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Submitted on 1 Jan 1985
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VIBRONIC INTERACTION IN THE EXCITED T2
ELECTRONIC STATE OF MnO-4 ION IN CRYSTAL
FIELDS VIA RAMAN EXCITATION PROFILE
STUDIES
L. Rebane
To cite this version:
VIBRONIC INTERACTION I N THE EXCITED T 2 ELECTRONIC STATE OF Mn04 ION I N CRYSTAL FIELDS V I A RAMAN EXCITATION PROFILE STUDIES
L.A. Rebane
I n s t i t u t e of Chemical P h y s i c s and B i o p h y s i c s , E s t o n i a n S . S. R. Academy of S c i e n c e s , 10 L e n i n i S t r . , T a l l i n n 200001, U. S.S.
R.
A b s t r a c t
-
The f i r s t helium temperature measurements o f t h e Raman e x c i t a t i o n p r o f i l e s were performed f o r \In04 i m p u r i t y i n KBr and KC104 c r y s t a l s . 'ata on v i b r o n i c m i x i n g o f t h e molecular e x c i t e d T 2 - e l e c t r o n i c s t a t e by v 4 ( t 2 ) mode were obtained. Luminescence and h o t luminescence were a l s o observed w i t h quantum y i e l dI
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INTRODUCTIONAccording t o t h e resonant secondary emission (RSE) t h e o r y t h e resonant Ranan s c a t t e r - i n g (RRS), and e s p e c i a l l y t h e Raman e x c i t a t i o n p r o f i l e (REP), p r o v i d e u s e f u l data on v i b r o n i c i n t e r a c t i o n i n t h e e x c i t e d e l e c t r o n i c s t a t e o f t h e i m p u r i t y c e n t e r /1,2/. I n t h i s study an experimental t e s t was performed f o r t h e sensitiveness o f t h e REPs t o t h e c e n t e r model. The Mn04 i m p u r i t y molecule i n KBr and KC104 s i n g l e c r y s t a l s i s o f i n t e r e s t because t h e molecular s t a t e s i n these hosts a r e a f f e c t e d by t h e c r y s t a l f i e l d s o f d i f f e r e n t symmetry. The c e n t e r has t o keep t h e Td molecular symmetry i n t h e Oh h i g h symmetry s i t e o f KBr, w h i l e i n KC104 one should expect the C, low sym- metry o f t h e c e n t e r and correspondingly s p l i t t i n g T2
- 2 4 '
+
A" and E-
I\'+
A" o f t h e degenerated s t a t e s . The low-temperature (5K) spectra o f RRS under t h e e x c i t a t i o n1 1
w i t h i n T2- A1 e l e c t r o n i c a b s o r p t i o n o f ~ n 0 4 and t h e REPs f o r t h e t o t a l l y symmetric s t r e t c h i n ? mode vl(al), i t s overtones, and f o r t h e degenerated bending v 4 ( t 2 ) mode were measured.
I 1
-
EXPERIMENTAL METHODSThe c r y s t a l s were e x c i t e d by ~ r ' l a s e r l i n e s and by t h e dye l a s e r l i n e s . The dyes Cumarin 47 tunable w i t h i n 525-550 nm and Rd 110 t u n a b l e w i t h i n 540-560 nm cover the
1
0-0 a b s o r p t i o n band o f t h e d i p o l e t r a n s i t i o n T ~ - - ' A ~ i n KBr and KC104 hosts. The RRS spectra were recorded w i t h a Spex 1402 double spectrometer (holographic g r a t i n g s , 1800 mm-l) and detected by photon c o u n t i n g system. The frequency dependence o f t h e s c a t t e r i n g cross s e c t i o n U(U) (REP) was obtained as a s e t o f t h e given RRS l i n e
i n t e g r a l i n t e n s i t i e s r e l a t e d t o t h e i n t e g r a l i n t e n s i t y a. o f an i n t e r n a l standard
C7-436
JOURNAL D E PHYSIQUE
Fig.
1-
Resonance secondary
emission spectra of
MnGi
molecule in the c r y s t a l s of
low symmetry KC104 ( a ) and
high symmetry KEr (b) a t 5K
and the excitation close t o
the vibronic level
v
=2;
w
ex
=19435 cm-l ( a ) and
19931 cm-l (b).
0 2 0 0 4 0 0 6 0 0 800 &-w, cm"
l i n e , when the excitation frequency was tuned point-by-point over t h e 0-0 vibronic
absorption band.
I11
-
RESULTS AND DISCUSSION
The t r a d i t i o n a l source of information about the excited s t a t e s of i m ~ u r i t v
centers
1
i s absorption spectrum. The T ~ - ' A ~
absorption bands in both c r y s t a l s have similar
vibronic s t r u c t u r e determined by ~ranck-condon progression of the v l ( a l ) molecular
vibration with only minor,differences in frequencies and Stokes s h i f t s - / 3 , 4 / .
A t 5K
f i n e s t r u c t u r e in the vibronic 0-0 band appears, revealing t h e c o ~ b i n a t i o n s
of the
purely electronic t r a n s i t i o n with the crystal phonons (which are considerably d i f f e r -
ent f o r the two hosts) and with t h e low frequency intramolecular modes (which depend
only s l i g h t l y on the host). The vibronic level energies and vibrational frequencies
were obtained from the absorptioi band s t r u c t u r e s a t 5K. There i s one e s s e n t i a l d i f -
ference: absorption of KC104:Mn04 reveals three components polarized along the
crystal axes a , b and c , while there i s no polarization in t h e case of
KBr host. I t
'corresponds t o
Tdsymmetry of the center in KBr with the t r a n s i t i o n dipole along
< I l l > c r y s t a l axes, while i n KC104 t h e center i s of
Cssymmetry with T2-state s p l i t -
ted into three components. Though low-temperature absorption i s more informative
than t h a t of the higher temperature one, Franck-Condon e f f e c t s a r e overwhelming and
more s u b t l e vibronic coupling e f f e c t s remain suppressed.
The resonant enhancement of the s c a t t e r i n g cross-section was observed f o r
the t o t a l l y symmetric vl vibration of Nn04 impurity and i t s overtones in KC104
/5/
coordinate. No evidence about v i b r o n i c i n t e r a c t i o n s was found from t h e hiah- 4 t e s p e r a t u r e RRS.
F i s . 1 shows t h e comparison o f t h e low-temperature (5K) FRS spectra o f KC104:Mn0; ( a ) and KBr:MnOq (b). (Only t h e f i r s t - o r d e r p a r t s o f s c a t t e r i n g a r e f,o
shown). The frequencies o f t h e detected Raman l i n e s a r e given i n /7/. For both c r y s t a l s t h e e x c i t a t i o n frequency i n Fig. 1 provides t h e c e n t e r s w i t h energy somewhat below t h e energy o f t h e v i b r o - n i c l e v e l v = 2. The energy d e f i c i t i s 4 cm-I i n KBr and 150 cm-I i n KC104. The l i n e o f t h e t o t a l l y symmetric b r e a t h i n g mode o f Mn0; i s t h e most prominent i n both RRS spectra (vl = 850 cm-I i n KBr 9s
and 853 cm-' i n KC104). I n KC104 i t s i n t e n s i t y i s about t h e same as t h a t o f t h e vl-line o f C10; showing t h e resonant
4 enhancement by a f a c t o r about 10
.
The RRS shows remarkable enhancement a l s o f o r t h e molecular s t e t c h i n g v3 and bending v4 v i b r a t i o n s o f t2-symmetry. As c o u l d be expected from t h e symmetryo f t h e centers, t h e t2-modes appear as 18,7 18,s
a s i n g l e 1 in e i n KBr and a r e s p l i t t e d i n t o t h r e e components i n KC104. The
Fig. 2
-
Resonance e x c i t a t i o n p r o f i l e s combined modes and overtones (up t o o f t h evl
mode a t 5K i n t h e r e g i o n o f 4vl and 2v3) were a l s o observed. No0-0 v i b r o n i c a b s o r p t i o n o f Mn04 n o l e c u l e enhancement was observed f o r t h e
molecular v 2 v i b r a t i o n o f e - s y ~ m e t r y , i n t h e c r y s t a l s o f low symmetry KC104 ( a ) whose frequency should be 360 cm-I i n and h i g h symmetry KBr ( b ) .
KBr and 355 cm-' i n KC104.
C7-438 JOURNAL
DE
PHYSIQUEo f t h e e l e c t r o n i c l e v e l s
rl
=r2
= = T3 = 15 c i l and t h e s c a t t e r i n g amp1 i t u d e s being Cl = 36 andC2
+
C3 = 66/
3 / . F o r Td-center t h e r e i s a s i n g l e p u r e l y e l e c t r o n i c resonance ( r O = 30 cm"), which o n l y b.y a f a c t o r o f 2.2 exceeds t h e v i b r o n i c resonance a t vO0 + 46 cm-l. There i s a l s o a resonance o f about 1:9 i n t e n s i t y a t t h e energyvO0
+
315 cm-1 which belongs t o t h e v 4 - e x c i t a t i o n . The p u r e l y e l e c t r o n i c resonance i n REP o f t h e vl modes i n case df Td c e n t e r cannot be i n t e r - p r e t e d i n Condon approximation and t h e r e i s evidence o f t h e Jahn-Teller m i x i n g w i t h v4-mode and a l s o w i t h t h e pseudolocal mode o f t h e center. The corresponding ground-state gap- mode a t 95 cm-' i s displayed i n Fig. 1 i n t h e r e g i o n o f c r y s t a l v i b r a t i o n s . The REP f o r v4 v i b r a - t i o n o f Td c e n t e r d i s p l a y s a reso- nance a t t h e p u r e l y e l e c t r o n i ct r a n s i t i o n (which i s 1:60 o f t h a t Fig. 3
-
Resonant e x c i t a t i o n p r o f i l e s o f f o r v,) and so confirms t h e i n - Mn04 i n KC104 h o s t i n t h e r e g i o n o f 0-0 volve~ent of v4 in the Jahn-Teller components o f t h e v i b r o n i c a b s o r p t i o n a t 5K f o r t h e t h r e emode: 407 cm-1 (a), coupl i n g o f T2-state.
396 cm" (b),and 393 cm" (F): I n s e r t
-
For Cs-center t h e v4 mode p a r t o f Raman spectra c o n t a i n i n g t h e i s s p l i t t e d i n t o t h r e e w i t h t h e v4 components a t t h e e x c i t a t i o nfrequencies 407, 396 and 393 cm-l uex = 18072 CK'.
(see i n s e r t i n Fig. 3). T h e i r REPs a r e presented i n Fig. 3 a, b and c,
correspondingly, and show e s s e n t i a l d i f f e r e n c e s , which a l l o w a more d e t a i l e d v i - b r o n i c m i x i n g c o n s i d e r a t i o n .
/a/.
The resonant i n p u t s o f t h e p u r e l y e l e c t r o n i c t r a n s i t i o n s i n t h e REPs were considered t a k i n g i n t o account t h e l i n e a r v i b r o n i c m i x i n g o f t h e t h r e e components, E ~ , E~ and E~ o f t h e T2-state by normal modes v 4 ( a 8 ) , v5(a1') and v6(a1), which a r i s e from t h e v4 molecular mode. T h i s coupl i n s i s governed by t h e well-known v i b r o n i c c o u p l i n g m a t r i x f o r a t e t r a h e d r a l molecule undergoing small c r y s t a l f i e l d perturba- t i o n . The v i b r a t i o n v4 i s i n v o l v e d i n m i x i n g and E s t a t e s , v 5
-
i n m i x i n g c2 and3
where v3 i s the t o t a l l y symmetric component of
t h e molecular e-mde.
The excitation of
N n ~ i
brings about also a weak broad background emission
(see Fig. 1) which i s composed of the ordinary luminescence (OL) and hot luminescence
(HL) of the centers. The OL bands of t h e ' t r a n s i t i o n s vO0
-
vl, vO0
-
vl
-
v4 and
vOO
-
2v1 were observed a t the predicted energies i n KC104. The relaxation time
TI
-
10-13 s and the OL y i e l d about
were estimated f o r T2 e l e c t r o n i c s t a t e in
t h i s host /7/. In case of KBr, t h e
HLt r a n s i t i o n s a r e more intense.
As e t of HL bands
was observed nearby the RRS l i n e s while excitation was within t h e width of t h e
vi broni c 1 eve1
.
Helium temperature measurements of the REPs performed f o r ~ n 0 4
in KBr and KC104
c r y s t a l s proved t o be s e n s i t i v e and s e l e c t i v e enough t o probe the symmetry of these
impurity centers and obtain information about r a t h e r s u b t l e vibronic interactions
in the excited e l e c t r o n i c s t a t e s .
Anumber of c h a r a c t e r i s t i c data on t h e centers has
been obtained.
REFERENCES
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