AC microcalorimetry of adsorbates on evaporated metal films: Orientational ordering of H₂ multilayers

Thesis/Dissertation · Fri Nov 01 04:00:00 EST 1991

DOI:https://doi.org/10.2172/10124761· OSTI ID:10124761

Phelps, Robert Bloss ^[1]

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1. Univ. of California, Berkeley, CA (United States)

We have improved and extended a novel ac calorimetric technique for measuring the heat capacity of adsorbates on evaporated metal films. Metallic substrates are of particular interest in current studies of the thermodynamics of adsorbed molecules. The method described in the present work is only calorimetric technique which allows measurements of molecules on simple metallic surfaces. Among other improvements, we have achieved significant progress in the preparation and characterization of the evaporated metal film. We have applied this novel technique to a study of hydrogen multilayers on gold and sapphire substrates. We have shown that samples of normal-hydrogen with a nominal coverage n of approximately 25 monolayers (ML) undergo a bulk-like orientational ordering transition. The transition is suppressed as the coverage is decreased, and no sign of the transition remains above 1.6 K for n ~ 1 ML. For n ≲ 8 ML, the peak in the heat capacity exhibits signs of finite-size effects. At higher coverages, finite-size effects are not observed, and the shape of the peak depends strongly on the substrate. We conclude that the peak is inhomogeneously broadened for n ≲ 8 ML. This work represents the first measurements of the heat capacity due to orientational ordering in adsorbed hydrogen. The results of an earlier experiment involving vibrational spectroscopy of adsorbed molecules are included in the Appendix. In this work, we have used infrared emission spectroscopy to study the spectral region in the vicinity of the C=O stretch vibration of bridge-bonded CO on Pt(111).

• Research Organization: Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 10124761
• Report Number(s): LBL--31576; ON: DE92008317
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360104
360606
ADSORPTION
CALORIMETRY
GOLD
GRAPHITE
HYDROGEN
LAYERS
MEASURING INSTRUMENTS
PHYSICAL PROPERTIES
SAPPHIRE
SPECIFIC HEAT
SPECTROSCOPY
THERMODYNAMICS