Transition surface interband

Pinchuk A, Kreibig U, Hilger A. 2004. Optical properties of metallic nanoparticles Infiuence of interface effects and interband transitions. Surface Sci. 557 269-280. 

Sometimes it is possible to distinguish surface and bulk plasmons by lowering Eq so that the bulk plasmon will decrease in intensity more rapidly than the surface plasmon. However both surface states and interband transitions can show the same behavior. 

The alkali metals, with only one free electron per atom, have lower plasmon energies than those of divalent free-electron metals such as Mg and A1 because the plasma frequency decreases with decreasing electron density. Thus, surface plasmon energies for alkali metals are in or near the visible, whereas they are in the far ultraviolet for Mg, Al, and Pb. Surface plasmon energies of the divalent metals Ag, Au, and Cu are shifted toward and into the visible because of interband transitions (see Fig. 12.9d) this is also the cause of the large values of c" for Au and Cu. 

The threshold of the photocurrent is at the energy of 1 eV. This value is less than the absorption edge enagy, what proves the existence of deep levels inside the forbidden gap. The absence of the structure at the beginning of the interband transitions excludes the possibility that the photocurrent is due to the surface exciton dissociation. Attempts to observe good photoconduction in ris-(CH) were unsuccessful. The photocunent in cis-(CH)n was three orders of magnitude lower than in trans-(CH) . 

One of the most important theoretical contributions of the 1970s was the work of Rudnick and Stern [26] which considered the microscopic sources of second harmonic production at metal surfaces and predicted sensitivity to surface effects. This work was a significant departure from previous theories which only considered quadrupole-type contributions from the rapid variation of the normal component of the electric field at the surface. Rudnick and Stern found that currents produced from the breaking of the inversion symmetry at the cubic metal surface were of equal magnitude and must be considered. Using a free electron model, they calculated the surface and bulk currents for second harmonic generation and introduced two phenomenological parameters, a and b , to describe the effects of the surface details on the perpendicular and parallel surface nonlinear currents. In related theoretical work, Bower [27] extended the early quantum mechanical calculation of Jha [23] to include interband transitions near their resonances as well as the effects of surface states. 

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