 Solution

Potassium is electropositive and transfers electron density toward the metal. The resulting surface dipole generally lowers the metal work function at such submonolayer coverages, reducing the tunnelling barrier and its inverse decay length $q\propto \sqrt{{\Phi }_{\mathrm{effective}}}$.

At the same specified $5\mathring{\mathrm{A}}$ separation, a smaller $q$ gives a larger $\exp (-2qs)$, so the expected tunnelling current is greater than $1\mathrm{nA}$ in the barrier-dominated comparison. No numerical value follows without the modified work function and electronic density of states. A real STM image also depends on the local density of states and adsorbate corrugation; equal geometric separation does not imply equal electronic barrier height.