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Photothermal desorption spectroscopy with IR lasers
A541 Surface Science 161 (1985) 245-254 North-Holland, Amsterdam PHOTOTHERMAL M. BUCK,
245
DESORPTION
B. S C H A F E R
SPECTROSCOPY
WITH
IR LASERS
a n d P. H E S S
Physikalisch- Chemisches lnstitut, Universit~t Heidelberg, Im Neuenheimer Feld 253, D- 6900 Heidelberg Fed. Rep. of Germany Received 6 March 1985; accepted for publication 20 May 1985
Spectroscopic properties of condensed films of trans-l,2-dichlorocyclohexane were determined by studying the dynamics of transient photothermal desorption and ablation. Maxwellian time-offlight distributions yield the translational temperature and the desorption yield as a function of laser wavelength. The optical absorption coefficient regulates energy deposition i n the condensed phase. The T O F temperature spectra and the desorption yield spectra display the high spectral selectivity of laser-induced desorption in coincidence with the infrared absorption spectrum of the liquid. The mechanism of photodesorption by laser excitation of internal vibrational modes is discussed.
Surface Science 161 (1985) 255-277 North-Holland, Amsterdam OXIDATION
AND
FACETING
255 OF
POLYCRYSTALLINE
TUNGSTEN
RIBBONS J. L E P A G E ,
A. MEZIN
and M. NIVOIT
Laboratoire ERMES, CNRS UA 865, 6 Rue du Joli Coeur, F-54000 Nancy, France Received 31 August 1984; accepted for publication 3 May 1985
We have measured the oxidation rate of tungsten and the evaporation rate of tungsten oxide in the temperature range from 900 to 1200 K at an oxygen pressure from 5 × 10 -4 to 5 × 10 3 Torr. The oxidation rate increases steadily with coverage in the whole range studied. The evaporation rate decreases at high pressure and is strongly dependent on the initial conditions of the experiments. These kinetic measurements support a qualitative model of oxidation. The surface is composed of oxide islands surrounded by oxide-free regions covered only by chemisorbed oxygen atoms. On the bare regions beside the chemisorbed oxygen atoms we suppose the existence of a dilute chemisorbed oxide layer which can either enter the condensed oxide phase or evaporate. The number of the growing islands is set up at the beginning of the reaction and does not increase further. This model, consistent with kinetic results during oxidation, has been proposed first to explain results obtained by Auger electron spectroscopy and thermal desorption spectroscopy under vacuum. Faceting is particularly important in the early stages of the experiment because it can hinder the nucleation of the oxide which is a necessary step for growth. In a narrow range of temperature and oxygen pressure this inhibited nucleation leads to an enhanced evaporation rate so that the growth rate is lower. Recording this growth rate allows us to follow faceting. The parameters studied are the oxygen coverage and the temperature, experimental results are in agreement with LEED and R H E E D results. Reconstruction and faceting are discussed and are believed to be caused by a smoothing of the surface during the chemisorption step.
245
DESORPTION
B. S C H A F E R
SPECTROSCOPY
WITH
IR LASERS
a n d P. H E S S
Physikalisch- Chemisches lnstitut, Universit~t Heidelberg, Im Neuenheimer Feld 253, D- 6900 Heidelberg Fed. Rep. of Germany Received 6 March 1985; accepted for publication 20 May 1985
Spectroscopic properties of condensed films of trans-l,2-dichlorocyclohexane were determined by studying the dynamics of transient photothermal desorption and ablation. Maxwellian time-offlight distributions yield the translational temperature and the desorption yield as a function of laser wavelength. The optical absorption coefficient regulates energy deposition i n the condensed phase. The T O F temperature spectra and the desorption yield spectra display the high spectral selectivity of laser-induced desorption in coincidence with the infrared absorption spectrum of the liquid. The mechanism of photodesorption by laser excitation of internal vibrational modes is discussed.
Surface Science 161 (1985) 255-277 North-Holland, Amsterdam OXIDATION
AND
FACETING
255 OF
POLYCRYSTALLINE
TUNGSTEN
RIBBONS J. L E P A G E ,
A. MEZIN
and M. NIVOIT
Laboratoire ERMES, CNRS UA 865, 6 Rue du Joli Coeur, F-54000 Nancy, France Received 31 August 1984; accepted for publication 3 May 1985
We have measured the oxidation rate of tungsten and the evaporation rate of tungsten oxide in the temperature range from 900 to 1200 K at an oxygen pressure from 5 × 10 -4 to 5 × 10 3 Torr. The oxidation rate increases steadily with coverage in the whole range studied. The evaporation rate decreases at high pressure and is strongly dependent on the initial conditions of the experiments. These kinetic measurements support a qualitative model of oxidation. The surface is composed of oxide islands surrounded by oxide-free regions covered only by chemisorbed oxygen atoms. On the bare regions beside the chemisorbed oxygen atoms we suppose the existence of a dilute chemisorbed oxide layer which can either enter the condensed oxide phase or evaporate. The number of the growing islands is set up at the beginning of the reaction and does not increase further. This model, consistent with kinetic results during oxidation, has been proposed first to explain results obtained by Auger electron spectroscopy and thermal desorption spectroscopy under vacuum. Faceting is particularly important in the early stages of the experiment because it can hinder the nucleation of the oxide which is a necessary step for growth. In a narrow range of temperature and oxygen pressure this inhibited nucleation leads to an enhanced evaporation rate so that the growth rate is lower. Recording this growth rate allows us to follow faceting. The parameters studied are the oxygen coverage and the temperature, experimental results are in agreement with LEED and R H E E D results. Reconstruction and faceting are discussed and are believed to be caused by a smoothing of the surface during the chemisorption step.