The phonon-assisted luminescence in AgBr:I is discussed in the framework of a conventional configuration coordinate model in which both optical and acoustic phonons are coupled to the excited electronic states. The authors derive the expression for the luminescence intensity using the coherent phonon description for the displaced oscillator (excited impurity) in a consequent way. As compared to the conventional derivation they gain in simplicity and transparency of the calculation. With the formula derived, the experimental results concerning the spectral features of the luminescence as well as their dependence on temperature are well reproduced. In particular the importance of the acoustical phonon contribution in determining the correct intensity and its temperature dependence is stressed. Furthermore the question of the interpretation of the experiments in terms of the introduced coherent phonon states is discussed.

Light emission from impurity states in solids: conventional and coherent phonon descriptions

Coherent states of phonons in the polarons problem: Optical transitions of the large polaron at rest

Creation and detection of states of the phonon field are discussed. I briefly compare phonon coherent states (PCS) with Fock and thermal states in two harmonic systems, the diatomic molecules and a non-conducting crystal with local centers. I analyze some of the phenomenology suggesting the cases where PCS are generated and report on their unmistakable fingerprint: the Poisson distribution among the Fock states and the oscillating Gaussian of the displaced oscillator generated in ultrafast processes. The PCS created in the “pump and probe” femtosecond spectroscopy are specially commented. The motion of PCS is seen to be responsible of the persistent oscillations discovered at ultrashort times, but also, in rare cases, of a very fast decay when a true acoustical PCS is created by the ultrafast excitation.

Signatures of phonon coherent states

The excited state of the quantum phonon field, generated by very fast processes is deduced. γ-ray emission, orabsorption, by unstable nuclei (occurring in Mossbauer experiments) and neutron scattering by a crystal are in particular considered. We show that coherent states of the phonon field are generated. The appropriate response functions are written in terms of these states. The phonon field generated by the scattering of a large number of neutrons is discussed, since phonons are found to be distributed according to an exponential distribution, heat is generated.

Quantum coherence among phonons generated by fast processesin elastic targets. II

The reflection coefficient determining the inelastic scattered intensity is defined and analysed theoretically. Various approximations including the Born approximation, the Distorted-Wave Born approximation (DWBA), the GR method and the eikonal approximation are discussed and illustrated with examples. From an expression for the DWBA inelastic reflection coefficient for n-phonon processes, factors determining the relative role of temperature for multiphonon processes and one-phonon scattering are analysed. Several approximate expressions for the Debye-Waller factor are derived and used to define experimental criteria favouring one-phonon scattering. Most of the examples dealt with are for insulator surfaces. Chapter 8 then describes the scattering theory for metal and semiconductor surfaces.

N. Terzi

Papers

Quantum coherence among phonons generated by fast collisions on elastic targets