Optical spectroscopy of the new laser materials, LiSrAlF6:Cr3+ and LiCaAlF6:Cr3+

Journal of Luminescence 44 (1989) 167-476 North-Holland, Amsterdam

167

OPTICAL SPECTROSCOPY OF THE NEW LASER MATERIALS, LiSrAlF6:Cr3~ AND LiCaA1F 3~ 6:Cr Stephen A. PAYNE, L.L. CHASE and G.D. WILKE University of California, Lawrence Livermore National Laboratory, P0 Box 5508, L-490, Livehnore, CA 94550, USA Received 9 February 1989 Revised 16 June 1989 Accepted 26 June 1989

We have obtained the absorption and emission spectra and the emission lifetimes, of Cr3 k-doped LiCaAIF 6 and LiSrAIF6. The spectral observations can be understood by carefully accounting for the small non-octahedral components of the crystal potential field existing at the substitutional Al site. The polarization properties of the absorption spectra are due to a static 2~-~~le component of the field, although the odd-parity dynamical 4T component also contributes significantly to the oscillator strength. An analysis of the spin—orbit components of the 2 state reveals the importance of a Jahn—Teller distortion in the e5 coordinate. Finally, the overall configurational displacement between the ground and excited states is found to be larger in LiSrAIF6, compared to LiCaA1F6.

1. Infroduction 3 + impurity is nearly always found in The Cr environments which are characterized by six nearest-neighbor (nn) ligands. This is the case because both the ionic radius and the “octahedral 3 + favor crystal stabilization” of +Cris nearly sixfold field coordination [1,2]. energy Since Cr3 always situated in octahedral sites, the absorption and emission spectra are similar in a large variety of materials, the main distinction being the strength of the octahedral field, rather than its exact In nature. addition to the octahedral field, however, small low-symmetry distortions are often present. These distortions may arise from dynamical vibrations, or from the static fields of the host medium, In this work, we will consider the effects of these small fields on the Cr3 + impurity incorporated on Al3~ sites of the hosts LiSrA1F 6 (LiSAF) and LiCaA1F6 (LiCAF). On the basis of the crystal structure determination [3] of LiCAF, it can be shown that static fields having t 2~-type character exist at the 3 + site [4]. t25 As and a result of the t Al 25 and t2u fields, the site symmetry is lowered from °h to D3. It is

shown that the static odd-parity t2~, field fully accounts the polarized of the absorption and for emission spectra, nature while the odd-parity dynamical vibrations provide an isotropic component to the transition strength. The 4A 4T 2, 2 states exhibit a greater relative offset in LiSAF pared to LiCAF (see sect. 3.2), giving rise cornto a larger Stokes shift. The totally symmetric a 15 coordinate is normally implicated as causing the greater Stokes shift [5], although the e5 coordinate has been found to have a significance comparable to that of the a15 mode due4T to states the influence of the (sect. 3.4). Jahn—Teller effect on the Cr-doped LiSAF and LiCAF have both recently been demonstrated to lase and the development of these solid-state lasers is likely to benefit from a comprehensive ui~derstandingof their electronic states [6—91.

2. Experimental The experimental methods used were conven-

tional and will only be briefly outlined. The absorption spectra were red~ordedwith a computercontrolled Cary-17 spectrophotometer. The enus-

0022-2313/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

168

S.A. Payne et aL

/

Optical spectroscopy of new laser materials

sion spectra were obtained by exciting with either a helium—neon laser or the 458 nm line of an with a GaAs photomultiplier tube. Lock-in and argon—ion laser, and monitoring the luminescence computer techniques were used to improve the signal-to-noise ratio. The spectral response of the monochromator— detector combination was not corrected for, since the spectral response is reasonably flat over the region of interest. To de-

8 6

Absorption spectra at T -20 K l\

2 4 0

~ ElIc ~FS:C~,/~!

V\~

I

I

I

3~

UCaA~’F

C,’ —

6: Cr

E

—

2~ 1~!

___________

C

termine the emission lifetime, a N 2 laser-pumped transient to average dye laser digitizer served as was the used excitation source, several and a

o 8 ~~ 4

hundred decay curves. In all the experiments, the sample temperature was varied between 20 K and room temperature with a close-cycle helium re-

3~

.2

L1SrAEF6: Cr

U’ U’

2

0

C.) C

L1CaAI’F 6: Cr 3+

frigeration system. The LiCaA1F6 and LiSrA1F6 crystals were both gro~ by the horizontal zone melting technique by Newkirk and Kway at Lawrence Livermore National Laboratory [7]. The initial charge contamed a stoichiometric mixture 3~concentration of the fluoride components and the actual Cr was measured by the inductively coupled plasma (ICP) technique.

3

400

500 600 700 Wavelength (nm)

3

2 1 3

Fig. 1. Low-resolution absorption spectra of Cr3~-doped LiSrA1F 6 and LiCaA1F6 at 20 K for the Eli c (ii) and E ±C (n) polarizations. Note the strong polarization dependence and that the cross sections of the LiSrA1F6 spectra are higher than those of LiCaA1F6.

3 : LiCAF. are about twice as strong as those of Cr (c) The 4A 12 to 14756 cm1 in passing ~T2 zero-phonon line from shiftsLiSAF from 14296 cm to LiCAF. (d) Cr3 : LiCAF exhibits much fine structure, while the vibrational features are mostly washed out for LiSAF. In the present section, we +

3. Results and discussion 3.1. Absorption spectra

—+

The absorption spectra of Cr-doped LiSAF and LiCAF at 20 K appear in fig. 1. Both the ir(E c) and a(E L c) spectra are shown since these host materials are uniaxial. Since LiSrA1F 6 and LiCaAlF~both have for the the colquiriite structure [3] 3 substitutes Al3 site, one might and Cr the spectra to be very similar, and superfiexpect cially they are. The band centered near 450 nm is due to the 4A 2 ~T1a 4A 4T transition, and that near 650 nm is the 4T 2 2. The sharp dips 2Esupenmand 2T posed on the general 2 band arise from the known [10].1 states. These features are well As the data in fig. 1 are examined further, it becomes apparent that several observations must be explained: (a) The relative intensities of the 4A 4T 2 ~ and 1 a transitions Eeverse, depending the polarization of the lightoffor both andonLiCAF. (b) The transitions ~ : LiSAF LiSAF +

+

—~

—~

—*

+

+

will primarily address observations (a) and (b) and deferemission a discussion of in (c)sect. and 3.2. (d) until we consider the spectra The question relating to the absolute and relative absorption strengths of the bands in fig. 1 are best reduced to the values of the oscillator strength,

~

f, as defined by [11,12]1 1[ mc~ f’ I ~+ 2)2 j[~jfa di,

(1)

where n is the refractive index, m and e are the electron mass and charge, a is the cross section, and i’ is the2/s~e2 frequency wavecm1. numbers. The = 1.13 in x 10~ Utilizing prefactor mc eq. (1) and the spectra in fig. 1, the oscillator

S.A. Payne et a!.

/

Optical spectroscopy of new laser materials

Table 1 4A 4T 4A Oscillator strengths of the 3 + impurity in 2several —~ 2hosts and 2 —‘ ~T1a transitions of the Cr Host Polarization Oscillator strength (units of 10-6) 4A

4T 2 -~

4A 2

sr o

109 53

45 98

LiCaAIF6

o~r

48 28

51 39

We now develop the group-theoretical descrip4A 4T This tion of derivation the 2 appears ~T~, elsewhere 1 a transition [4] but strengths. will be —+

given here in abbreviated form for completeness.

4T 2 -,

LiSrA1F6

169

1a

The transition rate R ij~iducedby the odd-parity field, V.,, can be described, to within a constant, by 2, (2) Ra=

I
where j~a is the appropriate form of the dipole moment for light of polarization a. The symmetry properties of the J’~, potential at the Al3 site is described with +

strengths have been calculated and are listed in table 1. The values are in the range of i05—i04, as is typical due to the forbidden nature of the

~

[~v(t

+ ~v(t2ufl)

2u~)

d—d bands of transition metal ion impurities. The main question want to address is why the 4A 4A we 4T 2 ~ ( 2 1a) transition is stronger in the ~i-(a) spectrum. Since, as will be seen, the polarization dependence is due to the static oddparity field present at the Al site, we now need to consider the details of the crystal structure of LiCAF. The unit cell of LiCaA1F6 is reproduced from the work of Viebahn in fig. 2 [3]. We3 now focus substituour attention on the site,The since Cr tionally replaces thisAlion. deviation of the A1F 6 cluster from perfect octahedral symmetry can be imagined by considering two symmetry-lowering operations on the hypothetically perfect A1F6 octahedron. Firstly, the three fluorines located above, and those located below the Al ion, must be moved slightly closer together. This distortion is of even parity (t 2g type) and introduce a 4T may states. The other trigonal field splitting into the involves the destrucsymmetry-lowering operation tion of the center of inversion, and therefore will induce oscillator strength into the d—d transitions. We may imagine that the effect of the Ca ions is slightly clockwise (as viewed from above) and the to rotate the upper equilateral triangle of fluorines lower three fluorines counterclockwise. The distortion can be described by the t 2,, representation according to group theory, and is anticipated to be responsible for the polarization dependence of the absorption band displayed in fig. 1. As a result of these two symmetry-lowering operations, the site symmetry is reduced from OhtO D3. —+

—*

(3)

+ ~~(T2~)],

where the Al—F bonds are near the x, y, and z axes, as is appropriate for the tetragonal basis. The ~~(t2~’r) operators describe the odd-parity field, and transform according to the r •‘1~ ~ row of the ~~ representation of the °h group. =

~,

(a)

+

•Ca

ØAI

(b)

~

Li

Q

F

L..

1

0

/(~Sc—~X~----®ca ~ ~LI4~F

14

36

0

3 4 Fig. 2. Crystal structure of the LiCaAIF6 lattice, reproduced from the work at Viebahn [3]. The numbers in the circles of part (b) are the percentage height of the ions relative to the full c axis dimension of the unit call in (a). The Al site exhibits small t~- and t2~-typedistortiotis from exact octahedral symmetly.

170

S.A. Payne et aL

/ Optical spectroscopy of new laser materials

Table 2 3~transition strengths resulting from the Calculation of the Cr static t 2,, crystal field distortion of LiSAF and LiCAF (all representations are gerade Transition 4A

4T 2(t~)—~

4A

Transition strength

Since there are four unknowns and four experimental measurements, the problem is not constrained but the information derived is neverthe-

E II c (er)

less interesting and reasonable. The results of the

4T 2(t~)—‘

E i. c (o)

41
2(t~e)

=

1(t~e)

and the data in table 1, we canfordetermine the dynamic and static contributions each crystal.

fstat(

2

2)

~iKt2iit~lIe> =

~l

0

=

fit are listed in table 3. First of all, the static t2~, distortion explains the observed polarization dependence. Equations (5) clearly indicate that the 4A 4A 2 ~T2 transition should predominate in the ~r polarization, and the 2 ~‘T~in a. Secondly, we see that the fstat values are a factor of 3.6 ±1 times larger for LiSAF and LiCAF, indicating a

2

~ fstat (4

2

T1)

—‘

—*

Similarly, the symmetries of the dipole moments for ~ and a polarized light fields are described with9 1/~/~[~(t ic— 1,,x) + ,(t1~y)+ ~~(t1~z)], (4a) is,,

—+

greater ~2u distortion for LiSAF relative to LiCAF. Thirdly, values for the ~1’2 4T we see that the and 1 states are roughly similar in the twostudied hosts. 3~(GFG), previously Also included in table 3 are athe fdyn values for Na3Ga2Li3F12:Cr material in which the Ga site is known to be centrosymmetric [8]. We therefore assume that the

1/~’~[~(t 1~x) ~,(t1~y)],

(4b)

entire oscillator strength of GFG is dynamically

where the ~ operators transform according to the representation and row indicated parenthetically. By substituting eqs. (3) and (4) into (2) and using the tables of Griffith couple the V., ~of and Vd~t~~ terms, and by [13] the todecomposition the 4A 4T 4T 2(t~), t2(t~e),and 1a(t~e)states into their constituent 25 and e54A orbitals4T[5], the polarization 4A dependences of the 2 2 and 2 ‘~T1a transition rates can be calculated to within a reduced matrix element. The results of these calculations appear in table 2. This analysis essentially accounts for the oscillator strength induced by the static field. We need also to include the dynamically-induced portion of the oscillator strength, fdyn’ In so doing, we can describe each of the four transitions with 4T 4T f~,,( 2)—f,jyfl( T2) +fstat( 2), (5a)

induced. As one may anticipate, table 3 shows that the fd~nvalues are similar for LiSAF, LiCAF, and GFG, as a result of the similar of the 3 site.nature The average dynamical motions at thefor Cr the three materials dynamical contributions 4Tl) are fdyn( T2) = (25 ±10) X 10_64Tand fdY~( (39 ±10) x 10_6; note that the 1a value is con4T sistently larger than that obtained for the 2 state. The temperature dependence of fdy~ was determined from the Cr: GFG absorption spectra, and gave an enabling mode frequency of 310 cm~. Similar temperature analyses were not as accurate for the LiCAF and L1SAF absorption

—

—

f,, (~i’2)

=

f,, (~T1)

=

—‘

4r

fdyn

(~i’2),

4T1),

(5b) (Sc)

fdYfl(

4T

4T,), 1)

+ fstat

=

Table 3

Calculated dynamic and static contributions to the oscillator

( 2) + ~fstat

Jo (~Ti)= fdyn (

+

(Sd)

(

where the definitions of the reduced matrix elements are indicated given in table 2, and the relevant states are parenthetically. With eqs.final (5)

strengths for several Cr-doped hosts (see eqs. (5)) Parameter Oscillator strength (units of 10 6) LiSrA1F 6 LiCaA1F6 Na3Ga2Li3F12 (For comparison) f 4T 4~n(4T 2) 34 21 19 foat( 2) 27 0 4T~ 75 45 39 33 fdY,~( ~tat(4T~ 53 12 0

S.A. Payne et aL

/ Optical spectroscopy of new laser materials

spectra, owing to the combination of both dynamic and static contributions. 3.2. Emission spectra

4T 4A 3 k-doped The and 2 LiCAF 2 emission LiSAF appearspectra in fig.of3.CrThe most salient observation is that the vibrational structure —#

Table 4 3 tdoped LiCAF and for the Vibrational elpasolite, Kassignments for Cr 2NaGaF6 1) Vibrational Energy assignment LiCaA1F(cm 6 K2NaGaF6 (ref. [15]) 561

575

tiu

452 308

480 330

~

217

200

~ e 5

is much more pronounced for LiCAF, than for LiSAF, as is the case for the absorption spectra in fig. 1. The Stokes shift is normally interpreted as being due to the relative displacement of the equilibrium position in the configuration coordinate for the ground and excited states. The emission spectra show that theLiCAF Stokes shifts are 1070respeccm’ 1 for and LiSAF, and 1450 cm tively, indicating a larger displacement for the LiSAF host. The displacement can be quantified by determining the Huang—Rhys factor, S, with [14] A (0—0) e A(sideband) (6) —

—

—

where the A’s are the areas of the 0—0 line or the

Emission spectra T=20K

III

~,

sideband. We can estimate that S(LiCAF) 4.2 and S(LiSAF) 5.9. This shows that there is a measurable difference the configurational displacement, and that theinLiSAF host allows greater relaxation than LiCAF. By dividing the Stokes shift by S, we can determine the average frequency of the configurational coordinate, which is 255 cm1 for LiCAF and 246 cm~ for LiSAF. The similar magnitude of the frequencies is reasonable, phonon since thedensity two hosts of states~ are ~xpected to have a similar The assignments for Cr3 : LiCAF are designated on the emission spectrum in fig. 3, and are listed in table 4, along with the assignments of 3 [15]. There are numerous data K2 NaGaF6 and analyses : Crwhich support the assignments in table 4 [15—24].These include infrared and Raman spectra, two-phonon spectra, magnetically circularly polarized emission studies, and lattice +

+

6: Cr 3+

tIIIIIIIIIIII

eg

~.

LiSrAQF

171

II

u~a1g

tlu

dynamics analyses calculations. the analogous. isoelectrbmc h~addition, vibrational out to beof quite TheMn4~impurity mainthepoint of turn this exercise is to note that the vibrational features of LiCAF can be assigned in analogy to a perfectly octahedral CrF 6 cluster, as exists for the elpasolite hosts. This indicates that the deviation from octahedral symmetry is probably 3not: LiSAF, large for on LiCAF. emission spectra of Cr the otherThe hand, defy this simple interpretation of its vibrational structure, due to the larger value of S and the less resolved nature of the emission spectra. We can estimate the niagnitude of the trigonal +

650

_______________________________ 700

750 800 Wavelength (nm)

850

900

Fig. 3. Emission spectra of Cr3 ‘-doped LiSrAIF 6 and LiCaAIF6 at 20 K. Note that the LiCaAIF6 host exhibits much more fine structure than LiSrAIF6. The vibrational assignments are indicated in the figure.

field splitting in LiCAF using the standard methods of crystal field theory 3’ and free-on the value point [25] charge of model [13]. Using the Cr

172

S.A. Payne et aL

/

Optical spectroscopy of new laser materials

that the cross section of eq. (7) should be averaged over all polarizations for non-cubic crystals. Using eqs. (7) and (8), and the data in fig. 1 and ref. [8], the emission lifetimes are calculated to be 156, 280, and 540 ~ss for LiSAF, LiCAF, and Na 3Ga2Li3F12 (GFG), respectively, to be compared with the measured low-temperature values of 67, 205, and 530 its. The agreement is reasonable for GFG and LiCAF, although the discrepancy ~s larger for LiSAF (156 vs. 67 its). The indicates nonradjative decay isof unlikely to lack of anythat temperature dependence Cr: LiSAF

250 Emie,ion lifetimedata

~

200

3

UCaAfF

6:Cr

0 1100

. -

-

~—_~__!

3~ // LISrA2F5: Cr

50

play a significant role in affecting the lifetime. The basis for this discrepancy remains uncertain.

__

Temperature (K) Fig. 4. Emission decay times for Cr3~-dopedLiSrA1F 6 and

LiCaA1F6 as a function of temperature. The data has been fit to a combination of statically and dynamically induced transition rates, see eq. (9).

2) = 0.695 A, and the necessa~structural data ~r ref. [3], we calculate that (T from 2~I This value T2n) =is 1 for the tetragonal basis. —220ancm only estimate, and shows that the splitting may be on the order of several hundred wavenumbers for LiCAF. There appears to be little evidence, however, for the trigonal field splitting in the

The temperature dependencies of fig. 4 can be explained by assuming that the emission lifetime is influenced by both the dynamic and the static transition rates, T [kstat + kdyfll ~

(9)

=

(Emission show thatmaterials nonradiative decay is not experiments important for these at room

6

Emission spectra of LiSrAi’F 6: Cr ~

absorption or emission spectra. 3.3. Emission liJetimes

T = 28K

-

30J\

The emission lifetimes of Cr in LiSAF and LiCAF are displayed in fig. 4. The main observalions which we wish to explain include the absolute magnitudes and the temperature dependences of the decay times. The emission lifetime, T, can be related to the spectrally-integrated erms-

2

~2~ ~4 I

C

[~J_l

sion 8cross x section, ircn2

,~

2

I GE,

with [26]

~J\50~

a)

and the integrated absorption and emission cross sections, GA and a~,of the 4A 2 ~T2 transition are related by

2

75K

—‘

gexfaE di’ = g5fldfaA

di’,

(8)

where the ground and excited state degeneracies, and

gex,

have been taken into account. Note

0 14.40 14.35 14.30 3cm~) 14.25 14.20 Fig. 5. Fine structureWavenumber(10 of LiSrA1F 3’ in the vicinity of the origin of the 4T 4A 6 : Cr 2 —~ 2 emission at several temperatures. The energies of the zero-phonon lines are indicated.

S.A. Payne et aL

/

Optical spectroscopy of new laser materials

temperature, or below, and need not be considered further.) The dynamical part is usually described by the hyperbolic cotangent law [14], kd~,,,= kd°~coth(h~~/2kT), (10) where hw is the phonon energy of the odd-parity enabling mode, and k is the Boltzmann constant. In order to limit the number of parameters, we utihze the values of hca= 310 cm and kd~= 2040 s1 previously derived for Cr3~in Na 3Ga2 Li3F12 (GFG). (Recall that the Ga site in GFG is centrosymmetric, and therefore, the transition rates are entirely dynamically induced.) The fits displayed in fig. 4 are the results of using these GFG values in eqs. (9) and (10). The fitted values of kstat are 12890 s_I for LiSAF and 2960 s~’for LiCAF. The important result here is that the absence of any temperature dependence for LiSAF and the small change observed for LiCAF can be explained by accounting for the appropriate magnitudes of the dyna.mically and statically induced contributions to the overall rate. 3.4. Zero-phonon lines The 4T 4A 3 in 2 2 emission spectra of Cr LiSAF and LiCAF are shown in fig. 5 and 6. For 3 : LiCAF in fig. 6, four lines are the case of Cr clearly observed. The spectra are shown for several temperatures to demonstrate that all four of the emission lines are associated with the same Cr3 center, since the relative emission intensity from the higher lying levels is seen to increase as the temperature is raised. Group-theoretically, we may expect four spin-orbit (SO) levels for the 4T 2 state in an octahedral field, since U X T2 E’ + E” + 2U; theinenergies of positions these lines indicated for LiCAF fig. 6. The of are the four lines are uncertain for LiSAF in fig. 5 and it is possible that the dominant line is actually composed of two unresolved components. +

—~

+

+

—~

In what follows, we will thoroughly analyze the four components of the LiCAF spectra, and will not consider3~environment the LiSAF dataisfurther. assume nearly We octahedral; that the Cr the vibrational assignments of sect. 3.2 and the unsplit nature of the U states suggest that the trigonal splitting is small, in spite of the estimate

173

-

3~

LiCaA2F

6: Cr 14

2

I

-

(~

1

4~ ______

~‘

—~

T = 23K

/ J

I

I

2 75K —

1 0 I

2

I

125K

1 0 I

I

I

14.90

14.85

14.80

I

14.75 3cnr1) 14.70

Wavenumber Fig. 6. Fine structure of LiCaAlF (lO3~in the vicinity of the 6:Cr origin of the ~1’2—~4A 2 emission at several temperatures. The 4T energies of the spin-orbitindicated. components of the 2 state are

of several hundred wave numbers obtained from the point charge model. We can estimate, on this basis of first-order SO coupling and the free-ion value [13] of ~ = 273 cm 1, that the maximum separation of the zero-phonon lines should be 2~/3= 180 cm This calculated result is1 much the in fig.greater 6. This than observaobserved value of 60 cm tion can be explained by utilizing the theory of Ham [27,28]. In 1965, Ham showed that the SO splitting of the T states of impurity ions is “quenched” as a result of the T x e 5-type Jahn—Teller effect (JTE). The main impact of this quenching is to reduce the splitting between the zero-phonon to much less than that which is expected on lines the basis of’ crystal-field theory and the free-ion value of The 4T 2 state is expected to be SO split into four states, describable by the °h double group as ~.

174

S.A. Payne et aL

/ Optical spectroscopy of new laser materials

F = E’, Ua, Ub, and E”, having degeneracies of 2, 4, 4, and 2, respectively. By including the effect of Ham quenching and by using first- and secondorder perturbation theory, we can calculate the energies of the four zero-phonon lines having the symmetries F with the equation [29—33] E(r)

I 160

-

Quenchingsplitting of spin-orbit

E’

ç 120

-

()

—

218 cm~

—.

a1~exp(—3Eff/2hwe) 4T 2Q(F;4T +~[b1( 2, I)~] 2,I)/E(I),

=

I

-

80 ~flw=452’cm~

(11) where E~is the JT energy, w,, is the frequency2E of the e mode, the summation includes I = ~A2, 2T 4T 1, ~T2 and 1a, and the symmetry coefficients, b1 and a1, were obtained from table A34 of 4T Griffith [13]. The Q(F; 2, I) of eq. (11) account for the Ham quenching of the off-diagonal, second-order values calculated matrix explicitlyelements. for eachThe stateQ for whichwere the second-order energy shift was > 2 cm1 (see ref. [34] for the values of Q utilized). By inserting the values of the energies relative to the 4T 2 state, E(I), from fig. 7, we obtain the curves describing the SO components, as is plotted in fig. 8 as a function of the Jahn—Teller energy. Here, we have

30

4T~a2T

-

quenching 92

-

2Tf~\~~ ~

2E

::~~ 1L—~i ~o~2~9

—

wC 10

I

10.5

-13.6

-

0

100

200

300

400

Jahn-Teller Energy (c&) Fig. 8. Calculation of the relative positions of the zero-phonon 4T lines of the 2 state, based on the theory of Ham [27],using eq. (11). The solid circles are the experimentally observed 3~,as obtained from fig. 6. positions of the spin—orbit components for LiCaA1F6 Cr

t’~’e 452 cm ~ as was derived from the emission used h spectra of sect. 3.2. We note that our handling of the second-order SO interactions differs from the method of Sturge [29] and others [30—32],in which the second-order SO interactions are fit to an approximate effective Haxmltonian, and then the parameters of the fit are quenched by the Jahn—Teller effect. We prefer the present method since the information concerning the of the second-order matrix elements is directly preserved, since othy those elements off4T diagonal the and 2 basis will experience gested by in Stephens Lowe-Pariseau [33]. the quenching. This type of calculation was first sugThe zero-phonon line energies are plotted for = 218 cm1 in fig. 8. Since the free-ion value is 273 cm1, the magnitude of ~ used in fig. 8 =

—

assumes that the effect of covalency leads to 20%

a

_______

.—.

Qualitative configurationsi coordinate

reduction from the free-ion value. We estimate from the Ham quenching of the spin—orbit components displayed in fig. 8 that EJT 250 cm~. In a study reported elsewhere, we measured the 4T 4T 2 3 :1LiCAF a excited state absorption and found that the(ESA) peak spectra of this ESA of Cr band occurs at a much higher energy than —

—

potential energyrepresentation surfaces of the Cr3’ impurity states in Fig. 7. Pictorial of the relative locations of the LiCaAIF 6 as a function of the Cr—F separation for aqualitative coordinate. The energies denoted in the figure were used to calculate the off-diagonal matrix elements of eq. (11).

+

that which is predicted to be the case on the basis of crystal field theory [4]. The difference between

S.A. Payne et aL

/

Optical spectroscopy of new laser materials

the measured and predicted peak positions was attributed to the JTE, and a value of EJT = 600 cm was derived from the spectrum. The present value of 250 cm~ differs substantially from the previous result obtained from the ESA spectrum. The reasons for this discrepancy are related to the approximate application of both the Ham theory and the analysis of the ESA. In any case, it is somewhat encouraging to find that both analyses suggest that the JTE is important and EJT is on the order of several hundred wave numbers. -

It was stated earlier that the trigonal static field at the Al3 site was estimated to have 4T2 matrix 1. Since the U spin-orbit elements states are of not220 split,cm however, we assume that the actual trigonal field is much smaller. In fact, it ~5 likely that the trigonal field splitting is quenched by the JTE, as is the SO splitting [33,35].

175

References [1] S.A. Payne, L.L. Chase and W.F. Krupke, J. Chem. Phys. 86 (1987) 3455. [2] For example, see F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemis~ry(Wiley, New York, 1962). [3] V.W. Viebahn, Z. Anorg. ~llg. Chem. 386 (1971) 335. [4] H.W.H. Lee, S.A. Payne, and L.L. Chase, Phys. Rev. B39 (1989) 8907. [5] S. Sugano, Y. Tanabe and H. Kamunura, Multiplets of Transition-Metal Ions in Crystals (Academic Press, New York;1970). [6] S.A. Payne, L.L. Chase, L.K. Smith, W.L. Kway and H.W. Newkirk, J. Appl. Phys. 66 (1989) 1051. The mea-

+

4. Conclusions We have shown in this work that the static t2u distortion at the Al Site of Cr3 tdoped LiCaA1F 6 and LiSrA1F6 gives rise to the polarization characteristics of the absorption and emission spectra, while the dynamical distortions also provide a contribution to the absorption strength and the emission lifetime. There also tends to be greater relaxation in LiSAF host compared to LiCAF resulting from the displacement between the equi4A 4T librium position of the 2 ground- and the 2 excited-states. Finally it was shown that the T2g X eg Jahn—Teller effect is active using Ham’s theory of spin—orbit quenching.

Acknowledgements We wish to thank Herbert Newkirk and Wayne Kway for providing us with the LiSrAIF6 and LiCaALF6 samples used in these experiments. This work was performed under the auspices of the Division of Materials Sciences of the Office of Basic Energy Sciences, US Department of Energy and the Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.

suredPayne, intrinsic efficiency to be L.K. 53%. Smith and [7] S.A. L.L. Chase, was H.W.found Newkirk, W.F. Krupke, J. Quant. Electron. 24 (1988) 2243. [8] J.A. Caird, S.A. Payne, P.R. Stayer, A.J. Rainponi, L.L. Chase and W.F. Krupke, J. Quant. Electron. 24 (1988) 1077. [9] J.C. Walling, 0.0. Peterson, H.P. Jenssen, R.C. Morris and E.W. O’Dell, IEEE 1. Quantum Electron. 16 (1980) 1302. [10] D.L. Wood, J. Ferguson, K. Knox and J.F. Dillon, J. Chem. Phys. 39 (1963) 890. [11] W. Kauzmann, Quantum Chemistry (Academic Press, New York, 1957). [12] W.T. Simpson, Theories of Electrons in Molecules (Prentice—Hall, New Jersey, 1962). [13] J.S. Griffith, The Theory of Transition Metal Ions (Cambridge Univ. Press, London, 1972). [14] A.M. Stoneham, Theory of Defects in Solids (Oxford Univ., Press, London, 1985). [15] L. Dubicki, J. Ferguson and B. von Oosterhout, J. Phys. C 13 (1980) 2791. [16] R.L. Chien, J. Berg, D.S. McClure, P. Rabinowitz and B.N. Perry, J. Chem. Phys. 84 (1986) 4168. [17] Flint, J. A.M. Mol. Spectrosc. (1971) 414. [18] C.D. S.L. Chudos, Black and 37 C.D. Flint, J. Chem. Phys. 65 (1976) 4816. [19] W.C. Yeakel, R.W. Schwartz, H.C. Brittain, J.L. Slater and P.N. Schatz, Mol. Phys. 32 (1976) 1751. [20] J. Ferguson, H.J. Guggenheim and D.L. Wood, J. Chem. Phys. 54 (1971) 504. [21] P. Greenough and A.G. Paulusz, J. Chem. Phys. 70 (1979) 1967. [22] R. Knochenmuss, C. Reber, M.V. Rajasekharen and H.U. Gudel, J. Chem. Phys. 85 (1986) 4280. [23] D. deViry, J.P. Dems, B. Blanzat and J. Grannec, J. Sol. St. Chem. 71 (1987) 109. [24] D. deViry, J.P. Deals, N. Tercier and B. Blanzat, Sol. St. Commun. 63 (1987) 1183. [25] Zhao Min-Guang, Xu(1983) Ji-An,1516. Bai Gui-Ru and Xie HuongSen, Phys. Rev. B 27 [26] W. Koechner, Solid-State Laser Engineering (Springer, New York, 1986). [27] F.S. Ham, Phys. Rev. A 138 (1965) 1727.

176

S.A. Payne et aL

/ Optical spectroscopy of new laser materials

[28] M.D. Sturge, in: Solid State Physics, Vol. 20, eds. F. Seitz, D. Turnbull and H. Ehrenreich (Academic Press, New York, 1967). [30] M.D. [29] P. Koidl, Sturge, Phys. Phys. Stat.Rev. Sol. BB 1(1970) 74(1976)477. 1005. [31) S. Muramatsu and N. Sakamoto, J. Phys. Soc. Jpn. 46 (1979) 1273. [32) 0. Pilla, E. Galvanetto, M. Montagna and G. Viliani, Phys. Rev. B 15 (1988) 3477.

[33] P.J. Stephens and M. Lowe-Pariseau, Phys. Rev. 171 (1968) 322. 4A [34] Thevalues are Q(Ua; ~1’3, 4T 2E) = 1/2 + ~y/2; Q(Ua; ~1’2,2T 2)=11/25+14y/25; 4y/25 Q(Ua; Q(Ub; 2, 4T 2E) = 1/2+ y/2; Q(Ub;2) 4T = 11/25 2T~) += 1 3/8+ 5?/8; Q(E’; 2, 4T 2T 2, 2, 1) = 1/3 + 2y/3, where ~ = exp(3E~-/hwe). [35] W.C. Scott and M.D. Sturge, Phys. Rev. 146 (1966) 262.