Abstract
The optical constants n, k and sigma ( omega ) of paramagnetic gadolinium and terbium have been determined from in situ reflection measurements on opaque films prepared in ultra high vacuum ( approximately 10-10 Torr). Optimized reflection ratio measurements at oblique incidence in the photon energy range 1.8 to 3.1 eV give evidence of direct optical transitions at 1.95, 2.15, 2.50, 2.75 and 3.1 eV for gadolinium and at 1.90, 2.20 and 2.45 for terbium. Deductions from electronic band structure calculations for the bulk materials predict optical transitions at 2.0, 2.2, 2.4 to 2.6, 2.8 and 3.1 eV for gadolinium, and at 1.9, 2.2 and 2.5 eV and other energies for terbium.