3.3. Some Special Classes of Luminescent Centers
`
`39
`
`::J
`d
`
`0
`
`2
`
`!.
`TIME(iJ.s)
`
`6
`
`8
`
`10
`
`Fig. 3.6. Decay curve of the Eu2+ emission of SrB 4 07 : Eu 2+ at room temperature. The
`luminescence intensity I is plotted logarithmically versus the time after the excitation pulse. In
`agreement with Eq. (3.3) a straight line is found
`
`recently been unravelled by modem experimental techniques. This example illustrates
`the process of relaxation, and simultaneously the power of the instrumental technique
`used, viz. femtosecond spectroscopy [5].
`Consider as a specific example KCl, a very simple compound indeed. In Chapter 2,
`its lowest optical absorption band was mentioned to be due to the 3p6 --+ 3p5 4s
`transition on the Cl -
`ion. The excited state can be considered as a hole on the Cl(cid:173)
`ion (in the 3p shell) and an electron in the direct neighbourhood of the Cl- ion, since
`the outer 4s orbital spreads over the K+ ions. Now we consider what happens after
`the absorption process. The hole prefers to bind two ci- ions forming a v K centre:
`this centre consists of a Ch- pseudomolecule on the site of two Cl-
`ions in the
`lattice. The electron circles around the V K centre. In this way a self-trapped exciton
`is formed. An exciton is a state consisting of an electron and a hole bound together.
`By the relaxation process (Cl-* --+ VK.e) the exciton has lowered its energy and is
`now trapped in the lattice.
`Up till a few years ago it was generally assumed that the luminescence of KCl
`was due to self-trapped exciton recombination, i.e. the electron falls in the hole of the
`V K center and the energy of the exciton is emitted as radiation. Recently, however, it
`was shown that the VK.e exciton can relax further: the Ch- pseudomolecule moves
`to the lattice site of one Cl -
`ion (this is called an H center), the electron to the other
`Cl- site which is now vacant (this is the well-known F center). The new relaxed state
`is an F.H pair. It has a lower energy than the V K·e relaxed state. The several steps
`in the relaxation process are depicted in Fig. 3.7. As a matter of fact such a large
`relaxation results in an enormous Stokes shift (several eV's for alkali halides). After
`emission, the F.H configuration relaxes back to the ground state, i.e. eCI.(Ch -)CI --+
`2CldCl-. The subscript indicates the lattice site.
`
`Vizio EX1018 Page 0050
`
`

`

`40
`
`3. Radiative Return to the Ground State: Emission
`
`a
`
`c
`
`Fig. 3.7. Schematic representation of relaxed excited states in an alkali halide. a: ground state;
`b : self-trapped exciton consisting of a VK centre and an electon; c: F.H pair centre. The electron
`is represented by its orbit (drawn line) marked by the letter e, the Ci2 pseudomolecule (i.e. the
`trapped hole) by Cl-Cl. See also text
`
`This example shows clearly that the emission process is very different from the
`(simple) absorption process. For all details the reader is referred to the literature
`[5]. Finally we draw attention to the fact that the life time of the relaxed self-trapped
`exciton in the alkali halides is longer ("""' 1 o- 6 s) than expected for an allowed transition
`(I o- 7 - 1 o- 8 s). This is ascribed to the fact that the emitting state contains an amount
`of spin triplet character. Such a triplet state arises when the spins of the electron and
`the hole are oriented parallel. The emission transition becomes (partly) forbidden by
`the spin selection rule (see Chapter 2).
`
`3.3.2 Rare Earth Ions (Line Emission)
`
`The energy levels of the trivalent rare earth ions which arise from the 4fn configuration
`are given in Figure 2.14. In a configurational coordinate diagram these levels appear
`as parallel parabolas (~R = 0), because the 4felectrons are well shielded from the
`surroundings. Emission transitions yield, therefore, sharp lines in the spectra. Since
`the parity does not change in such a transition, the life time of the excited state is
`long ( ,...__, 1 o- 3 s).
`The energy levels presented in Figure 2.14 are actually split by the crystal field.
`As a matter of fact the splitting is very small due to the shielding by the 5s2 and 5p6
`electrons: whereas the crystal field strength in case of transition metal ions (dn) is
`characteristically a few times 10 000 cm- 1 , it amounts in the rare earth ions (fn) a
`few times 100 cm-1.
`After these general considerations some special cases will be dealt with.
`
`Vizio EX1018 Page 0051
`
`

`

`3.3. Some Special Classes of Luminescent Centers
`
`41
`
`This ion has a half-filled 4fshell which gives a very stable gS712 ground state. The
`excited levels are at energies higher than 32 000 cm- 1
`• As a consequence the emission
`of Gd3+ is in the ultraviolet spectral region. The gS 7; 2 level (orbitally nondegenerate)
`cannot be split by the crystal field. This limits the low-temperature emission spectrum
`to one line, viz. from the lowest crystal field level of the 6 P 712 level to gS712 . However,
`usually the real spectrum consists of more than one line for several reasons.
`in the first place one usually observes weak vibronic transitions at energies be(cid:173)
`low that of the electronic 6 P7/2 ----+ 11 s7/2 transition. In the vibronic transition two
`transitions occur simultaneously, viz. the electronic one (in this case 6 P 7 12
`----+
`8 S 712 ) and a vibrational one. Therefore, the energy difference between the elec(cid:173)
`tronic transition (in this context often called zero-phonon transition or origin) and
`a vibronic one yields the vibrational frequency of the mode which is excited in
`the emission process. In Figure 3.5 a vibronic line occurs in the Gd3+ emission
`spectrum at 1350 cm- 1 below the electronic origin, which shows that vibrations
`of the borate group are involved. These vibronic transitions are observed in many
`rare-earth spectra (see, for example, Ref. [6].
`in the second place one may observe at higher energy than the electronic transition
`other transitions which are due to transitions from the higher crystal field levels
`of the 6 P 712 level. Since the crystal field splitting is small, even at 4.2 K their
`population may be sizable. In Figure 3.8 we show as an example the emission
`spectrum of Gd3+ in LuTa04 at room temperature: there are four lines in the
`6 P7/2 ----+ 8 s7/2 transition originating from the four crystal field levels of 6
`P7/2•
`and, in addition, three lines belonging to the 6 Ps;2 ----+ 8 s7/2 level which is also
`thermally populated at room temperature.
`if one excites with high enough energy (e.g. X rays) many more transitions are
`: Gd3+)
`observed. An example is given in Figure 3.9. The composition (LaF3
`shows under X-ray excitation, in sequence of increasing energy, emission from
`the 6 P, 6 I, 6 D, and 6 G levels [7].
`
`The emission of this ion consists usually of lines in the red spectral area. These
`lines have found an important application in lighting and display (color television).
`These lines correspond to transitions from the excited 5 D 0
`level to the 7 F 1
`(1 =
`0, 1, 2, 3, 4, 5, 6) levels of the 4f6 configuration. Since the 5 D 0 level will not be split
`by the crystal field (because J = 0), the splitting of the emission transitions yields the
`crystal-field splitting of the 7 F 1 levels. This is illustrated in Figure 3.1 0. In addition
`to these emission lines one observes often also emission from higher 5 D levels, viz.
`5 D 1, 5 D 2 and even 5 D 3 . The factors determining their presence or absence will be
`discussed in Chapter 4.
`7F 1 emission is very suitable to survey the transition probabilities of the
`The 5 D 0 -
`sharp spectral features of the rare earths. If a rare-earth ion occupies in the crystal
`lattice a site with inversion symmetry, optical transitions between levels of the 4f 0
`
`Vizio EX1018 Page 0052
`
`

`

`42
`
`3 . Radiative Return to the Ground State: Emission
`
`307
`
`311
`
`315 nm
`
`Fig. 3.8. Emi ssion spectrum of the Gd3+ luminescence of LuTa04:Gd3+ at room temperature.
`The 6 P 712 ~ Xs712 transition shows four components (longer wavelength side), the 6 P s12 ~
`8s 712 transition three (shorter wavelength side)
`
`v
`
`7.50E 03
`
`3
`
`2
`
`-
`
`1
`
`t
`
`190 A
`
`0.15E 03
`
`~ u
`
`260
`Wavelength (nm)
`
`\.
`
`J
`
`5
`
`\_
`
`330
`
`Fig. 3.9. The emission spectrum of the X -ray excited Gd3+ luminescence of LaF3 : Gd3+. Line
`I is the 6 P ~ Xs transition , line 2 6 1 ~ 8 S, line 3 6 D ~ xs, line 4 6 G ~ 8 S, whereas band 5
`is the self-trapped exciton emission of LaF3 (V K type, see Sect. 3.3. 1)
`
`configuration are strictly forbidden as electric-dipole transition (parity selection rule).
`They can only occur as (the much weaker) magnetic-dipole transitions which obey the
`
`Vizio EX1018 Page 0053
`
`

`

`3.3. Some Specia l Classes of Luminescent Centers
`
`43
`
`Lu. Eu
`1P
`o Na
`
`G Gd.Eu
`o Na
`
`Na(Lu.Eu)02
`
`Na(Gcf.EuJ02
`
`r
`
`580
`
`600
`) . -
`
`620nm
`
`620nm
`
`A.- -
`
`Fig. 3.10. The emission spectrum of Eu3+ in NaLu02 and NaGd02. In the NaLu02 : Eu3+
`spectrum the 5 D0 - 7 F 1 lines dominate, in the NaGd02 : Eu3+ spectrum the 5 D 0 - 7 F 2 lines. At
`the top of the Fig., a schematical presentation of the crystal structures of the host lattices is
`given. See also text
`
`selection rule .6.J = 0, ± 1 (but J = 0 to J = 0 forbidden) or as vibronic electric-dipole
`transitions.
`If there is no inversion symmetry at the site of the rare-earth ion, the uneven crystal
`field components can mix opposite-parity states into the 4j 0 -configurational levels
`(Sect. 2.3.3). The electric-dipole transitions are now no longer strictly forbidden and
`appear as (weak) lines in the spectra, the so-called forced electric-dipole transitions.
`Some transitions, viz. those with .6.1 = 0, ±2, are hypersensitive to this effect. Even for
`small deviations from inversion symmetry, they appear dominantly in the spectrum.
`
`Vizio EX1018 Page 0054
`
`

`

`44
`
`3. Radiative Return to the Ground State: Emission
`
`We consider again Figure 3.10 for an illustration of these statements . The figure
`shows the emission spectra of NaLu02 : Eu 3+ and NaGd02 : Eu 3+. Both host lattices
`have the rocksalt structure, but with a different superstructure between mono- and
`trivalent metal ions. In NaLu0 2 the rare earth ions occupy a site with inversion
`symmetry. In NaGd02 the rare earth ions are octahedrally coordinated but due to the
`superstructure there is a small deviation from inversion symmetry.
`: Eu 3+ the 5 D 0 - 7 F 1 emission transition is dominating. All other tran(cid:173)
`In NaLu02
`sitions occur only as very weak and broad lines. These are the vibronic transitions;
`the electronic origins are lacking. Since the starting level is 5 0 0 , the only possible
`7F 1, as observed experimentally. The trigonal crystal
`magnetic-dipole transition is 5 0 0 -
`field at the rare earth site in NaLu0 2 splits this transition into two lines.
`: Eu3+ the 5 0 0 - 7 F 2 emission transition dominates, but other lines are
`In NaGd0 2
`also present. The Eu3+ case is so illustrative, because the theory of forced electric(cid:173)
`dipole transitions [8] yields a selection rule in case the initial level has J = 0 . Tran(cid:173)
`sitions to levels with uneven J are forbidden. Further J = 0 ~ J = 0 is forbidden,
`because the total orbital momentum does not change. This restricts the spectrum to:
`5 0 0 -
`7 F 1, present as magnetic-dipole emission, but overruled by the forced electric(cid:173)
`dipole emission,
`5 D 0 - 7 F 2 , a hypersensitive forced electric-dipole emission, which indeed is dominat(cid:173)
`ing,
`5 D 0 - 7 F 4 , 6 , weak forced electric-dipole emissions.
`For applications it is required that the main emission is concentrated in the 5 D 0 - 7 F 2
`transition. This illustrates the importance of hypersensitivity in materials research.
`
`The emission of Tb3+ is due to transitions 5 D 4 - 7 F 1 which are mainly in the green. Of(cid:173)
`ten there is a considerable contribution to the emission from the higher-level emission
`5 D 3 - 7 F 1 , mainly in the blue. Figure 3.11 gives an example of a Tb3 + emission spec(cid:173)
`trum. Since the J values, involved in the transitions, are high, the crystal field splits
`the levels in many sublevels which gives the spectrum its complicated appearance.
`
`The emission of Sm3+ is situated in the orange-red spectral region and consists of
`transitions from the 4 G 512 level to the ground state 6 H 512 and higher levels 6 H 1 (J > ~ ).
`
`The emission of Dy3+ originates from the 4 F912 level. Dominating are the transitions
`to 6 H 15; 2
`( "' 470 nm) and 6 H 1312 ("' 570 nm). The latter one has dJ = 2 and
`is hypersensitive. The emission has a whitish color which turns to yellow in host
`lattices where hypersensitivity is pronounced.
`
`Vizio EX1018 Page 0055
`
`

`

`3.3. Some Special Classes of Luminescent Centers
`
`45
`
`100
`
`50
`
`OL-----------~------------~------------~------~
`500
`600
`400
`300
`
`f... ( n m ) ------
`Fig. 3.11. The Tb3+ emission spectrum of GdTa04 : Tb3+
`
`The emission color of Pr'+ depends strongly on the host lattice. If the emission
`3 H 4 ) like in Gd2 0 2 S : Pr, but red
`originates from the 3 P 0 level, it may be green eP0 -
`3 H 6 , 3 F 2 ) like in LiYF4
`: Pr. If the emission originates
`lines may also be strong eP0 -
`from the 1 D 2 level, it is in the red and near-infrared. The factors determining whether
`emission occurs from 3 P 0 or 1D 2 will be discussed later. The decay time of the 3 P 0
`emission is short for a rare earth ion (tens of J-LS). Not only is there no spin selection
`rule active, but also the 4forbitals are probably more spread out in the lighter rare(cid:173)
`earths (with lower nuclear charge) facilitating the mixing with opposite-parity states
`[9].
`Rare-earth ion emission is not necessarily sharp line emission, as we will see now.
`
`3.3.3 Rare Earth Ions (Band Emission)
`
`Several rare earth ions show broad band emission. In this emission transition we are
`dealing with an electron which returns from a 5dorbital to the 4forbital (see also
`Sect. 2.3.4). First we discuss trivalent ions (Ce3+, p~+, Nd3+), later divalent ions
`(Eu2+, Sm 2+, Yb 2+).
`
`a. Trivalent Ions
`
`The Ce3+ ion ( 4 f 1
`) is the most simple example, since it is a one-electron case. The
`excited configuration is 5d 1
`• The 4f 1 ground state configuration yields two levels,
`viz. 2F 512 and 2 F 712 , separated by some 2000 cm- 1 due to spin-orbit coupling. The
`
`Vizio EX1018 Page 0056
`
`

`

`46
`
`3. Radiative Return to the Ground State: Emission
`
`_/::~::::~ - - - --
`5d
`-------k~~:::~:::~~~------
`
`4f
`- - -- -- ·o::: ...... - -- - --
`
`so
`
`Fig. 3.12. The simplified energy level scheme of the Ce3+ ion (4/ 1 ). On the left hand side
`only the 4flevel and the 5dlevel are given without taking into account further interactions.
`On the right hand side the spin-orbit (SO) coupling has split the 4flevel into two components
`(about 2000 cm- 1 apart), and the crystal field (D.) has split the 5dlevel into five crystal-field
`components spanning together some I 5 000 em -I
`
`5d 1 configuration is split by the crystal field in 2 to 5 components. The total splitting
`amounts to some 15 000 cm- 1 (Fig. 3. 12).
`The emission occurs from the lowest crystal field component of the 5d 1 configu(cid:173)
`ration to the two levels of the ground state. This gives the Ce3+ emission its typical
`double-band shape (Fig. 3.13). Since the 5d-+ 4 /transition is parity allowed and spin
`selection is not appropriate, the emission transition is a fully allowed one. The decay
`time of the Ce3+ emission is short, viz. a few ten ns. The decay time is longer if
`the emission is at longer wavelengths: 20 ns for the 300 nm emission of CeF3 , and
`: Ce3+ . It can be derived that for a given
`70 ns for the 550 nm emission of Y 3 Al 5 0 12
`transition the decay time 7 is proportional to the square of the emission wavelength
`A [10]: 7"'"' A2
`.
`The Stokes shift of the Ce3+ emission is never very large and varies from a
`thousand to a few thousand wave numbers (medium coupling case). The spectral
`position of the emission band depends on three factors:
`covalency (the nephelauxetic effect) which will reduce the energy difference be(cid:173)
`tween the 4f 1 and 5d 1 configurations
`crystal field splitting of the 5d 1 configuration: a large low-symmetry crystal field
`will lower the lowest crystal-field component from which the emission originates.
`the Stokes shift.
`Usually the Ce3+ emission is in the ultraviolet or blue spectral region, but in
`Y 3 Al 5 0 12 it is in the green and red (crystal-field effect), and in CaS in the red
`(covalency effect).
`Under certain conditions 5d-4femission has also been observed for p~+ (4/2 )
`and Nd3+ (4/3 ). For example, LaB 3 0 6 : p~+ shows band emission around 260 nm
`and LaF3 : Nd3+ around 175 nm. Due to the 7 ""' .A2 relation, the decay time of the
`latter is only 6 ns [11 ]. However, these ions have an alternative way to emit, viz. by
`an emission transition in the 4/" configuration.
`
`Vizio EX1018 Page 0057
`
`(cid:173)
`

`

`3.3. Some Special Classes of Luminescent Centers
`
`47
`
`i
`
`300
`
`320
`
`340
`
`A (nm ) -
`
`Fig. 3.13. The Ce3+ emission spectrum of LiYF4 : Ce3+ at 4.2 K. The two bands correspond to
`the transition from the lowest 5dcrystal-field component to the two components of the 4fground
`state. The longer wavelength component shows vibrational structure
`
`b. Divalent Ions
`
`In this group the most well-known and widely applied example is the Eu 2+ (4j7 ) ion
`which shows a Sd ----+ 4f emission which can vary from long-wavelength ultraviolet
`to yellow. Its decay time is about 1 f.LS. This is due to the fact that the emitting level
`contains (spin) octets and sextets, whereas the ground state level ( 8S from 4j7 ) is an
`octet, so that the spin selection rule slows down the optical transition rate.
`The host lattice dependence of the emission colour of the Eu 2+ ion is determined
`by the same factors as in the case of the Ce3+ ion. If the crystal field is weak and the
`amount of covalency low, the lowest component of the 4j6 5d configuration of the
`Eu2+ ion may shift to such high energy, that the 6 P 7 12 level of the 4 j 7 configuration lies
`below it. At low temperatures sharp-line emission due to the 6 P 7 ; 2 ----+ 8 S 7 12 transition
`occurs. This has been observed for quite a number of Eu2+ -activated compounds. As
`an example we can mention SrB4 0 7 : Eu2+ [ 12].
`Figure 3.14 gives the emission spectrum as a function of temperature. At 4.2 K
`there is line emission from 6 P 712 (and a weak vibronic structure). At 35 K the thermally
`activated emission from the higher crystal-field components of 6 P 712 appears, together
`with a broad band due to the 4j6 5d----+ 4f7 transition. This band has a zero-phonon
`line, indicated 0. At 110 K the band dominates.
`
`Vizio EX1018 Page 0058
`
`

`

`48
`
`3. Radiative Return to the Ground State: Emission
`
`a
`
`I
`
`~
`
`b
`
`-
`
`I
`
`I
`
`t
`
`I
`
`6p
`71 2
`
`t
`
`~~.~
`
`c
`
`380
`
`A. (nm) 370
`
`360
`
`Fig. 3.14. The Eu2+ emission spectrum of SrB4 07 : Eu2+ as a function of temperature. See also
`text. a: 4.2 K (6 P7;2 --+ 8S7 ;2 line emission); b: 35 K (line emissions, and broad band emission
`due to the 4j6 5d--+ 4j7 transition; c: 110 K (band emission dominates)
`
`Vizio EX1018 Page 0059
`
`

`

`3 .3. Some Special Classes of Luminescent Centers
`
`49
`
`i
`
`E
`
`6p
`7/ 2
`
`t.. t sd
`
`---------- ----1
`
`---- ---- - -- - - -- -
`
`6E
`
`Fig. 3.15. Configurational coordinate model for Eu2 + in SrB4 0 7 . Figs . 3 .1 4 and 3 . 15 are de rived
`from A . Meijerink, thesis, University Utrecht, 1990
`
`o -
`
`Figure 3.15 gives the relevant configurational coordinate diagram with the four
`6 P 712 crystal-field levels and the lowest component of the 4f6 5dconfiguration (with
`a different equilibrium distance).
`Finally Figure 3.16 gives the decay time ·of the Eu2+ emission in SrB 4 0 7 . At
`low temperatures it is 440 f-LS (parity forbidden 6 P -+ 8 S transition), but at higher
`temperatures it decreases rapidly due to the occurrence of the faster Sd-+ 4femission.
`The Sm2+ ion (4/6 ) can show Sd-+ 4femission in the red. However, if the
`lowest level of the 4f5 Sd configuration is at high energy, the intraconfigurational 4/6
`emission is observed. This runs parallel with the case of Eu3+ , although the Sm2+
`transitions are at a much longer wavelength.
`The Yb2+ (4/ 14) ion can only show one emission, viz. 4f 135d -+ 4/ 14
`• It is
`observed in the ultraviolet or blue. An example is given in Fig. 3.17. In the case of
`this ion the spin selection rule is of even more importance, since the observed decay
`times are very long for a Sd -+ 4ftransition (a few ms, Ref. [13]) .
`
`Vizio EX1018 Page 0060
`
`

`

`50
`
`3. Radiative Return to the Ground State: Emission
`
`L.OO
`
`i
`
`CJ}
`
`~300
`
`Cl)
`
`E
`
`0 o~------~2~0--------4~0~------~6~0------~B~O~~
`T (K ) -
`
`Fig. 3.16. Decay time of the Eu2+ emission of SrB4 0 7 : Eu2+ as a function of temperature
`
`l
`
`Fig. 3.17. The Yb2+ emission spectrum of SrB4 07: Yb2+ at 4.2 K
`
`370
`
`360 nm
`
`3.3.4 Transition Metal Ions
`
`The luminescence of the transition metal ions will be discussed using the Tanabe(cid:173)
`Sugano diagrams (Sect. 2.3.1 ). First we will consider ions which have played or
`still play an important role in luminescent materials, viz. Cr3+ and Mn4+ with d 3
`configuration and Mn2+ with d 5 configuration. Then we will mention some ions
`which became more recently of interest. For a detailed account of this field the reader
`is referred to ref. [ 1].
`
`Vizio EX1018 Page 0061
`
`

`

`3.3. Some Special Classes of Luminescent Centers
`
`51
`
`a
`
`700
`
`800
`
`900
`
`1 000 nm
`
`Fig. 3.18. Emission spectra of Cr3+. a: AI203: Cr3+ eE ~ 4 A2
`Mg4 Nb2 0 9 :Cr3+ (4 T 2 ~ 4 A 2 band emission)
`
`line emission); b:
`
`The luminescence of CrH in Ah0 3 (ruby) has already been mentioned in Chapter 1.
`It formed the basis of the first solid state laser in 1960. This emission consists of two
`sharp lines (the so-called R lines) in the far red (see Fig. 3.18). Since it is a line, it
`must be due to the transition 2 E -+ 4 A 2 (Fig. 2.9); generally speaking the emission
`of transition metal ions originates from the lowest excited state. The life time of the
`excited state amounts to some ms, because the parity selection rule as well as the spin
`selection rule apply. The emission line is followed by some weak vibronic transitions:
`obviously this emission transition belongs to the weak-coupling case.
`Not always the 2 E level is the lowest excited state. For relatively low crystal
`fields the 4 T 2 level is lower. In that case the emission changes character. It is now
`4 A 2 emission in the infrared with a decay time of r"oJ 100 J.LS. As an
`broad-band 4 T 2 -
`example, Fig. 3.18 gives the emission spectrum of Mg4 Nb2 0 9 : c~+. This emission
`forms the basis of tunable infrared lasers. Figure 3.18 also illustrates how dramatic
`the crystal field influences the emission spectrum in the case of d 3 ions.
`In YA1 3 B 4 0 12 : CrH, the emission at 4.2 K is 2 E-+ 4 A 2 , but at higher temperatures
`4 T 2 -+ 4 A 2 . Although the 2 E level is below 4 T 2 , the latter can be thermally occupied
`at higher temperatures. Since the 4 T 2 -+ 4 A 2 transition probability is higher than that
`of the 2 E -+ 4 A 2 transition due to the spin selection rule, the 4 T 2 emission rapidly
`dominates the emission spectrum.
`The strength of the crystal field on the c~+ ion is therefore of imperative impor(cid:173)
`tance for its optical properties. Its color is red for high crystal field strength (ruby),
`
`Vizio EX1018 Page 0062
`
`

`

`52
`
`3. Radiative Return to the Ground State: Emission
`
`and green for low crystal field strength. Its emission is a narrow line in the red in
`the former case, but a broad band in the near infrared in the latter. In some cases the
`4T 2 ---+ 4 A 2 emission band shows vibrational structure. There is not only a progres(cid:173)
`sion in the symmetric stretching mode v 1 , but also in v 2 (see Fig. 3.4) indicating a
`tetragonally distorted excited state.
`
`This ion is isoelectronic with cr>+, but the crystal field at the higher charged Mn4+
`ion is stronger, so that the Mn4 + emission is always 2 E---+ 4 A 2 . Usually the vibronics
`are more intense than for Cr'+ [14].
`
`The Mn2+ ion has an emission which consists of a broad band, the position of which
`depends strongly on the host lattice. The emission can vary from green to deep red. The
`decay time of this emission is of the order of ms. From the Tanabe-Sugano diagram
`(Fig. 2.1 0) we derive that the emission corresponds to the 4 T 1 ---+ 6 A 1 transition.
`This explains all the spectral properties: a broad band due to different slopes of the
`energy levels, a long decay time due to the spin selection rule, and a dependence of the
`emission color on the host lattice due to the dependence on crystal field. Tetrahedrally
`coordinated Mn 2 + (weak crystal-field) usually gives a green emission, octahedrally
`coordinated Mn2+ (stronger crystal field) gives an orange to red emission.
`
`d. Other d 11 ions
`
`The Ti 3+ ion (3d 1
`) gives a broad-band emission in the near infrared due to the 2 E---+
`2T 2 transition. The titanium-sapphire laser is based on this emission.
`The Ni2+ ion (3d 8 ) gives a complicated emission spectrum due to the appearance
`of emission transitions from more than one level. Luminescence from K.MgF3 : Ni2+
`appears in the near infrared CT2 ---+ 3 A 2 ), the red ( 1T 2 ---+ 3 T 2 ) and the green ('T2
`---+ 3 A2).
`Giidel and coworkers, in recent years, have reported many near infrared emissions
`from transition metal ions with "unusual" valencies (V2+(3d 3 ), V3+(3d2 ), Ti 2+(3d 2 ),
`Mn5+ (3d 2 )
`[ 15]).
`
`3.3.5 d ° Complex Ions
`
`Complexes of transition metal ions with a formally empty d shell show often intense
`broad-band emission with a large Stokes shift ( 10 000-20 000 cm- 1 ). Examples are
`Vo~- , Nb06-, WO~ - and Wo~- [ 16]. The excited state is considered to be a charge(cid:173)
`transfer state, i.e. electronic charge has moved from the oxygen ligands to the central
`metal ion. The real amount of charge transfer is usually small, but a considerable
`amount of electronic reorganisation occurs, in which electrons are promoted from
`
`Vizio EX1018 Page 0063
`
`

`

`3.3. Some Special Classes of Luminescent Centers
`
`53
`
`Table 3.1. Decay times T of some luminescent compounds with d 0 metai ions at 4 .2 K.
`
`Compound
`YV04
`KVOF4
`Mg4Nb209
`CsNbOP207
`CaMo04
`CaW04
`
`T (p.,s)
`500
`33500
`100
`500
`250
`330
`
`bonding orbitals (in the ground state) to antibonding orbitals (in the excited state).
`The value of 11R is large, the Stokes shift is large, and the spectral bands broad.
`Especially the complexes with the lighter metal ions show long decay times of
`their emission. Table 3.1 gives some examples. Following early suggestions [ 16], Van
`der Waals et al. were able to prove that the emitting state is a spin triplet [ 17]. They
`showed also that the excited state is strongly distorted due to the Jahn-Teller effect.
`Here we meet another clear example of an excited state which is distorted relative to
`the ground state.
`Octahedral complexes of this type have a smaller Stokes shift than the tetrahedral
`ones. The important consequences of this will be outlined in Chapter 5. Although not
`understood, certain structural configurations seem to promote efficient luminescence,
`for example, edge or face sharing of octahedral complexes (Li 3 Nb04, Ba3 W 20 9 ),
`and the occurrence of one short metal-oxygen distance (CsNbOP20 7, Ba2Ti0Si 2 0 7,
`KVOF4 and vanadate on silica [18]).
`Although it was believed for years that the emission spectra of these species were
`fully structureless, in recent years vibrational structure has been reported for several
`cases. A beautiful example is given in Fig. 3.19. This relates to vanadate on a silica
`surface. The progression is in a vibrational mode with a frequency of 950 cm- 1 • This
`is the stretching vibration between vanadium and the oxygen pointing out of the silica
`surface.
`The presence of ions with low-lying energy levels, for example ions with
`s 2 configuration, influence the d 0 -complex luminescence drastically. For example,
`CaW04 : Pb2+ (6s2) shifts its emission to longer wavelength relative to undoped
`CaW04 and the quenching temperature goes up. A nice example is YV04 : Bi3+
`(6s2
`) which has a yellow emission, whereas YV04 has blue emission. Arguments
`have been given to ascribe the new emitting states to charge-transfer states in which
`the 6s electron is transferred to the empty dorbital (metal-to-metal charge transfer
`[19]).
`
`3.3.6 d 10 Ions
`
`The emission transitions of ions with d 10 configuration are of a complicated nature
`and are only partly understood. For clarity these ions are here divided into two classes,
`
`Vizio EX1018 Page 0064
`
`

`

`54
`
`3. Radiative Return to the Ground State: Emission
`
`R v
`
`/I\
`000
`I
`I
`I
`Si Si Si
`
`400
`
`600
`500
`Wavelength (nm)
`
`700
`
`Fig. 3.19. Emission spectrum of the vanadate group on silica at 4.2 K
`
`viz. the monovalent ones (Cu+ and Ag+) and the higher valent ones (for example
`zn2+, Ga3+, Sb5+, Te6+).
`
`a. Monovalent Ions
`
`Complexes with monovalent d 10 ions show often efficient emission at room tempera(cid:173)
`ture. For cu+ the reader can find a summary in Ref. [ 19]. The emission transition has
`been assigned to a d 9 s ~ d 10 transition, a ligand-to-metal charge-transfer transition,
`or a metal-to-ligand charge-transfer transition, depending on the ligands. In the mean
`time the first assignment has been put in doubt, since optically detected electron para(cid:173)
`magnetic resonance measurements on the excited state of cu + in NaF point to a very
`low spin density in the Cu 4s orbital [20]. As an alternative, the excited state may be
`thought of as a Cu2+ ion which distorts its surroundings due to the J ahn-Teller effect
`and an electron which has moved away from the hole in the d shell so that an exciton .
`state is formed. This would be another example of an impressive relaxation after the
`absorption process.
`The Stokes shift of the cu+ emission is usually large (:::: 5000 cm- 1
`) , indicating
`the strong-coupling scheme. About the Ag+ ion less is known, but what is known
`shows a similarity with the cu+ data.
`
`b. Higher Valent Ions
`
`Luminescence from higher valent ions with d 10 configuration has been questioned
`for a long time. Nowadays there exists strong evidence for such a luminescence.
`Examples are Zn4 0(B02 ) 6 , LiGa02 , KSiSb0 5 and LiZrTe06 . The Stokes shifts are
`very large. Table 3.2 gives some spectral data [21 ].
`The nature of this emission is not yet completely clear, but probably it is a
`charge-transfer transition, LMCT in absorption. However, also a transition on the
`oxygen ion (2p6 ·~ 2p5 3s) plays a role. Such an interpretation implies indeed a large
`
`Vizio EX1018 Page 0065
`
`

`

`3.3. Some Special Classes of Luminescent Centers
`
`55
`
`Table 3.2. Some data on the luminescence of complexes with a central d 10 metal ion. All values
`at 4.2 K and in cm- 1
`
`•
`
`Compound
`
`Z14B60l3
`LiGa02
`KSbSi05
`Li2ZrTe06
`
`Complex
`Zn(II)04
`Ga(III)04
`Sb(V)06
`Te(VI)06
`
`Emission max.
`22.000
`27.000
`21.000
`16.000
`
`Excitation max.
`40.000
`45.000
`41.500
`33.000
`
`Stokes shift
`18.000
`18.000
`20.500
`17.000
`
`Table 3.3. Stokes shift of the Bi3+ emission in several host lattices [23].
`Stokes shift (cm- 1)
`800
`1800
`2700
`7700
`8500
`10800
`16000
`17600
`19200
`20000
`
`Composition
`Cs2NaYC1 6 : Bi
`ScB0 3 : Bi
`YAl3B4012: Bi
`CaLaA104 : Bi
`LaOCl :Bi
`La20 3 : Bi
`Bi2Al409
`Bi4Ge3012
`LaP04 : Bi
`Bi 2 Ge309
`
`amount of relaxation in the excited state and has been confirmed by molecular-orbital
`calculations.
`Indeed the 0 2 -
`ion, when isolated in the lattice, can yield an emission of the
`type 2p5 3s --+ 2p6 . Examples are LiF: 0 2 -, CdF2 : 0 2 - and SrLa2 0Be04. Since the
`3s electron in the excited state has a widely spread orbital, this transitiOn on oxy(cid:173)
`gen, charge transfer in d 0 complexes and charge transfer in d 10 complexes can be
`considered as three members of one family.
`
`3.3. 7 s 2 Ions
`
`Ions with outer s 2 configuration are of large importance in the field of luminescence.
`In principle their spectroscopy is well understood [22]. The influence of the host
`lattice on these properties is drastic which fact is considerably less understood.
`Well-known luminescent ions in this class are TJ+, Pb2+, Bi3+ (all 6s2 ), and Sn2+,
`Sb3+ (both 5s2
`). The influence of the host lattice can be illustrated by comparing
`Cs2 NaYC1 6 : Bi 3 + where the emission consists of a narrow band with considerable
`vibrational structure and a small Stokes shift (800 cm- 1) and LaP04 : Bi 3+ where
`the emission consists of a broad band without any structure at all and