  We report the sympathetic cooling and Coulomb crystallization of molecular
ions above the surface of an ion-trap chip. N$_2^+$ and CaH$^+$ ions were
confined in a surface-electrode radiofrequency ion trap and cooled by the
interaction with laser-cooled Ca$^{+}$ ions to secular translational
temperatures in the millikelvin range. The configuration of trapping potentials
generated by the surface electrodes enabled the formation of planar bicomponent
Coulomb crystals and the spatial separation of the molecular from the atomic
ions on the chip. The structural and thermal properties of the Coulomb crystals
were characterized using molecular dynamics simulations. The present study
extends chip-based trapping techniques to Coulomb-crystallized molecular ions
with potential applications in mass spectrometry, cold chemistry, quantum
information science and spectroscopy.
