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Chapter 6 Photoluminescence, C a n d o l u m i n e s c e n c e , and Radical Recombination Luminescence of M i n e r a l s

Downloaded by UNIV OF PITTSBURGH on December 23, 2014 | http://pubs.acs.org Publication Date: November 29, 1990 | doi: 10.1021/bk-1990-0415.ch006

William B. White Materials Research Laboratory and Department of Geosciences, Pennsylvania State University, University Park, PA 16802 The UV and electron beam excited luminescence of many insulator minerals can be ascribed to transition metal and lone-pair-electron ions. Emission and excitation spectra can be related to mineral crystal structure by relatively simple crystal field models. Luminescence due to molecular species such as the uranyl ion, the disulfide ion, and organic molecules occurs occasionally. Some of these centers can be excited by recombination of high energy species in flames (candoluminescence), ion beams, and plasmas. These spectra are similar to, but not identical with, photoemission spectra. Candoluminescent and radical recombination luminescence spectra are strongly temperature dependent with the greatest efficiency often being obtained well above room temperature. Energetic species in the solar wind could excite mineral grains on planetary surfaces. Many minerals can be made to luminesce under various excitation sources, usually UV light, but in relatively few cases is the mechanism understood in detail. Best understood is luminescence due to transitions between localized states in the unfilled d-orbitals of transition metal ions and localized states in the unfilled f-orbitals of rare earth ions. Rare earth ions, important in the development of synthetic phosphor and laser materials, are uncommon among naturally occurring minerals. This paper reviews briefly the mechanisms responsible for mineral luminescence, then turns to less common modes of excitation, particularly ion and radical recombination luminescence, and finally discusses luminescence on the scale of remote sensing of planetary surfaces. The literature on mineral luminescence is somewhat bimodal in character. On one hand those mineral structures that have proved useful as phosphors or as crystal lasers have been studied in great detail and the physics of the energy transfer processes are often well known. The luminescence characteristics of many other minerals 0097-6156/90/0415-0118S06.00/0 ο 1990 American Chemical Society

In Spectroscopic Characterization of Minerals and Their Surfaces; Coyne, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Luminescence of Minerals

remain at the level of qualitative descriptions compiled by mineral collectors (e.g. 1). A special case is thermoluminescence which has been investigated in considerable detail (2). The invention of a cathodoluminescence attachment for the polarized light microscope (3) allowed wide use of cathodoluminescence for the examination of growth features in minerals (4). Marshall (5) has recently compiled the literature on cathodoluminescence and its application to petrology and the reader is also referred to the articles by Sears and Steele in this publication.


Background Molecular, Insulator, and Semiconductor Phosphors. Luminescent materials can be divided into three broad categories: molecular phosphors, insulator phosphors, and semiconductor phosphors (6). The absorption of energy, its internal redistribution, and emission as luminescent radiation all take place within a single molecule in molecular phosphors. Mostly these are organic compounds which are of relatively little importance in mineral luminescence. Exceptions are the fulvic and humic acids which are carried into sedimentary rocks by circulating groundwater and which give a characteristic phosphorescence to certain freshwater calcite deposits (Brennan and White, unpublished data). Important inorganic molecular activators include the uranyl ion which imparts the well-known green luminescence to many uranium minerals and the disulfide ion which is responsible for a yellow luminescence in scapolite and some other minerals (7). Most minerals fall into the class of insulator phosphors. The characteristics of the luminescence are usually defined by the electronic structure of an activator ion as modified by the crystal field of the host crystal structure. Although some energy transfer takes place between nearby ions, appearing as the phenomena of co-activation, luminescence poisons, and activator pair interactions, the overall luminescence process is localized in a "luminescent center" which is typically 2 to 3 nm in radius. From a perspective of band theory, luminescent centers behave as localized states within the forbidden energy gap. In semiconductor phosphors the energy band structure of the host crystal plays a central role. Some semiconductor luminescence arises from decay of exciton states, other emission arises from decay of donor states generated by impurity or defect centers. It is not the magnitude of the band gap itself that separates insulator from semiconductor phosphors; it is a question of whether the spectrum is characteristic of impurity energy levels as perturbed by the local crystal structure or whether the spectrum is characteristic of the band structure as modified by impurities. Thus the fluorescent nitrogen-doped diamond, with a 5.5 eV band gap, would be considered a semiconductor phosphor. Activators. It is convenient to classify activator ions for insulator phosphors in terms of their electronic configuration. Intra-Configurational Transitions f —• f d —* d

Eu3+, Tb3+, Nd3+ Mn2+, Fe3+, Cr3+, Mn*+


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SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES

Inter- Configurational Transitions d —» f 10 - > 9 d ° (anion ρ —¥ cation d) 2 —» p d


s

d

Eu2+, Ag , Ti* , Sn , +

p

+

2 +

S

Ce3+ Cu , Zn W , Mo Pb +

6 +

2 + 6+

2 +

By far the most important activators in mineral luminescence are the iron group ions which exhibit transitions between partly filled d-orbitals. These will dominate the discussion that follows. Luminescence arising from the trivalent rare earth ions occurs in some phosphate minerals but is dealt with elsewhere in this volume (Wright). The filled d-shell ions are activators for cathodoluminescence phosphors such as ZnS, however, most sulfide mineral phases contain too many luminescence poisons for the transitions from these ions to be observed. The empty d-shell ions are unusual in that they act as emitting centers even when present in high concentrations. This is in contrast with the transition metal ions which usually concentrationquench in the range of 1 - 5 mole percent of the activator. Minerals containing d ° ions such as benitoite, fresnoite, scheelite, and powellite are often fluorescent (Figure 1). These ions likewise do not fit neatly into the insulator category. Although the T i ion behaves as an isolated center, a molecular orbital or band model is needed to account for the energy transfer. Likewise the WO4 and M0O4 tetrahedra can be treated as molecules but further considerations of the crystal structure are needed to describe the energy transfer (8). The non-bonding lone pair s-electrons of such ions as divalent lead and divalent tin generate a series of strongly localized states which can be pumped and which emit a broad band luminescence typically in the range of 360 to 500 nm. 4 +

Energy Transfer Processes. Luminescence is a complex sequence of energy transfer processes. Figure 2 is a schematic of the most important of these for photoluminescence, cathodoluminescence, and candoluminescence. The ultimate source of energy is the excitation: UV light, electron beam, ion beam, or radical recombination excitation. We are not concerned here about the triggered release of previously trapped energy such as occurs in thermoluminescence and triboluminescence. The schematic phosphor grain for photoluminescence and cathodoluminescence is shown containing three kinds of centers: activators (A), sensitizers or co-activators (S), and poisons (P). The UV photon may be assumed to have an energy less than the band gap of the phosphor host so the absorption by A , S, and Ρ takes place at the centers. Many activators, particularly M n , have a low absorption cross-section so that direct pumping of the activator is inefficient. Sensitizers, such as P b , are selected to have large absorption cross-sections at UV wavelengths. They then transfer the excitation to the activator which re-emits it as luminescent radiation. Most co-activators have a luminescent emission of their own so that the efficiency of the system depends on the kinetics of energy transfer compared with direct emission of the co-activator. In many mineral systems, one observes both and thus the activator radiation R A and the sensitizer radiation Rs are 2 +

2 +


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Luminescence ofMinerals

τ

1

1

1

r BENITOITE BaTiSi 0


3

20,000

25,000

30,000 WAVENUMBERS

35,000

Figure 1. Photoluminescence emission and excitation for benitoite showing Ti^+ luminescence.

PHOTOLUMINESCENCE

CATHODOLUMINESCENCE

9

40,000

spectrum

H

CANDOLUMINESCENCE

Figure 2. Drawing i l l u s t r a t i n g energy transfer processes within a phosphor g r a i n . S = co-activator or sensitizer; A = activator; Ρ = poison center. Short wiggly arrows represent non-radiative transitions. Dashed line outlines the unknown depth to which energy from radical recombination excitation can be transferred.



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shown in Figure 2. Luminescence poisons are defects and impurities which promote the non-radiative decay of the excitation. In mineral systems, ferrous iron is the most important luminescence poison. Excitation by impinging electrons (or ions) takes place by a somewhat different mechanism. Energetic electrons penetrate the phosphor grains and the lose energy by multiple collision processes. The energy is sufficient to pump the conduction band of the phosphor host and the excitation can move anywhere in the crystal to pump the localized luminescence centers. Cathodoluminescence thus appears at lower concentration thresholds than photoluminescence but is more susceptible to the influence of defects and other luminescence poisons. Direct pumping of poison centers as well as energy transfer from co-activators, and energy transfer from activators all represent an energy loss in the system. In addition, activators and co-activators also have non-radiative decay routes. Because non-radiative decay is usually phonon-assisted, non-radiative decay is exacerbated by increasing temperature and manifests itself by a characteristic temperature at which luminescence is quenched. The crystallographic relations that provide optimum sensitizer activator energy transfer are outlined by Blasse (9) Crystal Structure and Luminescence of Insulator Mineral Phosphors The energy level structure of partly filled d-orbitals can best be described by crystal field theory as expressed in Tanabe-Sugano diagrams. These account for absorption and luminescence spectra and allow the spectra to be correlated with crystal structure. Transition metal ion activators that emit in the visible region of the spectrum belong mainly to the d-3 and d-5 electron configurations, d ions have a characteristic narrow band emission that is largely independent of the crystalline host, d^ ions are broad band emitters with emission wavelengths that are very sensitive to details of the crystal structure. 3

3

Narrow Band Emitters. The Tanabe-Sugano diagram for the d configuration is shown in Figure 3 for the specific case of the C r ion. The emitting level is the doublet state that is nearly independent of the crystal field parameter, Dq. If the crystal field is sufficiently strong to make the doublet state the lowest energy excited state, there will be a red luminescence that will not vary much between different crystal hosts. Because the doublet state belongs the the same orbital configuration as the ground state, the Stokes shift and Condon offset will be small. As a result, the line width of the emission is narrow and a considerable amount of phonon fine structure is observed even at room temperature (Figure 4). A similar red luminescence is observed in other minerals activated by chromium such as ruby, emerald, and alexandrite. Mostly C r appears in minerals as a trace constituent substituting for Al + in six-fold coordination. As such, the larger Cr ion is placed in a site with small metal-ligand distances, the crystal field is large - typically 1650 - 1750 wavenumbers, and only the doublet to quartet transition is seen. At weaker crystal fields, less then 1500 wavenumbers in the diagram in Figure 3, a short lifetime, broad band quartet-quartet transition appears (10). 3 +

3 +

3

3 +



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Dq (cm" ) 1

Figure 3. Tanabe-Sugano diagram for d for Β = 755 and C = 3257 wavenumbers.

3

configuration calculated

A l _ Cr Mg0 2

x

x

4

SPINEL

φ œ

001 ro CM 800

700 600 WAVELENGTH (nm)

500

Figure 4. Photoluminescence emission spectrum for natural spinel activated by C r . Mohler and White, unpublished data. 3 +


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Within the cross-over region there is more complicated mixing between doublet and quartet states and the luminescence band is broad with additional sub-bands (11). Because of the importance of ruby lasers and related C r activated systems, the physics of the doublet-quartet transition has been worked out in considerable detail as has the mechanism of non-radiative transfer and the temperature dependence of the luminescence(12). Although the C r narrow band emission is essentially independent of crystal field and thus of site size and symmetry, the energy of the doublet levels does depend on Racah parameters Β and C . The Β and C parameters are dependent on the covalency of the metal-ligand bond and thus there is some variability in the C r emission from host to host. The other common ion with the d configuration is M n which is important as an activator in some phosphors but it does not commonly occur in fluorescent minerals. 3 +

3 +


3 +

3

2 +

4 +

3 +

Broad Band Emitters. M n and F e are the two most widely occurring activators in fluorescent minerals. Figure 5 shows the Tanabe-Sugano diagram for the d configuration with a typical choice of the Racah parameters Β and C . Luminescent emission takes place from the quartet Τχ level to the sextet A\ ground state. Because these two levels belong to different strong field configurations there is a large Condon offset and a correspondingly large Stokes shift. Typical values for the Stokes shift of M n * a n d F e are sketched in Figure 5 by off-setting the emission levels. Because of the steep slope of the quartet Τ χ level, the emission band is highly sensitive to interatomic distances and to distortions in the activator site symmetry. To a first approximation, the emission wavenumber decreases linearly with Dq over the range of observed luminescence bands. Figure 6 shows a composite spectrum for M n emission from spinel, willemite, olivine (13) and a CaO-SrO solid solution. The red to green color change when M n is moved from six to four-fold coordination is a classic phenomenon and has been known at least since 1942. It is seen in Figure 6 as the comparison between the willemite spectrum and the other spectra. Less well recognized are the variations in emission band with crystal field strength as reflected in bond lengths. This small selection of minerals exhibits a range of 6-coordinated M n emission from yeUow to deep red depending on interatomic distance. Figure 7 shows more detail by comparing the emission bands of two solid solution series. The CaO-SrO solid solution data are instructive since the site is strictly cubic and the interatomic distance varies linearly with composition. The absorption spectrum was measured by diffuse reflectance to provide the wavenumber of the Τχ level. The Stokes shift was found to be about 1000 wavenumbers as sketched on Figure 5. Corresponding data for the calcite - magnesite solid solution series (14) show a similar linear trend. Interpretation of M n emission on the basis of a Tanabe-Sugano diagram has led to some confusion. If the site is non-cubic the triply degenerate T\ level will split into two or three components and only the lowest energy component wiU be an emitting state. The emission, however, will appear at lower wavenumbers than predicted by the average interatomic distance and coordination number. Likewise, the Racah parameters depend on the metal-ligand bonding and scale the entire diagram including the emitting level. 5

2

3 +

2 +

2 +

2 +

2 +



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500

1000 1500 Dq (cnrT )

2000

1

5

Figure 5. Tanabe-Sugano diagram of d configuration calculated for Β = 636 and C = 3678 wavenumbers. Typical emission levels for four and six coordinated M n and F e are sketched on the diagram. 2 +

ι

3 +

r\ JT\ /"V /o\/2i\ / A 1 A \ / \ f fio 10

\

1 y MgAI 0 2

/ · o V•4-Mg Si0 1i 2

Zn Si0 / / 2

4

I 450

500

\

/ \ A

S

4

\

f

0

4

β

/ CoO

1 550 600 650 WAVELENGTH (nm)

Figure 6. Emission spectra for M n olivine were extracted from (13).

2 +

700

750

in various hosts.

Data for



126


I8,000h

14,000' 2.0

2.1 2.2 2.3 2.4 2.5 METAL-OXYGEN DISTANCE (&) 2 +

Figure 7. Variation of Mn emission wavenumber with interatomic distance for the CaO-SrO solid solution and for the calcite - magnesite solid solution. Data for the latter plot were extracted from (14).


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Trivalent iron is an effective activator when it is free of the effects of ferrous iron. Because of the larger value of Dq, the emission is shifted to 700 - 750 nm in the deep red when F e is present on tetrahedral sites. F e in octahedral coordination is predicted to emit in the near infrared at about 900 to 1000 nm but is rarely observed and is thus indicated by a dashed line in Figure 5. Fe * luminescence in minerals is mainly observed when F e substitutes for A l in tetrahedral sites in aluminosilicates. Observations on feldspars (15) provide the transition sketched on Figure 5. In the orthoclase structure the Stokes shift is 1500 wavenumbers. Trivalent iron is the most likely activator for luminescence seen in many terrestrial feldspars and feldspar-containing rocks. The characteristic 700-750 nm band is weak or absent in lunar feldspars (16,17). 3 +

3 +

3

3 +


3 +

Candoluminescence and Radical Recombination Luminescence Qualitative Observations. Candoluminescence is excited by flames. The emission spectra are similar but often not identical to those excited by UV (18). The energy source is the recombination of free radicals that occurs in the flame and thus flame-excited luminescence is the same as the radical recombination luminescence observed when free neutral radicals from plasmas are used as an excitation source. A simple hydrogen diffusion flame is the simplest source for demonstrating the phenomenon. Figure 8 shows the recombination energy of various neutral species that can be extracted from nitrogen, hydrogen, and water vapor plasmas and the hydrogen flame. These energies are substantial and are comparable to the pumping energy available from the 365 and 253.7 nm lines of Hg-vapor lamps commonly used as UV excitation sources. Note that some of the interactions, notably OH*, require only that the excited species collide with the phosphor surface while others require two-body collisions (see sketch in Figure 2). An important question concerning neutral radical excited luminescence is the energy transfer mechanism. As sketched in Figure 2, the extraction of energy from the radicals takes place only on the surface. There must be some mechanism to transfer this energy to the A and S centers in the bulk. It has been argued (19) that OH* is the most important excitation source in the hydrogen flame. The most remarkable characteristic of candoluminescence is its temperature dependence. When a hydrogen flame is brushed over the phosphor surface, some time is required to warm up the surface before the luminescence appears. Each phosphor has a characteristic maximum in the temperature-intensity curve and at higher temperatures the intensity falls off again. Maxima for most transition metal ion- and rare earth ion-activated systems are in the range of 100 to 300 C . Candoluminescence thus occurs at much lower temperatures than the onset of purely thermal emission. Candoluminescence of Minerals. Figure 9 shows a simple experimental set-up for the measurement of candoluminescent spectra. The mineral sample is ground to a fine powder, slurried with water or organic solvent and painted over the surface of a Kanthal metal heating rod. The heating rod with an attached thermocouple allows the phosphor temperature to be controlled independently of the flame. The surface In Spectroscopic Characterization of Minerals and Their Surfaces; Coyne, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

128

SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES NITROGEN GAS

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a

s

>| >i «1 | v

ι

ι

|i

ii

Α

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8 II

1

WÂTEft VÂPOFl ι I 4 3 j 5.5Ο I

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X % I Ε 1 1 I I I 1 f I I '!vsj "' X HYDROGEN Gt.3 ι 1 1 1 1 1 « I

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2 3 4 5 6 7 8 9 1 0 ENERGY (eV)

Figure 8. Recombination energies for various active formed i n flames and i n microwave discharge induced Data taken from Corredor (22).

species plasmas.

Cory 14 Spectrometer

Ring of Candoluminescence

Figure 9. Sketch showing experimental set-up for observation and measurement of candoluminescence and candoluminescence spectra.



6. WHITE

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of the phosphor patch is brushed with a hydrogen flame and the luminescence spectrum observed by the spectrophotometer through the flame. Flame lines are superimposed on the luminescence spectrum but the narrow flame lines are easy to recognize against the broad band phosphor emission. Some effort must be taken to avoid flame flicker and indeed much of our early work used a spectrograph with photographic plates to avoid the flicker problem. With care, flicker noise can be reduced and useful spectra can be taken with a conventional spectrophotometer. The candoluminescent and radical recombination spectra of willemite are shown in comparison with the UV-excited spectrum in Figure 10. The envelope of the main emission band at 530 nm is the same in all spectra but the radical recombination excitation brings out another feature at 613 to 642 nm not seen in the photoluminescence spectrum. Similar behavior is seen in other M n activated candoluminescent spectra (20) The temperature dependence of the 530 nm band is strikingly different for the different sources of excitation. (Figure 11). The intensity of the UV-excited band simply falls off with increasing temperature. The hydrogen radical recombination luminescence spectrum peaks at 40 C and then falls off rapidly. Nitrogen and water vapor radical recombination luminescence peaks near 320 C while the candoluminescent spectrum peaks at 178 C . The temperature response curves are the combined results of adsorption-desorption of the active radical on the phosphor grain, the de-excitation of the active radical, and the transfer of the energy of de-excitation to the activator centers within the grain. Candoluminescence appears bright when observed in the laboratory. However, efforts to estimate the quantum efficiency of willemite using hydrogen ion bombardment (21) gave 4xl0" photons per incident hydrogen ion when extrapolated to zero accelerating voltage. This number was obtained at room temperature and not at the temperature of maximum candoluminescence. Judging by the dependence of luminescence intensity on temperature shown in Figure 11, the quantum efficiency could be 100 times higher at the temperature of peak brightness. This low quantum efficiency is not consistent with visual observations in flames although it is difficult to know the number of impinging active species in flame excitation. 2 +

6

Planetary Surfaces Many luminescent features are known in the natural environment on the earth's surface. Many are of biological origin, ranging from fireflies to the phosphorescent sea, and most of these have been known since ancient times (23). Luminescence from inorganic sources is much more limited due to lack of an effective excitation mechanism beneath the earth's atmosphere. Mineral collectors use portable ultraviolet lights to prospect quarries and tailing piles in the dark of night but the natural background of UV is too weak and of too low energy to effectively excite luminescence. Luminescence, however, may be observed on the surfaces of airless bodies such as



130


Zn Si0 2

4

: Mn

530

+ 2

• W A T E R V A P O R R.R.L • N I T R O G E N R.R.L HYDROGEN R.R.L 2 5 3 . 7 nm Hg >HYDROGEN

FLAME

613 ι

///

800

750

700

650 600 550 W A V E L E N G T H (nm)

500

450

400

Figure 10. Flame-excited and radical recombination excited emission spectra of willemite. Corredor and White, unpublished data.

mercury, the moon, the asteroids, and possibly the rocky satellites of the outer planets. The band of interplanetary dust responsible for the zodiacal light seems to brighten during magnetic storms possibly due to luminescence (24). Most of the observations concern lunar luminescence (25). Lunar luminescent phenomena are a fascinating contradiction. On one hand, there is some evidence for a general brightening of the lunar surface both under sunlight and during lunar eclipses as well as substantial observational evidence for transient and localized


In Spectroscopic Characterization of Minerals and Their Surfaces; Coyne, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990. 2

I ι ι ι ι I ι ι ι ι Iι ι 500 450 400

Figure 11. Temperature dependence of the intensity of the 530 nm band of willemite. Corredor and White, unpublished data.

e

200 250 300 TEMPERATURE ( C)

%

\

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132


patches of brightening, usually in the red region of the spectrum. On the other hand experimental work shows that the more obvious mechanisms are insufficient by some orders of magnitude to account for the observations. The general brightening of the lunar disc at times of total eclipse and excess brightness under sunlight has taxed experimental measurements. McCord (26) concluded that large portions of the lunar surface may continuously emit radiation at the level of 0.5 to 1 percent of the solar continuum. Solid state luminescence is one possible explanation. The general brightening of the lunar disc was once thought to be due to thermoluminescence. Trap centers were charged by solar radiation during the lunar day and the energy released by slow decay during eclipses or by heating at the lunar dawn. Analysis of the thermoluminescence explanation (27) shows it to be marginal. The other and much better established phenomena are transient events seen in specific limited areas of the moon and usually lasting for times on the order of hours or days. Any activity on the supposed "dead" surface of the moon is of interest and catalogs have been made which, as of 1978, had logged 1468 observations (2830). Many of the lunar transient phenomena take the form of patches of red luminescence, some blue luminescence, and various flashes and related features. A substantial percentage of the events are related to the crater Aristarchus. The red patches of luminescence have been associated with the arrival of energetic particles emitted during solar flares (31, 32). The lunar transient events could be excited by protons in the solar wind but experiments with silicate minerals in proton beams show that the process is inefficient, quantum efficiencies from lxlO" to l x l O " , and given the concentration of protons in the solar wind the mechanism cannot account for the intensity of the observed luminescence (33). Another possibility is that neutral particles in the background solar wind or associated with disturbances on the sun's surface provide the excitation source (34). This would be a process very similar if not identical to the candoluminescence and radical recombination luminescence observed in the laboratory. If the red emission events of the lunar transient phenomena are reaUy due to luminescence, the activator and host must be identified. Ferric iron in feldspars is the dominant red emitter in terrestrial igneous rocks. However, there is little if any ferric iron in the highly reducing environment of the lunar rocks. The samples returned by the Apollo missions were eagerly examined for luminescence (35-40) The results from all of the Apollo landing sites were much the same. The main luminescent phase was plagioclase which often exhibited three emission bands: a blue band at 450 nm thought to be due to defects in the silicate structure, a green band at 560 nm assigned to M n substituting for C a in the plagioclase structure, and a weak deep red band at 780 nm which might be due to ferric iron. The Fe + luminescence from the lunar plagioclases was much weaker than from terrestrial feldspars as expected both from the lower F e concentration and also the quenching effect of the dominant F e . Experiments by several teams 4

6

2 +

2 +

3

3 +

2 +


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of investigators included excitation by electron and proton bombardment over a range of energies. It was concluded that the efficiencies of any of these processes was too low to account for the astronomical observations. This review ends with a question mark. In spite of the intense research activity before and during the Apollo lunar missions, the origin of the supposed luminescence lunar transient phenomena remains in doubt. It is suggested that the recombination of neutral radicals, as a process for the excitation of luminescence, may play a more important role than has previously been ascribed to it. Acknowledgments This review draws on the work of former graduate students Rick L. Mohler and Andres Corredor and on the work of the late Arnost Bergstein who was at the time a visiting scientist from Czechoslovakia. The luminescence program was supported by the National Science Foundation. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

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