REFLECTANCE AND PHOTOEMISSION FROM Ag IN LIQUID NH3

Abstract

We have made simultaneous observations of reflectance and photoyield from Ag films in liquid NH3. Photoyield is at the angle for surface plasmon excitation. The yield varies linearly with laser intensity. The results can be interpreted in terms of the classical Fresnel equations. The effective dielectric constant for NHg is 1.95 instead of 1.75. Photoinjection thresholds in insulating fluids, such as CH3 or Ar, provide a measure of the location of the conduction band in the material and of photoelectron mobilities 11-31. There are two effects when a polar fluid is placed adjacent to a photocathode /4,5/. First, a potential shifts all electronic levels in the fluid relative to those in the electrode 151. The second effect is that the photo-threshold is reduced, as electrons are injected into electronic states of the fluid /I/ rather than the vacuum. Bennett, et al. 161 have documented these effects for Ag electrodes in liquid NH3. Disagreement exists as to the photoyield dependence on cell potential, and as to any effect of the dipole field on measurement of the position of the conduction band 14-12]. Watanabe and Gerischer /lo/ attributed the tail on the photothreshold curve to oxide on their Au electrode. In an oxide, charge is pulled from the metal to the oxygen In contrast, with Cs, charge is shifted from the atom toward the metal. Cs is well known to lower thresholds; therefore, one expects oxides to raise the threshold. Mee and coworkers 1141 have demonstrated an increase in threshold on Ag with the addition of 0 2 and S2. We prefer a tail on the density of states. The position of the surface plasmon polariton, SPP, resonance, es, is shifted by the dielectric constant of a semi-infinite fluid, such as NH3, placed in contact with the Ag: n2sin 2 e ~ = [&I (Ag) &I (NH3)M ~1 &I (NH3)I (i) where ~1 (Ag) and &1 (NH3) are the real parts of the Ag and NH3 dielectric constants, respectively, and n is the prism index of refraction 1141. The critical angle also depends on &I (NH3) and n. Measurements 1151 on Ag films in H20 show the electrode potential changes the SPP resonance energy, as well as the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1991508 C5-72 JOURNAL DE PHYSIQUE IV dispersion curve near the potential of zero charge. Potential extracts charge from the electrode and causes a shift in the metal surface dielectric constant. We have measured photocurrents in liquid NH3 in the attenuated total reflection, ATR, configuration. In contrast to the gas case 1161, we find the photoyield to depend only on the first power of the laser intensity. We have looked at the trends of photoyield with bias /6-8,101. Separately, we have also examined the dic?lectric constants derived from the reflectance data and inferred from them the nature of the interface between the metal and fluid 15,151 The photoyield may be calculated from the measured dielectric constants.