Chemistry Reference
In-Depth Information
a relationship with the microwave power
P
m
and the quality factor
Q
m
according to
the following equation:
p
Q
m
o
1
E
/
k
P
m
:
(12)
Thus a high power microwave source is required for DNP experiments. Cur-
rently, the microwave sources can be classified into two different types: solid-state
and vacuum electronic devices. Several excellent reviews of the introduction of -
and outlook for - microwave source technology are available [
41
-
43
].
Among these, gyrotrons and cyclotron resonance masers are high-frequency
vacuum electronic devices that have the ability to produce sufficient power in the
frequency range of 140-590 GHz for electrons (200-900 MHz for proton nuclei).
The electron cyclotron resonance maser can emit the coherent radiation near the
relativistic electron cyclotron frequency. The irradiation frequency is given by
eB
0
M
0
m
e
c
o
g
¼
f
(13)
and
1
1
M
0
¼
p
;
(14)
u
2
=
c
2
where
B
0
is the strength of the static magnetic field and
e
is the charge of the
electron.
f
is the harmonics of the operation mode,
m
e
the mass of the electron,
while
M
0
is a relativistic mass factor given by (
14
).
and
c
are the speeds of the
electron and light, respectively. From (
13
), the coherent radiation frequency
u
o
g
is
directly proportional to the strength of the static magnetic field
B
0
when the other
experimental parameters in (
13
) remain constant.
3.2 Microwave Waveguides
The microwave waveguide is a device which is applied to transmit the microwaves
from the source to the NMR probe. The efficient delivery of the microwave irradi-
ation is highly necessary for microwave waveguides. When the microwave
frequencies increase, however, the efficiency will decrease. Nowadays, with the
help of corrugated waveguides, the loss is almost negligible and the efficiency of
the microwave transmission is immensely increased. These devices have already
been used in high frequency EPR and DNP applications [
24
,
44
].
