A New Nuclear-Quadrupole Double-Resonance Technique based on Solid Effect

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1 A New Nuclear-Quadrupole Double-Resonance Technique based on Solid Effect J. Seliger 1, 2 and V. Žagar Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia "Jozef Stefan" Institute, Jamova 39, 1000 Ljubljana, Slovenia Z. Naturforsch. 52a, (1997); received October 9, 1996 A new nuclear-quadrupole double-resonance technique is described. It has a higher sensitivity and a higher resolution than the conventional nuclear-quadrupole double-resonance technique based on solid effect. The new technique involves magnetic field cycling between a high and a low static magnetic field and simultaneous application of two //magnetic fields when the sample is in the low static magnetic field. A strong rf magnetic field induces "forbidden" simultaneous transitions in a magnetic (usually 1 H) and in a quadrupole spin system and thus couples the two spin systems. A weak//magnetic field induces transitions between the energy levels of the quadrupole nuclei and simulates a fast spin-lattice relaxation of the quadrupole nuclei when its frequency matches an NQR frequency. The sensitivity and resolution of the new technique are discussed and test measurements in tris-sarcosine calcium chloride are presented. Key words: NQR, NMR, Double Resonance, Magnetic Field Cycling, Solid Effect Introduction A n / / m a g n e t i c field of a f r e q u e n c y v in solids c o n taining nuclear spin species I and S with the r e s o n a n c e f r e q u e n c i e s u l and v s, respectively, can i n d u c e transitions b e t w e e n the nuclear energy levels w h e n u u l or v u s, and also w h e n u u l ± u s. T h e transitions at v ux ± v s are allowed in solids d u e to the p r e s e n c e of the m a g n e t i c dipole-dipole interaction b e t w e e n the I and S nuclei [1]. T h e y are called solid-effect transitions. T h e solid-effect transitions are actually simult a n e o u s transitions b e t w e e n the energy levels of b o t h spin systems. Absorption of a q u a n t u m of energy at the f r e q u e n c y v v l u s causes an u p w a r d transition in the spin s y s t e m I at the f r e q u e n c y u l and an u p w a r d transition in the spin system S at the f r e q u e n c y u s, etc. T h e probability per unit time f o r a solid-effect transition, W S E, is at the s a m e a m p l i t u d e B, of the / / m a g netic field significantly lower than the probability per unit time for a direct transition at u vx or at v us. In case of a strong m a g n e t i c dipole-dipole interaction b e t w e e n the nuclei I and S, as f o r e x a m p l e w h e n the atoms I and S are covalently b o n d e d, the solid- e f f e c t rate 2 W S E is o f t e n of the order of ( m s ) - 1 w h e n B «1 t n T and the a m p l i t u d e B, of the / / m a g n e t i c field is e q u a l to a r e a s o n a b l e value of several mt. In such a case the solid-effect transitions m a y be used for an indirect detection of u n k n o w n resonance f r e q u e n c i e s of the S nuclei via their effect on the N M R or N Q R signal of the I nuclei. A nuclear-quadrupole double-resonance (NQDR) t e c h n i q u e b a s e d on the solid effect [ 2, 3, 4 ] is o f ten used f o r an indirect detection of low nuclear q u a d r u p o l e r e s o n a n c e ( N Q R ) f r e q u e n c i e s z/ QS. T h e t e c h n i q u e involves m a g n e t i c field cycling. A purely m a g n e t i c spin s y s t e m I (usually protons) is first polarized in a high static m a g n e t i c field B0. T h e n the external m a g n e t i c field is adiabatically red u c e d to a low value B, B «1 mt. T h e s a m p l e is kept in the low m a g n e t i c field B for a time r, shorter than the spin-lattice relaxation time T n ( B ) of the spin s y s t e m I in the low m a g n e t i c field B. T h e n the external m a g n e t i c field B0 is adiabatically restored and the N M R signal Sx(T) of the I system is measured. It is a p p r o x i m a t e l y e q u a l to 5i(r) 5,oexp(-r/r,i(B)), Reprint requests to Prof. Janez Seliger, Seliger@fiz.uni-lj.si. w h e n B «C B0. (1) H e r e S I 0 is the N M R signal of the / 97 / $ Verlag der Zeitschrift für Naturforschung, D Tübingen

2 338 J. Seliger and V. Zagar A New Nuclear-Quadrupole Double-Resonance Technique 338 spin s y s t e m I prior to the reduction of the external m a g n e t i c field. W h e n during the time r spent in the low m a g n e t i c field B an rf m a g n e t i c field of an a m plitude B x, B 1 mt, is applied at the solid effect f r e q u e n c y v VQS v x, the s i m u l t a n e o u s transitions in both spin systems establish in several m i l l i s e c o n d s a quasi equilibrium, w h e r e N u / N 2 l N 2 S / N X S. H e r e N l s and N 2 S are the p o p u l a t i o n s of the u p p e r and lower q u a d r u p o l e energy levels separated by h u q S, w h e r e a s N u and N 2 l are the p o p u l a t i o n s of the upper and lower Z e e m a n energy levels of the I nuclei, respectively. D u r i n g the process of establishing the quasi equilibrium, the m a g n e t i z a t i o n of the I s y s t e m usually decreases and therefore a lower N M R signal S j - A S j is observed at the e n d of the m a g n e t i c field cycle. T h e ratio A S j / S I 0 is of the order of Ns/N{, w h e r e Ns Nls N2S and N{ Nu N2l. W h e n the rf m a g n e t i c field is applied at another solid-effect f r e q u e n c y v VQS - u x, a quasi equilibanc rium is established w h e r e Nu/N2l ^ a d r o p A S l of the N M R signal of the s a m e order of m a g n i t u d e as in the previous case is observed. A d o u b l e t around v UQS is thus o b s e r v e d in the ^ - d e p e n d e n c e of Sv T h e doublet is in general a s y m metric and not very well resolved. T h e solid-effect rate 2WSE, namely, strongly decreases on increasing the low m a g n e t i c field B, w h e r e a s the spin-lattice relaxation time TU(B) o f t e n strongly increases on increasing B. T h e r e f o r e an optimal L a r m o r f r e q u e n c y 7 B is usually chosen as being equal to a f e w w i d t h s of the N M R line of the spin s y s t e m I. W h e n dealing with polycrystalline samples, the external m a g n e t i c field also b r o a d e n s the N Q R line. T h u s in practice a reasonable value of B is a p p r o x i m a t e l y 1 m T or less. T h e m a i n disadvantage of this N Q D R t e c h n i q u e is its relatively low sensitivity as c o m p a r e d to the sensitivities of other N Q D R techniques, and also its low resolution. T h e wih of a line within the d o u b l e t is n a m e l y a p p r o x i m a t e l y equal to the wih of the N M R line of the spin system I. T h e low resolution is not alw a y s a disadvantage since it allows us to d e t e r m i n e the N Q R f r e q u e n c i e s in a short time. S o m e recent experi m e n t s [ 5, 6 ] show that the sensitivity of the N Q D R technique based on the solid effect significantly increases w h e n the spin-lattice relaxation rates of the q u a d r u p o l e nuclei are high. In this case, n a m e l y, after establishing the quasi equilibrium, the c o u p l e d spin s y s t e m s relax towards the thermal e q u i l i b r i u m with the crystal lattice faster than the I spin s y s t e m alone. T h i s led us to an idea of simulating the fast spin-lattice relaxation of the q u a d r u p o l e nuclei by applying a seco n d w e a k rf m a g n e t i c field of an amplitude B2 and f r e q u e n c y u 2, v 2 ~ z/ QS, during the time w h e n the first strong rf m a g n e t i c field at the f r e q u e n c y vx VQS ± z/j is s w i t c h e d on. In the u 2 - d e p e n d e n c e of the N M R signal Sj at the e n d of the magnetic field cycle w e n a m e l y e x p e c t a relatively sharp dip at u 2 z/ QS. T h e w i d t h of the dip d e p e n d s on the h o m o g e n e o u s and i n h o m o g e n e o u s b r o a d e n i n g of the N Q R line as c a u s e d by the local electric and m a g n e t i c fields, as well as on the solid-effect rate 2WSE. In general w e e x p e c t a w i d t h of a f e w k H z w h i c h is a p p r o x i m a t e l y an order of m a g n i t u d e smaller than the wih of the solid-effect doublet. In the f o l l o w i n g w e discuss the sensitivity of the N Q D R t e c h n i q u e b a s e d on the solid effect as well as the sensitivity of the new technique. Finally w e present s o m e e x p e r i m e n t a l results obtained by the n e w t e c h n i q u e in tris-sarcosine calcium chloride. Sensitivity of the N Q D R t e c h n i q u e s I m m e d i a t e l y after the adiabatic reduction of the external m a g n e t i c field is c o m p l e t e d (t 0) the populations N j! a n d N 2 l of the u p p e r and lower energy levels of the m a g n e t i c nuclei I, ivn(o) i j V i ( l - a*0)), N2,(0) ij\ti(l * ( 0 ) ). ( 2 ) do not differ significantly f r o m their initial values N^ ) and ivj, ijv,( 1 So) prior to the the reduction of the external m a g n e t i c field Cc(O) «S 0 ). H e r e S 0 hjb0/2kbt. Let us f o r the sake of simplicity a s s u m e that a q u a d r u p o l e n u c l e u s S exhibits only t w o nuclear q u a d r u p o l e energy levels with the energy separation hvqs. I m m e d i a t e l y after the adiabatic reduction of the external m a g n e t i c field is c o m p l e t e d, the populations Nls and N2S of the u p p e r and lower q u a d r u p o l e energy levels m a y b e written as N i s ( 0 ) ^ s ( l - y ( 0 ) ), iv 2 S (0) i i V s ( l y ( 0 ) ). ( 3 ) H e r e yq d e p e n d s on the effectiveness of the level crossing [7] during the adiabatic decrease of the external m a g n e t i c field. If the level crossing is fully effective, i. e. if the ratios of the populations of the energy levels of the t w o spin systems equalize w h e n

3 339 J. Seliger and V. Zagar A New Nuclear-Quadrupole Double-Resonance Technique the r e s o n a n c e frequencies m a t c h, then x(0) y(0). If this is not the case, then y(0) < x(0). ^ - -2Wlx-WSE (x T h e p o p u l a t i o n s of the e n e r g y levels of the nuclei I a n d S are d u e to the spin-lattice relaxation g o v e r n e d by the rate equations ^ -2Ws(y-yo)-WSE(x dr C[NN d Nis W\(Nu - 7V2I), -W Nis WgN2 (4) W» V Ws(\-hvQs/2kBT), (5) Wi Ws(\hvQs/2kBT). H e r e W s is an average of and VF^. In case of the I nuclei w e assume that the energy separation of the Z e e m a n energy levels is low and w e d o not d i s t i n g u i s h b e t w e e n the probability per unit t i m e for an u p w a r d transition and the probability per unit time f o r a d o w n w a r d transition: M7,11 W\. Let u s f u r t h e r assume that the n u m b e r N s of cryst a l o g r a p h i c a l l y equivalent q u a d r u p o l e nuclei is lower t h a n the n u m b e r iv, of the purely m a g n e t i c nuclei, a n d that there are Ns strongly coupled I-S pairs. If the rf m a g n e t i c field is applied at the f r e q u e n c y v UQS i/j, then only the pairs w h e r e both nuclei sim u l t a n e o u s l y occupy either the upper e n e r g y levels or the l o w e r energy levels participate in the solid effect. T h e n u m b e r s of these pairs are N ^ N ^ N ^ and N 2 S ( N 2 l / N { }, respectively. T h e rate e q u a t i o n s gove r n i n g the populations of the nuclear energy levels as a c o n s e q u e n c e of the solid e f f e c t transitions read (7) y). H e r e N i S / N u a n d y0 hvqs/2kbt. T h e steady-state solutions x* and y* of the Eqs. (7) are s. Here is the probability per unit time for an u p w a r d transition a n d is the probability per unit time f o r a d o w n w a r d transition in the spin s y s t e m S. In the h i g h t e m p e r a t u r e a p p r o x i m a t i o n the t w o transition p r o b a b i l i t i e s per unit time r e a d y), Vo x 2WsWi 2W$W\ -y WSWSE WSWSE WiWSE' (8) WSEe 2 Wi WSE ' H e r e x* is p r o p o r t i o n a l to the N M R signal of the d y n a m i c a l l y polarized I spin system. A d y n a m i c polarization of h y d r o g e n nuclei by the q u a d r u p o l e 7 5 A s nuclei using the solid e f f e c t has been recently observed in K H 2 A s 0 4 [8]. In a usual d o u b l e r e s o n a n c e e x p e r i m e n t ( 0 ) is m u c h larger than y0. W e m a y therefore neglect the steady-state solutions x* and y* and a s s u m e that both x a n d y relax t o w a r d s zero. T h e relaxation is twoe x p o n e n t i a l with the relaxation rates W and W _ : W± WI -(1 )Wse ± \fa, (9) A [2(Ws - W\) (1 - e)wse] 4eW^E. S i n c e w e are interested in the sensitivity of the double r e s o n a n c e t e c h n i q u e, w e a s s u m e that e 1 and W$E -C Wi. In this case, the expressions (9) s i m p l i f y : W 2VFsVFWSE, 2W{ 2WSITSE 2WS.(10) ITSE T h e p o p u l a t i o n d i f f e r e n c e of the energy levels of the I nuclei is p r o p o r t i o n a l to x(t), x(t) xexp(-wt) x-exp(-w-t), (11) where divis 7 d Nu -7 w AT -WseNIS Nu Ni WSEN2S N2i Ni.(6) H e r e W S E is the probability per unit time f o r a solide f f e c t transition. C o m b i n i n g (4) and (5), writing the p o p u l a t i o n s N u, N 2 l, N l s and A r 2 S in the f o r m N u 1^(1 - x), N2l iivjd x), Nls IJVS( 1 - y) a n d N2S y) a n d assuming that x, / «1 w e obtain the following rate equations f o r x a n d y: X- (12) (0), x H^SE 2Ws VFse y(0) ^SE 2Ws WSE x(0). T h e m a g n e t i z a t i o n M j of the I spin s y s t e m thus d r o p s in a short t i m e ( «Wx) for A M i, AM\/M\ x/x(0), and then the rest of the m a g n e t i z a t i o n relaxes t o w a r d s zero with the spin-lattice relaxation rate.

4 J. Seliger and V. Zagar A New Nuclear-Quadrupole Double-Resonance Technique T h e m a x i m u m value of x is o b t a i n e d in case of a slow spin-lattice relaxation of the S nuclei (W$ Wse) w h e n x (o-(o) y(0)). W h e n the spin lattice relaxation of the q u a d r u p o l e nuclei is slow, W$ < W\ WSE, the m a x i m u m d o u b l e resonance signal is o b t a i n e d if W\T 1. In this case AS, Sm ^ x ( 0 ) y(0)_ x(0) x(0) If, on the other hand, the q u a d r u p o l e nuclei relax fast, i. e. w h e n svfs > W\, the m a x i m u m d o u b l e r e s o n a n c e signal is obtained w h e n the d i f f e r e n c e WlT Aa- x(0)e~ wt - xe~ - w T x-e~ ~ (14) *x(0) (e~wlt - e~w-t) is m a x i m u m. In this case Ax m a y be of the order of x ( 0 ) ( A S / S i o ~ 1), w h i c h is m u c h m o r e than A S i / S i o ~ e (expression (13)) as o b t a i n e d w h e n the q u a d r u p o l e nuclei relax slowly. W h e n the f r e q u e n c y v of the rf m a g n e t i c field is equal to another solid effect f r e q u e n c y, v z/qs v\, the relaxation rates W and W- are still given by (9) and (10), is still a p p r o x i m a t e l y equal to x(0), w h e r e a s x is in case of slow spin-lattice relaxation of the q u a d r u p o l e nuclei a p p r o x i m a t e l y equal to (x(0)y(0)). T h e d o u b l e resonance d o u b l e t is thus in case of a slow spin lattice relaxation of the q u a d r u p o l e nuclei asymmetric: stronger at v vq$ v\ and w e a k e r at In case of a fully effective level crossing v - z/qs (X(0) y(0)) the line at v z/qs v\ d i s a p p e a r s. T h e doublet obtained in case of a fast spin-lattice relaxation of the q u a d r u p o l e nuclei is stronger and nearly symmetric. In the new N Q D R t e c h n i q u e w e apply t w o rf magnetic fields: a strong one, say, at the f r e q u e n c y vx vqs u\ and a w e a k o n e at a f r e q u e n c y z/2, u 2 ~ z/qs. W h e n the w e a k rf m a g n e t i c field is switched off, the largest part of the I-spin m a g n e t i z a tion, which is proportional to, relaxes t o w a r d s zero with the spin-lattice relaxation rate W -. W h e n, on the other hand, the w e a k / / m a g n e t i c field is s w i t c h e d on at Z/2 Z/QS, the transition probability per unit t i m e between the q u a d r u p o l e energy levels increases, and as a c o n s e q u e n c e the m a g n e t i z a t i o n of the m a g n e t i c spin system relaxes towards zero value with a h i g h e r relaxation rate than W-. If a large e n o u g h a m p l i t u d e of the w e a k rf m a g n e t i c field is c h o s e n so that the transition probability per unit time b e t w e e n the q u a d r u p o l e energy levels e x c e e d s VFSE, then the magnetization of the I spin s y s t e m relaxes t o w a r d s zero with a relaxation rate W0, W0 W-(WS -> oo) 2W1 EWSE. T h e d o u b l e resonance signal AS of the t w o f r e q u e n c y irradiation technique is the difference bet w e e n the N M R signals of the I system at the e n d of the magnetic field cycle as o b t a i n e d under the singlef r e q u e n c y and u n d e r the t w o - f r e q u e n c y / / i r r a d i a t i o n. It m a y be a p p r o x i m a t e l y expressed as AS S\o (e~w~t e~w T). (15) T h e sensitivity of the t w o - f r e q u e n c y irradiation t e c h n i q u e critically depends on the spin-lattice relaxation rate 2W$ of the q u a d r u p o l e nuclei, and on VFSE- T h e highest sensitivity is obtained w h e n the q u a d r u p o l e nuclei relax slowly, W$ <C W$e- In this case AS is of the order of Sio w h e n W$E <C W\. W h e n, on the o t h e r hand, the q u a d r u p o l e nuclei relax very fast, W$ -C WSE, the relaxation rate is already u n d e r the single-frequency rf irradiation e q u a l to Wo a n d the double r e s o n a n c e signal is zero. T h e solid effect doublet is in this case symmetric and strong but the resolution can not be improved. T h e new t w o - f r e q u e n c y irradiation technique is thus applicable w h e n Wse < ^ and W$ < W S E, w h e r e a s in the low-resolution N Q D R b a s e d on the solid e f f e c t the best results are obtained w h e n WSE "C W\ a n d > WseExperimental Results A s a test of the new N Q D R technique w e m e a sured 35 C1, 3 7 C1 and 1 4 N N Q R frequencies in trissarcosine c a l c i u m chloride ( T S C C ) with the c h e m i cal f o r m u l a ( C H 3 N H 2 C O O - ) 3 C a C l 2. All m e a s u r e m e n t s were p e r f o r m e d at r o o m temperature. T h e p a r a m e t e r s of the m a g n e t i c field cycle w e r e B0 0.8 T, B 0, and r 0. 3 s. The cycles w e r e repeated every 10 seconds. T h e ' H C 1 solid-effect d o u b l e t as o b t a i n e d by the single-frequency irradiation is shown in Figure la. T h e amplitude B\ of the / / m a g n e t i c field w a s a p p r o x i m a t e l y 2 mt. T h e d o u blet in zero external magnetic field ( B 0) is the c o n s e q u e n c e of a finite dipole wih hvd of the proton N M R line (SZ^D ~ 30 khz). T h e relative c h a n g e ASh/Sh P r c, t o n N M R s i g n a l is at Z/Q ± DUD larger than A^( 3 7 Cl)/iV( 1 H) 0.033, what s h o w s that the spin-lattice relaxation rate W 0 of the chlorine

5 J. Seliger and V. Zagar A New Nuclear-Quadrupole Double-Resonance Technique 1 H Cl NQDR IN TSCC (a) o o ^ T 23"C v [ M H z ] (b) 1.6 B0 Bi ~ 2mT B20.2mT B2:0.1 mt V2[MHZ] Fig H- 37 C1 NQDR spectra in TSCC as measured by the single-frequency irradiation technique (a) and by the new two-frequency irradiation technique (b). nuclei multiplied by e is higher than the proton spinlattice relaxation rate WH in zero m a g n e t i c field (VF H ~ 4 s _ l ). T h e doublet is slightly a s y m m e t r i c and centered at a p p r o x i m a t e l y 1.7 M H z. A similar result was o b t a i n e d in a previous experiment [9]. T h e effect of the t w o - f r e q u e n c y irradiation is s h o w n in Figure lb. T h e strong / / m a g n e t i c field with an a m p l i t u d e B x of approximately 2 m T was applied at the f r e q u e n c y z/, 1670 k H z and the m a g n e t i c field cycles were repeated at different f r e q u e n c i e s v 2 of the w e a k / / m a g n e t i c field with the a m p l i t u d e B-,. T w o f r e q u e n c y scans are shown: one at B2 zz 0.2 m T a n d another at B2 «0.1 mt. T h e second scan gave a significantly narrower line than the first one. A t still lower values of B2 the intensity of the double r e s o n a n c e line decreases whereas the line wih d o e s not c h a n g e significantly. T h u s the 3 7 C1 N Q R freq u e n c y Z / Q ( 3 7 C 1 ) is in T S C C at 2 3 C equal ^ Q ( 3 7 C 1 ) ( ± 3 ) khz. T h e N Q R f r e q u e n c y of the m o r e a b u n d a n t 3 5 C I nuclei w a s also m e a s u r e d by the new N Q D R techniques. T h e p a r a m e t e r s of a magnetic-field cycle as well as the a m p l i t u d e s of the / / m a g n e t i c fields were the s a m e as b e f o r e. A s a result w e obtained Z / Q ( 3 5 C 1 ) ( ± 4 ) k H z. T h u s the ratio of the nuclear q u a d r u p o l e m o m e n t s Q ( 3 5 C 1 ) / Q ( 3 7 C 1 ) of the two chlorine isotopes is - if w e neglect the influence of the isotope m a s s e s on the m o t i o n a l averaging of the electric-field-gradient t e n s o r - equal to Q ( 3 5 C 1 ) / Q ( 3 7 C 1 ) ± A precise m e a s u r e m e n t of the 1 4 N ( 1 1 ) N Q R f r e q u e n c i e s v, z/_ and UQ in T S C C m a y also be p e r f o r m e d. T h e r e are t w o positions of the sarcosine m o l e c u l e s in the crystal structure with the o c c u p a t i o n 341 ratio 2:1 [9]. W e t h e r e f o r e expect t w o sets of three 14 N N Q R lines. T h e secular part of the p r o t o n N dipole-dipole interaction is in zero external magnetic field equal to zero w h e n the a s y m m e t r y p a r a m e t e r rj of the electricfield-gradient tensor at the nitrogen site differs f r o m zero [10]. T h e 1 4 N N Q R lines are therefore narrow, but the highly sensitive n u c l e a r - q u a d r u p o l e doubler e s o n a n c e t e c h n i q u e involving resonant coupling of nitrogens in the rotating f r a m e and protons in the laboratory f r a m e [11] can not b e used. H o w e v e r a strong rf m a g n e t i c field with the f r e q u e n c y close to a 1 4 N N Q R f r e q u e n c y still i n d u c e s the solid-effect transitions [ 3, 4 ] a n d t h u s c o u p l e s the t w o spin systems. In the s i n g l e - f r e q u e n c y N Q D R e x p e r i m e n t a broad line b e t w e e n k H z and k H z and t w o narrower lines a r o u n d 1 M H z w e r e o b s e r v e d, indicating that there are indeed t w o crystallographically inequivalent nitrogen sites in the crystal structure and that the a s y m m e t r y p a r a m e t e r rf is for both sites close to 1 [9], T h e large value of rj is in a g r e e m e n t with the electric-charge distribution within a C - N H 2 - C group. M o r e precise m e a s u r e m e n t s of the l 4 N N Q R freq u e n c i e s in T S C C have been d o n e by the new technique. T h e p a r a m e t e r s of a m a g n e t i c field cycle were the s a m e as b e f o r e. T h e a m p l i t u d e B x of the s t r o n g / / m a g n e t i c field w a s a p p r o x i m a t e l y 2 mt, w h e r e a s the a m p l i t u d e B2 of the w e a k rf m a g n e t i c field was app r o x i m a t e l y 0.1 mt. T h e results of the m e a s u r e m e n t s of the 1 4 N N Q R f r e q u e n c i e s u, u and u 0 for the m o r e a b u n d a n t sarcosine m o l e c u l e s are s h o w n in Figure 2. T h e o b s e r v e d N Q R lines are of different wihs and shapes, w h i c h is p r e s u m a b l y the effect of the proton-nitrogen d i p o l e - d i p o l e interaction. T h e lowest 1 4 N N Q R f r e q u e n c y v 0 was o b s e r v e d at u 0 (430.1 ± 0. 4 ) k H z, the intermediate N Q R f r e q u e n c y v_ at //_ ( ± 1 ) k H z and the highest N Q R freq u e n c y v b e t w e e n k H z and khz. T h e q u a d r u p o l e c o u p l i n g c o n s t a n t eqq/h is thus equal eqq/h ( ± 2 ) k H z, and the a s y m m e t r y p a r a m e ter r) is equal 77 ( ± ). For the o t h e r nitrogen site w e obtained by the s a m e t e c h n i q u e the f o l l o w i n g N Q R f r e q u e n c i e s : u 0 (484.6±0.5) khz, ( ± 0. 5 ) k H z and ( ± 1 ) k H z. T h e q u a d r u p o l e coupling constant eqq/h is e q u a l to ± 2 ) k H z and the a s y m m e t r y p a r a m e t e r 77 is ± T h e 1 4 N N Q R f r e q u e n c i e s as m e a s u r e d by the new t e c h n i q u e are well resolved and d e t e r m i n e d with a m u c h h i g h e r precision than by the single-frequency

6 342 J. Seliger and V. Zagar A New Nuclear-Quadrupole Double-Resonance Technique H - U N NQDR IN TSCC T 23 C B v0 o o o v1 380kHz JL v2[khz] 435 o o oo v, 410kHz o OOoOoOn 475 v o o o V2[kHz] v< 835kHz 905 v2[khz] I4 Fig. 2. 'H- N NQDR spectra in TSCC as measured by the new two-frequency irradiation technique. irradiation technique. T h e above m e a s u r e m e n t s also m a n i f e s t the p o w e r of the new t e c h n i q u e in case of overlapping solid-effect spectra. n i q u e involves m a g n e t i c field cycling b e t w e e n a high and a low 1 m T ) static magnetic field. D u r i n g the time spent in the low static magnetic field, a strong rf m a g n e t i c field of an amplitude B x applied at a solideffect frequency ± VQS couples the m a g n e t i c spin s y s t e m I and the q u a d r u p o l e spin system S. A twoe x p o n e n t i a l relaxation of the I-spin magnetization M x f o l l o w s the application of the strong / / m a g n e t i c field. In a short time Mx drops to approximately M l N s / N l, and then the rest of M, relaxes towards zero with the relaxation rate VF_. T h e relaxation rate is high w h e n the spin-lattice relaxation rate W s of the q u a d r u p o l e spin system is high. If this is not the case, a high spin-lattice relaxation rate W s can b e simulated by a second rf m a g n e t i c field B2 applied in r e s o n a n c e at i/2 vqs- T h u s in the v 2 - d e p e n d e n c e of the N M R signal of the I nuclei at the end of the m a g n e t i c field cycle a sharp line is observed around V2 VQS. T h e n e w t e c h n i q u e gives the best results w h e n the q u a d r u p o l e nuclei relax slowly: W s WSE- If this is not the case, then the new technique m a y be still applied until W s VFSE, but the sensitivity is strongly reduced. We present a new nuclear q u a d r u p o l e d o u b l e resonance technique b a s e d on the solid effect. T h e tech- T h e e x p e r i m e n t s p e r f o r m e d in T S C C at r o o m t e m perature show that the resolution of the new t e c h n i q u e is indeed by m o r e than one order of m a g n i t u d e higher than the resolution of the single-frequency N Q D R t e c h n i q u e. Moreover, the new technique is particularly u s e f u l w h e n the solid-effect spectra as obtained by the single-frequency N Q D R technique overlap. [ 1 ] M. Goldman, Spin Temperature and Nuclear Magnetic Resonance in Solids, Chapt. 7., Oxford UP, London [2] R. E. Slusher and E. L. Hahn, Phys. Rev. 166, 332 (1968). [3] J. Seliger, R. Blinc, M. Mali, R. Osredkar, and A. Prelesnik, Phys. Status Solidi a25, K121 (1974). [4] J. Seliger, R. Blinc, M. Mali, R. Osredkar, and A. Prelesnik, Phys. Rev. B 11, 27 (1975). [5] J. Seliger, V. Zagar, R. Blinc, R. Kind, H. Arend, and F. Milia, Z. Phys. B - Condensed Matter 67, 363 (1987). [6] J. Seliger, V. Zagar, R. Blinc, R. Kind, H. Arend, G. Chapuis, K. J. Schenk, and F. Milia, Z. Phys. B - Condensed Matter 69, 379 (1987). [7] R. Blinc, M. Mali, R. Osredkar, A. Prelesnik, J. Seliger, I. Zupancic, and L. Ehrenberg, J. Chem. Phys. 57, 5087 (1972). [8] J. Seliger, V. Zagar, and R. Blinc, Phys. Rev. B 48, 52 (1993). [9] R. Blinc, M. Mali, R. Osredkar, and J. Seliger, J. Chem. Phys. 63, 35 (1975). [10] G. W. Leppelmeier and E. L. Hahn, Phys. Rev. 141, 724(1966). [11] See for example J. Seliger in Proc. Ampere Summer Inst. On Advanced Techniques in Experimental Magnetic Resonance (Portoroz 1993), ed R. Blinc, M. Vilfan and J. Slak, J. Stefan Institute, Ljubljana 1993, p. 55. Conclusions