SIMBAD references

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 H₂ O molecules from amorphous solid water (ASW) by combining temperature-programmed desorption and resonance-enhanced multiphoton ionization (REMPI). REMPI spectra of desorbed H₂ O molecules were measured at ∼150 K from vapor-deposited ASW at 8 K. The nuclear-spin temperature of desorbed H₂ O molecules exhibits almost an upper limit to the ortho-to-para ratio of close to 3. No discernible change was observed in the H₂ O REMPI spectrum, neither after leaving the deposited ASW for 9 days in a vacuum chamber at 8 K, nor in the presence of O₂molecules, 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 H₂ O molecules thermally desorbed from ice do not necessarily reflect the surface temperature at which H₂ O molecules condensed or formed under some laboratory conditions. We discuss the possibility of nuclear-spin conversion of H₂ O 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.