~

r

NMR in the Rotating Reference Fralne. Apparatus and

Some Preliminary Results

Takehiko CHIBA and Takeshi ITO

(19 January 1968)

SYNOPSIS

A detailed description of an apparatus for the Hartmann-Hahn-Lurie-Slichter type

'double resonance in the rotating frame and for the measurement of spin-lattice relaxation

-times 7lp and T ADRF is given. The apparatus is primarily intended to use for the study

of quadrupole interactions which are too weak to be detected by the standard techniques.

Operating frequency of the observing nuclei is about 30 MHz. A frequency sweep is

used to lock spins in the rotating frame by the adiabatic process. A double resonance

in the completely demagnetized state is employed. Abnormally short proton TIADRF

values are observed in ammonium salts having quadrupole nuclei. Double resonance

spectra of 14N, 37Cl in NH4CI, and D in (NH4)2S04 are presented. Resolution in the double

resonance spectra is about 5 kHz.

I. INTRODUCTION

In solids NMR Iines are usually broad due to the dipolar interactions among nuclear

spins- This feature together with its low frequency nature as compared to the majority

of other spectroscopic means and a long spin-lattice relaxation time, Tl' frequently en-

countered makes the signal intensity of NMR to be weak. Thus the application of NMR

to many interesting systems has been hampered by the insufficient signal-to-noise ratio.

A double resonance in the rotating frame introduced by Hartmann and Hahnl) is con-

sidered to be a breakthrough to this difficulty by improving sensitivity of the NMR

detection to a great extent. This technique is suited for the measurement of the reso-

nant frequency rather than the detailed line shape, and therefore, it will be particularly

valuable for the study of quadrupole interactions.

The detection of NMR signal of a nuclear species (B spin system) which is too weak

to be observed by the standard techniques is made by observing the effect of double

resonance on the strong signal of another nuclear species (A spin system) present in

the same sample. The A spins are first brought to an equilibrium magnetization in

a strong dc field Ho . By a suitable procedure such as applying 90' phase shift after a

15

90'-pulse or by rotating the effective field adiabatically from the z direction (HO direction~

to the Hl direction, the A spins are locked in a reference frame rotating with the angular

frequency (v=rHo along H1 ' (L' being the angular frequency of the rf field. When the

latter procedure is used the rate of rotation of the effective field must satisfy the follow-

ing equation,

l dho 1

l --~r : <<HI ( I ), where

o) (2) h0=Ho~~ T

A thermodynamic presentation of the double resonance by Lurie and Slichter2)'

enables one to follow this process in a relatively simple manner. The spin temperature

Os m the rotating frame for the locked spins are given by

6)s~ ~lll2 +HI.2)1 !i eL ( 3 ) ~ (H02+HL2)1/2

where el, is the temperature of the lattice, and HL is the appropriately defined local field_

strength at the spin 2) In an ordinary experimental condition. H0>>HI ' Therefore, es for

the A spins is very small. A strong resonant rf field HIB is applied nonadiabatically to

bring the B spins to the infinite spin temperature in their rotating frame. If a rapid

cross-relaxation process is provided between the A spins and B spins referred to their

Tespective rotating frames, heat flows from the B spin system to the A system, warming

up the latter, until finally the two spin systems attain a common temperature. Thus the

A spin magnetization is reduced. As treated thermodynamically, e)f, the spin temperature

of the A spins at the final state is related to ei, that before the heat contact with the

B splns by the following equation,

Oi CA [HI A2 + HL2] _ I ( 4 ), ~)f~CA[HIA2+HL2]+CBHi~~ 1+a '

CA > the heat capacity of the A spin system is given by

CA= rA2fi21.~ (IA+ 1) NA/3 k , ( 5 )

where NA is the number of the A spins per cc Similar equation holds for CB .

In cases where the double resonance detection is needed, rB or NB or both are small.

Then e cannot be a large factor and the diminution of the A spin magnetization by the

double resonance will be only slight However, by applying HIB in the form of on-off

rectangular pulses, or HIB with its rf phase shifted 180' periodically, the process of heat-

16

,~

1

i~

(

~

*

,,

ing the A spins can be successively repeated so that the cumulative effect is large enough

to be detected by the attenuation of the A spin magnetization. After n times on-off

of HIB, one gets at maximum efHciency,

6)i 1 ( -" = ) ef I +E ( 6 ) For quick heat transfer from the B spin to the A spin system it is necessary that 1)

the dipole coupling between the A and B system is strong and 2) the following Hahn

condition is satisfied.

rAHIA= rBHIB ( 7 ) When rA/TB is large the latter condition may be difficult to realize. A Iong mixing time

is required to get high sensitivity, which means a large amount of power to be put intO

the sample coil. Such experimental difficulties are greatly reduced as demonstrated by

Lurie and Slichter2) if the double resonance is performed in the completely demagnetized

state in the rotating fr.".me (ADRF statc) of the A spins. This ADRF double resonance

is performed as follows.

After locking of the A spin is done, HIA is reduced to zero adiabatically. Now.

the order of the A spin system is completely transfered to the dipole system. HIB is

pulsed on as before. Heat transfer takes place between the B spin Zeeman system and

the A spin dipole system in their respective rotating frames The required condition irL

place of Eq. (7) is now a less stringent,

TAHL-TBHIB ( 8 ) A{ter HIB is turned off HIA is slowly turned on again to resume the A spin magnetiza-

tion along Hl'~ and the signal is observed as before,

So far we have only considered the decay of the A spin magnetization by the double

resonance In the spin-locked state the magnetization decays with a relaxation time Tlp

which is comparable magnitude with TI in the high field. Tlp is a function of Hl ' and

roughly speaking, it corresponds to the spin lattice relaxation time in a very low field,

a field of the order of Hl (typically 10 G).

The measurement of Tlp has the importance that it provides information about the

random atomic motions in the region of correlation frequency which is not obtained from

the high fleld Tl - Especially when Hl=0, Tlp (=TIADRF) can be affected by the "ultra-

slow motion" as analysed theoretically and demonstrated experimentally by Slichter and

Ailion.3),4) In the ADRF state spins ar~ ordered to the local field HL . The atomic motion

which takes place with a rate much faster than rHI. will not contribute appreciably t~

the relaxation. On the other hand, if a slow atomic motion changes the local field ex-

17

perienced　by　a　spin，the　order　in　the　spin　system　is　destroyed　by　such　a　motio11，There－

fore，a　strong　relaxation　of　spin　magnetization　can　be　expected　even　by　motions　of　an

extremely　slow　rate，contrary　to　the　high五eld　relaxation，where　ordinar逓y　the　effect　of

atomic　motions　can　only　be　a　small　perturbation．

　　　In　this　report　we　descr孟be　details　of　our　apparatus　for　the　double　resonance　experi－

ment　in　the　rotating　frame　primarily　intended　for　the　measurement　of　quadrupole　interac－

tions　which　are　di伍cult　to　observe　in　the　conventional　NMR　techniques．　The　main　rf

unit　will　be　a　variable　frequency　type　so　that　it　can　also　be　used　for　the　PΩR　study．

1n　order　to　alleviate　requirements　oHarge　rf　power　and　the　strict　Hahn　conditlon，the

doub！e　resonance　hl　the　ADRF　state　will　be　used．The　apparatus　can　also　be　used　for

the　measurement　of　T】ρand　TIADRF．Results　of　some　preliminary　measurements　by　this

apParatus　are　also　presePted．

1

II．　APPARATUS

2

6

A　block　diagram　of　an　apparatus　for　the　double　resQnance　is　shown　in　Fig，1．Main

　　　　・MAIN　RF　PULSE　UNIT　　　「　　　　　　　　　　　　　　　　　　　「　　　I　　　　　　　　　　　　　　　　　　　I

　　　l　cw　　　　l　　　　　　　　　　PHASESHIFT　　　l　　　　　OSCILLATOR　　　　　l　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　CIRCUIT　　　l　　　　　　　　　　　　　　　　　　　　I

RF

GATE　CIRCUIT

2一［6

3

RF　POWER　AMP　　　　　＆

PULSE　SHAPING

L＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿」

RFPROBE

PRE－AMPLIFIER

ぜ⑤

MAINAMPLIFIER

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l　　　　　　　　　　　　　　　ll　　　　　CW　　　　　　　　　　lIl　　　　　　O　SCILLATOR　　　　　I

l　　　　　　　　　　　　　　　　　　　lL＿＿＿一曹＿＿＿＿＿＿＿＿＿＿＿＿＿＿三

　SECOND　RF　PULSE　UNIT　　　　　　　Fig．1，　Block　diagram　o

　　　　　　　　　　　　　　　　　　　　6

FREQUENCY

　COUNTER

the　double　resonance　apParatus

CRO

18

レ

調

爆

へ

e

,

RF Unit furnishes rf pulses necessary for the double resonance at the resonance frequency

of the A spins. A sequence of rf pulses for I nl lr

the observation of the double resonance is

depicted in Fig. 2 Spin-locking and adiabatic

delnag'netlzation is performed by an rf pulse I Fig 2 Schematic illustration of rf pulses

and remagnetization by a pulse 11 Between fof the do~ble re~0nance in the

ADRF state the tlvo pulses the second rf pulse (pulse 111) -at the resonance frequency of the B spins is applied. At the end of puls~ 11 rf power

is turned off nonadiabatically to observe induction decay slgnal of the precessing mag-

netization After amplifled by a prealrrplifier and a wide-band aniplifier, the signal is

detected with or without the superposition of a r(~fer'ence rf voltage from a phase shift

circuit and displayed on an oscilloscope The whole cycle is repeated with varying the

second rf frequency. The double resonance is detected by a diminution of the free

PG- I ~ 162

#1 ¥r

~

161

#1

JL

TL

(~

161 +#2

161

#3

~)

JrL~)

pc-1r ~_r~

@

PG-HI

Fi~~ 3A. Block diagram of pulse generator

Series generators

162

#2

~JJ:L 161

#4

l 61

#5

JL

JuUuL

R

arrangement.

~;'

161 and 162 are Tektronix 160-

R ~:b

R bc

(~],

d le

~)

Fe :f

,

(~~

e If

~

'-=~ Fig, 3B,

tilne

Pulses 1-6.

g h

19

induction decay signal. The double resonance experiment, and Tlp and TIADRF measure-

ments, which can be made by slight modifications in the operation, require a series of

programmed pulses shown in Fig. 3B.

1. Pulse Generators

A combination of pulse generators shown in Fig. 3A furnishes the required pulses.

PG-1 is a self-running simple pulse generator whose interval between pulses can be

varied between a few sec and about 15 sec. A pulse from PG-1 initiates the whole cycle

of the double resonance. This pulse triggers Tektronix 162 wave form generator (#1)

(operated as trigger mode) to form a sawtooth wave. Period of the sawtooth is adjusted

to be longer than the time interval between a and e of Fig. 3B. Pulses l, 2 and 3 are

initiated by Tektronix 161 pulse generators each l;1vith an appropriate delay time from the

start of the sawtooth rundown, Pulse 2 triggers PG-II at the time e to form pulse 4.

PG-II is a pulse generator similar to Tektronix 161 with a variable pulse width of 6

msec to 15 sec. Pulse 4 triggers PG-III at the time f. PG-III is similar to PG-11 with a

variable pulse width between O 3 msec and O 5 sec. PG-lll forms a pulse with width fg

which triggers Tektronix 161 (#4) to produce pulse 6. Pulse 4 is also used to gate

Tektronix 162 (#2) (operated as gate mode). The pulse or sawtooth output from this

generator is used to trigger Tektronix 161 (#5) to generate a train of pulses 5.

2. Main RF Pulse Unit

i) CW Oscillator and Frequency Sweep Circuit In order to lock spins by sweeping

+250V

Q o 5k rok 200k

frequency sweep c~l pulse in Isa4 Q c:; I ~~ ~~ ~~ o.ol o ~) o o co ~ ~

o o.ol ~~ ~ ~ D* -;~ ~ = ~ - I S4R o roop -" p~ o (:;

D rook 50p ~2, 150 o.ol rf out o o o

F~ f2. o ~ a o cs ----~ l -1 I~I o Q ~ f~ '~ D, ~ ~~ o ~ p~ ~) o ~ 1 S4D Q " * 9V 1.5V o V1 eeeB

Fig 4 CW osclllator clrcult

20

V, 12AT1

1

1

,

,

r

lr

,

the frequency to be described later, a variable frequency oscillator of the Clapp circuit is

used- The circuit is shown in Fig. 4- Vl (6688) generates an rf oscillation of about

10 MHz. The rf voltage is fed into the Gate Circuit through a cathode-follower stage,

V2 (12AT7). For spin-locking we first shift the rf frequency a little off resonance, then

Hl is pulsed on and the frequency is slowly brought back to the exact resonance value.

Pulse I for sweeping the frequency is fed into the "frequency sweep pulse input". In

the absence of this pulse, 10.5V is applied across the voltage sensitive capacitor Dz '

When the pulse voltage higher than 10.5V is applied this voltage changes. At the time

_b, the pulse voltage decays exponentially with a time constant of Cl Rl by discharge

through Dl but the voltage across D2 is brought back to 10.5V at time b! before the

(iischarge of pulse voltage is completed as illustrated in Fig. 5-A. This prevents a

slow drift of the frequency at the tail of the exponential decay. Fine adjustment of

the oscillation frequency is made by varying voltage applied to another voltage sensitive

capacitor Ds ' Rf output is about 2.5 rmsV.

A ___ ___ 1.5V -~-~-~ "' 9VT ~~~~~~~~~~r~ l*, a b b'

B

,t C Il b b'

D

==~---E

b d d' e

Fig 5 Schematic of pulse shaping

ii) Rf Gate Circuit. To produce rf pulses the Blume's gate circuit5) is used as shown

in the circuit diagram of Fig. 6. 10 MHZ rf input is tripled by V3 (6AU6) and applied

to the cathode of V5 (7077) througe a cathode-follower stage of V4 (6DJ8). The rf gat-

ing pulse 2 of about 15V is applied to the "gate pulse input " (Fig. 3B)_ This pulse is

supplied to the plate of V5 after dc-amplified by V6 (12AT7). Gated rf pulse is cathode-

21

十250V

3，3k

090

O卜寸

470k

〇一

to　Vg尺rid0．O

NN

0．01

　　一　一『

80H

ODo

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図。っoっ

ぶoっ．oう

整

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一二丁－⊥

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OHエ

マ

起O旨

20P

68k

Xoo．O

80H

250V80N

一〇．O

に曽

》

話．一

100

　　　ref．o

麹ONN》

誘oっ

0．0

出OH

OqQo

80H

一〇．O

国OOH

0．O

一〇．O

一〇．O工

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一一10MHzrf　in◎

図oっ○っ

目O．O

OO一

H．OO

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V鳶12Aマ

OO一

蕾oっ

9一

　　一　一一

瀕ゆ一

き簿

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D

マga亡ePulse功

当NN

OOH

一170V

エマ

め

RF　gate　circu6Fl9

マ一→一ト

憎ω

~

followered by V? (12AT7), amplified by Vs (5763), and fed mto the Mam RF Power

Amplifier

to V13

+500V +_'50v +000v eathQde o-500v unreg' o-_?kV unreg'

o o H .H RFC RFC o o l~il~= 0.01 RFC rf ~)' ulse o o c¥sl RFC o ~,:

~ o.ol c¥~TTo~~ ~ (:) ~:o I --' c) ~~: o out c; ~~ ~~ Ltr=0J ~ Iok ~ oo o*1 *~

o.ol 2R4CP Ioo 2B2gp o.O1

2E2G :?_2 V9 RFC V11 irom V8 Vlo o RFC 47 ~ o ~~ Lr:~ 'H 40 0'O1 'i ~,: o o c~ ~r~ (:) 20k ~ ~ 'H co o 50k ~,: o H (~ J~~~ o ~o ~~ ~(Lr) 50k - o ~i~ - Lr) o .H .H cf) - 170V ~ 340V - 170v

Fig. 7 Main rf power amplifier circuit.

iii) Main RF Power Amplifier. This amplifier consists of three stages of tuned

-amplifier operated as Class C, the tubes being V9 (2E26). Vlo (2B46P) and Vll (2B29P).

The circuit is shown in Fig 7. Output from the final stage is of low impedance type.

Rf pulse shaping for the adiabatic demagnetization and remagnetization is effected by

'controlling the screen-grid voltage of Vlo

+250V + 500V

o o o o o.O1 (161) o.OI ~!~ -~;~1 l

~~l _ i_}{J!1!iDRF pulse in _

_"s 'o V12 ~12ATI ~i~ o::sc¥1 __ Vi3 12R4 c~

(161) modulation pul~e in ~c~ ~~ 220k rectangular to Vlo screen grid

~ i c¥] shaped *~ c:)

l ook o oc~,I ~~ o Lr) made Selection ~ ~~ -f H ~j

~~ J~ LiJ Vt2'1.,_212AT7 o c¥]

o o c)

c) o o a - 1 70V 340v

Fig 8 Pulse shaping circuit

23

iv) Pulse Shaping Circuit. The circuit to shape rf pulse for the adiabatic processe~

is shown in Fig. 8. In the absence of an input pulse, V12' (1/2 12AU7) is conducting and

the cathode of V13 (12B4) is at the ground potential. Pulse 2 (Fig. 3 B) which is used

to gate the rf is also applied to the "modulation pulse input ". The cathode voltage of

Vlz and V1" rises with the pulse, cutting off V12' and raising the cathode of V13 to about

250V. By a tinre constant determined by C=iR3 the voltage at the cathode of V13, and

therefore, that of the screen grid of Vlo have a shape shown in Fig. 5-C. With the

screen grid voltage of the form of Fig. 5-C, Vlo transforms a rectangular rf pulse input

of Fig. 5-B into an output pulse at the final stage of the shape shown in Fig. 5-D.

Frequency shift is effected at bb' to the rf pulse Fig. 5-D to achieve spin locking. The

pulse shaping for the ADRF is effected as follows. About 50V positive pulse 3 (Fig-

3 B) is applied to the "ADRF pulse input" V1,, which has been in the cut-off state

becomes conducting, and the screen grid of Vlo is lowered to the ground potential. The

voltage drop is governed by a time constant of R2C= _ Voltages at the screen grid of

Vlo and the rf output of Vl* are shown in Fig. 5-E and 5-F, respectively. Since the

poT~'er amplifier are operated as Class C it is not always possible to get a desired pulse

shape of Fig 5-D or 5-F. However, it can be adjusted in a trial-and-error fashion by

various turning capacitors and grid-bias setting potentiometers in the Main RF Unit.

The rf output power is such that with the plate voltage of 500V, H1 of about 10 G is

obtained with an rf coil of dimensions 1.2cmc x I cm. When rectangular pulses are

needed as in short pulse experiment they are obtained by connecting the screen grid of

V1,, to a 250V power supply (by the mode selection switch). Tektronix 163 pulse gene-

rators are used in place of the 161's for the short pulse experiment.

3. Second RF Unit

Th.･. Second RF Unit furnishes an rf pulse of the searching frequency, whose rf

phase can be shifted by 180' periodically at a given time interval. This periodic 180'

ph_ase shift is accomplished by gating alternately two rf signals of 180' out-of-phase to

ec~ch other. If on-off pulses are to be used it is only necessary to pull out one of the

gating tubes. A circuit for supplying necessary gate pulses is shown in Fig. 9. A

variable-frequency cw oscillator, a gating circuit, and a power amplifier of rf pulses are

sho~vn in Fig. 10. Pulse 4 (-45V) (Fig. 3B) which sets the total length of the second

rf pulse is fed into the "negative pulse input " In the absence of this pulse, the cathod

of ¥,'14' (1/. 12AU7) is kept at -30V. When the pulse is on it rises to 200V and V15

(12AU7) starts to function A train of pulses (pulse 5) of 40V (Fig 3B) for 180' phase

shift is fed into the "c shift pulse input " In the absence of pulses the cathodes of V16.

24

(

,

(161) I ~D~r ~i negative pulse

------ol/~ 150k

~

I

+ 250v

~~ c ~ ~P

In

V14 v' ,4

,~ ~ c¥] o ": c¥~

,H

=340v (161) l

j cshift Pulse in

o-------~=-l

~~l (h(:,

O

- 1 70V

~] J

~: V16 c~l.

C~~

vl6

1 50k

35ymJuL V15 ,~ o

Vt4_ t6 : 1 2AU7

Fig 9 Second rf

to V16 cathode to V16'

~,S C¥l C¥,

:~ ,H

V15

~ If~.

to VIB grid

250v 500v

- 1 70V

unit-pulse circuit.

~~LrD ~~ t!)

to V19 grid

6.8k

VRl50

lOOp F~ V17

Lf)

O 0~4 ~ ~F

~ o o o~~

V17 H c lc

c athode

Co

:~:

~ C¥!

V ra

'I: o 0.05 -

vt9

~~ eO c(S~

c:)

O ,H Lt)

0.01 1: l

~l

l]

cv)~ ><,H 'H :~: 100 c~1 Lr)~:~~.0.01X3 o ? ce

~,;~

Lr)

C $:~ c:~ O a C¥~

~~ ~:)

o c~] Lr)

o o

~2,

o o H

;~

o o

V20

~ Q o ~ ~ o o

,~ Q H

" ~'occ; CW output~~cc;

Vi7: 1 2Adl

Fig 10 Second

E1;

1.5k lk 15k

Vi8,Vt9:57e3

rf unit=0scillator, pulse

25

5k 1 5k

- 170V V20: 21 4er

gate and power amplifier.

,(12AU7) are kept at the ground potential. When the train of pulses is on, 195V positive

pulses are generated alternatelv_ at the two cathodes. An rf voltage generated in a

Hartley tvpe oscillator V17 (1/2 1_?AU7), through a buffer stage of V1,-･ (1/2 1?_AU7), is

fed to an rf transformer. Through the transformer rf voltages of out-of-phase to each

other are supplied to the control grids of Vls and V1g (5763). Gate pulses at the cathodes

of V16 are supplied t6 the screen grids of Vls and V19' A periodically 180' ph~seishifted

rf pulse is generated! at the common plate of V18 and V19 and power amplified b_v V20

,(Class C operation), jAn rf exciting coil is incorporated in a plate tuning circuit. The

rf output was about' 15 G in field strength in a typical double resonancc experiment.

The tuning variable capacitors of the rf oscillator, the gating circuit, and the power

amplifier are ganged so that a single tuning control can be made for a wide frequency

range This feature is important for the detection of a broad spectrum a~ in the case

of quadrupole interaction in powder samples from the following reason. The signal is

not completely free froTn the effect of second rf power, that is, due to the overload of

the receiver the signal size becomes a little smaller when a strong rf pulse is applied.

To detect weak ~double resonance signal one has to observe a change in the signal size

resulting only from the change in : the second rf frequency In a typical case, a frequency

range of 3 to 5 MHZ is covered by one set of coils. By injecting an rf signal from the

"cw output " into a frequency counter, the rf frequency is directly read off.

3. RF Probe

To avoid complicated tunings and unwanted couplings among various components a

single coil arrangement similar to that of McKa_~~' and Woessner6) for the excitation-

detection networks hown in Fig. 11 is. employed. -~evere overload of the receiver is avoided

**n*1']e c*)il

~~F~~]

rf l'"]*~ i** 201* *ig"*1 .)**t 5 6k R*

l : D~'~,J~ j~~ I '-'.-.･-,-...~'-

D Is218 :

i j L Fig ll RF probe network

by a net~;~rork of R4 ~nd back-to-back diodes D~ and D5 ' The rf voltage at the receiver

input when the pulse is on is less than a few volts.

26

~

In order to minimize rf

the main rf and the second

field as possible it is found

12 can be used equally well

couplings it is preferable to use a cross-coil arrangement for

rf inputs. However, in order to apply as large second rf

that a probe of coaxial coil ,.-rangement as shown in Fig-

for the double resonance. rf connectors

4. Signal Detection Circuit

The signal is amplifled first by a preamplifier then by

an Arenberg Wide Band Amplifier WA-600-D (the Main

Amplifier). Preamplifier (Fig. 13) consists of two stages of

rf amplifiers and a cathode-follower output stage, power be-

ing supplied by the WA-600-D. Band width of the amplifica-

tion can be varied by selecting shunt resistances R5 and R6 '

An rf reference signal for the phase coherent detection is

injected into the "Auxiliary Tuning " jack** of the WA-600-

D arnplifier. The signal detected in the Main Amplifier

(video detection or dc detection) is displayed on a long-

persistence 12 cm oscilloscope (Iwasaki Communication Ap-

paratus Co., Model SS-5051~=0r on a memory-tube oscilloscope

(Iwasaki Communication Apparatus Co., Model MS-5103B).

lk lk

teflon

spacer

l

~*

If :l

l:

ll l II l

lll l

Il

l~ l

cross

/ section

O

Fig. 1_9 A double nance probe

reso-

~ a :Ilc5 ~ c lc;

+ 208¥r

~

signal in

O OOI ~i c¥].

c¥~

vt

10

~~ oLr)

Q c¥: (:5 ce

~~

'~~ C¥]

~; ~~:

~~ Q ~4 c¥~ c¥,

R5

~~*

O LO

~~ c

H o rH lc ~ ,~i

e~-c¥J

v2

O ,H Lt) O H O <:;

0.005

RG O ~:) LO ;,:LS)

H ~te ,O Lr;

~:: Lr:~

,~(

H O C:;

47

'H (:)

Io 0'005

+ 125V

04 ,::)

o H

v3

signal

out

1 S21 B

~: o

~~~~'

V* GDJe

'Auxiliary Tuning "

low-gain multistaged

V2 BCV5 v3 GBK5 Fig 13 Preamplifier circuit

j,ack is connected to a grid of a 6AK5 which is situated in the middle

6AK5 ~vide-band amplifier.

~

** The of a

27

Phase Shift Circuit is shown in Fig- 14. A whole circuit including a battery is

shielded in a copper box to minimize rf leakage Back-to-back diodes are provided across

tuning cshift ll' * IS2le 2SA51 ' loop -rf input 4.7k

_ ~ lk rf output ~ * ~ - o " " 9V~ . cs c; -Fig 14 Phase shift circuit.

the output tank circuit to cllp output voltage. Regulated voltage power supplies of -340

V, -170V, ?_50V and 500V are provided for the Main RF Pulse Unit and the Second

RF Unit. To be able to change output po¥ver of the main rf pulse, variable-voltage

unregulated power supplies of up to 500V and up to 2kV are provided for the screen

grid and the plate of V11, respectively-

III. PERFORMANCE AND SOME PRELIMINARY EXPERIMENTS

1. Short Pulse Experiments

Results of a few examples of ordinary short pulse experiments are shown in Fig,

15. The recovery time is about 15 ~~ sec. The strength of rf field Hl (~10G) is not

sufficient for the PQR of 35Cl where r is small and T2* is short. Therefore, the 90'

pulse signal of good quality is not obtained.

2. Rotatil]g Frame Experiments

i) Spin-locking, adiabatic denl:tgnetization and remagnetization in the rotating frame.

As described in the previous section the rf frequency is swept in order to achieve spin-

locking In the rotating coordinate (x,y,z), the effective field in the z direction at the

time b rotates in the xz plane during the time bb' until it is parallel to the x direction

at b!***. The rotation of the effective field should be sufficiently slow in order to satisfy

adiabatic condition, Eq. (1). In most cases the time for spin locking is set to about I msec.

In the experiment with protons of various ammonium salts, however, it is found that this

*** In the present arrangement of frequency shift circuit (Fig. 4), the nuclear magnetization is

aligned antiparallel to Hi (corresponding to a negative temperature state) instead of parallel

to it. Thi3 is, however, immaterial because the polarization at the lattice temperature in

the rotating frame is negligibly small

28

ギ

（a） （b）

（c） （d）

（e）

　　　　　　　　　　　　Fig．15

（a）90。一180。pulse　spin　echo　of】H　in　Cu＋＋

　　doped　water，

（b）Examples　of　T亘measurement　by　Carr－　　Purcell　l800－900pulsesequenceforIHin　　　rubber．

　　UpPer　trace　by　phase　coherent　detection・

　　　Lower　trace　by　dlode　detection．Time　　scale，10msec／div，

（c）　PΩR　free　induction　signal　of　35Cl　in　　　NaCIO3．　　（phase　　coherent　　detection）．

　　Time　scale　O，3msec！div．　　　UPPe「t「ace：　at　exact　resonance，　（30，2　　　MHz）

　　Lower　trace：　o鉦resonance．

（d）PΩR　spin　echo　of35Cl　in　NaClO3，Time　　scale　O．3msec／div．

（e）　PΩR　free　induction　signal　of35CI　in♪一　　dichlorobenzene．Time　scale　O．3msec、／div、

　　UPPe「t「ace；　at　exact　resonance　（34，4　　MHz），

　　Lower　trace＝off　resonance．

29

time (time bb/) can be made as short as O.1 msec without appreciable loss in the mag-

netization. An oscilloscope presentation of the rf pulse shape and frequency modulation_

for the spin locking is given in Fig. 16. A rectangular pulse may be used with the

frequency sweep to lock spins. It is, however, more efficient to use a pulse of the gradu-

all_v~ growing shape of Fig. 5-D, because, then the effective field direction at the instant

of pulse on is more nearly parallel to the z direction than with the rectangular pulse.

In the double resonance experiment it is important to set the field and frequency in the

exact resonance condition If spins are demagnetized (H1~~0) in the off re~onance condi-

tion, the effective field ends up parallel to the z direction, restoring part of the magnetiza-

tion in the z direction (depending on the relative magnitudes of ho and HL) Thus only

a small fraction of the spins are brought to the completely demagnetized state- Adjust-

ment of the field to exact resonance is conveniently made by letting the frequency sweep

pulse off and observing ~L point where the induction decay signal vanishes.?) If thi~

signal is observed it means a tilt of the effective field toward the z direction.

1,

)

Fig 16 Upper trace shows an rf pulse Fig 17 Free induction decay signal after shape (rectified) for 'spin locking spirr locking of IH in NH4Cl a,t 30 Frequenc"v s~veep for the spin lockin*" MHZ is shown In the lo~ver trace as a change Upper trace: without reference signal.

in beat frequency produced by mixing Middle trace: with external reference

with a constant frequencv rf Time signal. scale, I msec'div Lower trace: spin locking not effected

(frequency sweep pulse off) Time scale, 30 p sec,/div.

The measurement of Tl,, Is made by observing a change in the induction decay singal.

heibaht when the rf pulse length is varied (by measuring the pulse length increment for

the signal size to attenuate to l/2).

Accurate phase adjustment for the coherent detection is sometimes difHcult. In order

to secure line~r detection in th~_ relaxation time measurement it is more convenient to

inject an external rf signal of a little off-resonance for the reference and observe arL

30

f

(

Free induction signals after spin locking for IH in induction signal of beat pattern.?)

NH4Cl are shown in Fig. 17_

ii) Adiabatic demagnetization and remagnetization in the rotating frame. Adiabatic

demagnetization in the rotating frame is accomplished by tapering off the tail of the

locking pulse as shown in Fig- 5-F (dd'). Remagnetization of ADRF state is made by

applying a gradually growing pulse (Fig. 5-D). The time for demagnetization or remag-

netization used in the case of ammonium salts is typically I msec.

In order to be able to perform double resonance in the ADRF state it is necessary

that the magnetization does not decay much during the ADRF state. Since the sensitivity

of the Hartmann-Hahn's experiment depends substantially on the time of mixing, it is

,desirable for T,ADRF to be sufficiently long. It is found that in some ammonium salts,

the observed T],~DRF is abnormally short compared with the higher field Tl ' The result

,of the measurement at roorn temperature is shown in Table I. In these ammonium salts

Table I_ Spin lattice relaxation times. Tl and Tl.~DRF, in

some ammonium salts

Ti (sec) = T1*~DRF (sec)

NH4Cl 0.36 O 017 NH4Br 0_5 <0 002 (NH4)2SnC16 3 6 O 043 NH4CI04 19 3 3 NH4103 O 7 O~l NH4N03 ! 36 14 (NH4)2S04 3 6 1.4 NH4H2As04 0.29 O 004 NH4H2P04 Q 52 0>35 < 1

the motion of NH4 Is sufficiently rapid. Therefore, the diminution of TIADRF is not

ascribed to the mechanism of "ultra-slow motion'". It is easily seen that the anomaly is

observed in compounds containing nuclei of large qtiadrupole moment. When the dipole

system of protons at low spin temperature is brou*"ht into contact with the dipole system

of quadrupole nuclei which is kept at the lattice temper~ture due to a rapid spin lattice

relaxation, through a dipole interaction between th~se two systems in their respective

rotating frames, spin t~mperat~re of the protons shouiu reach the lattice temperature

much quicker than in the absence of quadrupole nuclei. In such cases, Tlp is expected

to be longer than TIAl)RF , since in the presence of H1 ' the Zeeman system of protons,

･due to its large level separation, does not as easily exchange energy with the dipole

.system of quadrupole nuclei as does the dipole system of protons. In fact longer T]p

31

com pared

in which

neighbor

with TIADRE is observed in these cases as shown

the observing nuclei have other nuclei of large

a high sensitivity of the ADRF double resonance

Table 11 Spin lattice relaxation time Tl~ in

f rame

in Table II

quadrupole

may not be

the rotating

In a compound_

moment in their~

ex pected

Tl~ (msec)

H* (G)

O

13 2

35 5

NH+Cl

l 7

60

90

190

350

NH*Br

<2

4

22

iii) Double Resonance Experiment. To observe double resonance the rf pulses (pulse'

III in Fig. 2) is applied to the sample during the ADRF state In Fig. 18 is given an

oscilloscope presentation of such rf pulses. In Fig. 19, detection of signals of 14N and

37Cl in NH4Cl, and D in (NH4)2S04 containing I atomic % D by the double resonance is,

sholvn For the detection of the double resonance a reference signal is not employed in

most cases.

By plotting the induction si*~nal height while the second rf frequency is varied a

double resonance spectrum is obtained. Fig. 20 shows an example of such spectrum.

Du'. to the cubic symmetry at N and Cl, single line spectra are observed for 14N, 3~Cl_ ,

Fig 18 A CRO presentation of the rf pulse shape for ADRF double reso-

nance Upper trace: the main rf (rectified)

Lo~ver trace: the second rf Time scale> 3 mseddiv.

32

and 37Cl in NH4Cl. Ordinary steady-state spectrum of D in (ND4)2S04 powder has a

structure due to a small quadrupole coupling.8) In the present method such structure was

not detected. Because of large HIB used in this procedur{= the resolution is inferior to

the steady-state experiment. Estimated resolution is about 5 kHZ in our experiment.

f

,

lr

Flg (a) (b) 19 (a) IH signal at 30 MHZ in NH4Cl showing ADRF double resonance of 37Cl

(Diode detection). Mixing time 23 msec

Upper trace: off resonance

Lower trace : at resonance (2 448 MHz)

(b) ADRF double resonance as seen from IH signal of I atomlc "/o D in (NH4)2S04 Mixing time 660 msec Time scale 30 p sec,*div.

Upper trace : second rf frequency 4.590 MHZ (off resonance)

Lower trace: second rf frequency 4.610 ~/IHz (at resonance)

(External reference signal injected)

,e$:; ~;:~ ~3>~ ~;~ FHnj

$::~:~ eJOS4 U)a~.-

2. 15 MHZ

Fig 20.

Frequeney uB

Double resonance spectrum (NHl)2S04 Single crystal.

2.20MH2::.

of 14N in

We including

wish to thank Professor T.

the method of spin locking

Hashi of Kyoto University for

by frequency sweep

valuable suggestions

/ Department of Chen~istry

Nihon University

Sakurajosui, Setagaya-ku

Tokyo.

33

References

12345678

S．R．Hartmann　and　E．L．Hahn，Phys，Rev．128，2042（1962）・

F．M．Lurie　and　C．P．Slichter，Phys．Rev．133，AllO8（！964），

C．P，Sllchter　and　D．Ailion，Phys，Rev，135，AIO99（1964）．

D．C．Ailion　and　C．P．Slichter，Phys．Rev．137，A235（1965）．

R．Blume，Rev・Sci，Instr・32，554（1961）．

RA．McKay　and　D．E　Woessner，j，Sci．Instr．43，838（1966），

C　P．Slichter　and　W．C．Holton，Phys．Rev．122，1701（1961）・

TChiba，JChem．Phys．36，ll22（1962），

つ

F奪

レ

34