SPIN TEMPERATURE OF WATER MOLECULES DESORBED FROM THE SURFACES OF AMORPHOUS SOLID WATER, VAPOR-DEPOSITED AND PRODUCED FROM PHOTOLYSIS OF A CH_4/O_2 SOLID MIXTURE

摘要

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 H_2O molecules from amorphous solid water (ASW) by combining temperature-programmed desorption and resonance-enhanced multiphoton ionization (REMPI). REMPI spectra of desorbed H_2Omolecules were measured at～150 K from vapor-deposited ASWat 8 K. The nuclear-spin temperature of desorbed H_2O molecules exhibits almost an upper limit to the ortho-to-para ratio of close to 3. No discernible change was observed in the H_2O REMPI spectrum, neither after leaving the deposited ASW for 9 days in a vacuum chamber at 8 K, nor in the presence of O_2 molecules, nor upon exposure of ASW to vacuum-ultraviolet (VUV) photons. Desorbed H_2O molecules were also investigated from ASW produced by VUV irradiation of a CH_4/O_2 mixture at 8 K. The resulting H_2O spectrum showed a high nuclear-spin temperature. These results suggest that the nuclear-spin temperatures of gaseous H_2O molecules thermally desorbed from ice do not necessarily reflect the surface temperature at which H_2O molecules condensed or formed under some laboratory conditions. We discuss the possibility of nuclear-spin conversion of H_2O 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.