Cross-Luminescence

Recently there has been a lot of interest in the luminescence of Bap2. Its crystals have a potential as a scintillator material (detection of gamma rays, see Chapter 9). They show a luminescence at 220 nm with a very short decay time, viz., 600 ps. This luminescence is of a new type (cross-luminescence). iLs nature has been unraveled by Russian investigators [28], Excitation with about 10 cV excites anion excitons i.e. excitons of which the hole is trapped on P. Upon recombination these anion excitons show an emission at about 4.1 eV (300 nm). This is an emission of the type 

Energy level scheme of BaFs showing cross luminescence (0. The exciion luminescence is indicated (s). See also text 

Other compounds for which this phenomenon has been found are CsCI and CsBr, and KF, KMgF,. KCaF,. and K2YF5. 

---

It allows the existence of the scintillating properties of BaF2 crystals a wide forbidden band is necessary for avoiding reabsorption of the ultraviolet 2pF —> 5pBa emission produced when holes are formed in the 5pBa core levels by X-rays or y-rays (cross-luminescence) [3],... 

The discovery of the fast emission of BaF2 [46,47] has brought about a large activity in the field of cross-luminescence materials. The cross-luminescence (CL) (or... 

Fluoride crystals are also well suited for fast detectors for high-energy radiation. Scintillators based on the Ce3+ emission have been particularly investigated. The discovery of the very fast cross-luminescence of BaF2 has led to the investigation of the luminescence of fluorides containing K, Rb, Cs and Ba under excitation by ionizing radiation. 

Short decay times can be obtained by using luminescent ions with allowed emission transitions. In the Held of inorganic materials the best examples arc the 5d 4/ transitions (t 10 ns) (Sects. 2.3.4 and 3.3.3) and the cross luminescence (t 1 ns) (Sect. 3.3,10) [11]). The afterglow is governed by the presence of traps in the host lattice as described in Sect. 3.4. 

In view of the strong interest in very fast scintillator emission, it is not surprising that many other compounds have been investigated for cross luminescence. It is essential, of course, that the cross-luminescence emission energy is smaller than the bandgap energy, since otherwise the cross luminescence cannot be emitted (see also Fig. 3.27). This is illustrated in Table 9.9 [9]. The table shows excellent agreement between prediction and observation. 

Table 9.10, finally, shows some compounds for which cross luminescence has been definitely observed [9J. All decay times are of the same order of magnitude ( 1 ns), whereas the light yields do not reach the level of 2000 photons MeV . It is, at this time, too early to predict whether cross luminescence will have an important application or not. 

FMNH2 requirement in bacterial luminescence Crystallization of Cypridina luciferin Crystallization of firefly luciferin Cypridina luciferin in fishes the first cross reaction discovered Structure of firefly luciferin Discovery of aequorin and GFP (green fluorescent protein) Structure of Cypridina luciferin Concept of photoprotein Structure of Latia luciferin Dioxetanone mechanism proposed in firefly and Cypridina luminescence... 

The active state of luminescence spectrometry today may be judged ly an examination of the 1988 issue of Fundamental Reviews of Analytical Chemistry (78), which divides its report titled Molecular Fluorescence, Phosphorescence, and Chemiluminescence Spectrometry into about 27 specialized topical areas, depending on how you choose to count all the subdivisions. This profusion of luminescence topics in Fundamental Reviews is just the tip of the iceberg, because it omits all publications not primarily concerned with analytical applications. Fundamental Reviews does, however, represent a good cross-section of the available techniques because nearly every method for using luminescence in scientific studies eventually finds a use in some form of chemical analysis. Since it would be impossible to mention here all of the current important applications and developments in the entire universe of luminescence, this report continues with a look at progress in a few current areas that seem significant to the author for their potential impact on future work. 

The luminescent centers require a range of properties that include a large cross-section for the collision excitation to occur, an ionic radius and valency to fit the lattice and be stable under the applied high electronic fields, and the capability to display high luminous efficiency when excited.11 Metal ions suitable for EL devices include Mn, Tb, Sm3+, Tm3+, Pr3+, Eu2+, and Ce3+.12-17 ZnS lattices doped with Mn2+ (yellow-orange emission at ca. 585 nm) have proved to be one of the best phosphors for EL devices. 

In radiolysis, a significant proportion of excited states is produced by ion neutralization. Generally speaking, much more is known about the kinetics of the process than about the nature of the excited states produced. In inert gases at pressures of a few torr or more, the positive ion X+ converts to the diatomic ion X2+ very rapidly. On neutralization, dissociation occurs with production of X. Apparently there is no repulsive He2 state crossing the He2+ potential curve near the minimum. Thus, without He2+ in a vibrationally excited state, dissociative neutralization does not occur instead, neutralization is accompanied by a col-lisional radiative process. Luminescences from both He and He2 are known to occur via such a mechanism (Brocklehurst, 1968). 

In a nonattaching gas electron, thermalization occurs via vibrational, rotational, and elastic collisions. In attaching media, competitive scavenging occurs, sometimes accompanied by attachment-detachment equilibrium. In the gas phase, thermalization time is more significant than thermalization distance because of relatively large travel distances, thermalized electrons can be assumed to be homogeneously distributed. The experiments we review can be classified into four categories (1) microwave methods, (2) use of probes, (3) transient conductivity, and (4) recombination luminescence. Further microwave methods can be subdivided into four types (1) cross modulation, (2) resonance frequency shift, (3) absorption, and (4) cavity technique for collision frequency. 

Scheme 8 gives species with extended 109 or starbust -type structures, which are strongly luminescent even with the large ligand L = PCy3.85 The diphenylfluorene derivative shows a remarkable heavy atom effect on the intersystem crossing rate.78... 

---