CHAPTER THREE SPECTROMETER DEVELOPMENT
The FPI plates were adjusted until a weak sideband signal was observed. The
transmission peaks from the sidebands and the FIR could be distinguished from each
other because their intensities and waveforms were different, (figure 3.5). On 60dB gain,
the initial sideband intensity was about 50mV peak-to-peak at the oscilloscope. The
diplexer position was adjusted until one of the sidebands was clearly observed. The
signal authenticity was tested by scanning the microwave frequency by 1GHz. All the
optics were adjusted until the sideband signal was 700 to 800mV on the oscilloscope
with 60dB gain. The FPI was then used to check the frequencies of the sidebands and the
FIR laser radiation by measuring 11 consecutive transmission maxima for each signal.
Once the sideband frequencies had been verified, the required sideband was optimised.
To alter the spectrometer’s operating frequency, the whole system did not necessarily
have to be realigned: suitable procedures are outlined in table 3.2.
Frequency Change
Alignment Procedure

none – maintain sideband at maximum
intensity


leave system to reach thermal  for 2
hours
optimise FIR laser output every ½ hour
minor alignment adjustments on a daily
basis
single sideband (upper or lower) by
100MHz
(FIR laser frequency unchanged)
no change required since the sideband
intensity only fell to half its original value
at these frequency limits
single sideband (upper or lower) by
>100MHz
(FIR laser frequency unchanged)


manually adjust FPI plate separation
>1GHz also adjust diplexer
pathlength difference
switch between upper and lower sidebands
(FIR laser frequency and microwave


adjust FPI plate separation
adjust diplexer pathlength difference
frequency unchanged)

all optics may require some minor
adjustments
switch FIR laser line

stages 2 and 3 of the alignment process,
i.e. realign FIR beam completely
through system, establish TuFIR
sidebands and optimise
re-establish sidebands after optics repair or
laser system alignment

full system re-alignment
(i.e. stages 1-3)
Table 3.2: Summary of the various alignment procedures that were undertaken when
the TuFIR sideband frequency was altered.
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CHAPTER THREE SPECTROMETER DEVELOPMENT
The maximum sideband intensity measured on this system was an upper
sideband signal, generated from the 761.6GHz laser line in HCOOH (maximum laser
power 37mW) and 27GHz microwaves (equivalent to 2mW microwave power at the
cube). After amplification (40dB) and bandpass filtration (1kHz to 10kHz), the signal
measured 523mV at the LIA. The same signal measured 36mV peak-to-peak, when fed
directly from the detector to the oscilloscope. Ignoring signal transmission losses, these
values correspond to a maximum sideband power of 12W at the detector. Up to 50% of
the incident radiation was not coupled onto the detector element, so the true sideband
power was probably in the region of 25 to 28W.
3.2 Removal of the FPI from the Spectrometer
In Chapter 2 it was shown that most of the incident TuFIR radiation was actually
reflected from the front face of the first FPI mesh (section 2.3.3) and that the FPI
transmission peaks had a finite bandwidth, governed by the finesse of the FPI [10]. It
was not possible to identify or align the TuFIR sidebands without the FPI plates in place.
In both spectrometer configurations the FPI was retained for this purpose. However, the
FPI limited both the range and the sensitivity of the TuFIR spectrometer. When scanning
a spectrum, the FPI plate separation was fixed at a distance corresponding to that
required for maximum sideband transmission at the centre frequency of the scan. Figure
3.6 shows how the intensity of a single TuFIR sideband varies with frequency at a fixed
FPI plate separation. The sideband intensity fell to one half of its original value around
125MHz from the central ‘transmission’ frequency, and fell to 1/eth of its original value
around 155MHz from this point. Consequently, each TuFIR spectrum had to be
constructed from consecutive scans, each covering a maximum frequency range of
200MHz. Between scans, the spectrometer had to be re-optimised at a new sideband
frequency. This precluded the spectrometer’s use in wide frequency-range searches for
novel transient species whose spectra were entirely unknown. When the FPI plates were
removed, all the incident sideband power was transmitted to the detector and the scan
range was extended to 2GHz. (This limit was imposed by the path difference in the
diplexer, which was also wavelength dependent and still affected the sideband intensity).
There were two disadvantages to operating the TuFIR spectrometer this way:
81
CHAPTER THREE SPECTROMETER DEVELOPMENT
1.0
0.9
Peak-to-peak Intensity (V)
0.8
0.7
0.6
0.5
P=Po/2
0.4
P=Po1/e
0.3
0.2
0.1
0.0
-0.1
700.00
700.25
700.50
700.75
701.00
701.25
701.50
701.75
702.00
Frequency (GHz)
Figure 3.6: The frequency variation of the intensity of the lower sideband, optimised at
700.9128GHz. The sideband was generated from the 729.9328GHz laser line in
CD3OD and 29.02GHz microwaves. The green and red lines indicate the half-power
and 1/e power points respectively.
1. it was not possible to attribute spectral features to the upper or lower sideband
unless the transition origin was checked by replacing the FPI plates,
2. the intense FIR radiation was not completely filtered from the sidebands and
therefore reached the detector. The FIR radiation was at a fixed frequency
and only partially modulated but it dominated the background radiation
signal. Consequently, the source noise (originating from the laser system or
the mixer diode) occasionally exceeded the detector noise level.
If the laser system was tuned so that the source noise was eliminated, the spectrometer
sensitivity was improved by at least one order of magnitude, (figures 3.7 and 3.8). It was
possible to make rapid, extensive searches for radical spectra at very high sensitivity, and
then focus in on a narrow region of interest. Provided that a spectral feature was still
visible when the FPI plates were replaced, its frequency could be determined absolutely.
This modification was also advantageous in the pressure broadening experiments where
the line-profile could be measured at greater S:N ratios and higher pressure limits than
the corresponding ‘plates in’ spectra.
82
CHAPTER THREE SPECTROMETER DEVELOPMENT
98,2  87,1
Intensity (arb units)
0.4
0.2
0.0
feature due to
lower sideband
-0.2
-0.4
936.25
936.26
936.27
936.28
936.29
936.30
plates out
plates in
Upper Sideband Frequency (GHz)
Figure 3.7: A comparison between the relative intensities of two 1st derivative SO2 TuFIR
spectra recorded with the FPI plates in and out. The transition was recorded using the
upper sideband, generated from the 902.0016GHz laser line in CH2CHF. The sideband
was FM modulated at 75kHz, with a modulation depth of 1.3MHz.
0.3
Intensity (arb units)
194,16  183,15
382,36  373,35
0.2
Upper Sideband
757.8199GHz
0.1
328,14  327,15
0.0
-0.1
702.05
702.10
702.15
702.20
702.25
Lower Sideband Frequency (GHz)
702.30
plates out
plates in
Figure 3.8: A comparison between the relative intensities of two 2nd derivative SO2
TuFIR spectra recorded with the FPI plates in and out. The transition was recorded
using the 729.9328GHz laser line in CD3OD. The sideband was FM modulated at
45kHz, with a modulation depth of 500kHz.
83
CHAPTER THREE SPECTROMETER DEVELOPMENT
3.3 Absorption Cell Design
The introduction of an appropriate absorption cell is one of the most important
aspects of improving the sensitivity of an absorption experiment. Beer-Lambert’s law
shows that the absorption intensity depends on the gas concentration and the radiation
pathlength through the sample, as well as the intrinsic quantum-mechanical properties of
the transition (equation 3.4). Unlike studies of stable molecules where the gas
concentrations in the cell are controllable, the concentrations of transient species are
ultimately limited by their chemistry, i.e. the rate at which they are produced and their
lifetime in the cell. In spectroscopy, transient species are usually formed in electric and
magnetic discharges, either in a ‘side arm’ of a cell or in situ within a cell, e.g. hollow
cathode discharge cell, velocity modulation cell, [11]. The pumping speed and reactant
concentrations are adjusted to optimise the transient concentration. Sometimes these
adjustments affect the lifetime of the transient species and therefore also change the
radiation pathlength through the sample. In most IR absorption experiments the
pathlength is increased by using a multi-pass cell, e.g. Fabry-Perot cell, White cell,
Herriot cell [11]. Alternatively, the incoming beam can be expanded or defocused so that
it will interact with a larger sample volume, e.g. collimating cell [11].
The original absorption cell used on the Cambridge TuFIR spectrometer is shown
in figure 3.9. The cell was 70cm long and 4cm diameter. It was sealed at each end with
two Teflon windows, angled at 5o from the vertical, to prevent standing waves from
building up inside the cell cavity. The radicals were generated in a microwave discharge
in a sidearm attached to one of the cell ports. The discharge induced a small amount of
Pump
* = cell ports
*
*
*
*
TuFIR beam
Teflon windows
sample region
angled at 5o
Figure 3.9: The simple absorption cell.
84
*
CHAPTER THREE SPECTROMETER DEVELOPMENT
noise on the TuFIR sideband, which was mostly rejected by the bandpass filter on the
PSD. De-tuning the discharge, or positioning it further from the main cell body reduced
the noise, although this also affected the radical concentration. The cell was attached to a
Roots Blower, with a pumping speed of 240dm3min-1.
In configuration A, the TuFIR beam made a ‘double pass’ of the absorption cell,
(figure 2.1). When the spectrometer was rebuilt in configuration B, it was originally setup again as a ‘double pass experiment’. Once the ‘cavity-like’ behaviour of the TuFIR
beam had been ascertained, it was postulated that the sensitivity of this system did not
necessarily increase with the radiation pathlength through the sample. To verify this, two
TuFIR spectra were recorded under identical scan conditions, except that in one case the
TuFIR beam made a single pass of the absorption cell, and in the other a double pass,
(figures 3.10 and 3.11). The single pass experiment produced significantly better S:N
ratios with both 1st and 2nd derivative absorption lineshapes. The spectrometer sensitivity
was estimated in terms of the minimum molecular concentration, N/V, that could be
detected at a S:N ratio of 1.5:1. In configuration A, the sensitivity was around
5x1014molec.cm-3. In the double pass set-up in configuration B, N/V was 3x1013
molec.cm-3: in the single pass case the sensitivity was improved by one order of
magnitude to 3x1012 molec.cm-3. There were three reasons for this improvement:
1. in the single pass experiment the beam passed directly along the central axis
of the cell, and was unaffected by the cell walls or interference effects,
2. the total sideband intensity reaching the absorption region was greater in the
single pass case as the FPI was positioned after the absorption cell. The beam
intensity was more evenly distributed inside the cell and less severely
attenuated over the whole pathlength,
3. in the absence of a gas sample, the sideband intensity at the detector in the
single pass case was 1V peak-to peak on 50dB gain, compared to 63mV
peak-to-peak on 50dB gain in the double pass case. Consequently, it was
much easier to detect a change in the absorption intensity in the single pass
set-up.
85
CHAPTER THREE SPECTROMETER DEVELOPMENT
single pass
double pass
3 3,15
194 4,16<-18
19
18 15
16
702.05
702.10
702.15
3836
37
38
<37 3,35
2,36
35
2
702.20
702.25
702.30
3
702.35
Frequency (GHz)
Figure 3.11: TuFIR spectra of SO2 recorded using the lower sideband from the
729.9328GHz laser line in CD3OD and 1st harmonic detection. The sideband was
FM modulated at 75kHz, with a 1.32MHz modulation depth (equal to the Doppler
width of the line). Transitions are labelled as JKa,Kc.
Even in the single pass configuration around 40% of the incoming TuFIR
radiation did not reach the detector. Typically the sideband intensity dropped from
450mV (on the 40dB gain scale) to 900mV (on the 50dB gain scale) once the cell was in
place. This effect was ascribed to the cell windows. This intensity drop had to be reduced
to improve the spectrometer sensitivity further. The cell windows were therefore the
primary focus in the re-design of the TuFIR absorption cell.
Polyethylene and Teflon are the most common window materials in FIR
spectroscopy. Both are cheap, strong, inert, temperature stable below 333K, and can be
polished or machined flat. Their optical properties have been discussed extensively
elsewhere, e.g. Chantry [12] and Kimmit [13], and some of them are summarised in
Table 3.3. A beam of unpolarised light is usually partially transmitted and partially
86
CHAPTER THREE SPECTROMETER DEVELOPMENT
Teflon (PTFE)a
FIR Absorption

rises exponentially
between 100GHz and
2THz
absorption coefficient, 
Polyethyleneb


0.27
absorption increases
rapidly below 20m
absorption less efficient
in high density material
0.095
@ 850GHz
(nepercm-1)
FIR Transmission


transmission intensity
highly dependent on
sample thickness
transmits up to 90% of
the incoming radiation
refractive index, n
1.391


independent of sample
thickness below 1THz
(to a maximum value of
95%)
above 1THz
transmission drops
rapidly with wavelength
and sample thickness
1.461
@ 850GHz
Brewster Anglec
54.3o
55.6o
a. Ref. [14]
b. Ref. [12,13]
c. see equation 3.12
Table 3.3: Relevant optical properties of Teflon and Polyethylene at FIR frequencies
reflected by a dielectric interface. The relative amplitudes of the reflected and
transmitted components of the beam can be calculated from the Fresnel Equations [15].
At the interface between two dielectric media, the Fresnel equations for an unpolarised
beam are given by [15]:
r 
ni cos  i  nt cos  t
ni cos  i  nt cos  t
(3.8)
t 
2ni cos  i
ni cos  i  nt cos  t
(3.9)
r| | 
nt cos  i  ni cos  t
nt cos  i  ni cos  t
(3.10)
t|| 
2ni cos  i
ni cos  t  nt cos  i
(3.11)
87
CHAPTER THREE SPECTROMETER DEVELOPMENT
where r and t are the amplitude reflection and transmission coefficients perpendicular
to the plane of incidence (parallel to the surface), r║ and t║ are the amplitude reflection
and transmission coefficients parallel to the plane of incidence (perpendicular to the
surface), nt is the refractive index of the interface medium (e.g. dielectric), ni is the
refractive index of the external medium, (usually air), i is the angle of incidence, and t
is the transmittance angle. The reflectance, R, is defined as the ratio of the reflected beam
intensity to the incoming beam intensity. For an unpolarised beam [16]:
R
I r||  I r
Ii


1
R||  R   1 r||2  r2
2
2

(3.12)
Similarly, the transmittance, T, is defined as the ratio of the transmitted beam intensity to
the incoming beam intensity, and for an unpolarised beam is given by [16]:
I t||  I t 
1
T||  T   1  nt cos t  t||2  t 2
(3.13)
Ii
2
2  ni cos i 
The absorbance, A, denotes the fraction of the beam that is attenuated as it passes
T



through the dielectric medium, such that [16]:
R T  A 1
(3.14)
A simple computer programme was written in LabView, using equations 3.12 and
3.13, to show how R and T vary with incidence angle at a Teflon or Polyethylene
interface, (figure 3.12). When the beam is incident at Brewster’s Angle, B, only those
components of the beam whose E-field vector lies parallel to the surface, i.e.
perpendicular to the plane of incidence, are reflected [13]. Brewster’s Law states that
[16]:
nt
(3.15)
ni
At B, none of the incoming radiation is reflected from the dielectric interface if the beam
tan  B 
is linearly polarised perpendicular to the surface (i.e. parallel to the plane of incidence).
Given that the TuFIR radiation in this experiment was linearly polarised, the
windows in the new cell were set at Brewster’s angle to minimise the reflected intensity
and maximise the sideband power entering and leaving the cell. The reflectance and
transmittance were re-derived for a linearly polarised beam, propagating along the z-axis,
whose E-field vector was tilted at 10o from the x-axis in the xy plane (i.e. equivalent to
the polarisation of the TuFIR beam). These equations were used in a second computer
programme to establish the most suitable window material and orientation, assuming that
88