U.S. patent number 3,784,938 [Application Number 05/216,207] was granted by the patent office on 1974-01-08 for microwave spectroscopy.
This patent grant is currently assigned to Cambridge Scientific Instruments Limited. Invention is credited to John Cuthbert, James Albert Stow.
United States Patent |
3,784,938 |
Cuthbert , et al. |
January 8, 1974 |
MICROWAVE SPECTROSCOPY
Abstract
In a Lide type of absorption cell for use in Stark modulation
microwave spectroscopy the cell is in two mutually insulated halves
which are secured together by longitudinally spaced releasable
clamps allowing the cell to be taken apart readily for cleaning but
holding the halves accurately in their correct relative positions
when assembled. The clamps can be of C-shape with disengageable
leaf springs fitting in their open sides.
Inventors: |
Cuthbert; John (Bottisham,
EN), Stow; James Albert (Ely, EN) |
Assignee: |
Cambridge Scientific Instruments
Limited (Cambridge, EN)
|
Family
ID: |
9720355 |
Appl.
No.: |
05/216,207 |
Filed: |
January 7, 1972 |
Foreign Application Priority Data
|
|
|
|
|
Jan 12, 1971 [GB] |
|
|
1,343/71 |
|
Current U.S.
Class: |
333/239;
356/316 |
Current CPC
Class: |
G01N
22/005 (20130101) |
Current International
Class: |
G01N
22/00 (20060101); H01p 003/12 () |
Field of
Search: |
;333/95 ;356/74,86 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1,515,958 |
|
Sep 1965 |
|
DT |
|
883,460 |
|
Jul 1969 |
|
DT |
|
Primary Examiner: Rolinec; Rudolph V.
Assistant Examiner: Chatmon, Jr.; Saxfield
Attorney, Agent or Firm: Samuel Scrivener, Jr. et al.
Claims
1. An absorption cell for use in Stark modulation microwave
spectroscopy comprising a rectangular section wave guide of
cross-section having longer sides and shorter sides and split in a
central plane midway between and parallel to the planes of said
longer sides to form two channel-section members, opposed blocks of
electrically insulating material extending along the length of said
wave-guide and having portions of predetermined width projecting
between the mutually opposed flanges of said channel section
members to mutually space said members the correct distance apart
while mutually insulating one from the other, and a series of
longitudinally spaced apart releasable clamps each engaging a
portion of each of the opposed blocks and releasably holding said
blocks and said members in assembled condition.
2. The cell set forth in claim 1 wherein said clamp comprises a
C-shaped member having a web and limbs surrounding three sides of
said waveguide, said limbs engaging the portions of said blocks,
and a spring spanning the fourth side, said spring bearing on one
of said channel-section members to urge it towards the other said
channel-section member.
3. The cell set forth in claim 2 wherein said spring is a leaf
spring.
4. The cell set forth in claim 2 including a pad of electrically
insulating material interposed between said spring and said one
channel-section member.
5. The cell set forth in claim 3 wherein the free end of at least
one of said limbs of the C-shaped member includes a notch
detachably engaged by said leaf spring.
6. The cell set forth in claim 1 wherein said portions of the
blocks engaged by said clamps are grooved to be located by said
clamps.
7. The cell set forth in claim 1 wherein each said clamp has a
periphery containing points lying on a circle centered on the
longitudinal axis of said waveguide.
8. The cell set forth in claim 2 wherein the external surface of
the web portion of one of said channel-section members carries
longitudinally spaced transverse grooves each capable of being
engaged by, and thereby locating, the inner edge of said web of one
of said C-shaped members.
9. The cell set forth in claim 1 wherein the opposed blocks are
arranged in longitudinally spaced apart, opposed pairs of blocks.
Description
This invention relates to the construction of the absorption cell
used in the microwave spectroscopy of gases. Such a cell takes the
form of a waveguide of rectangular cross-section, usually a metre
or up to several metres in length, which contains the gas under
examination and a microwave source feeds in power at one end while
a detector at the other end examines the resulting absorption
spectra.
In the Stark modulation spectrometer an intermittent transverse
electric field is applied to the gas in the absorption cell to
shift the resonant frequency of the gas molecules by the Stark
effect and this field, which must be of the order of at least
hundreds of volts per centimetre to achieve an adequate frequency
shift, is applied in the known arrangements by means of a flat
strip electrode extending along the middle of the cell midway
between the two broader (H-plane) opposing faces. This electrode
must be located with great precision since, if it is slightly
nearer to one face than the other at any point it will give a
greater electric field strength on that side, and any
non-uniformity of the field will result in a broadening of the
shifted line, and the non-uniformity of the resulting electrostatic
forces on the electrode may cause it to oscillate mechanically, the
result of which may be an apparent microwave absorption signal at
the appropriate frequency and phase. Yet at the same time the
electrode must be electrically insulated from the guide. Also, the
presence of this Stark electrode introduces appreciable attenuation
and tends to produce unwanted reflections despite care in its
shaping and location. The problem of mounting the electrode in a
cell usually a metre or more in length is such that its removal for
cleaning is generally a skilled job which may require its return to
the manufacturers. This can be a serious drawback where
contamination from the gas sample occurs, e.g., due to inadvertent
electrical breakdown across the electrode-supporting insulation,
which results in a conductive or semi-conductive path across the
insulation.
It has been proposed by D.H. Baird and others (Review of Scientific
Instruments Vol 21 No. 10 1950 page 881) to apply the Stark field
between two opposite walls of the cell by splitting the cell into
two mutually insulated halves, the plane of the split being along
the axis of the cell and parallel to the plane of the main electric
field (the E-plane). Lide improved on this (Review of Scientific
Instruments Vol 35 1964 page 1226) by increasing substantially the
dimension in the E-plane, so as to improve the uniformity of the
Stark field. However a drawback of this cell was its rigid
construction, with two halves permanently connected together by
strips of insulating material. This was liable to distortion when
heated and also was difficult to take apart for cleaning.
The aim of the present invention is to provide an absorption cell
of this type, which can be readily taken apart for cleaning
purposes but which at the same time is rigid when assembled, and
keeps the two halves accurately in their correct relative
positions, as well as being substantially free from distortion with
changes in temperature.
According to the invention there is now proposed an absorption cell
for use in Stark modulation microwave spectroscopy comprising a
rectangular-section wave-guide split in a central plane midway
between and parallel to the planes of the longer sides of the
cross-section to form two mutually insulated halves, the two halves
being releasably held spaced apart by a series of releasable clamps
spaced apart along their length.
Each clamp is preferably in the form of a C-shaped member
surrounding three sides of the waveguide, with a spring (which may
be a leaf spring) spanning the fourth side and bearing on one of
the channel-section members to urge it towards the other.
The invention will now be further described by way of example with
reference to the accompanying drawings, in which;
FIG. 1 is a simplified block circuit diagram showing a Stark
modulation microwave spectrometer;
FIG. 2 is a perspective view of the cell according to the
invention; and
FIG. 3 is a cross-section through the cell according to the
invention, illustrating the construction of one of the clamps.
Referring first to FIG. 1, a microwave spectrometer operating on
the principle of Stark modulation, that is to say, modulation by
the Stark effect, comprises basically an oscillator O, an
absorption cell C and a detector D. The oscillator may for example
be a Klystron oscillator but is preferably a backward wave
oscillator since the latter has a greater ability to be varied in
frequency over a range of as much as 1.6 to 1 purely electrically,
without requiring any mechanical movement such as a change of
cavity size. In the example under consideration the oscillator
operates over the Q band (called the R band in the U.S.A.) which
covers 26,500 to 40,000 Megaherz. The signal from this oscillator
is passed through the cell C which may be a metre long, and into
which the substance under examination is introduced. Where the
substance is a gas it can be fed in at normal temperature, but at a
low pressure of the order of 10.sup..sup.-1 Torr. Where it is a
liquid at normal temperatures it will normally vaporise anyway at
this pressure, but it may be introduced at an elevated temperature,
and solids may likewise be heated to vaporise them. The frequency
of the oscillator is varied slowly over the whole band, or over a
part of the band, by a sweep-frequency oscillator S giving a
saw-tooth signal of which the sweep period can be varied between 10
seconds and 90 minutes, and as the frequency of the oscillator O
passes through the frequency of one of the resonant modes of a
molecule present in the substance under examination (which may be a
mixture) the energy is absorbed to some degree and the signal in
the detector D (which is a crystal diode) falls. As the absorption
is, in absolute terms, very small (only perhaps one millionth of
the total energy passing through the cell, it is impossible to
detect directly the fall in signal as the oscillator sweeps through
a resonant frequency. Instead a modulation is applied, and in the
Stark spectrometer this is applied to the substance itself, making
use of the Stark effect by which an electric field applied to the
molecules will shift the resonant frequency slightly. The
modulation signal is, as far as possible, a square wave signal with
as short a rise time as possible, to shift the resonant frequency
back and forth between two distinct values. Thus with the
oscillator at a given frequency (the sweep being slow and therefore
to be ignored in relation to the modulation frequency) which is a
resonant frequency for the molecules in the absence of an electric
field, a signal will appear in the diode D at the modulation
frequency, as the resonant frequency of the molecules shifts
between its natural value, to which the oscillator is tuned, and
the shifted value. As shown in FIG. 1, the output of the diode D is
fed to an amplifier A tuned to the modulation frequency (which may
be 40 kiloherz) and used to control the Y deflection of a cathode
ray oscilloscope CRO or an X-Y recorder, the X deflection or
time-base of which is provided by the saw tooth signal from the
sweep oscillator O.
Additional signals will appear in the diode D as the osciallator O
passes through frequencies differing by the amount of the
modulation frequency from the Stark frequency, and these will
appear as so-called Stark lobes on each side of the main Stark
signal on the CRO screen or recorder chart. By including a
phase-sensitive detector we make these lobes of opposite sign to
the main signal, to produce a spectrum (for one particular resonant
frequency) of the kind indicated on the CRO screen in FIG. 1.
The absorption cell C according to the invention is shown in FIGS.
2 and 3. A waveguide for the Q band (in U.S.A. the R band),
operating in the normal TE.sub.01 mode, would have a cross-section
of 0.702 cm. by 0.315 cm., the shorter dimension being that of the
opposed walls that lie parallel to the E-plane. To apply the Stark
electrostatic field by splitting a cell of such dimensions in a
central longitudinal plane midway between these two shorter walls
and applying the signal between the resulting two mutually
insulated halves would, as indicated earlier, produce
unstatisfactory results, since the electrostatic field produced
would be non-uniform. The degree of Stark shift of the resonant
frequency is proportional to the field strength, which needs to be
of the order of two or three thousand volts per centimetre for a
worthwhile shift of one Megaherz. If this field strength is not
uniform the shift will be spread over a range and result in a loss
of resolution. In the cell according to the invention, therefore,
we increase substantially the dimension of the E-plane, so that it
is greater than the other dimension, that is to say, the width of
each of the opposing walls of the cell between which the Stark
voltage is applied is greater than the spacing between those walls.
In the example shown the width is 2.54 cm., over three times the
theoretically correct value. The spacing between them is 0.702 cm.,
which is the normal dimension in this direction for a waveguide in
the Q band, and there is a gently tapering transition section T at
each end of the cell to change over the internal dimension in the
E-plane (but not the H-plane) from the standard waveguide
cross-section to that of the cell.
The two rectangular channel sections F that face each other to form
the cell are machined by milling or electro-forming from drawn
metal channel sections. They are held in their correct relationship
by a series of C-clamps at intervals along their length, two of the
clamps being visible in FIG. 2. Each clamp comprises a flat rigid
C-shaped member G extending around three sides of the cell and a
leaf spring L of flattened V-shape that engages notches in the
inner faces of the two ends of the member G. Opposed pairs of B of
PTFE of Tee-shaped cross-section fit between the flanges of the two
channels F to keep them at the correct spacing apart (0.15 mm. in
the example shown) and have their outer faces grooved to engage and
be located by the inner edges of the member G. The outside face of
the web of one of the channels F (the right-hand one on FIGS. 2 and
3) can have shallow transverse grooves machined in it at
longitudinally spaced intervals as shown at I in FIG. 3 to receive
the inner edges of the webs of the members G of the clamps and
thereby locate the clamps axially. The clamps are thus at the
electric potential of this channel, normally the grounded channel,
and the clamp is insulated from the other channel, by a pad P of
PTFE.
The ends of the leaf springs L are themselves notched to locate
them in the members G. It will be appreciated that the resulting
clamp can be readily assembled and dismantled by hand and that,
when assembled, it urges the two halves of the cell tightly and
resiliently together. The simple shape of the parts, including the
channels F, enables their dimensions to be closely controlled
during manufacture, so that the two halves of the channel are
accurately and rigidly located in the correct relative positions
and minimum spacing (consistent with the voltage to be applied
between them) to provide a smooth waveguide offering a minimum of
reflections and unwanted modes. At the same time, the simple manual
way of releasing the clamps ensures that the user can quickly take
them apart to dismantle the cell for cleaning purposes. This is
particularly useful in a cell intended for use in routine chemical
analysis, where unexpected condensation or decomposition, within
the cell, of (possibly unknown) constituents may occur.
The rigid construction resulting from the channels F and the clamps
G,L allows the cell to be heated or cooled without significant
distortion. The cell will normally be mounted in a close fitting
round cylindrical housing to which the gas or vapour under
examination is admitted, and the members G, as well as forming
parts of the clamps, also form spaced supports for the cell within
the housing, which is indicated in broken lines at H in FIG. 3. For
this purpose at least the lower limb of each member can, as shown,
be of part-circular profile, or at least have as an outer periphery
containing points lying on a circle centered on the axis of
symmetry of the cell, so that the cell is then located centrally in
the housing. We claim:
* * * * *