Two image-potential states

Figure 3.2.4.5 Two-photon-photoemission signal for the lowest two image-potential states on Cu(OOl) as a function of pump-probe delay.

The image-potential states on (001) surface of copper have been studied by several groups in considerable detail. The inelastic decay rates for the image-potential states on Cu(OOl) are given in the first two rows of Table 6.5. The agreement between experiment [84, 120] and calculations [73, 82, 121] is almost perfect. This holds for the various scattering rates F at ky = 0 as weU as for the increase with energy denoted by the slope dF /dE. 

The surface electronic states are typically probed with PES and I-PES (photoemission measures occupied states inverse photoemission, unoccupied states) described in Section 7.3.1, or for more accuracy with two-photon photoemission (2PPE), which is particularly suitable for analyzing the image potential states. In the 2PPE technique, a first laser hght pulse (pump) excites an electron from its initial state below the Fermi level Ep into an unoccupied intermediate state (of Shockley or image potential type). A second pulse (probe) lifts the electron to the final state above the vacuum level E c. so it can be detected as a photoelectron. One of the possible data acquisition modes is to record energy-resolved spectra at a particular delay between pump and probe pulses [51]. 

Figure 3.2.4.7 Time-resolved spectra for the n = 1 image-potential state on Cu(OOl) for two different excitation photon energies The dashed line indicates the cross-correlation curve (Adapted from Ref [27]).

Figure 3.2.4.S Two-photon photoemission-spectra of image-potential states from Cu(OOl) for various pump probe...

Marchf et al. have applied image potential arguments to predict the orientation of a diatomic molecule of H2 type adsorbed in a precursor slate with respect to a metal surface. Their arguments are illustrated in Fig.(2.60). They use the valence bond picture of chemical bonding in H2 that we discussed in section 2.2. Bonding in H2 is considered to be the interaction of a covalent neutral state 0 nd the two... 

Fig. 6.11. The bound, continuum, and resonance (metastable) states of an anharmonic oscillator. Two discrete bound states are shown (energy levels and wave functions) in the lower part of the image. The continuum (shaded area) extends above the dissociation limit i.e., the system may have any of the energies above the limit. There is one resonance state in the continuum, which corresponds to the third level in the potential energy well of the oscillator. Within the well, the wave function is vecy similar to the third state of the harmonic oscillator, but there are differences. One is that the function has some low-amplitude oscillations on the right side. They indicate that the function is non-normalizable and that the system will dissociate sooner or latex.

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