SPIN TEMPERATURE OF WATER MOLECULES DESORBED FROM THE SURFACES OF AMORPHOUS SOLID WATER, VAPOR-DEPOSITED AND PRODUCED FROM PHOTOLYSIS OF A CH{sub 4}/O{sub 2} SOLID MIXTURE

Journal Article · Thu Sep 01 00:00:00 EDT 2011 · Astrophysical Journal Letters

DOI:https://doi.org/10.1088/2041-8205/738/1/L15· OSTI ID:21562442

Hama, T; Watanabe, N; Kouchi, A ^[1]; Yokoyama, M ^[2]

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1. Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido 060-0819 (Japan)
2. Department of Molecular Engineering, Kyoto University, Kyoto 615-8510 (Japan)

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{sub 2}O molecules from amorphous solid water (ASW) by combining temperature-programmed desorption and resonance-enhanced multiphoton ionization (REMPI). REMPI spectra of desorbed H{sub 2}O molecules were measured at {approx}150 K from vapor-deposited ASW at 8 K. The nuclear-spin temperature of desorbed H{sub 2}O molecules exhibits almost an upper limit to the ortho-to-para ratio of close to 3. No discernible change was observed in the H{sub 2}O REMPI spectrum, neither after leaving the deposited ASW for 9 days in a vacuum chamber at 8 K, nor in the presence of O{sub 2} molecules, nor upon exposure of ASW to vacuum-ultraviolet (VUV) photons. Desorbed H{sub 2}O molecules were also investigated from ASW produced by VUV irradiation of a CH{sub 4}/O{sub 2} mixture at 8 K. The resulting H{sub 2}O spectrum showed a high nuclear-spin temperature. These results suggest that the nuclear-spin temperatures of gaseous H{sub 2}O molecules thermally desorbed from ice do not necessarily reflect the surface temperature at which H{sub 2}O molecules condensed or formed under some laboratory conditions. We discuss the possibility of nuclear-spin conversion of H{sub 2}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.

• OSTI ID: 21562442
• Journal Information: Astrophysical Journal Letters, Vol. 738, Issue 1; Other Information: DOI: 10.1088/2041-8205/738/1/L15; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

74 ATOMIC AND MOLECULAR PHYSICS
DESORPTION
ICE
INTERSTELLAR SPACE
METHANE
MIXTURES
MOLECULES
OXYGEN
PHOTOIONIZATION
PHOTOLYSIS
SPIN
SURFACES
VAPOR DEPOSITED COATINGS
WATER
ALKANES
ANGULAR MOMENTUM
CHEMICAL REACTIONS
COATINGS
DECOMPOSITION
DISPERSIONS
ELEMENTS
HYDROCARBONS
HYDROGEN COMPOUNDS
IONIZATION
NONMETALS
ORGANIC COMPOUNDS
OXYGEN COMPOUNDS
PARTICLE PROPERTIES
PHOTOCHEMICAL REACTIONS
SORPTION
SPACE