CHAPTER SIX CONCLUSIONS
CHAPTER SIX
CONCLUSIONS AND FUTURE
WORK
This thesis explored the historical and theoretical background to Tunable Far
Infrared (TuFIR) Spectroscopy, and focused in particular on the applications of this
technique to studies of transient and stable species. This last chapter gives an overview
of the development work that was undertaken by the author and then summarises the
results that have been presented in the thesis. In the following section, the current status
of the TuFIR spectrometer is assessed and a number of design modifications are
discussed. Finally, the results from primary searches for halogenated transient species
and other recent work are used to suggest future candidates for TuFIR spectroscopic
studies.
6.1 Summary
The development work undertaken by the author aimed to improve the
spectrometer performance to a level where it could be used to study:
1. FIR spectra of novel transient species,
2. highly accurate pressure broadening data.
Prior to this, the spectrometer was precluded from searches for novel transient species by
both its range and sensitivity. Over 30 modifications were made to the original
spectrometer. These are summarised in Table 3.5. Most significantly, in 1997 the whole
instrument was dismantled and rebuilt on a new optical bench. Combined with many
other minor changes, this made the ‘new’ spectrometer much more mechanically stable
(thereby reducing the laser noise) and increased the throughput of the instrument. The
TuFIR sidebands were generated using a microwave synthesiser in place of the Klystron
source, reducing the uncertainty associated with the TuFIR frequency. It was found that
the signal intensity could be improved by exploiting the ‘cavity-like’ behaviour of this
spectrometer, and that the S:N ratio in the spectra increased by almost one order of
220
CHAPTER SIX CONCLUSIONS
magnitude when the FPI plates were removed. A programme was written in LabView
to control the instrumentation and data acquisition [1]. The final version of the
programme enabled the user to set all the parameters on the SFS and the LIA directly
from the computer. The scan routine could be switched between single and multiple
scans, over a single narrow frequency range, or over a wider range that was broken down
into shorter, higher resolution scans. The data acquisition cycle was constructed to
minimise collection errors; the final data reading was averaged over five consecutive
points and time delays were incorporated into the cycle to ensure that this value was truly
related to the TuFIR frequency at each point. The ‘interactive’ sections of the
programme enabled the user to assess the data as it was collected, and adjust subsequent
scans accordingly. The data files were saved in a format that could be imported onto
many different operating systems. Typically the scan data were processed in Microcal
Origin (Window’s ’95) and fitted using SPECFIT [2], a Fortran programme written by
Dr. K.V.Chance of Harvard University, run on the Cambridge Unix server. More
recently, the TuFIR absorption cell was redesigned with Brewster windows. This also
increased the throughput of the spectrometer. Finally, the whole spectrometer was
maintained and accurately realigned on a regular basis, to sustain its performance.
Overall, the optimum spectrometer sensitivity was 1x1011 molec.cm-3, which was
obtained with the Brewster window cell and the FPI plates out. The maximum-recorded
sideband power was estimated as 25 to 28W, on the upper sideband signal, generated
from the 761.6GHz laser line in HCOOH and 27GHz microwaves. The spectral
resolution was ultimately limited by the intrinsic linewidth of the absorbing species, and
ranged from around 500kHz at low sample pressures (1 to 5mT) to at least 1.5MHz at
high pressures, (>300mT). By careful selection of the scan parameters, the instrumental
line broadening could always be reduced below the Doppler and pressure broadening
levels. The nominal precision of this spectrometer was only limited by the uncertainty in
the FIR laser frequency i.e. between 300 and 500kHz [3]. Consequently, TuFIR spectra
were only recorded with sidebands that had been generated from FIR laser lines for
which accurate frequency measurements had been published [7]. If a reference spectrum
was used to calibrate the frequency scale, the precision was improved to around 100kHz.
221
CHAPTER SIX CONCLUSIONS
The Cambridge TuFIR spectrometer was then used to accurately measure
pressure-broadening data for BrO and CHF3. These results demonstrated that the
improvements to the Cambridge TuFIR spectrometer should enable it to be used to study
the high-resolution spectra of transient species, in addition to the pressure-broadening
studies for which it is most commonly employed.
The FIR spectrum of BrO had not previously been recorded above 600GHz [4,5].
This pressure broadening study focused on the J=55/253/2, F=+1 transitions in
81
BrO, at around 700GHz. They were observed using the lower sideband from the
CD3OD laser line at 729.9328GHz. BrO is thought to play a significant role in Ozone
depletion, and this BrO transition coincidentally lies in an atmospheric window that is
suitable for remote sensing and, possibly, future atmospheric observations of BrO.
Consequently, the data presented in Chapter 4 will be directly relevant in the
interpretation of future stratospheric data. The pressure broadening coefficients of 81BrO
in Ar and O2 were determined from over 50 2nd derivative spectra. The final values
were:
Ar = 1.69 (0.09) MHzTorr-1
O2 = 2.89(0.14) MHz Torr-1
These results are in very good agreement with those obtained by Wagner et al [6].
The FIR spectrum of CHF3 has been ivestigated extensively by a number of
groups, notably Bocquet et al [7] and Cazzoli et al [8], who have both studied this
molecule using a TuFIR spectrometer. No pressure-broadening studies of CHF3 have
ever been reported. This pressure broadening study focused on the J=3433, K-band
transitions in CHF3, at around 700GHz. They were observed using the lower sideband
from the CD3OD laser line at 729.9328GHz. CHF3 is thought to have a large Global
Warming Potential because it has a strong IR absorption band between 1100 and
1200cm-1 [9]. To date, no direct spectroscopic observations of atmospheric CHF3 have
been made, although it was recently detected in the stratosphere for the first time using
gas-chromatography mass spectroscopy [10]. However, its spectra are intense and
remote sensing of CHF3 is a future possibility, particularly in the FIR where it exhibits a
strong, distinctive rotational spectrum. Consequently, the data presented in Chapter 5
will be directly relevant to the interpretation of any future stratospheric data. Over 150 1st
222
CHAPTER SIX CONCLUSIONS
and 2nd derivative spectra were used in this pressure broadening study. The air pressurebroadening coefficient for CHF3 at 298K was determined as:
air = 3.22 (0.037) MHzTorr-1
Since the pressure broadening did not appear to depend on the K-value of the transition,
this result was averaged over all the observed K components of the J=3433 rotational
transition. This result is very similar to the air pressure-broadening coefficient of
CHF2Cl, measured by Cazzoli et al [11]. The temperature dependence was also
measured using a single K component of the J=3433 rotational transition. The
temperature dependence of air is given by:
 300 
 (T )  3.32

 T 
1.25
MHzTorr 1
(6.1)
This relationship is important when the effects of stratospheric temperature are
considered in the CHF3 remote sensing spectra. The self-broadening coefficient for
CHF3 was:
self = 20.96 (0.28) MHz Torr-1
Both these studies provide an insight into pressure broadening effects in TuFIR
spectra. It would be interesting to extend the CHF3 work to include other transitions and
investigate the J-dependence of air. The pressure-broadening coefficient should decrease
with J. Three other transitions lie within the range of this TuFIR spectrometer:
1. J=4241, K-band (lower sideband from the 902.0016GHz line in CH2CHF),
2. J=4342, K-band (upper sideband from the 858.0533GHz line in CH3Cl),
3. J=4948, K-band (upper sideband from the 980.5916GHz line in CH3OD).
It is not possible to investigate any lower J-value transitions, but their pressure
broadening coefficients could be predicted from the relationship between J and air. It
would also be useful to determine the N2 pressure-broadening coefficient in BrO. This
work was originally planned as part of this thesis, but had to be curtailed due to increased
noise effects originating in the mixer diode, (see next section). The same 81BrO transition
can be used to obtain this data and then the N2 value could be combined with the O2
value to calculate a value for air at 298K for the J=55/253/2, F=+1 transitions.
Finally it would be interesting to investigate the isotopic dependence of the pressure
broadening. It is not excepted that there will be a significant difference between the two
isotopes,
79
BrO and
81
BrO, since they are chemically identical and there is hardly any
223
CHAPTER SIX CONCLUSIONS
difference between their reduced masses. If the FIR laser stability is improved further,
and line competition between FIR laser lines reduced, the most suitable transitions for
this study would be the J=57/253/2, F=+1 transitions, which can be accessed with the
lower sideband from the 761.6083GHz laser line in HCOOH. There are no O3 transitions
that directly overlap with these BrO transitions, so it would also be feasible to generate
BrO from the Br+O3 reaction in this instance.
6.2 Design Modifications
Despite the many improvements that were made to the TuFIR spectrometer over
the course of this PhD project, there are some further modifications that could be
introduced to improve the spectrometer performance. In comparison to FIR LMR, TuFIR
spectroscopy is:
1. significantly less sensitive (by at least four orders of magnitude due to the
lower source power),
2. offers comparable resolution (governed by the intrinsic linewidth of the
absorbing species),
3. much more precise (as the uncertainty associated with a transition frequency
is derived from the uncertainty in the FIR laser frequency and not the
magnetic field strength) [12].
Consequently, these modifications aim to advance the TuFIR spectrometer’s sensitivity
closer to the sensitivity associated with FIR-LMR, (106 to 107 molec. cm-3 [13]).
Currently, the TuFIR spectrometer is not operating at its optimum sensitivity.
Firstly, the laser system requires a full realignment, as the CO2 laser output is highly
erratic and mode-hops between different transverse modes. The realignment has not been
undertaken for at least 14 months, but should improve the long-term stability and power
in the TuFIR spectrometer. Secondly, the support struts in the detector need to be
replaced. They support the He dewar and stop the detector crystal from vibrating.
Without these struts, standing waves build up in the detector light pipe, and the incoming
TuFIR signal appears to oscillate. Finally, at the end of the BrO pressure broadening
work, it was necessary to record all the spectra with the FPI plates in place, due to
excessive source noise that originated from the mixer diode. This noise has now
deteriorated to a level that preludes data acquisition with the FPI plates removed.
224
CHAPTER SIX CONCLUSIONS
The principle source of noise in a Schottky Diode is Shot Noise. Shot Noise
depends on the mean current flow in a conductor, and arises because the individual
charge carriers have a random velocity distribution that is superimposed on their drift
velocity along the conductor. The r.m.s. Shot Noise contribution to the total diode
current is given by [14]:
i s2  2qI o  
1
(6.2)
2
where q is the carrier charge, Io is the forward bias current, and  is the system
bandwidth. An ideal ‘noisy’ diode, with negligable series resistance and junction
capacitance, is equivalent to a lossy network at a ‘noise temperature’, Td, given by [14]:
Td  
T
2
(6.3)
where  is the ideality factor of the diode and T is its physical temperature. Td is used to
express the diode noise level and is typically 300 to 500K at RTP. The Td of the
Cambridge diode was originally around 338K.
There are two reasons why the Shot Noise in a Schottky diode increases [15]:
1. excessive temperature or electrical damage to the diode, indicated by a high
ideality factor, 
2. poor contacts between the diode and the whisker antenna, or the diode and the
support post, indicated by a high series resistance, Rs.
Both these parameters can be determined from the diode I-V characteristics, which,
under forward bias conditions are given by [15]:
  qV  I (V ) Rs  

I (V )  I o exp
kT
(6.4)

 
where V is the bias voltage. In the low current regime where IRs<<V, equation 6.4 can be
rewritten as:
 I  qV
ln  
(6.5)
 I o  kT
Consequently, if the full IV curve is plotted in this form over 4 decades,  can be found
from the gradient of the linear part of the curve, and Rs can be estimated as [15]:
Rs  100V4  V3   V2  V1 
(6.6)
where for example V1 is measured at 1A, V2 at 10A, V3 at 1mA and V4 at 10mA. The
original and current I-V characteristics of the Cambridge diode are compared in figure
6.1. The diode properties are compared in Table 6.1. This data clearly suggests that the
225
CHAPTER SIX CONCLUSIONS
Current Parasitics
Original Parasitics [16]

2.872
1.135
Rs ()
5
18.1
Td (K)
856
338
Table 6.1: Comparison of Schottky Junction Parasitics
diode junction has been electrically damaged in some way, and the Td values uphold the
original assumptions that were made regarding the noise source in the BrO spectra. It is
likely that the diode junction has gone ‘soft’ i.e. some of the carriers have migrated back
to the semiconductor from the metal so the carrier concentration has reduced in the
depletion region. Consequently, the current flow through the epitaxial layers is restricted,
and the Shot Noise increases [17]. This noise is re-radiated from the diode by the
whisker antenna, and is therefore superimposed on the TuFIR and the FIR signals. When
the diode is repaired, the junction parasitics will be returned to their original values, and
ln Current (mA)
this source of noise will cease to be significant.
e
2
e
1
e
0
e
-1
e
-2
e
-3
e
-4
e
-5
e
-6
e
-7
I-V characteristics
(after re-charging bias supply)
I-V characteristics
(prior to re-charge)
original I-V characteristics
fit to linear part of I-V curve
0.4
0.6
0.8
1.0
1.2
Voltage (V)
Figure 6.1: The I-V Characteristics of the Cambridge Schottky Barrier Diode. The
linear section of this curve is fitted to equation 6.5.
226
CHAPTER SIX CONCLUSIONS
In addition to these ‘maintenance’ modifications there are two further
improvements that should be made to this TuFIR spectrometer in the near future:
1. replacement of the diplexer with a Martin-Pulpett Interferometer,
2. introduction of frequency doubler and tripler units to the microwave source.
A Martin-Pulpett Interferometer (MPI) is a polarising version of the Michelson
Interferometer and operates in a very similar fashion to the diplexer described in section
2.3.2. The theoretical basis of its operation has been described in detail by Lesurf [18].
Basically, the Melinex beamsplitter in the diplexer is replaced with a Grid Polariser. The
incoming beam is separated into two components according to the orientation of its Efield vector with respect to the grid wires. The component whose E-field vector lies
parallel to the wires will be reflected; the component whose E-field vector lies
perpendicular to the wires will be transmitted. The reflected and transmitted components
of the beam will be 180o out of phase. If the wires are set at exactly 45o to the
polarisation of the incoming radiation, the incoming beam will be split into two beams of
equal intensity. The E-field vector from each beam is rotated through 90o after it is
reflected from the roof top mirror. Consequently the component that was originally
transmitted through the grid polariser is now reflected and visa-versa. When the two
beams recombine, the beam exiting the interferometer will always have the same
intensity as the beam that entered the interferometer, but its polarisation will differ. The
exact polarisation of this beam depends on the phase difference between the E-field
vectors of the re-combining beams. This phase difference depends on the path difference
between the two arms of the interferometer. Consequently, the polarisation of the
outgoing beam is also wavelength dependent.
Blake et al have described the use of a MPI on a TuFIR spectrometer [19]. The
MPI has three main advantages over the diplexer that is currently in use:
1.
a grid polariser exhibits a uniform response over a wide frequency range
unlike a Melinex beamsplitter whose absorption, reflection and transmission
coefficients are frequency dependent,
2.
the intensity of the outgoing radiation is always equal to the intensity of the
incoming radiation in an MPI. Consequently, in the TuFIR spectrometer, the
FIR power transmitted to and from the cube will always be optimised, as will
227
CHAPTER SIX CONCLUSIONS
TuFIR power transmitted from the cube, independent of the pathlength
difference between the two interferometer arms,
3.
the polarisation of the transmitted TuFIR sidebands and the FIR beam will all
differ, as the phase difference between the recombining beams will be
different in each case.
Consequently, when combined with a second grid polariser, the MPI is a very powerful
way to separate the FIR and TuFIR beams completely.
A suitable spectrometer configuration is shown in figure 6.2. The FIR radiation is
focused into the spectrometer through a FPI then directed towards the mixer-diode by a
MPI. The grid polariser in front of the FIR laser is set to transmit 100% of the incident
radiation: the grid polariser in the MPI is set to transmit 50% of the incident radiation.
The FIR radiation is focused onto the Schottky diode and combined with microwaves
from the microwave synthesiser to generate TuFIR sidebands as before. Both the
sidebands and FIR signal are then re-radiated from the antenna and directed back
towards the diplexer by the parabolic mirror. The MPI is tuned so that one of the TuFIR
sidebands is linearly polarised and reflected by the grid polariser in front of the FIR
laser. The other sideband and the FIR beam will be elliptically polarised: there will
always be a path difference at which the major component of the elliptically polarised
beams will be orthogonal to the linear polarisation of the sideband signal. Consequently
a single sideband will be transmitted into the absorption cell without being contaminated
by the FIR signal or the other sideband signal. Since the FPI will no longer be required to
separate the signals, it can be moved in front of the FIR laser to prevent feedback from
the second sideband signal. The sideband power reaching the detector should increase
significantly. In the absence of the intense FIR radiation, it will be much easier to detect
small changes in the sideband intensity, and it is anticipated that the spectrometer
sensitivity could be improved by at least one order of magnitude.
228
CHAPTER SIX CONCLUSIONS
pump
CO2 Laser
ZnSe Lens
FIR
Laser
Hot Electron
InSb Detector
Absorption Cell
Teflon Lens
TuFIR = FIR  -wave
FPI
only one
Preamplifier
FIR
Spectroscopic
Signal
Swept Frequency -wave
Microwave
Corner Cube Mount
Synthesiser
& Schottky Barrier
GPIB-PC
Diode
Interface
(800-1000GHz)
Signal
Generator
Reference
Lock-in
Amplifier
Grid Polariser
MPI
GPIB-PC
Interface
PC
CO2 laser
FIR
beamradiation
TuFIR radiation
Figure 6.2: New configuration of the Cambridge TuFIR Spectrometer, incorporating
the Martin-Pulpett Polarising Interferometer.
229
CHAPTER SIX CONCLUSIONS
Currently the frequency coverage of the Cambridge TuFIR Spectrometer is
limited by the availability of strong (>10mW), stable, and unique FIR laser lines, i.e.
those that are not in competition with other laser lines on the same CO2 pump transition.
At least 23% of the frequencies between 600 and 1200GHz are completely inaccessible
from FIR laser lines that meet the above criteria. If the laser lines from isotopic and
prohibitively expensive laser gases are also removed, this figure rises to 40% of the total
spectrometer range. Obviously these limits are not well suited to extensive spectroscopic
searches for novel transient species. In Chapter 2 it was shown that the sideband power is
clearly dependent on the FIR laser power. Consequently, sideband powers differ greatly
according to the FIR laser line from which they are generated. These difficulties could be
overcome by introducing microwave doublers and triplers to the spectrometer. The
TuFIR sidebands could then be generated at frequencies given by:
 TuFIR   FIR  n   wave
(6.6)
where n is1,2 or 3.
Due to the nature of microwave instrumentation, it would not be possible to cover
the full range of doubled and tripled microwave frequencies [20]. In the doubler case, a
broadband passive multipler with an input frequency of 25 to 37.5GHz and an output
from 50 to 75GHz would cost in the region of £2300. A similar multiplier with an input
frequency of 20 to 30GHz and an output from 60 to 90GHz would cost around £3300.
Two different waveguides would be required to transmit the microwaves from the
multipliers to the diode mixer. Furthermore, it would not be possible to use the current
diode at microwave frequencies beyond 40GHz. A suitable diode and corner cube
mount, with a waveguide port to feed the microwaves onto the diode and a tunable
backshort to optimise the microwave power reaching the diode, would cost in the region
of £5000. Although the microwave power in the multipliers would be attenuated, the
synthesiser power could be increased to maintain the power at the diode at its current
level, (4dBm). With these units incorporated onto this particular spectrometer the
sideband frequencies (in GHz) would range from:
 TuFIR   FIR  26.5  40
(6.7)
 TuFIR   FIR  53  75
(6.8)
 TuFIR   FIR  79.5  90
(6.9)
to:
to:
230
CHAPTER SIX CONCLUSIONS
Consequently, far fewer FIR laser lines would be required to generate sidebands to cover
the whole frequency range between 600GHz and 120GHz and only the most powerful
FIR lines would need to be used. Around 10% of the total frequency range would remain
uncovered.
6.3 Spectroscopic Investigations of Transient Species
The latest version of the TuFIR spectrometer could be used to study many
transient species, particularly those that are of interest to the atmospheric community.
Following the improvements mentioned above, the scope of this instrument would be
expanded even further. Two studies are suggested below bearing this in mind. The first
experiment focuses in detail on the FIR spectroscopy and pressure broadening of IO and
could be executed with the spectrometer in its current state. The second study focuses on
the FIR spectra of halagonated radicals. It would be better to attempt these experiments
after the above modifications have been implemented.
It was shown in Chapter 4 that free radicals are significant in many of the
chemical processes in the stratosphere. In particular, the halogen oxides play a pivotal
role in Ozone destruction cycles [21]. In recent years, it has been realised that Iodine
chemistry may also be important, although stratospheric Iodine chemistry is not well
understood. Models have shown that if only 1ppptv of CH3I is added to the stratosphere,
the rate at which other halogenated species deplete O3 will increase [22]. Atmospheric
Iodine is almost exclusively of biogenic origin, and is mostly released into the
troposphere in the form of CH3I as a result of oceanic algal activity [23]. The CH3I is
rapidly photylised to produce IO, i.e.:
CH 3 I  h  CH 3  I
I  O3  IO  O2
IO itself is thought to play only a minor role in tropospheric O3 loss. However, even if a
small percentage of this IO were transported to the lower stratosphere, its reactions with
ClO, BrO and HO2 would be important catalytic pathways for the destruction of lower
stratospheric O3 [24]. Iodine is removed from the atmosphere in stable inorganic
molecules such as HI. Recently Wennberg et al [25] measured the stratospheric IO
concentration from its solar absorption spectrum around 445nm, using the FTS at the
National Solar Observatory at Kitt Peak, Arizona. They were able to resolve the
rotational fine structure in the absorption band. Furthermore, the spectral lineshapes in
231
CHAPTER SIX CONCLUSIONS
the IO spectra exhibited a very strong pressure broadening dependence [25]. They
estimated the upper limit to the current IO concentration at around 0.5pptv at all altitudes
in the stratosphere.
The spectroscopy of IO is essentially identical to that of BrO, except that Iodine
has only one stable isotope (mass 127) and its nucleus has a spin of 5/2. Consequently its
high-resolution spectra exhibit more hyperfine structure than the BrO spectra. Vaidya,
who studied the emission spectrum from a CH3I flame source, made the first
spectroscopic observations of IO in 1937 [26]. He observed over 35 bands between 410
and 630nm that were later assigned to the A2iX2i transition. There have been many
further UV/visible spectroscopic studies of this radical, most notably by Bekooy et al
who observed the LIF from a microwave-optical double resonance experiment [27].
They resolved the hyperfine structure of the lowest rotational transitions, and determined
a number of the molecular constants for the =0,1,2 vibrational levels of the ground
electronic state, (23/2). Saito has studied the pure rotational spectrum of IO. He
observed the hyperfine structure from the two lowest rotational transitions in the ground
electronic state [28]. To date this is the only zero field pure rotational spectrum of IO to
have been reported.
These facts all suggest that there is a need for spectroscopic and pressure
broadening data relating to IO. The IO radical is relatively short-lived [29], but has a
large dipole moment (2.3D [30]) so it should be possible to detect this species with the
Cambridge TuFIR spectrometer. IO radicals can be generated from the reaction between
I+O, where the O atoms are generated from a microwave discharge of O2 in excess He.
Wennberg et al have shown that the IO concentration is enhanced if a small amount of
O3 is also passed through the discharge [25]. The rotational transition frequencies of the
IO radical have been predicted between 600 and 1200GHz using the B and D constants
derived from Saito’s microwave study [28] and these values are given in table 6.2.
Although these predictions do not show the exact transition frequencies of each of the
hyperfine transitions that will be observed in the TuFIR spectrum, they can be used to
pinpoint the frequencies at which it will be most useful to make preliminary searches.
Two of these transitions can be observed using the upper and lower sidebands from the
232
CHAPTER SIX CONCLUSIONS
729.9328GHz laser line in CD3OD. This would be the most logical region in which to
start a searching for spectra because:
1. the sidebands from this laser line are relatively strong and have been used to
obtain TuFIR radical spectra previously, (see Chapter 4),
2. it is known that an atmospheric window exists in the FIR at around 700GHz,
so in the future transitions in this region could be studied by atmospheric
remote sensing.
Transition
61/259/2
63/261/2
65/263/2
67/265/2
69/267/2
71/269/2
73/271/2
75/273/2
79/275/2
81/279/2
83/281/2
85/283/2
87/285/2
89/287/2
91/289/2
93/291/2
95/293/2
97/295/2
99/297/2
101/299/2
103/2101/2
105/2103/2
107/2105/2
109/2107/2
111/2109/2
113/2111/2
115/2113/2
117/2115/2
119/2117/2
Frequency (GHz)
618.560
638.771
658.974
679.170
699.360
719.542
739.716
759.883
780.041
800.191
820.332
840.465
860.588
880.703
900.808
920.903
940.989
961.064
981.128
1001.185
1021.229
1041.262
1061.284
1081.295
1101.294
1121.282
1141.257
1161.220
1181.171
Table 6.2: Predicted IO rotational transition frequencies in the range of the
Cambridge TuFIR Spectrometer. Calculated using spectroscopic constants from ref.
[28].
233
CHAPTER SIX CONCLUSIONS
This would constitute the first FIR spectroscopic study of IO. No pure rotational
transitions of IO have been observed above J=7/25/2. The air pressure-broadening
coefficient of IO could also be measured using similar methods to those described in this
thesis. It might also be possible to measure the temperature- and the J-dependence of air.
and this data would be useful in the interpretation of stratospheric IO spectra.
Another group of species that could be studied with the Cambridge TuFIR
Spectrometer are the radicals with the formula CFy or CHxFy, e.g. CF2, CF3, CH2F,
CHF2. These radicals are formed in the stratosphere by photolysis of various HFC’s,
(CFC replacements) [31]. Consequently, there is great interest in both their spectroscopy
and kinetics. From a spectroscopic viewpoint, the structures of the CHxFy radicals are
also interesting, since CF3 is a pyramidal molecule, but CH2F is essentially planar.
Recent FIR-LMR work by Nolte et al suggests that CHF2 is also non-planar [32]. These
radicals have been the focus of theoretical [33] and experimental studies, particularly
0.08
0.06
discharge on
discharge off
Intensity
0.04
0.02
0.00
-0.02
-0.04
30.5
31.0
31.5
32.0
Microwave Frequency (GHz)
(True Transition frequency =FIR +/- this value)
Figure 6.3: Preliminary spectra from a microwave discharge of CF4 (top trace),
and the same gas sample with the microwave discharge switched off (bottom
trace). Recorded using sidebands generated from the 729.9328GHz laser line in
CD3OD and 2nd derivative lineshapes.
234
32.5
CHAPTER SIX CONCLUSIONS
in the microwave and IR regions of the spectrum [34] but none of these species has been
studied at zero field in the FIR. Their FIR transition frequencies can be predicted from
the molecular constants derived in microwave studies, e.g. Inada et al [35] (CHF2), Endo
et al [36] (CH2F), Endo et al [37] (CF3), and Charo et al [38] (CF2). They are generated
from the ‘stripping’ reactions of F atoms with fluorinated or partially fluorinated
hydrocarbons. In this case F atoms are produced in a microwave discharge of F2 in an
excess of He or Ar. CF2 and CF3 can be generated by direct microwave discharge of CF4.
Preliminary work on this spectrometer, prior to its reconstruction in configuration B,
produced some interesting spectral lines from a direct discharge of CF4, (figure 6.3). The
sidebands were generated from the 729.9328GHz line in CD3OD. Since this scan was
recorded with the FPI plates removed, it is not known whether the features that arise in
the spectrum with the discharge on, are due to absorptions on the upper or the lower
sideband. Consequently, the spectra are plotted in terms of the microwave frequency, and
this value has to be added to or subtracted from the laser frequency to get the true
transition frequency. Obviously the spectrometer performance has been greatly enhanced
since these spectra were recorded, so it might be possible to assign these transitions to a
particular transient species in the future.
235