The optical transmission of the recently discovered switchable yttrium hydride films is determined spectroscopically as a function of hydrogen content. This is done during electrochemical loading of Pd-capped Y film electrodes, thereby continuously changing the hydrogen concentration. The effect of the Pd cap layer on the film transmission is determined from measurements on a series of films with varying Pd layer thickness. The results are in good agreement with transmission measurements of in situ gas phase loaded, uncapped Y films. Both data sets can be consistently described with simple optical decay lengths such as 277.8 nm for YH 3Ϫ␦ and 15.1 nm for Pd at បϭ1.96 eV. The hydrogen concentration dependence of the optical transmission is discussed and compared with previous optical measurements on bulk samples and band-structure calculations.

In recent optical experiments for YH 3 , a large band gap of almost 3 eV has been observed. In contrast, calculations within the local density approximation ͑LDA͒ result in a semimetallic band structure with a band overlap near the Fermi level of more than 1 eV. This unusually large discrepancy between the LDA results and experiment has led to suggestions that strong local correlation effects, similar to those found in transition metal oxides, are important in YH 3 . Here we conclude from parameter-free quasi-particle calculations within the GW approximation that YH 3 is essentially a conventional semiconductor. We argue that the experimental results can be fully understood in band structure terms without invoking strong local correlations. The unusually large error in the band structure made by LDA is traced to its poor description of the electronic structure of the hydrogen atom. Our GW results predict a fundamental band gap of only 1 eV and an optical gap of 2.6 eV, the difference being due to vanishing matrix elements for optical transitions at lower energies. Our prediction of a small fundamental gap could be experimentally confirmed by a combination of photoemission and inverse photoemission experiments.

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