Static and Dynamic luminescence effects of Cr3+-Tm3+ pairs in YAG

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Static and Dynamic luminescence effects of Cr3+-Tm3+

pairs in YAG

V. Lupei, L. Lou, G. Boulon, A. Lupei, C. Tiseanu

To cite this version:

V. Lupei, L. Lou, G. Boulon, A. Lupei, C. Tiseanu. Static and Dynamic luminescence effects of Cr3+-Tm3+ pairs in YAG. Journal de Physique I, EDP Sciences, 1993, 3 (5), pp.1245-1254.

�10.1051/jp1:1993269�. �jpa-00246794�

J. Phys. J Franc-e 3 (1993) 1245-1254 _MAY 1993, PAGE 1245

Classification

Physics

Abstracts

78.50 78.55

Static and dynamic luminescence effects of Cr3+-Tm3+ pairs

in YAG

V.

Lupei

_(~>

*),

L. Lou

(~),

G. Boulon

(~),

A.

Lupei (2)

and C. Tiseanu

(2)

(1) Laboratoire de

Physico-Chimie

des Matdriaux Luminescents(**), Universitd

Lyonl,

Bit. 205, 69622 Villeurbanne Cedex, France

(2) Institute of Atomic

Physics,

76900 Bucharest, Romania.

(Received 9 November 1992, revised 4 January 1993,

accepted

14 January 1993)

Abstract. Luminescence emission of Tm~+ in YAG codoped with Cr~+ under selective (into

~ECr~+ levels _or ~F~ Tm~+ levels) and nonselective (into ~Y~Cr~+ level) excitation is dominated by three _new centers. Based _on structural and spectral characteristics, these _{centers are} associated with the three

possible perturbations produced by

^C~+ ions at the _nearest

neighbour

Tm~ + ions. A selective _energy transfer within these

Cr~+

Tm~+

pairs

is observed.

1. Introduction.

A _common way to

improve

the laser

efficiency

of

rare-earth-doped

systems ^{is the}

codoping

with transition metal

ions, Cr~+ being

the most used _so far. The sensitization process is

especially important

for

high

threshold, low

gain

systems

doped

with

Tm~

+

or

Tm~

+

:

Ho~

+.

A condition for effective sensitization via energy transfer between the transition metal ion

(which

_{acts as an energy}

donor)

and the active rare-earth acceptor ^is

(besides

the

overlapping

of emission and

absorption bands)

_a short distance between these ions in the

crystalline

lattice.

However the

proximity

of _a substitutional transition ion which differs in

size,

electronic structure and sometimes in

charge

from the substituted host cation _can

perturb

the

crystal

field at the active rare-earth ion site. This _can

produce

shifts of the active ion energy levels, manifested _as satellites of the lines

corresponding

to the

unperturbed

ion. Likewise the active ion _can

perturb

the energy levels of the sensitizer.

Since the

crystal

field

perturbation depends

on the distance from the sensitizer _to the active

ion,

the closest

pairs (that

show the most effective energy

transfer)

will be

usually

the _most shifted in energy this leads _{to a}

spectral sensitivity

^of the energy transfer which may

modify

the

spectral composition

of emission in sensitized

crystals.

(*) Permanent address Institute of Atomic

Physics,

76900 Bucharest, Romania.

(**) Unitd de Recherche associde _au CNRS _n 442.

JOURNAL DE PHhs10UE -T 3 N' ~ MAY 1993

An

important

active ion for which the sensitization is necessary is

Tm~ +,

with emission in

the two micron range.

Tm~+

_has

sharp

and weak

absorption

bands in the infrared and the

visible which cannot be

efficiently pumped by

_gas

discharge lamps.

Under such

pumping,

^the

excitation collects _on the ~H~ ^levels

(Fig, I).

At

large Tm~

⁺ concentrations _a very efficient

cross relaxation

involving

_~H~ and ~F~ ^{levels takes}

place, leading

to the

population

^of the

~F~ ^{metastable level. This excitation from} ~F~ can also be transferred _to the ~I~

Ho~

+ level.

20

18 'b

id ^~E 14 m12

~fi10

~ 3

~

6

-l- z5(247cW)

, 2~m0cj)

2 'A2 $~~$')~

~

~/

-

2f10)

Cr3' Tm3'

Fig,

I. Main Cr~+ and

Tm~+

_energy levels in YAG involved in 2 _~m emission.

The

codoping

of

Tm~

+ activated YAG

crystals

with transition ions such

Cr~

+

improves

the laser

efficiency,

_as

previous

studies

[1, 3]

have shown.

Unfortunately,

the

spectroscopic investigations generally neglect

the static effects of

codoping

and site

selectivity

in the energy

transfer

[4].

An

early attempt [5] pointed

out the

existenie

of _two

nonequivalent

Tm~

+ and

Cr~

+ sites in YAG that also

play

_an

important

role in the

dynamic

behaviour. A

similar

approach

is also involved in

Cr~

+ -Tm3+

codoped Gd~Ga50j~ single crystals

in which

a detailed

study

has been undertaken _on thulium and chromium first

neighbors

sites

[6].

The aim of this paper is _to

investigate

in detail the

origin

and characteristics of the

changes

induced in the

Tm~

+ emission spectra

by C~

⁺ in Cr _: Tm _: YAG and their

possible

effects in Cr to Tm energy transfer processes.

2.

Experimental

methods.

Single crystals

of Cr:Tm:YAG have been grown

by

the Czochralski method _at the

Laboratory

of Laser Active Media of the Institute of Atomic

Physics

Bucharest with various Cr

(from

0,I to 0.4

atfb)

_or Tm

(from

² to 5

atfb)

concentrations.

The

absorption spectra

^have been measured with _a

Cary

17

spectrophotometer

or with a one-

meter monochromator and

phase

sensitive detection. The luminescence measurements have

been

performed by using

the second harmonic of the YAG :Nd laser for nonselective excitation in the

Cr~

+ ~T~ ^{band and}

dye

lasers for selective excitation in the ~F~

absorption

band of

Tm3

+

or in the R lines of

Cr3

_{+. For the} luminescence

decay

measurements various

experimental set-ups

^with a

digital oscilloscope

_or transient recorder have been used.

N° 5 LUMINESCENCE OF Cr-Tm PAIRS IN YAG 1247

3.

Experimental

results.

As

pointed

out earlier

[7, 8], codoping

with

Cr3

+ manifests in the

absorption _spectra by

^the

modification of the

Tm~

+ satellite structure. Due to the dominance of the

Tm~

+ main lines and the presence of nonstoichiometric defect satellite

lines,

these

spectra

^do not offer _a sufficient

resolution for _a proper selection of these _new lines. Because selective excitation is

possible

the luminescence emission is _more informative.

Excitation into the

2E Cr~

+ levels

(Ri

and

R~ lines)

^leads to an efficient emission from the

~H~Tm~+ multiplet.

At low

temperatures

^the

emission,

unlike the _case of

singly doped samples,

is dominated

by

three _new

Tm~

+ centers C~ as illustrated in

figures 2a-2c,

where

part

of the ~H~ _- ~H~ spectrum at lo K is

presented. Although

a

dependence

of the relative

intensities _on the excitation

wavelength

inside the

R~

line is

obvious,

_a

complete selectivity

could _not be obtained

by

this way of

excitation,

due to the

C~

+ linewidths. The emission

wavelengths corresponding

_to these _new C~ ^centers for the

~H~(Wi)- ~H~(Zi)

transition

(between

the lowest Stark

levels)

_as

compared

with those of the

Tm~

+ main line N _are

given

ⁱⁿ table I. We

keep

on the notation from

[9]

for the Stark components.

Table I. The

position of

satellite lines

of C,

centers in

Tm~

+

~H4 (WI

_-

~H6(Z,

transition and

of

excitation

peaks

inside R lines

of Cr~

+

~~4(~'~1)

~

~~i6(~i

~~

~ ~J~2

Center A

(nm)

~

(rim) (RI) (R2)

Ci

793.43 687.45 686.55

C2

793.27 687.42 686.51

C3

793.05 687.29 686.45

Main line 793.35

In

Cr~

+

codoped crystals,

not

only

are the

Tm~

+

spectral

characteristics

modified,

but those of the

Cr~

+ ion too. Some shoulders _{are seen} in the

absorption spectra,

^{but the existence of} a

satellite _structure induced

by Tm~+

is best proven

by

the excitation spectra ^of

C,

centers emission when

pumping

into

Ri

_or

_R~.

In table I _we

give

the maxima in the excitation

spectra (in Ri

and

R~ regions)

for each of the C~ centers.

Excitation into the

Tm~

+ ~F~ ^level at lo K, _as illustrated in

figures 3b-3d,

enables _a clear

separation

of C _centers. In

figure

3

only

part ^{of the} ~H~ - ~H~ emission,

corresponding

to the transitions from the first Stark component

Wi

of the ~H~

multiplet

_to several Stark components Zi of ~H~

(see Fig, I),

is

presented

because the

selectivity

is lost for transitions to

higher Z, components.

^One can observe that if the emission of

Ci

and

C~

centers is similar _to that of

the main

Tm~+

_center

(Fig. 3a),

the

C~

center emission is _more

complex.

Our nonselective excitation with 532 _nm

light

into the

~T2Cr~+ band, investigated

for

comparison

with

previous

studies, leads also to the ~H~

Tm~

+ emission dominated

by

the three

C,

centers

(Fig. 2d).

If the emission spectrum ^{is recorded with various}

delay

times after the laser

pulse,

^the ratio of the intensities

changes (Fig. 4). Therefore,

the relative emission

intensities arise from different luminescence kinetics and

possibly

from different oscillator

strengths

and

they

cannot be taken _{as a measure} of relative concentrations. At 77 K the

3H~(Wi

-

~H~(Zi

emission

(Fig. 4)

is

complicated by

hot band contribution.

>ex.=686.55nm

aJ

Aex=686.snm

U

bJ

42~

b

#

~

2i

h

(

ex. = 686. 45 nm G~Q

/~

CJ

Aex

=5$2nm

dJ

700 800 900

A(nmJ

Fig.

2.

= Part of the ~H4 _- ~H~

Tm~

+ emission of _a Cr (0.2 atffi) _: Tm (3 atffi) _; YAG sample at lo K at selective excitation into the ~E C~ + level (a, b, c) and nonselective excitation into the ~T~ C~ + level (d).

N° 5 LUMINESCENCE OF Cr-Tm PAIRS IN YAG 1249

N center

Aex_=

682.25nm

~

aJ ^~

~

Ci-

center

Aex=680.I

_nm

~4~

aT

p bJ ~

(

( t~

fl

m

~

C2 Center

~j

j~

Aex₌ 6719 _nm

CJ $

~

'

C3

Nt Aex.= 679, 65 nm

dJ

_~~ ~n~

w

700 800 900

A(nm)

Fig. 3. Part of the ~H4

- ~H~Tm~ + emission of _a Cr (0.2 at%) Tm (3.2 at%) _; YAG sample at 10 K at selective excitation into ~F~ Tm~ + levels.

C~

c3

~ c~

C,

Ci G

12620 12610 12600 12620 12610 12600 1262012610 12600

Firm-') E(cW'/ Firm-')

ai bi c)

Fig.

4. ~H~

(Wj

)

-

~H~(Z,

^{Tm~ +} emission (77 K) of _a Cr _; Tm

: YAG sample under nonselective excitation at 532 _nm with different delay ^times a) 0 _~s, b) 50 _~s and c) 100 _~s.

When

pumping

into the

Cr~ +, R~

line, the luminescence kinetic is different for the three

C~ ^centers in _a

given sample,

at a

given

temperature.

Strong dependence

_on

temperature

^and

Tm~+

_{content has been observed. When}

pumping

is

performed

into

Cr~+

_{levels, the}

luminescence behavior of the

~H~

emission for

Cj

and

C~

centers shows _a risetime

(dependent

on

Tm~+

_{content and}

temperature)

followed

by

a

nonexponential decay.

The risetime is

evident in

figure

5 which shows also the

beginning

of the

decay

for the

Cj

center. Such _a

behavior is absent when the

Cj

emission is excited

directly

into the

corresponding

~F~Tm~+

lines. In

figure 6,

we present ^the

~H~(Wj)- ~H~(Z,) _decay

_at 77K for each

C~ ^center in _a YAG

sample

with 0.2 atfb Cr and 3.2 atfb Tm _{; one} must note the

long

tails in

the

decays.

This data demonstrates _a selective energy transfer from

Cr~

+ to

Tm~

+ The difference in

luminescence

decays

_can then

explain

the

dependence

of

global

emission

(under

532 _nm

excitation)

on the

delay

time.

4. Discussion.

The observation of the _new

Tm~

+ centers induced

by

the presence of

Cr~

+ in Cr Tm _: YAG

crystals

can be accounted for

by starting

from the

crystalline

structure of gamets. ^{The ideal}

yAG structure of

composition Y~A1501~

^{contains three} types ^of oxygen coordination for

cations dodecahedral

(c-sites)

for

Y~+,

_{with local symmetry}

_D~,

octahedral

(a-sites)

for

N° 5 LUMINESCENCE OF Cr-Tm PAIRS IN YAG 1251

<oo iooo

rj~s)

Fig.

5. Early part of the Tm Cj luminescence decay with pump into the Cr R2 band, 8 K.

o

Q' '

"

'

o

$

$ '

~

'-

~.,

$_ '~

-4

0

(~Js)

Fig.

(3.2

at%) sample.

Al~+,

_{with local symmetry}

_C~,,

_and tetrahedral

(d-sites)

for

Al~+,

_{with local symmetry}

D~.

In the ideal structure, the rare-earth ion

usually occupies

the

large

dodecahedral

c-sites,

while

Al~+ occupies

^that a- and d-sites. However, in the

high-temperature melt-grown

crystals,

_part of the octahedral a-sites _can be

occupied by ^Y~

⁺

^ions,

^this

leading

to

departures

from the ideal chemical

composition.

The difference between

Y~

+ and

Al~

+ ionic radii _can

produce

a

displacement

of the

surrounding

_oxygen ions from their normal

positions

and leads to the

perturbation

of the

crystal

field _at the

adjacent

cations.

It _was mentioned earlier

[lo,

I

I]

that each _« anomalous _»

Y~

+ ion in _an octahedral a-site

can

produce

different

perturbations

at the

neighboring

dodecahedral sites _on the _same coordination

sphere

around the

a-site, depending

on the direction of the

perturbation

with respect to the local

symmetry

axes of these sites. Such

perturbations destroy

^the

equivalency

of

the rare-earth sites from such _a coordination

sphere although

the distance

Y~

+

(a )- (R-E

_)~ ⁺

(c

is the _same. These _{centers are}

usually

called

P,

centers and their number for each coordination

sphere

is determined

by

the

crystalline

structure. Since the

crystal

fields at these sites differ from that _at the

normal, unperturbed c-sites,

the P centers are seen in the

optical

_{spectra as} satellites of the main lines. For

Tm~+

in YAG such centers have been observed

recently [7, 8].

In

YAG, Cr~

+ ions substitute

only

in octahedral

Al~

+ ion sites. The

probability

of such occupancy in the nearest

neighbourdhood

of

Tm~

+

can be calculated for various models of activator distribution in

crystals [12]. Thus, assuming

_a random

equiprobable substitution,

the

probability

of

having

_n such ions in _a coordination

sphere

of _m available sites around

Tm~

+ is

given by

P~~

#

'~~

,

C~(I

C

)~~~ (1)

n.

(m n).

where C is the relative concentration of

C~

+ with respect to

Al~

+ in a-sites.

In the _case of YAG there _are four a-sites in the first coordination

sphere

around a c-site

(m

=

4

)

: thus the

probability

of

having

_a

^C~

⁺ ion in such _a site

is,

for low concentrations

C, approximately proportional

to 4 C. It is evident that the relative

probability

of

having

_n-n- Cr- Tm

pairs,

with

respect

to the isolated Tm

ions,

is finite _even at very low Cr concentrations.

Thus in the _case of _our

crystals,

where the

C~+

concentration

(with

respect to

Al~+

_is

0.2

atfb,

the relative concentration with respect to Al a~sites is 0.5 atfb

(I,e.

^C ₌

0.005)

And

the relative concentration of _n-n- Cr-Tm

pairs compared

to isolated Tm centers

(m

=

4,

n =

0 in

Eq. (I))

is

equal

to 2 fb. We note that the existence of such Cr-Tm

pairs

_was observed

recently by

the presence of _a

clearly

manifested shoulder of the luminescence lines

Rj

and

R~

^of

Cr~

+ in Cr

: Tm _: YAG

[13].

Since

C~+

_{ions occupy}

only a-sites,

due _to differences in ionic size with

respect

to

Al~+, _they

can induce lattice distortions similar to those

produced by

the defective

Y~

+

(a)

ions. Each octahedral site is sourrounded

by

six dodecahedral c~sites _at 3.354

h

_and

by

six such sites in the second coordination

sphere

at 5.408

h.

_The

perturbation produced by

a

C~

+ ion at the

surrounding

c-sites

(for

_a

given

coordination

sphere)

will be

dependent

_on the direction of the

perturbation

with respect to the local _axes of

D~

_group of each c-site

(x-parallel

with cubic unit cell _axes,

z and y

along

face

diagonals).

Since the substitution of

Al~

+ with

Cr~

+ preserves the inversion at

a-sites,

the

perturbation

^will be similar for _c-centers

connected

by

inversion with

respect

to this site.

Thus,

_a

single ^Cr~

⁺ ion could

produce

three different

perturbations

: a

perturbation

in the local _x

=

0

plane,

a

perturbation

ⁱⁿ the

plane

that bisects the dihedron formed

by planes (x,

_{± y,}

0)

and

(x, 0,

_±

z),

and _a

perturbation

in the

plane containing

the x-axis and

perpendicular

to the

previous

one. The local symmetry at c~sites is

lowered from

D~

to C~ in the first _case and to C

j in the last two cases

(but

the

perturbation

could

be

different).

Statistically

_each

C~

+ ion could

produce

three

equally probable crystal

field

perturbations

at the nearest

neighbor

rare-earth

c-site,

manifested

by

the appearance of three satellites in the

optical

_spectra. Even if the concentration of each type ^of

perturbed

centers is the _ame, the line intensities could be different due to differences in the oscillator

strengths.

N° 5 LUMINESCENCE OF Cr-Tm PAIRS IN YAG 1253

Thus,

we propose to

assign

the three _new

Tm~

+

centers observed in

Cr~

+

codoped

YAG to

such

Cr~+ (a)-Tm~+ (c) near-neighbour pairs

at the _same distance

(3.35 h),

_{but with}

a

different orientation with respect to the local symmetry axes of the

Tm~

+ site. These

pairs

have their _own energy levels of the

type Ec~

+

E~~

₊ AE. The shifts

AE,

_as well _as the transition

oscillator

strengths, depend

_on the

symmetry

^and

perturbation strengths.

In table

I,

_{we present} the

energies

of Cr

(a)-Tm (c) pairs

in _two

important spectral regions ~H~(Zj)

-

~H~(Wi) Tm~

+ transition and ~A~ _- ~E

(Ri, R~ ) Cr~

⁺ transitions.

In

D~

symmetry ^the

Tm~

+

multiplets

are

split

into

singlets

that _can be associated with the four irreducible

representations r~.

The electric and

magnetic dipole

transitions

r,

-

r~

are

forbidden for I

=

j

and

large

differences in intensities could appear even for I _#

j.

The selection rules for C~ or

Ci symmetries

show that all the transitions _are allowed.

However,

_one

would expect ^{that for} centers with

slight

distorsions the transitions

corresponding

to

r~

_-

r~,

forbidden in

D~,

would have _a very low

probability.

The

analysis

of the data

given

in

figures

3a-3d shows that the emission spectra ^of

Ci

and

C~

centers

corresponding

to

~H~(Wi)- _~H~(Z~)

_are

quite

similar to that of the

unperturbed ^Tm~

⁺ N center

(Fig. 3a),

while that of the

C~

center is _more

complex. Especially

we notice the presence in the C~ spectrum ^{of the line}

Wi

-

Z~ corresponding

to a forbidden

ri

-

ri

transition for the N _center. The shifts of the satellite C~ lines from the main lines

depend

on the transition and the

Ci

and

C~

satellites _are closer _to main _center lines.

Thus,

in the

~H~(Wi)- _~H~(Z~) transition,

the shifts from the main line N _are

-1.3cm~~

_for

Ci,

+1.2

cm~~

for

C~

and ₊ 4.8 _cm~ for

C~.

These facts suggest ^{that in the} case of the

C~ center the

crystal

field

perturbation

is much

stronger

^{tha for}

Ci

and

C~

; this could lead _to

differences in the oscillator

strengths.

The ~H~

decays

for

C, ^Tm~

⁺ centers

(as

shown for instance in

Fig. 6)

_{suggest a} selective

C~+-Tm~+

energy transfer within each center, with _a much

larger

transfer rate for the

C~

center. The numerical evaluation of

C~

+

-Tm~

+ transfer _rates

by using

this type ^{of data is}

difficult since the ~H~ de-excitation is also affected

by

radiative and nonradiative

(multiphonon

or cross

relaxation)

_processes. Possible energy transfers between different

Tm~

+ centers as

well _as the

uncomplete

resolution of emission when

pumping

into

Cr~+

_{lines could also}

complicate

this

problem.

The _use of

C~

+ emission is also limited

by

the resolution.

One should mention

that,

_{even at} nonselective excitation into the ~T~

C~+ _level,

_the

3H~Tm~+

emission in Cr:Tm:YAG is dominated

by

the

C,

centers,

although

their concentration is much smaller than that of the isolated Tm _centers. This suggests ^{that the} transfer from

C~

+ to

Tm~

+ near

neighbours

is

prevailing

at low temperatures ^{in YAG. The}

long

tails

(dependent

_on

^Tm~+

emission

spectra suggest, however,

the existence of _a less

effective energy transfer to more distant ions too.

The observation of the selective energy transfer for

C,

centers attracts attention _to the

complex

behaviour of the

Cr~+ Tm~+

: YAG system _; the

selectivity

must be taken into

account for

describing

the

global

^effect of the energy transfer and sensitization of

Tm~

+

by ^Cr~

⁺ in YAG _or in other gamets. ^Work to get a better resolution and

selectivity

and to obtain _accurate data for luminescence

decays

^and _energy ^transfer parameters ^{is in} progress.

S. Conclusion.

This work,

together

with several

previous

_papers, shows that the

codoping

of garnets ^with transition metals ions _can lead to the modification of the static

(crystal field)

and

dynamic (luminescence decay) properties

of the rare-earth activator ions. Thus in the Cr Tm _: YAG

laser system we were able to

distinguish

the presence of three _new types ^of centers in

Tm~

+ spectra induced

by

_a

^Cr~

⁺ ion in _one of the

adjacent

octahedral sites. These centers,

corresponding

to

Cr3+ (a)-Tm3+ (c)

nearest

neighbour pairs,

have their _own energy levels and

preferential

_energy transfer with different transfer rates. This leads to _a

spectral sensitivity

of the energy transfer processes and of the sensitized luminescence emission. The

global

C~+-Tm3+

_energy transfer

investigation

must thus take into _account this

selectivity

in excitation and energy transfer.

Acknowledgments.

Part of this work _was

performed

while _one of _us

(V.L.)

was at the

University

^of

Lyon

under _a

TEMPRA

fellowship

offered

by Rdgion Rh6ne-Alpes.

He is indebted _to these

organizations

and to the staff of Laboratoire de

Physico-Chimie

des Matdriaux Luminescents

(Lyon I)

for

their kind

hospitality.

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