J. 705, L195–L198 (2009). et al.
- Supplementary Information (500k)