Isotopic Mass and Lattice Constant: X-ray Standing Wave Measurements
Alexander Kazimirov,
Jörg Zegenhagen,
*
Manuel Cardona
The molecular volume of crystals depends on their isotopic
masses. This influence originates from the zero-point motion and the
resulting small differences in lattice constants. This effect was
measured with high precision by using an x-ray standing wave. The
standing wave is generated during Bragg reflection and thus is in phase
with the planes of the substrate crystal, which is covered with a
homoepitaxial film that has a different isotopic composition than the
substrate. The positions of the surface planes of the film with respect
to the substrate planes are revealed by the photoelectrons excited by
the maxima of the standing wave. For germanium-76 on natural
germanium(111), a difference in lattice constant of 
1.1 × 10
5 and 2.5 × 105 at 300 and 54 kelvin, respectively, was found. The results are in good agreement with
theoretical predictions.
Max-Planck-Institut für Festkörperforschung
Heisenbergstrasse 1, D-70569 Stuttgart, Germany.
*
To whom correspondence should be addressed. E-mail:
jorg{at}tunux2.mpi-stuttgart.mpg.de
