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Vacuum ultraviolet spectroscopy of high-pressure helium microbubbles in metals
A95 66
Surface Science 126 (1983) 66-79 North-Holland Publishing Company
VACUUM ULTRAVIOLET SPECTROSCOPY HELIUM MICROBUBBLES IN METALS A.A. LUCAS,
S.E. DONNELLY
OF HIGH-PRESSURE
and J.P. VIGNERON
Institute for Research in Interface Sciences, FNDP, B - 5000 Namur, Belgium
and J.C. RIFE Naval Research Lnboratory, Washington, DC 20375, USA Received
15 September
1982
This paper reviews recent investigations and presents new results on the spectroscopic properties, in the vacuum ultraviolet, of composite materials made of metal containing inert gas bubbles of microscopic size. The basic interest of such systems, from the point of view of Surface Science, lies in the often important role played by the gas metal interface on account of the small size of the bubbles. Three techniques have been used, namely Absorption, Electron Energy Loss and Fluoresence Spectroscopies to study thin film samples. Most of the data presented here concern the helium/aluminium composite containing a few at% of helium. The spectral range explored, between 5 and 30 eV, covers regions where spectra1 features are assignable either to the metal matrix or to the helium gas. Between 5 and 20 eV, EELS of thin He/AI films reveals, in addition to the usual Al bulk plasmons and AI/Al,O, (boundary) plasmons of the metal film, two new loss continua, in the range 8.5 to 12.5 eV. The first band between 12.5 and 10.5 eV is assigned to spherical bubble plasmons. The assignment of a second, similar broad loss peak between 8.5 and 10.5 eV as being due to oxidized bubble plasmons is still tentative. Between 20 and 25 eV, Absorption Spectroscopy allows detection of the helium resonance line strongly broadened and shifted towards higher energies. The detailed line shape information, interpreted with the help of a theoretical model which will be summarized here, gives access to the density and pressure state of the gas in the bubbles. Such information is relevant to the understanding of the mechanical and other properties of gas containing materials of nuclear technological interest. The paper also reports new data, obtained for the first time, by Fluorescence Spectroscopy in the range 13-25 eV. The fluorescence of the He/Al composite is excited by an electron beam of a few keV energy. The spectrum consists of two broad continua, reminiscent of the emission spectra of high pressure helium gas and of low-temperature liquid helium: an intense band between 600 and 1000 A (21 to 13 eV) and a weaker band from 500 to 600 A (25-21 eV). The strong band and the long-wavelength part of the weaker band are assigned to delayed molecular fluorescence from He; excimers while the short-wavelength part of the weak band originates from prompt, non-self-absorbed atomic emission from the blue shifted resonance level series. The quantum efficiency of the electron-excited bubble fluorescence of He/Sri and He/AI specimen has been found adequate to be exploited in the construction of a new, solid state, VUV light source. Finally the paper presents new spectroscopic resuits obtained by VUV absorption from annealing and cooling experiments on thin He/Al composites. In the annealing experiments, the resonance lure spectrum exhibits spectacular evolution traceable to drastic alteration of the size distribution of the bubbles as a result of their thermal growth. In the cooling experiment, we have observed, for the first time. spectral changes which appear to be correlated with a gas-solid phase transition in the bubbles. If confirmed by experiments currently underway, this observation opens the perspective of studying high pressure phenomena in inert gases without elaborate high-pressure equipment.
Surface Science 126 (1983) 66-79 North-Holland Publishing Company
VACUUM ULTRAVIOLET SPECTROSCOPY HELIUM MICROBUBBLES IN METALS A.A. LUCAS,
S.E. DONNELLY
OF HIGH-PRESSURE
and J.P. VIGNERON
Institute for Research in Interface Sciences, FNDP, B - 5000 Namur, Belgium
and J.C. RIFE Naval Research Lnboratory, Washington, DC 20375, USA Received
15 September
1982
This paper reviews recent investigations and presents new results on the spectroscopic properties, in the vacuum ultraviolet, of composite materials made of metal containing inert gas bubbles of microscopic size. The basic interest of such systems, from the point of view of Surface Science, lies in the often important role played by the gas metal interface on account of the small size of the bubbles. Three techniques have been used, namely Absorption, Electron Energy Loss and Fluoresence Spectroscopies to study thin film samples. Most of the data presented here concern the helium/aluminium composite containing a few at% of helium. The spectral range explored, between 5 and 30 eV, covers regions where spectra1 features are assignable either to the metal matrix or to the helium gas. Between 5 and 20 eV, EELS of thin He/AI films reveals, in addition to the usual Al bulk plasmons and AI/Al,O, (boundary) plasmons of the metal film, two new loss continua, in the range 8.5 to 12.5 eV. The first band between 12.5 and 10.5 eV is assigned to spherical bubble plasmons. The assignment of a second, similar broad loss peak between 8.5 and 10.5 eV as being due to oxidized bubble plasmons is still tentative. Between 20 and 25 eV, Absorption Spectroscopy allows detection of the helium resonance line strongly broadened and shifted towards higher energies. The detailed line shape information, interpreted with the help of a theoretical model which will be summarized here, gives access to the density and pressure state of the gas in the bubbles. Such information is relevant to the understanding of the mechanical and other properties of gas containing materials of nuclear technological interest. The paper also reports new data, obtained for the first time, by Fluorescence Spectroscopy in the range 13-25 eV. The fluorescence of the He/Al composite is excited by an electron beam of a few keV energy. The spectrum consists of two broad continua, reminiscent of the emission spectra of high pressure helium gas and of low-temperature liquid helium: an intense band between 600 and 1000 A (21 to 13 eV) and a weaker band from 500 to 600 A (25-21 eV). The strong band and the long-wavelength part of the weaker band are assigned to delayed molecular fluorescence from He; excimers while the short-wavelength part of the weak band originates from prompt, non-self-absorbed atomic emission from the blue shifted resonance level series. The quantum efficiency of the electron-excited bubble fluorescence of He/Sri and He/AI specimen has been found adequate to be exploited in the construction of a new, solid state, VUV light source. Finally the paper presents new spectroscopic resuits obtained by VUV absorption from annealing and cooling experiments on thin He/Al composites. In the annealing experiments, the resonance lure spectrum exhibits spectacular evolution traceable to drastic alteration of the size distribution of the bubbles as a result of their thermal growth. In the cooling experiment, we have observed, for the first time. spectral changes which appear to be correlated with a gas-solid phase transition in the bubbles. If confirmed by experiments currently underway, this observation opens the perspective of studying high pressure phenomena in inert gases without elaborate high-pressure equipment.