Phonon-assisted process

As to the relaxation of the excitons in surfaces II and III by the bulk effect, a consistent discussion is as follows The bulk relaxation probability at the energy level s, PV(ES2), has been evaluated, at low temperatures, as few reciprocal centimeters and attributed to acoustical phonons (cf. Section II). With JSK 2cm"1, the direct probability (3.29) is evaluated as 0.2 cm"1, compatible with the observed value,141 while the phonon-assisted process (3.30) is drastically depressed and becomes comparable to direct transfer. 

From the preceding section it is clear that certain important questions must be answered in characterizing host sensitized energy transfer in a specific material. The first question is whether the transfer is a single-step or multistep process. If it is a single-step process, the type of interaction mechanism must be identified and the strength of the interaction determined. The latter is usually characterized by the critical interaction distance Rq. Finally, it is important to determine whether the energy transfer is a resonant or phonon-assisted process. If the transfer is a multistep process, two distinct sets of parameters must be determined ... 

Energy Migration Resonant and Phonon-Assisted Processes. 83... 

Fig. 2 a Energy migration, that is multi-step energy transfer between chemically identical chromophores. b Within the inhomogeneous distribution it can proceed via an resonant process (straight arrow) or via a phonon-assisted process whereby the energy mismatch between the donor and the acceptor is made up by lattice phonons (curly arrows)... 

At 4.2 K the spectrum does not show any narrowed features, that is at 4.2 K, energy migration is dominated by the phonon-assisted process in accordance with previous observations on chromium(III) containing systems [14]. As the temperature is lowered to 1.8 K, sharp features appear in the FLN spectrum. Such sharp features are indicative of energy selective and thus of resonant processes. However, the spectrum observed at 1.8 Kwith its... 

We only consider the one-phonon-assisted process in the weak-coupling limit, where D and A are taken to be the same species. For an isotropic crystal, the donor-acceptor ET rate is given, for a large energy mismatch (-100 cm1) between the donor and acceptor excitations, AE (also equal to the phonon energy), under the Debye phonon model, by the expression [368]... 

On the basis of the dressed photon theory and the important role of electric dipole-forbidden molecular vibrational excitations, recent theoretical studies have proposed a simple model to describe atom or atom-cluster desorption due to the dressed photons from a nanometric particle deposited on a substrate [49]. Assuming an anharmonic potential for each atomic binding, an effective atom-nanodot potential was evaluated to determine the desorption energy and the stabilized dot size. The model shows that electric dipole-forbidden molecular vibrational excitations play an important role in the phonon-assisted process, which could potentially lead to a novel fabrication method, in addition to controlling the size and position of nanostructures [51]. 

Fig. 1.24 Photolithography by phonon-assisted process, (a) Experimental set up. (b) Dependence of the cross-sectional profiles of the patterned photoresist areas on the exposure time, (c) and (d) are AFM images of the OFPR photoresist patterned by visible and UV light, respectively...

In addition to the high resolution obtained with a visible light source, this method based on the phonon-assisted process has several advantages, including ... 

It is possible to excite electrons by a two-step phonon-assisted process for creating electron-hole pair (Fig. 1.42) The first step is the transition from the initial state in the HOMO ( Eg el)(S) EeTi,tbenmi, phonon)) to the intermediate state Eg el)(S> EcjT,phonon)) by the DP-CP. Here, Eg-,el) represents the ground state of the electron, E x,thermauphonon) represents the thermal equilibrium state of a phonon whose energy is determined by the lattice temperature, and Eex,phonon) represents the excited state of a phonon whose energy depends on the energy of the DP-CP. The DP-CP is indispensable for this transition because it is electric dipole-forbidden. 

The phonon-assisted process is used twice once for efficient photocurrent generation in the wavelength range beyond the one limited by Eg, and once for fabricating a metallic electrode for the device, in which the morphology of the electrode surface... 

Step 1 Under light illumination, an optical near-field is generated on the Ag surface. This optical near-field excites a coherent phonon at the />n-junction, resulting in generation of a DP-CP. By the two-step phonon-assisted process, electrons can be excited to create electron-hole pairs at the /injunction even though the photon energy of the incident light is lower than Eg. 

Fundamental concepts such as the behavior of photons in nanometric space, excitation transfer and relaxation in a nanometric space, and the primary mechanisms of the phonon-assisted process should be delved into more deeply. Such fundamental studies are expected to bring about further discoveries involving the coupling of dressed photons with various elementary excitations besides coherent phonons, leading to the discovery of novel applications. This marriage of basic studies and technological applications is expected to establish a new field of dressed photon science and technology in the near future. 

---