Appl. Magn. Reson. 10, 263-279 (1996)
Applied
Magnetic Resonance
~~ Springer-Verlag 1996
Printed in Austria
Probeheads and Instrumentation for Pulse EPR and ENDOR
Spectroscopy with Chirped Radio Frequency Pulses and
Magnetic Field Steps
J. Forrer 1, S. Pfenninger 1., G. Sierra ~, G. Jeschke 1, A. Schweiger 1,
B. Wagner 2, and Th. Weiland 2
I Laboratorium fª Physikalische Chemie, Eidgen6ssische Technische Hochschule,
Zª
Switzerland
2Institut ¡ Hochfrequenztechnik, Technische Hochschule,
Darmstadt, Germany
Received June 19, 1995
Abstract. Probeheads and instrumentation for modero X-band pulse EPR and ENDOR experiments with chirped radio-frequency pulses and rapid B0-field pulses are described. The resonant
frequency, the quality factor and, for the first time, the response of a pulse ENDOR resonator
strueture to a microwave pulse in the subnanosecond time scale have been calculated. The performance of the probeheads for time-domain chirp ENDOR and electron Zeeman-resolved EPR is
demonstrated.
1. Introduction
Pulse E P R is a w e l l - e s t a b l i s h e d spectroscopic technique with an i m p r e s s i n g
repertoire o f experimental schemes. However, m a n y o f the ingenious concepts
introduced in the last few years d e m a n d for sophisticated instrumentation. In
this c o n t r i b u t i o n we describe pulse E P R p r o b e h e a d s t o g e t h e r with the apparatus for E N D O R experiments with chirped r a d i o - f r e q u e n c y (rf) pulses and E P R
e x p e r i m e n t s with m a g n e t i c - f i e l d pulses. Particular attention is given to the
h o m o g e n e i t y o f the m a g n e t i c - f i e l d pulses, to the creation o f chirped r f pulses
with flat frequency characteristics, and to the spatial m i c r o w a v e (mw) field
distribution in the E N D O R r e s o n a t o r during the first few n a n o s e c o n d s o f an
m w pulse.
* Current address: Institut fª Physikalische Chemie, Universit~R Basel, 4056 Basel, Switzerland
264
J. Forrer et al.:
2. Probehead for Pulse EPR/ENDOR Experiments
The probeheads used for pulse EPR and ENDOR experiments are made of two
parts, the probehead support and the interchangeable resonator (Fig. 1). The
latter consists of the bridged loop-gap resonator (BLGR), the mw shield, the
mw coupling loop, and the resonator housing [1-4]. The BLGR is made of Ag
and/or Au layers (thickness 0.5-4 q
on the inner and outer surface of a
quartz tube (5 mm i.d., 6 mm o.d., length 4-12 mm). The layer inside the quartz
tube is split by two gaps (width 1 mm), which are bridged by two metallic
stripes (width 1.5 mm) on the outer surface [1, 5, 6]. For critical coupling or
overcoupling the mw coupling loop (silver-plated brass wire, diameter 0.8 mm,
inner loop diameter 4.4 mm) is moved along the sarnple axis. The dimensions
of the loop influence the mw coupling range and the spatial electromagnetic
field distribution in the BLGR considerably. The BLGR is surrounded by ah
b
/
QUARTZ
TUBE
/
METALLIC LA YER
(gold a n d / o r si/ver -p/ated)
MW COAXlAL CABLE
EZ 86Cu (sitver -plated)
RF FLAT
COIL
TEFL ON
SUPPORT
SAMPLE TUBE
r
REXOLITE
/ " HOLDER
/
HELMHOL TZ
COIL
SOLENOIDAL
MW SHIELD
- - REXOLITE TUBE
COPPER DISC
COPPER (a,b) / REXOL/TE (c)
HOUSING
COUPUNG LOOP
(si/ver -plated brass)
MW SHIELO
HOLDER OF HELMHOLT-Z
COIL AND MW SHIELD
BLG RESONA TOR
SAMPLE
CLAMPING RINGS
RESONATOR HOLOER
(Rexo/ite)
Fig. 1. Pulse EPR/ENDOR resonator. Longitudinal cross-seetion with bridged loop-gap resonator,
Helmholtz cofl for magnetic-field pulses and details of the exehangeable m w sh[eld, a Metallic
layer on quartz tube. b Rf flat coil. e Winding of solenoid and Helmholtz eoit.
Probeheads and Instrumentation for Pulse EPR and ENDOR
265
mw shield. For standard putse E P R experiments the shield consists o f a quartz
tube (length 32 mm, 9-11 m m i.d.) which is gold and/or silver plated on its
outer surface (detail a in Fig. 1). For pulse E N D O R experiments the m w shield
and rf coil are combined and formed a flat solenoid with rectangular wire
cross-section (detail b in Fig. 1, length 32 mm, 10.3 m m i.d., wire cross-section
1x0.5 mm, 2 x 0 . 5 mm and 4 x 0 . 5 m m for rf coils with 26, 13, and 6 turns,
respectively) [2, 3]. Finally, an m w shield made o f copper wire (detail c in
Fig. 1) is used in EPR experiments with B0-fielct pulses to prevent the formation
o f eddy currents in the shield. The copper housing (26 m m i.d., length 38 mm)
is used as rf radiation shield and ground for the rf matching circuir.
Numerical computations o f the spatial distribution o f the electromagnetic fietd
veetors in the sample area as well as the calculation o f the quality factor, the
resonant frequencies and the time response of the resonator are o f considerable
importance for the design o f new resonator structures. In particular, the homogeneity o f the mw B 1 field is a prerequisite for optimum performance of many
pulse E P R experiments like spin-lock and decoupling sequences, transient nutation experirnents and schemes where coherence transfers are involved. The time
dependence o f the mw B~-field distribution in the sample area during the transition time o f the resonator has not yet been considered. This transition time
b
~
zi
j
3
Fig. 2. Geometric structure of the pulse ENDOR resonator d[scretized onto a grid of 160000 cells.
qhe strueture consists of the mw resonator with the coupling loop, a 6-turn rf coil (mw shield), the
sampte tube and the resonator holder, a Full structure; (t) coupling loop, (2) sample tube, (3)
BLGR, (4) rf coil, (5) BLGR hotder, (6) copper hous[ng, b, e D[scret~zed structure of the bridged
loop-gap resonator.
266
J. Forrer et
al.:
depends on the loaded quality factor Qt and amounts often up to 50% of the
entire pulse length.
The computations have been carried out with the software package MAFIA
(Solution of MAxwell's equation by the Finite Integration Algorithm) which is
based on the finite integration theory for solving the integral form of Maxwell's
equations [7-9]. The first step in the simulation is to discretize the geometric
structure orito a grid with the aid of the mesh generator MAFIA-M. The model
of the ENDOR resonator (including BLGR, 6-turn rf fiar eoil, coupling loop,
resonator holder, sample tube and copper housing), which eonsists of 160000
cells, is shown in Fig. 2. The mŸ
cell side length of the Cartesian grid
amounts to a quarter of a millimeter which is more than 100 times shorter than
the vacuum wave length. In spite of this small step size the modelling of the
metallic layers o f the BLGR is quite coarse, and one may therefore not expeet
to get absolute resonant frequencies. Nevertheless, we found that the discretization
is sufficient to determine the shifls o f the resonant frequency for different resonator dimensions. In contrast to earlier investigations [1, 2] that were based on
the eigenmode solver MAFIA-E, the present ealculations have been carried out
with the time domain solver MAFIA-T3 which uses the "leap-frog" scheme for
time integration.
Two different signals have been used for the computations, a broadband Gaussian pulse a n d a sine function which grows up from zero to its maximum
amplitude during the first three periods. The first computation delivers the
modes of the resonator (Fig. 3) with the EPR-active mode at an mw frequency
Vo
1
i
6
.
.
.
.
i
7
.
.
.
.
v/GHz
i
8
.
.
.
.
i
9
Fig. 3. Microwave resonant modes of the ENDOR resonator. Only the mode at 9.22 GHz is EPR
active.
Probeheads and Instrumentation for PuIse EPR and E N D O R
267
t
lE
2
b
4
6
ttns
8
i'"~
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i.......
~'~'I'I-imi:i
"........
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--
,,
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iii
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.
Bf-field
~
i.,
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..
_
lm..J
t2
ti
i i-i~-i~-Z
-:
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:-" ::
llil;-Ÿ
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'-11
i
E Ffietd
i ......
i!!
*:......"..i.i.. .__J
- ~ 1
. - -
.#
. . . .
a , o
i
i
i
9
I. ~III
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'
9
1Ir
9
l l r
r
' I'i~~ I ! y
'I'11IIT ! !
i
:I:IIILLLL
i
,,
i
k. . . . . . . . . . . . . . . . . . . .
i-i~iil!"
,i,~
t~
~
Fig. 4. Microwave fields in the ENDOR resonator (weakly overcoupled), a Response (B~ component
in the center of the resonator) to a m w laulse with a rise time o f 0,3 ns, ~: time r
t,, t 2, t3
mark times used for the p]ots in b. b B,- and El-field distfibufion in the xz-plane of the ENDOR
resonator at three different times aiter the m w putse is turned on (marked in I): t I =0.22ns, (2
pe¡
maximum arrow: B~ = 0.13 toT, E, = 70 V/mm; t 2 = 0.54 ns, (5 periods), maximum arrow:
B 1 = 0.29toT, El = 100V/mm; t 3 = 5.42 ns, (50 periods), m a x i m u m arrow: B 1 = 1.93 toT, E l =
= 140 V/mm. (1) coupling Ioop, (2) sample tube, (3) BLGR, (4) rf coil, (5) BLGR holder.
26~
J. Forrer
e t al.:
/ . 2
E
E
9
9
. 9
.J
]
I
!
L
t
%~
9
~ ~ ~ I T T T T f P
~,~,~ ~ t I ! T r T P P
k ~_~_'~_LI_~_I_I_LL_t_
-3
0
x/mm
3
Fig. 5. Microwave fields in the ENDOR resonator (weakly overcoupled). Detail of the Bl-field
distribution at time t3 (numbers see Fig. 4b).
of about v0 = 9.22 GHz (QL = 1 8 0 , weakly overcoupled). The second computation (Fig. 4a) shows the evolution of the B~ field in the center of the BLGR
characterized by the time constant r = QL/2nv0. Fig. 4b shows the spatial distribution of the magnetic and electric component of the mw field in the x z plane (defined in Figs. 2b and 2c) of the ENDOR resonator at times t I =
= 0.22 ns, t2 = 0.54 ns, and t3 = 5.42 ns after the pulse is turned on, corresponding to 2, 5 and 50 periods (maximum Bl-field amplitude, see Fig. 4a).
The length of the arrows is a measure for the B t- and El-field strength. Note
that the Bl-field vectors change direction between coupling loop and upper
end of the BLGR. A detail of the Bl-field distribution (coupling loop area) for
time t3 is plotted in Fig. 5. The influence of the resonator elements on the zcomponent B~ of the Bl-field vector along the three resonator axes for a distante between coupling loop and BLGR of 1 mm, is shown in Fig. 6. Along
the y-axis (Fig. 6a), the homogeneity within the BLGR is quite high. Along
the x-axis (Fig. 6b) however, B~ varies by about 15% between the center and
the gap of the BLGR. Finally, along the z-axis (Fig. 6c) the inhomogeneity, in
particular in the upper part of the BLGR, is strongly influenced by the dimension o f the coupling loop and its distante from the BLGR, as is also found
for a critically coupled resonator [1].
Another important aspect of the numerical computations refers to the study of
the influence of the flat rf coil on the resonator characteristics. We found that
the resonance frequency is reduced by about 30 MHz if the metallic shield is
replaced by the rf coil, in agreement with measurements. In addition, the flat
coil causes a slight asymmetry of the B 1 field inside the BLGR, which however
has no influence on the homogeneity in the sample area. Finally, Fig. 4b indicates that the strength of the electric field between the windings of the coil is
substantial.
Probeheads and lnstrumentatiort for Pulse EPR and ENDOR
269
1.5
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P=--=H
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0
z/mm
Z-component B, of the B~-field distribution along the resonator x-, y-, and z-axis for weak
overeoupling, QL = 180 (coordinate system, see Fig. 2). a B, along the y-axis. (1) inner diameter of
BLGR, (3) shield diameter, b B~ along the x-axis. (1) irmer diameter of BLGR, (2) outer d~ameter
of BLGR, (3) shield diameter, c B, along the z-axis. (1) Coupling loop, (2) BLGR.
Fig. 6.
270
J. Forrer et al.:
3. ENDOR Experiments with Chirped Radio-Frequeney Pulses
In a chirp ENDOR experiment the rf is rapidly swept in a time trf shorter than
the spin-lattice relaxation time of the eleetron spins and the phase memory time
of the nuclei. Time-domain (TD) chirp ENDOR [10] in particular is a technique
which is closely related to the TD-ENDOR experiments introduced by H6fer
et al. [11] and Cho [12]. In contrast to these approaches broadband excitation of
the nuclear transitions is achieved by sweeping the rf linearly during the pulse
over the entire spectral range. It has been demonstrated that optimum flip angles
can be achieved with this ENDOR instrumentation. Chirped rf excitation drastically reduces the measuring time, and power broadening is absent in the spectra. Moreover, it has been demonstrated that for many samples, sensitivity and
spectral resolution in TD-chirp ENDOR ate superior as compared to DaviesENDOR [.13] or Mims-ENDOR [14], and that the approach can easily be extended to a two-dimensional hyperfine sublevel correlation experiment [10].
3.1. E N D O R
Unit
The btock diagram o f the chirp ENDOR unit is shown in Fig. 7. The LeCroy
9100 arbitrary function generator (AFG) is the key part of the setup. Ir generates the chirped rf pulses with a digital resolution of 5 ns. Sweep time, frequency range and wave forms of the r f signals are programmed on an AT 486
computer and transferred via an IEEE 488 bus (Keithly Metra Byte KPC-488.2
AT) to the AFG. A power amplifier stage (Amplifier Research, Modet 100L
MB, 1-200 MHz, 100 W; and Transworld Electronics, Model T1000, 2-30 MHz,
pulse power 2.5 kW) amplifies the chirped rf pulses up to a power level of
2.5 kW. To avoid rf excitation outside the frequency region of interest, the
50 f91COAXtAL
CABEL RG 214
8nd SEMI-RfGtO
JN50141
5O
LO,lO
RF COIL
RF POWER
AMPLIFIERS
7 POL-FtLTER
ARRAY
50 l i COAXIAL
CABEL RG 2f4
~md SEMf-RIGtO
JN50141
Fig. 7. Bloek diagram of the chirp ENDOR unit with a cross-seetion of the ENDOR probehead
and the matching capacitors C] and C2.
Probeheads and Instrumentation for Pulse EPR and E N D O R
27t
trailing and falling edge of the chirp pulses are formed as sine-quarter and
cosine-quarter waves, respectively. The AFG also allows the selection of subranges to realize bandstops that exclude preselected spectral regions from excitation. The output of the power amplifier T1000 is equipped with six 7-pole
elliptic low-pass filters with cut-off frequencies of 3, 5, 8, 13, 20 and 30 MHz
(overlapped octave bandwidth) for harmonic suppression. Filter switching is
remote controlled vŸ the IEEE 488 interface (Iotech Digital 488) anda selfmade
relay board. This automatic filter switching is essential for pulse ENDOR methods relying on selective excitation of nuclear transitions.
For a rough impedance matching of the rf power amplifier to the 50 f91 dummy
load in the frequency range 5-30 MHz, a circuit is used [15, 16] that consists
i
I
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2.5
2.0-
i.~=
x
x
1.5O.
8..,.,o~ o - - - - - o - o --o.--------o ~ o \
o
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"~
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.
0
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0
9
0
0.5-
i'o
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ffo
4
v / MHz
i
i
i
i
i
4
b
2.0-
1.6lE
1.2-
zx
Zx
Zk
8'0
9~0
(1.4
3'0
4'0
5'0
6'0
7'0
v / MHz
Fig. 8. B: fields (rotating frame) a s a function o f the radio frequency with (solid line) and without
(dotted line) matching capae[tors C 1 and C 2 for three different r f coiis, a ( x ) coil with 26 tunas,
L = 4,1 ixH, Cj = C: = 134 pF; (O) eoil with 13 turns, L = 2 ~tH, C 1 = C 2 = 84pF. h Coil with 6
tunas, L = 1 ~tH, C t = 47 pF, C 2 shorted.
272
J. Forrer et al.:
of the rf col1 (L = 2.1 }aH) and two high-voltage ceramic capacitors C~ and C 2
(84 pF each) outside the copper housing (Fig. 7). This matching circuit shifts
the B2-field drop caused by the impedance of the rf coil to higher frequencies,
as is shown in Fig. 8a. The rf field strength B 2 (rotating frame) at the sample
position is measured with a pick-up coil (L = 10 nH, 1 mm i.d., cw rf power
50 mW), which is connected via twisted copper wires (length 20 mm, diameter
0.4mm) to the input of the 10:1 attenuator (hpl0214A, Zin = 1 MŸ191
shunted
by 1 pF) of a vector in voltmeter (hp8405A). The B 2 fields are up-converted to
1 kW pulse power. For the rf coils with 26 and 13 turns, the B2-field characteristic is considerably flatter with matching capacitors (Fig. 8a). For frequencies
>30 MHz a fiar B2-field characteristics is obtained with a 6-turn coil already
without impedance matching (Fig. 8b). However, higher field strengths have
been achieved between 30 and 80 MHz with C~ = 47 pF (C2 shorted) a n d a
coaxial cable between amplifier and probehead of 3 m (RG 214, Huber+Suhner,
CH-9100 Herisau, Switzerland) a n d a coaxial cable inside the cryogenic probehead of 25 cm (semi-rigid JN5014I, Precision Tube Company, North Wales, PA
19454, USA). Note that the characteristics may be influenced by the temperature coefficient of the capacitors.
3.2. Performance: Time-Domain Chirp ENDOR
We demonstrate the performance of the chirp ENDOR tmit on a single crystal
of triglycine sulfate doped with 0.5% Cu(II), in an orientation where the two
crystal sites contribute to the ENDOR spectrum with comparable intensities.
The ENDOR spectrum of this sample consists of a large number of lines covering a broad frequency range. Davies-based chirp ENDOR with the pulse sequence shown in Fig. 9a was employed. The basic principles of the experiment
have been explained elsewhere [10]. The pulse lengths of the three selective
mw pulses were 100, 100, and 200 ns. Chirp pulses of 5 kts duration with an rf
sweep from 5 to 28 MHz were used. The experiment has been performed at a
static field of 329.7 mT a n d a temperature of 15 K. A four-step phase cycle
[0,0]-[0,~]-[r~,0]+[Ÿ
has been applied to the two chirp pulses by recalling
sequentially stored wave forms with the required phases from the AFG using an
appropriate trigger scheme. The time-domain trace was sampled with 4096 data
points and 20-ns time increments, baseline-corrected by a constant offset, apodized
with a Hamming window and Fourier transformed.
The magnitude spectrum in Fig. 9b corroborates that a uniform excitation of
nuclear transitions is achieved in the frequency range 8 to 20 MHz. Though we
do not discuss here the assignment of the lines ir should be noted that both
proton and nitrogen transitions contribute to the ENDOR spectrum with comparable intensities. Phase cycling by recalling digitally stored chirped rf wave
forms is very effective in removing any distortion due to electron spin echo
crossings as can be implied from the flat baseline. A detail of the spectrum is
shown in Fig. 9c to demonstrate the resolution of the method. The linewidths
Probeheads and Instrumentation for Pulse EPR and ENDOR
273
ow_~
i
I
~~~~~~WlllllUr
rf
i
iIIIIIIk I,
b
[
i
8
i
i
10
i
i
i
i
12
14
i
i
16
i
J
,
18
i
20
i
22
VENDOR / MHz
i
16
i
i
17
i
18
VENDOR / MHz
Fig. 9. Time-domain chirp ENDOR. a Pulse sequence, b Single crystal chirp ENDOR spectrum of
Cu(II)-doped triglycine sulfate, arbitrary o¡
temperature 15 K. e Detail of the spectrum in b.
are essentially determined by proton dipole-dipole couplings, power broadening
is not observed.
4. Experiments with Rapid Magnetic Field Switching
Many pulse EPR schemes make use of rapid B0-field steps or pulses along one
of the directions of the laboratory coordinate system. Pulsed magnetic fields
along the x- and y-axis are applied to improve spectral resolution [17], while
field steps along the z-axis are used for example in ELDOR experiments [18,
19], hyperfine selective ENDOR [20], or electron Zeemann (EZ)-resolved EPR
[21]. In the following, we describe a simple device for the generation of magnetic-field pulses of high homogeneity, and variable lengths and amplitudes, and
demonstrate the performance of the setup by an EZ-resolved EPR experiment.
274
J. Forrer et al.:
4.1. Switching Electronics
Fast high-voltage high-current switches, as well as remote controllable high-voltage power supplies that are specially designed to charge capacitors are now avaitabte [22]. Fig. 10 shows a current-pulse circuir that was designed for current
pulses up to 30 A (voltage 400 V) with rise and fall times of 20 ns [23]. The
capacitor C (470 ~aF/450 V) at voltage Uc (0 to - 3 0 0 V, 300 mA) is discharged by
a high-power transistor switch (HEXFET IRF 730, n-channel) which generates the
pulse current lp = Uc/(R + RL) that passes through the Helmholtz coil, where R L
(1.4 ~ ) is the resistance of the coil and R (60 f 91 is the resistance of an array
that consists of ten "noninductive" wirewound 5 W resistors. The discharge time
constant of the capacitor is z"c = C(R + RL) = 29 ras. A feedback loop to stabilize
the pulse current is not needed because of the short current-pulse width (<15 q
used in this expefiment. However, the circuit provides current-pulse width and
duty-mtio control to avoid overload of the power supply and to protect the resistor array and the Helmholtz coil against overheating or burnout damage.
For a current pulse of lp = 5 A with length tp = 15 ~ts a n d a maximum repetition frequency v~p(max) = 500 Hz (duty ratio 0.75%), a current drop of 0.05%
is measured. The maximum duty ratio is determined by the maximum available
current of the high-voltage supply. Since the common (~) o f the circuit is on
high voltage (up to - 3 0 0 V), the Helmholtz coil is only at high voltage during
the transition time o f the current pulse. The 15-V supply is also on high voltage and has therefore to be isolated. Chassis mount and shield of the coaxial
cables are on ground. An optical coupler bridges common level and pulse input
J5. The transistor push-pull circuit (2N2219 and 2N2205) generates the current
drive for the HEXFET array which is essential for proper high-current switching [23]. Three large copper arcas on the circuit board forro the common (highvoltage output J4), the high-current leads between pulse connector J1 and J4,
and the power return lead between J3 and the shield of J1. These arcas also
minimize radiation caused by mutual inductance and capacitance between the
high-current leads and the low-power control and driver circuit. A fast recovery
power diode (FEES 8GT, 8 A/400 V) shorts the high-voltage spikes induced by
the Helmholtz coil. Connection between output J1 and the upper part of the
probehead is made by a coaxial cable (RG 223, length 3 m). The feed through
the probehead to the coils is made of flexible copper cord (tength 250 mm,
diameter 0.3 mm) with additional teflon insulation. Voltage Uc is set mechanically with a resetability better than ___0.1V by a stepper motor which is controlled by the computer. The pulse current is measured at the high-voltage
divider output J2, that serves f o r a computer-controlled setting and sweeping.
4.2. Probehead
Magnetic-field pulses with a high homogeneity over the sampte volume are
created by pulsing a current through a Helmholtz coil (Fig. 1). The field horno-
Probeheads and Instrumentation for Pulse EPR and ENDOR
275
t
!
i|
~+r-
..................................
LU
ji
~d
i
r
)-.
O
o
1
iI
-b
i
!
276
J. Forrer et al.:
geneity is however limited by eddy currents generated in the metallic parts near
the sample area. To minimize the generation of eddy currents the resonator
housing is made of Rexolite, the metallic radiation shield of the quartz dewar is
removed in the coil area, and an open ended wirewound solenoidal mw shield
(copper wire, diameter 0.2 mm, pitch 0.5 mm) is used (Fig. l c). The groove for
the wire (width 0.25 mm, depth 0.3 mm) is cut into the outer surface of a
Rexolite tube (11 mm i.d., l l . 8 m m o.d., length 32mm). The BLGR which is
placed in the center of the Helmholtz coil is virtually transparent to highfrequency fields and has originally been developed for experiments with rapid
magnetic-field steps [17].
The two flat coils with a mean diameter d = 24 mm are positioned outside the
mw shield, a = d/2 = 12 mm apart from each other. They consist of ten tums
of insulated copper wire (diameter 0.2 mm) each, wound uniformly in one
layer in circular grooves (23 mm i.d., 0.22 mm thick) cut parallel to the sample
axis into the Rexolite holder (Fig. l c). The coils are electrically parallel connected (L = 11.3 q
and fit into the liquid-helium gas flow dewar (homemade, 43 mm i.d.). Neglecting eddy currents the fietd in the center o f the
Helmholtz coil is given by [24]
~~,,,: ~~oo~i_~)EI_ex~I-~) l
(1)
with
413)q
(2)
N denotes the total number o f turns and r L = L/(R + RL) = 183 ns is the time
constant of the coil. Since Ÿ << TC, the shape of the pulses is mainly determined by Ÿ
The application o f field-jump EPR experiments is limited by the inhomogeneity
of the additional field B 0 which consists o f a time-independent part determined
by the geometrical arrangement of the Helmholtz coil, a n d a time-dependent
part caused by the eddy currents generated in the metallic elements close to
the sample. Experimentally, the homogeneity has been determined via the FID
of a phenalenyl sample [25] (room temperature, standard sample tube, 3.5 mm
i.d., effective length ~ 6 mm determined by the BLGR) observed after a mw
re/2 pulse and 0 < A B 0 < 3 . 6 m T with B 0 + A B 0 = const. To allow for the
eddy currents to decay, a time delay between the B0-field step and the mw n/2
pulse is introduced. Afler a time delay of 6 q an inhomogeneity of the AB0
field of about 0.2% is calculated from the broadening of the lines in the FTEPR spectrum.
Probeheads and Instrumentat[on for Pulse EPR and ENDOR
277
4.3. Performance: Electron Zeeman-Resolved EPR
A typical application o f magnetic-field steps in a pulse EPR experiment is
(EZ)-resolved EPR [21] which altows the separation o f overlapped spectra by
resolving a field-swept EPR spectrum into a second dimension that represents
the electron Zeeman interaction. There exist several pulse schemes for EZresolved EPR. One of the experiments is based on the FID-detected hole-burning approach [26]. In this e x p e r i m e n t a selective m w Ÿ pulse bums a narrow
hole into the inhomogeneously broadened EPR line. Then the magnetic field is
a
1 O0 m T
b
"a
299
100
Offset frecluency
125
303
e~c'~u'-
Fig. 11. Electron Zeeman-resolved EPR. a Field-swept sing[e crystal EPR spectrum of Cu(II)-doped
Mg(NH4)2(SO4)296H20, one line of site 1 overlaps with one line of site 2. b Two-dimensional EZresolved EPR spectrum of the region where the two lines overlap.
278
J. Forrer et al.:
changed from B 0 to B 0 + AB 0. This causes a shifl o f the EPR line together with
the hole by the frequency .Q = (gfle/h)AB o which is recorded via an FID that
follows a nonselective mw xi2 pulse. By repeating the experiment for a set o f
B 0 values and Fourier transformation with respect to time a two-dimensional
plot S(Bo,12 ) is obtained, where the EPR spectrum is disentangled owing to gvalue differences. The frequency separation along the s
o f two paramagnetic species k and l with g values gk and g~ is given by
A,(2 = $-2k - .(21 = (gk - g t ) @ A B 0
9
(3)
AZ2 increases with the difference in the g values, and is proportional to the
amplitude AB o o f the field jump.
EZ-resolved EPR is demonstrated on a single crystal of Mg(NH4)2(SO4) 2- 61-120
doped with 0.5% Cu(II) [27]. There are two magnetically nonequivalent copper
sites in the crystal, resulting in two spectra with four copper transitions each
(icu= 3/2) and g values gl = 2.153 and g2 = 2.325. For the crystal orientation
used in this experiment, the m c u = 3/2 transition o f site 1 overlaps with the
mc u = - 3 / 2 transitions o f site 2 in the field-swept EPR specmLrn (Fig. lla). Fig. llb
shows the EZ-resolved 2D EPR spectrum in the field range 299 toT < B0 < 303 mY;
the two lines are now fully separated from each other. The experiment has been
carried out with a preparation pulse o f length t~ = 3 gs a n d a Ÿ pulse with
t,~~ = 20 ns. A magnetic-field jump o f AB 0 = 3.6 mT is used, and the B 0 field is
incremented in steps o f 0.05 toT.
Acknowledgements
This research has been supported by the Swiss National Science Foundation.
The processing and preparation o f many o f the computer plots by Walter J~iggi
is gratefully acknowledged.
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Authors's Iddress: Dr. J6rg Forrer, Laborato¡
nische Hochschu|e, CH-8092 Zª
Switzer[and
fflr Physikalische Chemie, Eidgen0ssische Tech-