Dynamics of electronic relaxation in characteristic x-ray radiation processes in wide-gap insulators: Separation of luminescence from Raman scattering

Electronic relaxation dynamics on the separation of luminescence from Raman scatterings are theoretically studied in the x-ray radiation process from a shallow core level to a deep one in a wide-gap insulator. A four-band mod el composed of dispersionless deep and shallow core bands, and conduction a nd valence ones, is taken as one of the simplest examples. The Coulomb inte ractions among the conduction electrons and the valence holes are taken int o account by the lowest-order perturbation theory. By these interactions, a conduction electron is scattered together with creation of a valence-hole- conduction-electron pair. Using this model, we calculate a second-order opt ical process composed of an excitation of an electron from the deep core le vel to the conduction band by an incident x-ray, and a subsequent transitio n from a shallow core level to a deep one by radiating another x ray. When the incident x-ray energy is below the absorption edge, the resultant radia tion spectra have only one peak due to a Raman scattering. On the other han d, when the incident energy is above the edge, the radiation spectra separa te into two peaks: a luminescence peak, and a Raman scattering peaks on the higher-energy side. The luminescence is considered to occur after the foll owing electronic relaxation. The photoexcited electron enters the conductio n band by creating a valence-hole-conduction-electron pair through the Coul omb interactions. Afterwards, this pair goes away from the original site, a nd it results in a dissipation. From this scenario, the calculated x-ray ra diation spectra agree with experimental results. We have concluded that the conditions to obtain luminescence is that the conduction- and valence-band -widths are finite, and the lifetime of the deep core hole is long enough. With this scenario, we also qualitatively explain the characteristic x-ray radiation spectra in metals, semiconductors, and atoms. Furthermore, we dis cuss the spectral shapes in the Auger decay process competitive with the ra diation one. [S0163-1829(98)01448-9].