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Pseudodielectric functions (ε) = (ε1) + i (ε2) of GaAs were measured by spectroscopic ellipsometry (SE), in the range of 1.W.45 eV, at temperatures from room temperature (RT) to ~610 ° C. A very clean, smooth surface was obtained by first growing an epitaxial layer of GaAs on a GaAs substrate and immediately capping it with a protective layer of arsenic. The cap prevented surface oxidation during transport to the measurement chamber, where it was evaporated under ultrahigh vacuum at ~350 °C. Room-temperature SE results from this surface are in good agreement with those in the literature obtained by wet-chemical etching. A quantitative analysis of the (ε) spectrum was made using the harmonic-oscillator approximation (HOA). It is shown by the HOA that the E1 and E1 + Δ1 energy-band critical points shift downward ~300 meV as temperature increases from RT to ~610 °C. An algorithm was developed, using the measured optical constants at a number of fixed temperatures, to compute the dielectric function spectrum at an arbitrary temperature in the range of 22-610 °C. Therefore, the ellipsometer can be utilized as an optical thermometer to determine the sample surface temperature.