Spectroscopy of the Wigner crystal on helium and color center spectroscopy*

Electrons on the helium surface form a strongly correlated nondegenerate liquid or a Wigner crystal. In the both phases they  display sharp resonant absorption lines related to the transitions between the subbands of quantized motion transverse to the helium surface. A magnetic field parallel to the surface strongly affects the absorption spectrum. We show that the effect results from admixing the intersubband transitions to the in-plane quantum dynamics of the many-electron system. Our many-electron theory of the interband absorption spectra and the experimental observations are in full quantitative agreement, with no adjustable parameters. The results refer to a broad range of the electron densities, temperature, and the coupling strength of the in-plane and out-of-plane motions. The in-plane motion was quantized by the magnetic field normal to the surface, which is advantageous for revealing nontrivial aspects of the many-electron dynamics. The admixture of the in-plane excitations to the electron transition is similar to the admixing of phonons to electron transitions in color centers. The system provides a unique setting where both the effective coupling strength and the spectrum of elementary excitations coupled to the electron transition can be varied in situ by varying the in-plane and out-of-plane magnetic fields.