SPIN TEMPERATURE OF WATER MOLECULES DESORBED FROM THE SURFACES OF AMORPHOUS SOLID WATER, VAPOR-DEPOSITED AND PRODUCED FROM PHOTOLYSIS OF A CH4/O2 SOLID MIXTURE

The nuclear-spin temperature of molecules observed in interstellar space or cometary coma is an important key to understanding physical and chemical histories of the molecules. The present Letter reports measurements of nuclear-spin states and rotational temperatures of thermally desorbed H2O molecules from amorphous solid water (ASW) by combining temperature-programmed desorption and resonance-enhanced multiphoton ionization (REMPI). REMPI spectra of desorbed H2O molecules were measured at ∼150 K from vapor-deposited ASW at 8 K. The nuclear-spin temperature of desorbed H2O molecules exhibits almost an upper limit to the ortho-to-para ratio of close to 3. No discernible change was observed in the H2O REMPI spectrum, neither after leaving the deposited ASW for 9 days in a vacuum chamber at 8 K, nor in the presence of O2 molecules, nor upon exposure of ASW to vacuum-ultraviolet (VUV) photons. Desorbed H2O molecules were also investigated from ASW produced by VUV irradiation of a CH4/O2 mixture at 8 K. The resulting H2O spectrum showed a high nuclear-spin temperature. These results suggest that the nuclear-spin temperatures of gaseous H2O molecules thermally desorbed from ice do not necessarily reflect the surface temperature at which H2O molecules condensed or formed under some laboratory conditions. We discuss the possibility of nuclear-spin conversion of H2O in water ice. The present study advocates the importance of further studies to give an interpretation of nuclear-spin temperatures of molecules observed in interstellar space or cometary coma.