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THE NMR SPECTRUM OF ORTHO-H2 IN DILUTED SOLID ORTHO-PARA MIXTURES
R. Schweizer, S. Washburn, H. Meyer
To cite this version:
R. Schweizer, S. Washburn, H. Meyer. THE NMR SPECTRUM OF ORTHO-H2 IN DILUTED SOLID ORTHO-PARA MIXTURES. Journal de Physique Colloques, 1978, 39 (C6), pp.C6-95-C6-96.
�10.1051/jphyscol:1978643�. �jpa-00217902�
JOURNAL DE PHYSIQUE
Colloque C6, supplkmenr au no 8, Tome 39, aoat 1978, page C6-95
T H E NMR SPECTRUM OF ORTHO-H2 I N D I L U T E D S O L I D ORTHO-PARA M I X T U R E S
R. Schweizer, S. Washburn and H. Meyer
Department of Physics, Duke University, Durham, N . C. 27706, U. K.
R6sumd.- Le s p e c t r e d e r&sonance magnbtique n u c l d a i r e d e o-Hg e t d e s impuretgs d e HD dans un mono- c r y s t a l d e H2 s o l i d e avec une c o n c e n t r a t i o n X(o-H2)
'
0,015 e s t d b c r i t pour 0,02 < T < 1 K. Les s i g n a u x d e s p a i r s d e o-H a u s s i b i e n dans l e p l a n b a s a l qu'en d e h o r s , s o n t d d c r i t s a i n s i que l e s signaux d e s moldcules i s o l b e s de 2 H2 e t d e HD. Le r e t r 6 c i s s e m e n t d e l a l i g n e HD e s t d i s c u t b .A b s t r a c t . - The NMR spectrum of o-H2 and HD i m p u r i t i e s i n a s i n g l e c r y s t a l of s o l i d H w i t h a con- c e n t r a t i o n X(o-H ) = 0.015 i s d e s c r i b e d f o r 0.02 < T < 1 K. The s i g n a l s from t h e n e a z e s t o r t h o p a i r s , b o t h in-plane an8 o u t of t h e b a s a l p l a n e , and of i s o l a t e d o-H and HD a r e d i s c u s s e d . Motional nar-
rowing of t h e HD i m p u r i t i e s i s observed. 2
We r e p o r t a s t u d y o f t h e NMR spectrum of d i - l u t e d o-H i n s o l i d p-H2 w i t h a mole f r a c t i o n
2
X(o-H ) = 0.015 and o f t h e p r o t o n s i g n a l of HD impu- 2
r i t i e s , by means of c o n t i n u o u s wave t e c h n i q u e s a t 12 MHz. I n a d d i t i o n , t h e r e l a x a t i o n times T1 and Tp of s e l e c t e d p o r t i o n s of t h e spectrum were measured a t 9.5 and 25 MHz. The sample was grown around t h i n Cu w i r e s t h a t were i n thermal c o n t a c t w i t h a d i l u - t i o n r e f r i g e r a t o r . Temperatures o f Q 20 mK were r e a c h e d , a s measured from t h e N M R s i g n a l i n t e n s i t y of t h e Cu w i r e s and t h e HD. A spectrum of v e r y s h a r p l i n e s was o b t a i n e d o v e r a narrow r a n g e of d i r e c t i o n s of t h e a p p l i e d m a g n e t i c f i e l d . I n o t h e r d i r e c t i o n s , t h e l i n e s were b r o a d e r and d i f f i c u l t t o r e s o l v e .
We have s t u d i e d i n d e t a i l t h e spectrum n e a r t h e Larmor frequency and t h e p a i r spectrum. I n f i g u r e 1 , we show t h e i n t e g r a t e d spectrum t a k e n with
~ - 5 0 -&----% -20
-1b 6
tbFrequency (kHz1
frequency (kHz1
There was a s t r o n g a n g u l a r v a r i a t i o n of t h e f r e q u e n c i e s .
F i g . I : The i n t e g r a t e d NMR spectrum of H- (X- 0.015) It is known that the N M R a t suchlow a s a f u n c t i o n of t e m p e r a t u r e upon cooling: The l a -
b e l s of t h e v a r i o u s l i n e s a r e d i s c u s s e d i n t h e t e x t . c o n c e n t r a t i o n s i s mainly made up o f s i g n a l s from The e x a c t shape of t h e c u r v e s i s s u b j e c t t o s y s t e - isolated o-H (namely those molecules with no nea- m a t i c e r r o r s caused by s m a l l b a s e l i n e d r i f t s of t h e
2 r e c o r d e d d e r i v a t i v e spectrum. The l i n e C i s p a r t i a l -
r e s t o-H neighbour) and from i s o l a t e d n e a r e s t - l y s a t u r a t e d and no c o r r e c t i o n was made !?or t h e l i n e 2
neighbour o-H p a i r s /1,2/. The c r y s t a l l i n e f i e l d broadening through t h e a p p l i e d f i e l d inhomogeneity 2
-
t h a t s p l i t s t h e r o t a t i o n a l J = I t r i p l e t i n t o a d o u b l e t J Z = f 1 and a s i n g l e t J = 0 c a n b e deduced from t h e t e m p e r a t u r e dependence of t h e NMR s i g n a l width of t h e i s o l a t e d o-H
131.
I n t h e p r e s e n t2
experiment t h e s p l i t t i n g was found t o b e
V / k
-
0.025 K , The c r y s t a l l i n e e l e c t r i c f i e l d t h a t i n f l u e n c e s t h e energy l e v e l s of t h e o-H p a i r s i s2
d i f f e r e n t / 4 / f o r t h e i n - p l a n e p a i r s ( I P ) and t h o s e whose a x i s i s o u t of t h e b a s a l p l a n e (OP). I t a p p e a r s from t h e a n a l y s i s of microwave / 4 / e x p e r i - ments t h a t t h e ground s t a t e energy i s lower f o r OP t h a n f o r I P by about A/k = 0.035 K.
m a g n e t i c f i e l d a l o n g t h e d i r e c t i o n where t h e p a i r spectrum i s s h a r p e s t . The sequence of c u r v e s was recorded upon c o o l i n g from 0.6 t o 0.030 K. The l i n e Cb ( c e n t e r , broad) r e p r e s e n t s t h e i s o l a t e d o-H and
2 t h e a n g u l a r v a r i a t i o n o f i t s w i d t h d e t e r m i n e s t h e d i r e c t i o n of t h e c r y s t a l l i n e f i e l d . On t h i s b a s i s , we have concluded t h a t t h e t r i p l e t P' i s t h e s i g n a l
from OP airs, w h i l e P r e p r e s e n t s t h e I P p a i r s . For t h e spectrum of f i g u r e 1 , t h e OP and I P p a i r a x e s a r e v e r y n e a r l y p a r a l l e l and p e r p e n d i c u l a r , respec- t i v e l y , t o t h e d i r e c t i o n of t h e a p p l i e d f i e l d . Their s p l i t t i n g s and i n t e n s i t i e s a r e t h e n i n e x c e l l e n t
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1978643
agreement with the expressions derived by Harris (equation (13) in reference / I / ) . We interpret the broad maxima D as the unresolved spectrum of the OP and IP pairs that are not parallel or perpendicular to the applied field.
As the temperature is decreased, a dramatic change in the spectrum is observed. The line C
b representing the isolated o-H2 splits into a well- resolved doublet under the influence of the crys- talline field and disappears because of the pair formation 151. The peaks of the pair spectrum P broaden an eventually disappear, The broad maximum D is displaced. In the middle, a sharp peak Cs gradually emerges. At the lowest temperatures the only sharp features are the P ' and the C spectra.
Upon warming up, there is a hysteresis in the spec- trum intensity. The P and Cb spectra reappear at a higher temperature than they have disappeared upon cooling.
The disappearance of the P spectrum as T decreases suggests that by means of jumps, IP pairs can change into OP pairs whose ground state is lo- wer. It is interesting that the effects of this chan- ge-over manifests itself already at temperatures well above A/k=35 mK. The thermal hysteresis mentio- ned above is presumably linked with the various time constants of the configurational changes.
We have investigated the properties of the C line which we identify as the proton signal from the H3 impurities, because its intensity corresponds to a concentration compatible with the natural abundalce of HD in H2. We find T = 11 f 2 ms for
2
X = 0 . 0 1 , while the calculated T2 resulting from intermolecular dipolar interaction with the o-H is
2 about T2 = I ms. Hence there appears to be conside-
rable motional narrowing, possibly by vacancy wave motion 161. The time T1 is a strong increasing func-
tion of T-l, with T1 5 25 s at
T
= 0.02 K, and also increases with frequency. Finally there is a fre- quency shift which at 12 MHz is u (H )-v (HI))+ 0 . 2 WL 2 L which corresponds to a chemical shift of about 20 ppm.
This work has been supported by a grant from the National Science Foundation, We thank
Mr. Michael Hamrick and Dr. Moses Chan for their collaboration in constructing the cryostat and Pr. A.B. Harris for stimulating discussions.
References
/ I / Harris, A.B., Amstutz, L.I., Meyer, H. and
Myers, S.M., Phys. Rev.
175
(1968) 603 / 2 / Constable, J.H., Shi, Y.C. and Gaines, J.R.,Solid State Comun.
18
(1976) 689/ 3 / Buzerak, R.F., Chan, M. and Meyer, H., J. Low.
Temp. Phys.
2
(1977) 415/ 4 / Hardy, W.N., Berlinsky, A.J. and Harris, A.B., Can. J. Phys.
2
(1977) 1150151
Amstutz, L.I., Thompson, J.R. and Meyer, H., Phys. Rev. Lett.2
(1968) 1175/ 6 / See the articles by R.A. Guyer and by
