On the absence of energy transfer between F and M centers in KCl

The absence of energy transfer between F and M centres in alkali halides: the case of NaF

“…In conclusion, there was no evidence of energy transfer between F and M centres [9]: indeed, such a process should imply a change in the M-luminescence efficiency connected to the F-centre de-excitation mechanism. For the same reason, an energy transfer involving the M-centre triplet state is disregarded: indeed, excitation to the triplet level induces [11] M 1 luminescence because of the fast relaxation to the singlet state.…”

“…For the same reason, an energy transfer involving the M-centre triplet state is disregarded: indeed, excitation to the triplet level induces [11] M 1 luminescence because of the fast relaxation to the singlet state. The suggestions of Bosi and Nimis [9] were also supported by theoretical considerations which are essentially based on the well known Dexter theory for the energy transfer processes presented in [12] and widely discussed in review papers [13,14]. Dexter showed that the energy transfer efficiency is proportional to R −6 (R is the distance between the centres) and approximately proportional to the amount of overlap between the emission and the absorption spectrum of the transmitting and receiving centre respectively; thus Van Doorn [15] and subsequently Bosi and Nimis [9] showed that F and M centres would be very close together (R < 10 Å), i.e., a situation that is hardly likely.…”

“…The latter authors, although quoting the results of Bosi et al [3] concerning the lifetime of the F centre in KCl, avoided taking into account the de-excitation mechanism proposed for the F centre by Bosi in the same paper [3] as well as in subsequent works concerning the same subject; what is more, they completely disregarded the proof of absence of energy transfer between F and M centres in KCl presented by Bosi and Nimis [9]. They examined [16] the possible decay mechanism of excited F centres, and made their claim on the basis of time-resolved ground-state recovery measurements and optical absorption data, but they were not able to obtain excitation and emission spectra.…”

“…On the other hand, Bosi and Nimis [9] showed that the authors of [8] were not able to accurately separate the M luminescence from that of the F centre. In contrast, by using a single-photon technique, Bosi and Nimis accurately separated the M from the F luminescence (indeed there is a large difference between the F-and the M-centre lifetimes) and found that the relative efficiencies for the M-luminescence process by light excitation in M 1 and M F bands follow the same law as a function of T (see figure 1 of [9]).…”

MA-type centers in alkaline-earth doped KC1

“…1~ Our statement was made14 on the basis of: It can only be said that there are slight differences (see Table I) of the emission energies in the Briginets and Nishimaki experiments; in our opinion, they are accidental or more probably connected with the incomplete M+M(Sr) conversion always observed in their experiments as well as in ours. 14 Let us now discuss the lifetime (T) values we obtained (see Table I) for the perturbed and unperturbed M-centers. In a former paper, we showed15 that the M-center in alkali halides can be considered as an H2-like molecule: this is because the perturbing effect of the host lattice can be taken into account via a modification of the inter-ion distance and via the introduction of an effective dielectric constant.…”