Observation of Surface Plasmons and Measurement of the Optical Constants for Sodium and Potassium

The optical constants of sodium and potassium from 1.75 to 4.5 eV are measured using a highly sensitive ellipsometric technique developed by Schnatterly and Jasperson. Measurements are made on the vacuum interface of films vacuum deposited at 77 °K. Surface plasmons are readily observed, and their decay with time and temperature gives estimates of the activation energy and lattice-vibrational frequency associated with the annealing properties of the metallic surface. The plasma frequency and electronic "optical" mass (at visible and near-ultraviolet frequencies) are calculated from the real part of the dielectric constant and found to be 3.8 ± 0.1 eV (5.4 ± 0.2 eV) and $(1.08\mathrm{}\pm 0.02)m_e\left[\right(1.00\mathrm{}\pm 0.02\left)m_e\right]$ for potassium (sodium). The energy-loss function for potassium is given, and the room-temperature optical constants measured for sodium and potassium are tabulated for future use.