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| Titel |
The metastable HCl · 6H2O phase – IR spectroscopy, phase transitions and kinetic/thermodynamic properties in the range 170-205 K |
| VerfasserIn |
S. Chiesa, M. J. Rossi |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 13, no. 23 ; Nr. 13, no. 23 (2013-12-09), S.11905-11923 |
| Datensatznummer |
250085863
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| Publikation (Nr.) |
 copernicus.org/acp-13-11905-2013.pdf |
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| Zusammenfassung |
| In this laboratory study a multidiagnostic experimental approach including
Fourier transform infrared (FTIR) absorption of 1 to 2 μm thick
polycrystalline ice films, residual gas mass spectrometry (MS) and total
pressure measurement were employed. Both amorphous HCl–H2O and
crystalline HCl hexahydrate (HCl · 6H2O) have been
investigated. After controlled doping with HCl and evaporation of excess
H2O from the ice film, transmission FTIR of pure
HCl · 6H2O films and use of calibrated mass spectrometry
enabled the measurement of differential (peak) IR cross sections at several
mid-IR frequencies, for example σ = (6.5 ± 1.9)
× 10−19 cm2 molec−1 at 1635 cm−1. Two types of
kinetic experiments on pure HCl · 6H2O have been performed
under SFR conditions: (a) evaporation of pure HCl · 6H2O over
a narrow T range after evaporation of excess H2O, and (b) observation
of the phase transition from crystalline HCl · 6H2O to
amorphous HCl–H2O under H2O-rich conditions at increasing T. The
temperature dependence of the zero-order evaporation flux of HCl in pure
HCl · 6H2O led to
logJev molec cm−2 s−1 = (36.34 ± 3.20) –
(80 810 ± 5800)/2.303 RT with
R = 8.314 JK−1 mol−1, which turned out to be rate-limiting
for evaporation. HCl · 6H2O has a significant intrinsic
kinetic barrier to HCl evaporation of 15.1 kJ mol−1 in excess of the
HCl sublimation enthalpy of 65.8 kJ mol−1 at 200 K but is kinetically
unstable (metastable) at T ≥ 173 K. The atmospheric importance of
HCl · 6H2O is questioned in view of its large nucleation
barrier and its dependence on T and P(HCl) compared to the amorphous
HCl–H2O phase at upper tropospheric–lower stratospheric (UT/LS)
conditions. |
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