Chemistry Reference
In-Depth Information
o
0
E
while sweeping the magnetic field through the entire EPR line. In the latter
case, if the microwave power used for inversion of the electron spin polarization is
sufficient, electron spin will go through an adiabatic fast process. This is considered
as adiabatic-ISE. It is reported that ISE experiments were performed at low
magnetic fields (9 GHz) using either electrons to polarize
29
Si nuclei or transferring
the high polarization of a photoexcited triplet to surrounding protons [
28
-
30
].
A. Henstra et al. show an ISE build up curve for the adiabatic-ISE. In this experi-
ment, the
29
Si signal in p-type Si at 1.2 K is detected and a factor of about 20 larger
enhancements is obtained in an adiabatic-ISE rather than DSE [
17
].
There are certain limitations in the application of both the ISE and the adiabatic-
ISE. The ISE requires high-power microwave pulses for the excitation of a large
amount of electrons, while the adiabatic-ISE requires an adiabatic magnetic field
sweep. On the one hand, the appropriate microwave sources are not commercially
available at high microwave frequencies. On the other, the adiabatic magnetic
sweep is difficult to perform at high magnetic fields. Although having these
limitations, the ISE has a brilliant future in DNP applications because of the large
sensitivity enhancement it can achieve.
2.2.2 NOVEL
Nuclear spin orientation via electron spin locking (NOVEL) is a DNP experiment
based on coherent pulses. In NOVEL experiments, the electron magnetizations
are locked the in electron rotating frame via electron spin-lock sequences.
The Hartmann-Hahn condition (rotating frame or lab-frame) can be satisfied if
the field strength of the spin-lock pulse meets the condition
o
0
E
ΒΌ o
0
N
; then the
polarization can be transferred from the electrons to the nuclei. van den Heuvel
et al
.
[
31
] show the nuclear polarization buildup curve for a NOVEL experiment in
which the electron polarization from pentacene guest molecules in a photoexcited
triplet state is transferred to
1
H of the naphthalene host crystal. An enhancement of
220 is achieved [
31
]. To date, NOVEL experiments have only been performed at
9 GHz microwave frequencies [
30
-
33
]. Much higher microwave field strengths will
be required at higher magnetic fields.
2.2.3 Dressed-State Solid Effect
Dressed-state solid effect (DSSE) is a DNP mechanism based on simultaneous near
resonant microwave and RF irradiations, which can be established even in the
absence of nonsecular hyperfine coupling terms. This experiment was first
introduced by Weis et al
.
[
34
]. The mechanism of DSSE is illustrated in Fig.
3
.
o
1
E
are two effective fields which have unequal influence at high fre-
quency DNP polarization, because the microwave power is currently limited. It
indicates that the EPR transitions cannot be achieved with the same microwave
field strength. The matching condition for DSSE polarization is given by
1
E
and
o
