EXCITED STATE ELECTRON SPIN RESONANCE USING NON-RESONANT FLUORESCENCE LINE NARROWING

(1)

HAL Id: jpa-00225118

https://hal.archives-ouvertes.fr/jpa-00225118

Submitted on 1 Jan 1985

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

EXCITED STATE ELECTRON SPIN RESONANCE

USING NON-RESONANT FLUORESCENCE LINE

NARROWING

D. Sox, S. Majetich, J. Rives, R. Meltzer

To cite this version:

(2)

JOURNAL

DE PHYSIQUE

Colloque C7, supplément au nOIO, Tome 46, octobre 1985 page C7-493

EXCITED STATE ELECTRON SPIN RESONANCE USING NON-RESONANT FLUORESCENCE

LINE NARROWING

D.J. Sox, S. Majetich, J.E. Rives and R.S. Meltzer

Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602, U.S.A.

Abstract. Non-resonant fluorescence line narrowing (FLN) is used to examine resonances between excited states of ions. We discuss resonances between crystal field states in ruby, alexandrite and L ~:pr3+ and between Zeeman F ~ sublevels in LaF3:~r3+. We show that the latter is equivalent to excited state ESR, over which it can have many advantages.

1. INTRODUCTION.

The linewidths in non-resonant fluorescence line narrowing (FLN) experiments are often inhomogeneously broadened because their crystal field energy levels depend on many crystal field parameters, in a different way for each state, so that while a pair of sites in slightly different environments may have one transition energy in coincidence, it is unlikely that they will have more than one in coincidence/l/. An

exception occurs when a pair of excited levels arise £rom the same crystal field state as for example the sublevels of a Kramers ion split by a magnetic field. In this case, the inhomogeneous contribution to the non-resonant FLN spectra will con- tain only the contribution to the resonance width due to the distribution of g- values over the ions probed with the laser. In this paper we discuss the use of non- resonant FLN to examine two types of excited state zesonances: (1) between crystal field levels of an ion, in order to obtain the phonon resonance widths which occur in excited state relaxation, and (2) between the Zeeman sublevels of Kramers ions, which is the equivalent of high field ESR.

II. EXPERIMENTS

.

A single frequency scanning cw dye laser is used to excite either one of the higher lying crystal field levels above a metastable state or the upper of a pair of Zeeman levels of an ion in a magnetic field. The excited ions relax to the lower level, typically with a relaxation time T1:lOOps (upper crystal field level) or Tl-10ps (upper Zeeman sublevel). Fluorescence from the lowest metastable level is analyzed with a Fabry-Perot interferometer as shown in Fig. 1. A single fluorescence line is isolated with a 0.75m monochrometer and is detected with a photomul- tiplier. Single photon events are registered on a counter and the number of events in a lms time interval is stored sequentially in 250 memory locations of an LSI-11 computer as the interferometer is scanned over about two free spectral ranges. The spectrum is scanned about 100 times to signal average the data.

(3)

JOURNAL

DE

PHYSIQUE

Fig. 1 Schematic of the non-resonant FLN apparatus.

III. EXAMPLES OF CRYSTAL FIELD RESONANCES.

A. ~ 1 ~ 0 ~ : ~ r ~ + ( r u b ~ ) . The E+2A excited state resonance of the 2~ state of ruby is of great interest as a monochromatic source of phonons. Its resonance width has been measured from studies of the phonon dynamics in a magnetic field/2/ and by excited state far infrared absorption/3/.

We excited the

4 ~ 2 2 ~

transitions in a magnetic field of 29.75kG as shown in Fig. 2. The instrumental resolution was 310MHz. Pumpinq the E level and viewing the luminescepce to the 4 ~ 2 (-3/2) highest ground state Zeeman level (middle spectrum) gave an additional linewidth of 370MHz, the 3/2+-3/2 inhomogeneous resonance width. This resonance is inaccessible in ESR since it is a AMs=3 transition. When the lower Zeeman sublevel of 2A was pumped, an increase in the linewidth of 450MHz was observed (right spectrum). This yields, for a Gaussian 2A+E resonance, a width of 450MHz; for a Lorentzian, 780MHz. For a Voigt profile, Our measurement yields a width between these values and is consistent with the 570MHz result of Lengfellner/3/. Our data does not justify a more complete lineshape analysis at this time.

ruby for two non-resonant transitions Fig.3 Non-resonant FLN SpeCtrUm of (finesse = 8 and FSR = 2.5GHz). alexandrite.

B. BeA1204 :cr3+ (alexandrite). We obtained FLN spectra of alexandrite in zero magnetic field. Goossens/4/ had obtained a value for the 2A+Ë resonance width of 2.1GHz.from a study of the phonon dynamics in a small field. Our FLN results k'iq.3) yield a much larger value of 13.9GHz. The discrepancy may result either from sample differences or the occurrence of energy transfer during the 1.32mÇ radiative life; time. Also seen is the 4 ~ 2 qround state

splitting which we measure to be 6=18.9GHz compared-to 16.2GHz reported by Wallinq/5/.

BeAl2O4;cr3+ O.lO/o

T=1.4K

H=O

1-FSRz40GHz --i . .

LASER

. * _. i - 2 , 9 G H z , : ,

.

FLN 16.8 GHz

(4)

-

1 Fig. 4 Excitation spectrum of the 3 ~ 4 (I)+ D2 (II) transition of pr3+ in LaF3 (solid circles) and non-resonant FLN linewidth (open circles) viewing fluorescence £rom '~~(1) as a function of laser frequency relative to vo, the l~necenter of the l ~ î (II) transition.

C. L ~:pr3+. 23cm-1 F ~ phonons have been generated and detected with the 1 ~ 2 ( 1 1 1 - S ~ ~ (1) resonance of pr3+/6/. An excitation spectrum of the 3 ~ 4 (I)+

'

D II) transition shown in Fig.

2 !

4 gives am optical inhomogeneously broadened linewidth of 3GHz (solid circles). We measured the excited state resonance width 11'1 using non-resonant FLN. Fhen 1 ~ 2 (II) is excited at vo, line center, a width of 2GHz is observed which is three times the homogeneous width obtained by Erickson/7/. However as the laser is tuned to the high energy side of the line, the inhomogeneous contribution to the linewidth rapidly increases £rom 2GHz to over 7GHz (open circles). When the laser frequency is tuned more than lOGHz from line center the FLN spectrum splits into a doub- let. At this point we are look- ing at hiqhly perturbed sites. For this system there is little line narrowing in the non-reso- IV. HIGH FIELD ESR. nant transition.

As an example of the application of FLN to high field ESR we consider L ~ F ? : E ~ ~ + . The single frequency cw dye laser was tuned to excite the 4~15/2+4~9/2 transition of ~ r in a large magnetic field parallel to the c-axis (inset Fig. 5). The excitation ~ + spectrum of the absorption to the lower and upper Zeeman levels have inhomogeneous widths of 1.68 and 2.08 GHz, respectively.

LaF3 is complicated by the presence of six magnetically inequivalent sites. The non-resonant FLN spectrum under low resolution (FSR 10 GHz) is shown in Fig. 5. Because of the large anisotropy of the g-tensors of the ground and

29.8kG

-

-300 MHz

-

..,

-

-210 MHz

Fig.5 Non-resonant FLN SpeCtra (FS% Fiq.6 High resolution non-resonant FLN LOGHz) of L ~ F ~ : E ~ ~ + as a function of spectra of LaF3 : ~ r ~ + (FSR=l.SGHz) for two angle between the magnetic field and different magnetic fields viewing fluores- c-axis. Also shown is the relative cence to the lower Zeeman ground state(A) orientation of the C? axes of the and upper Zeeman ground state (B)

.

(5)

C7-496 JOURNAL DE PHYSIQUE

excited states only a small misalignment splits the spectrum into three resolvable groups of sites.

The FLN spectrum under high resolution (FSR 1.5GHz, instrument resolution 70MHz) appears in Fig. 6. The direction of the maqnetic field was carefully aligned along the c-axis to minimize the effect on the FLN linewidth of the inequivalent sites. Fluorescence to the 1o;er (spectrum A) and upper (spectrum B) Zeeman levels of the ground state were isolated with the spectrometer. The width of the FLN spectrum of fluorescence to the upper level is broader than that to the lower level, and this additional width of 90MHz is the resonance width of the ground state, corresponding to an ESR linewidth of 7.8G.

The magnetic field dependence of the linewidth for the fluorescence to the lower Zeeman level, after subtraction of the instrumental width, is shown in Fig. 7. This is the 4~g/2 (1) Zeeman resonance width and extrapolates to 70MHz at low fields. There are several factors which can contribute to the increase in linewidth with field: (1) inhomogeneity of the magnetic field over the O .lm3 viewing volume, estimated as <100MHz at 30kG, ( 2 ) slight misalignment of the field relative to the c-axis producing a small residual splitting due to the magnetically inequivalent sites, estimated as <50MHz at 30kG, and (3) an actual increase in the linewidth at higher fields. Thus the major portion of the observed increase in linewidth (130MHz) is probably experimental.

The low field (3.2kG) g-values of the ground state for HIC was measured to be 8.22k.07, which com ares with values in the literature of 8.12+.09/8/ and 7.5?.4/9/. The g-value of the GgI2(T) excited state was determined with great accuracy at high fields (-30kG) as 3.650I.002. With FLN one probes a subset of ions within the optical inhomogeneous line. In Fig. 8 we show the small but systematic dependence of the g-value on location within the inhomogeneous profile.

V . CONCLUSIONS.

Non-resonant FLN has been used to do ground and excited state resonance spectroscopy with resolution comparable to ESR but at frequencies outside the range of standard techniques. The advantages are major and include (1) the ability to work at very high resonance frequencies without the need of ultra-high frequency micro- waves, (2) the sensitivity inherent in optical detection techniques (especially

t

1

,

, -

N

O IO 20 30

MAGNETIC FIELD (kG)

Fig.7 Magnetic field dependence of the Fig.8 Shift in the 4~9/2 (1) spin reso- Zeeman resonance width of the 4~g/2 (1) nance frequency (Aures) at 29.75kG as state of ~ r in LaF3 (obtained from ~ + a function of excitation frequency spectrum A of Fig. 6)

.

relative to the absorption line center,

(6)

u s e f u l f o r e x c i t e d s t a t e ESR), and ( 3 ) t h e a b i l i t y t o s t u d y a s e l e c t group of i o n s w i t h i n t h e inhomogeneous a b s o r p t i o n p r o f i l e and w i t h i n a s m a l l s p a t i a l volume o f t h e sample.

We found f o r L ~ F ~ : E ~ ~ + t h a t t h e high f i e l d s p i n resonance l i n e w i d t h s a r e very

narrow. A s a r e s u l t , experiments i n s t i m u l a t e d phonon emission between t h e Zeeman

sublevels/lO/ must be c a r r i e d o u t on samples o r i e n t e d t o b e t t e r t h a n 0.1' i n o r d e r t h a t phonons g e n e r a t e d on one of t h e s i x m a g n e t i c a l l y i n e q u i v a l e n t s i t e s a r e i n resonance w i t h t h e o t h e r s i t e s .

This work was supported by t h e U.S. Army Research O f f i c e . We thank D r . Lai of

A l l i e d Corporation f o r t h e use of t h e a l e x a n d r i t e sample.

REFERENCES

W.M. Yen and P.M. S e l z e r , i n Laser Spectroscopy of S o l i d s , ed. W.M. Yen and

P.M. S e l z e r (Springer-Verlag, B e r l i n , 19811, p. 117.

J . I . D i j k h u i s , A. van d e r P o l and H.W. dewijn, Phys. Rev. L e t t .

37,

1554 (1926).

H . L e n g f e l l n e r , J . Hommel, H . N e t t e r and K.F. Renk, Opt. L e t t .

8,

220 ( 1 9 8 3 ) . R . J . G . Goossens, J . I . Dijkhuis and H.W. dewijn, i n Phonon S c a t t e r i n g i n Con- densed M a t t e r , ed. W. Eisenmenger, K. Lassmann, and S . D o t t i n g e r ( S p r i n g e r - Verlag, B e r l i n , 1 9 8 4 ) , p. 112.

J . C . Walling, O.G. P e t e r s o n , H.P. J e n s s e n , R.C. Morris and E.W. O ' D e l l , IEEE

J . Quantum E l e c t r o n .

QE-16,

1302 (1980).

L. Godfrey, J . E . Rives and R.S. Meltzer, J . Lumin.

18/19,

929 ( 1 9 7 9 ) .

L.E. Erickson, Opt. Commun.

g,

246 (1975).

M.B. Shulz and C.D. ~ e f f r i e s , Phys. Rev.

149,

270 ( 1 9 6 6 ) .

H.P. Sandner, H . Wolfrum, W. E i s f e l d and K.F. Renk, Phys. Rev.

z,

79 ( 1 9 8 3 ) .