Thermal surface phonons

In recent years there is a growing interest in the study of vibrational properties of both clean and adsorbate covered surfaces of metals. For several years two complementary experimental methods have been used to measure the dispersion relations of surface phonons on different crystal faces. These are the scattering of thermal helium beams" and the high-resolution electron-energy-loss-spectroscopy. ... 

The choice of methods is a matter of convenience. Both will capture the essential features of the GLE, namely frictional energy loss from the primary atoms to the secondary atoms and thermal energy transfer from the secondary atoms to the primary atoms. Both will provide a reasonable description of the bulk and surface phonon density of states of the solid. Neither will provide the exact time-dependent response of the solid due to the limited number of parameters used to describe the memory function. 

Surface phonon bands along symmetry lines of the SBZ are given for fee metals in Figs. 5.2-49-5.2-55 and in Table 5.2-20. In all figures the horizontal axis is the reduced wave vector, expressed as the ratio to its value at the zone boundary. Table 5.2-21 gives the surface Debye temperatures for some fee and bcc metals, as well as the amplitudes of thermal vibrations of atoms in the first layer p as compared with those of the bulk pb-In the harmonic approximation, the root mean square displacement of the atoms is proportional to the inverse of the Debye temperature. 

It should be mentioned that the magnitude of the vibrational energy of the oscillator generated by interaction with surface phonons is greater than the thermal vibration energy of the oscillator. 

Process (1) was first observed in the evaporation and desorption of SF molecules from NaCl surfaces induced by resonant CO2 laser pulses of 200 nanoseconds duration in 1978.(1) To understand process (2) we bear in mind that desorption takes place, when the instantaneous vibrational energy in the bond, which ties the molecule (adsorbate) to the surface (adsorbent), exceeds the binding energy. In thermal desorption phonon energy is transferred from the adsorbent to the vibration of the adsorptive bond. (2,3)... 

Thermal excitation of this mode leads at room temperature to the flipping of the buckled dimer such that temporarily averaging techniques such as scanning tunneling microscopy (STM) observe a symmetric dimer. For further discussion of the surface phonons, which are localized mainly within the first two atomic layers, it is illustrative to consider that in a simpHfied approach one expects 12 different modes at a given wave vector. This results from the three degrees of freedom, namely, motion in x, y, and z direction, per four atoms (in the first and second Si layer) within the (2x1) unit cell. The 12 phonon modes at the F point are sketched in Figure 8.2.25 [58]. Four modes, Aj — are polarized shear horizontally with respect to the mirror plane, which contains the Si-Si dimer bond and the surface... 

This chapter has given an introduction to vibrations at bare elemental surfaces, namely, surface phonons. Owing to the low energy of surface phonons, thermal excitations at surfaces nearly always include a significant thermal population of surface phonons. Consequently, the thermal properties are closely linked to phonon properties. 

Thermal properties of overlayer atoms. Measurement of the intensity of any diffracted beam with temperature and its angular profile can be interpreted in terms of a surface-atom Debye-Waller factor and phonon scattering. Mean-square vibrational amplitudes of surfece atoms can be extracted. The measurement must be made away from the parameter space at which phase transitions occur. 

In addition to the photoluminescence red shifts, broadening of photoluminescence spectra and decrease in the photoluminescence quantum efficiency are reported with increasing temperature. The spectral broadening is due to scattering by coupling of excitons with acoustic and LO phonons [22]. The decrease in the photoluminescence quantum efficiency is due to non-radiative relaxation from the thermally activated state. The Stark effect also produces photoluminescence spectral shifts in CdSe quantum dots [23]. Large red shifts up to 75 meV are reported in the photoluminescence spectra of CdSe quantum dots under an applied electric field of 350 kVcm . Here, the applied electric field decreases or cancels a component in the excited state dipole that is parallel to the applied field the excited state dipole is contributed by the charge carriers present on the surface of the quantum dots. 

If the two solids are of the same (or similar) materials and the depth of surface impurities (e.g. oxides) is thin in comparison with the heat carrier wavelength, the expected contact thermal resistance is Rc oc T 1 (see eq. 3.36) for metals, and Rc oc 7 3 (see eq. 3.33) for dielectric material. For a dirty contact between metals (heat conduction by phonons only) Rc oc T 2 (see eq. 3.35). These dependences of Rc have been observed experimentally. 

Figure 15.8 shows the thermal scheme of one detector there are six lumped elements with three thermal nodes at Tu T2, r3, i.e. the temperatures of the electrons of Ge sensor, Te02 absorber and PTFE crystal supports respectively. C), C2 and C3 are the heat capacity of absorber, PTFE and NTD Ge sensor respectively. The resistors Rx and R2 take into account the contact resistances at the surfaces of PTFE supports and R3 represents the series contribution of contact and the electron-phonon decoupling resistances in the Ge thermistor (see Section 15.2.1.3). 

The observations of complex dynamics associated with electron-stimulated desorption or desorption driven by resonant excitation to repulsive electronic states are not unexpected. Their similarity to the dynamics observed in the visible and near-infrared LID illustrate the need for a closer investigation of the physical relaxation mechanisms of low energy electron/hole pairs in metals. When the time frame for reaction has been compressed to that of the 10 s laser pulse, many thermal processes will not effectively compete with the effects of transient low energy electrons or nonthermal phonons. It is these relaxation channels which might both play an important role in the physical or chemical processes driven by laser irradiation of surfaces, and provide dramatic insight into subtle details of molecule-surface dynamics. 

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