CHAPTER FIVE CHF3 PRESSURE BROADENING
Added air pressure=321mT
Intensity
0.010
0.005
0.000
-0.005
-0.010
701940
701945
701950
701955
701960
701965
Intensity
Frequency
0.04
0.03
0.02
0.01
0.00
-0.01
-0.02
-0.03
Added air pressure=138mT
701940
701945
701950
701955
701960
701965
Frequency
0.10
Added air pressure=14mT
Intensity
0.05
0.00
-0.05
-0.10
701940
701945
701950
701955
Frequency
701960
701965
experimental data
fitted spectrum
residuals
Figure 5.40: Examples of experimental and Fitted 1st derivative Spectra for
K=0-3 components of the J=3433 rotational transition in CHF3.
Pressure Broadening of 5mT CHF3 with air in a static cell.
208
CHAPTER FIVE CHF3 PRESSURE BROADENING
Added air pressure=606mT
Intensity
0.001
0.000
-0.001
-0.002
-0.003
702030
702035
702040
702045
702050
702055
702060
Intensity
Frequency
0.015
0.010
0.005
0.000
-0.005
-0.010
-0.015
702030
Added air pressure=303mT
702035
702040
702045
702050
702055
702060
Frequency
0.10
Added air pressure=29mT
Intensity
0.05
0.00
-0.05
-0.10
702030
702035
702040
702045
702050
702055
Frequency
702060
experimental data
fitted spectrum
residuals
Figure 5.41: Examples of experimental and Fitted 1st derivative Spectra for
K=9 components of the J=3433 rotational transition in CHF3. Pressure
Broadening of 5mT CHF3 with air in a static cell.
209
Intensity
CHAPTER FIVE CHF3 PRESSURE BROADENING
Added air pressure=496mT
0.004
0.003
0.002
0.001
0.000
-0.001
-0.002
-0.003
702375
702380
702385
702390
702395
702400
Frequency
Added air pressure=228mT
Intensity
0.010
0.005
0.000
-0.005
702375
702380
702385
702390
702395
702400
Intensity
Frequency
Added air pressure=14mT
0.06
0.04
0.02
0.00
-0.02
-0.04
-0.06
702375
702380
702385
702390
Frequency
702395
702400
experimental data
fitted spectrum
residuals
Figure 5.42: Examples of experimental and Fitted 1st derivative Spectra for
K=19 components of the J=3433 rotational transition in CHF3. Pressure
Broadening of 5mT CHF3 with air in a static cell.
210
CHAPTER FIVE CHF3 PRESSURE BROADENING
K=19 Press ure Broadening Data
1st deri vati ve S tat ic data
5mT CH F3 in AIR
2.0
K=0 Press ure Broadening Data
1st Deri vati ve S tat ic data
5mT CH F3 in AIR
1.2
1.1
1.8
1.0
1.6
0.9
1.4
1.2
HW HM = 3.16 (+/- 0.0413) * P( T)
+ 0.1023 (+/- 0.00436)
R=0. 99745
1.0
0.8
HWHM Lo re ntzian
HWHM Lo re ntzian
0.8
0.7
0.6
HW HM = 3.26 ( +/-0. 128) * P( T)
+ 0.1589 (+/- 0.00827)
R=0. 99166
0.5
0.4
0.6
0.3
0.4
0.2
0.2
J = 34 < -- 33
J = 34 < -- 33
0.1
0.0
0.0
0
100
200
300
400
500
600
0
20
40
60
80
Pressu re (mT)
K=1 Press ure Broadening Data
1st Deri vati ve S tat ic data
5mT CH F3 in AIR
1.1
1.1
1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.6
H WH M = 3. 34 (+/ - 0.0944) *P( T)
+ 0.1538 (+/- 0.00601)
R=0. 99602
0.5
0.7
0.6
0.4
0.3
0.3
0.2
0.2
J = 34 <--33
0.0
H W HM = 3.40 ( +/- 0. 109) * P( T)
+ 0.1611 (+/- 0.00769)
R=0. 99434
0.5
0.4
0.1
K=2 Press ure Broadening Data
1st Deri vati ve S tat ic data
5mT CH F3 in AIR
1.2
HWHM Lo re ntzian
HWHM Lo re ntzian
1.2
100 120 140 160 180 200 220 240
Pressu re (mT)
0.1
J = 34 < -- 33
0.0
0
20
40
60
80
100 120 140 160 180 200 220 240
0
Pressu re (mT)
20
40
60
80
100 120 140 160 180 200 220 240
Pressu re (mT)
211
CHAPTER FIVE CHF3 PRESSURE BROADENING
K=+3 Pr essur e Br oadening D at a
1st Derivative St ati c dat a
5mT CHF 3 in AIR
1.2
K= - 3 Pr essur e Br oadening D at a
1s t D erivative st ati c dat a
5mT CHF 3 in AIR
1.8
1.1
1.6
1.0
1.4
0.9
HWHM Lorentzian
1.2
0.7
0.6
0.5
HW HM = 3.18 (+/-0.105) * P(T)
+ 0.1604 ( +/- 0. 00679)
R=0.994
0.4
1.0
0.8
HW HM =3.01 (+/- 0. 0444) * P(T)
+ 0.1653 ( +/- 0. 00302)
R =0.99805
0.6
0.3
0.4
0.2
0.2
J=34 <-- 33
J=34 <-- 33
0.1
0.0
0.0
0
20
40
60
80
1 00 1 20 1 40 1 60 1 80 2 00 2 20 2 40
0
1 00
Pressure (mT)
2 00
3 00
4 00
Pressure (mT)
K=9 Pr essur e Br oadening D at a
1st Derivative St ati c dat a
5mT CHF 3 in AIR
3.0
2.5
2.0
HWHM Lorentzian
HWHM Lorentzian
0.8
1.5
HW HM = 3.29 (+/- 0. 0463) * P(T)
+ 0.1263 ( +/- 0. 01735)
R =0.99656
1.0
0.5
J= 34 <-- 33
0.0
0
1 00
2 00
3 00
4 00
5 00
6 00
7 00
8 00
Pressure (mT)
Figure 5.43: Determination of air for individual K components of the J=3433
transition. 5mT CHF3 used in a static cell.
212
5 00
CHAPTER FIVE CHF3 PRESSURE BROADENING
As in the flowing experiments no clear K dependence could be observed. The
data from individual K values was closely correlated, exhibiting much less scatter than
any of the previous set-ups, (figure 5.44). The K=9 and K=19 spectra exhibited high S:N
ratios, no convolution with other transitions, and were observed at much higher pressures
than the K=0-3 scans. Their fits were particularly accurate and certainly better than the
previous experiments. A mean air value of 3.22 (0.037) was obtained from a weighted
linear regression fit of all the data, independent of the K value.
2.5
HWHM Lorentzian
2.0
1.5
K=0
K=1
K=2
K=+3
K=-3
K=9
K=19
1.0
0.5
0.0
0
100
200
300
400
500
600
700
Pressure (mT)
Figure 5.44: Comparison of all 1st derivative pressure broadening data
colour coded by K value for 5mT CHF3 broadened by air under static
conditions.
The flowing and static data from experiments 1 and 4 are compared in table 5.8.
air was significantly higher in the static case at almost every K value. The mean air
values lie within 2 of each other. Given the accuracy and correlation of the static data, it
is likely that the cell pressure measurements account for these discrepancies. In
experiment 4 (static case) the cell was in equilibrium and each of the Baratron gauges
read similar values. (Due to individual gauge calibration and accuracy the readings were
213
CHAPTER FIVE CHF3 PRESSURE BROADENING
K=
0
1
2
+3
-3
9
19
Mean

2.70
3.39
2.65
2.72
2.63
2.92
3.25
3.07
flowing
0.34
0.14
0.18
0.095
0.17
0.050
0.047
0.056

3.26
3.34
3.40
3.18
3.01
3.29
3.16
3.22
static
0.13
0.094
0.11
0.11
0.044
0.046
0.041
0.037
Table 5.8: Comparison of air values for flowing and static cells.
Mean value is obtained from weighted linear regression of all data together.
never exactly equal). The cell pressure was therefore determined as an average of the
gauge readings. In experiment 1 (flowing case) the central Baratron gauge was used to
determine the cell pressure. As previously stated a small pressure gradient did exist along
the cell: this would affect the results if the gradient changed at higher total cell pressures
or if the true sample pressure was different from the value recorded. The air value
derived from the static experiments should therefore be considered most accurate. The yintercepts (i.e. self-broadening contribution) are similar in both cases, (figure 5.45).
2.4
2.2
2.0
HWHM Lorentzian
1.8
HWHM = 3.22 (+/-0.03703) * P(T)
+ 0.10706 (+/- 0.00407)
R=0.99101
1.6
1.4
1.2
1.0
HWHM=3.07 (+/- 0.0562)*P(T)
+0.1049 (+/-0.00349)
R=0.98558
0.8
0.6
0.4
flowing data
static data
0.2
0.0
0
50
100 150 200 250 300 350 400 450 500 550 600 650 700
Pressure (mT)
Figure 5.45: Comparison of Pressure broadening data for flowing and static
experimental configurations. (Error bars gave been removed for clarity).
214
CHAPTER FIVE CHF3 PRESSURE BROADENING
In the final experiment the temperature dependence of the pressure broadening
was investigated. Following all the previous work, the cell was used in a static
configuration and only the K=9 component of the transition was scanned. Five cell
temperatures were used, from 254 to 298K. Approximately 30 spectra were scanned at
each temperature and fitted individually as in previous experiments. Most spectra
iterated between 4 and 18 times: none were rejected. Examples of these spectra are
shown in figure 5.46. At each temperature air was determined from a weighted linear
regression fit of the HWHM vs. pressure data. The errors in the HWHM values were
between 2 and 5% of the total value and the error on the pressure axis was taken as the
standard deviation of the average pressure reading from the three gauges. As figure 5.47
shows, the intercepts of each of the plots are very similar, and are also close to the selfbroadening linewidth expected for a 5mT sample (0.102MHz). This indicates that the
line broadening occurring in each experiment is wholly attributable to additional air in
the system, and that any differences between the experiments must be due to the
temperature change.
2.0
1.8
HWHM Lorentzian
1.6
1.4
1.2
1.0
0.8
T=297.8K H=62%
T=283.8K H=57%
T=274.0K H=67%
T=261.7K H=67%
T=254.7K H=67%
0.6
0.4
0.2
0.0
0
50
100
150
200
250
300
350
400
450
500
Pressure (mT)
T=
254.7
261.7
274.0
283.8
297.8

4.23
3.74
3.70
3.68
3.31
0.079
0.066
0.065
0.052
0.0.042
0.096
0.099
0.107
0.098
0.105
0.012
0.008
0.0.006
0.0.006
0.0.006
intercept
Figure 5.47: Temperature Dependant Pressure Broadening from K=9 spectra
(Error bars have been removed for clarity).
215
CHAPTER FIVE CHF3 PRESSURE BROADENING
Add ed a ir p ressu re= 221 mT
T =25 5K
0.008
Add ed a ir p ressu re= 230 mT
T =26 2K
0.010
0.006
0.004
0.005
Inten sity
Inten sity
0.002
0.000
- 0.002
0.000
- 0.005
- 0.004
- 0.006
- 0.010
- 0.008
702030
702035
702040
702045
702050
702055
702060
702030
702035
702040
F req uen cy
702045
702050
702055
702060
702055
702060
F req uen cy
Add ed a ir p ressu re= 260 mT
T =27 4K
Add ed a ir p ressu re= 215 mT
T =28 4K
0.015
0.02
0.010
0.01
Inten sity
0.00
0.000
- 0.005
-0.01
- 0.010
-0.02
702030
702035
702040
702045
702050
702055
702060
- 0.015
702030
702035
702040
F req uen cy
702045
702050
F req uen cy
Add ed a ir p ressu re= 236 mT
T =29 8K
0.015
0.010
0.005
Inten sity
Inten sity
0.005
0.000
- 0.005
exp erim enta l data
f it ted s pectr um
re sidua ls
- 0.010
- 0.015
702030
702035
702040
702045
702050
702055
702060
F req uen cy
Figure 5.46: Examples of Experimental and Fitted spectra for K=9
component of the J=3433 transition in CHF3. Spectra shown are all at
similar pressures but different temperatures.
216
CHAPTER FIVE CHF3 PRESSURE BROADENING
The temperature dependence of the pressure-broadening coefficient is clearly
observed in this experiment. The temperature variation of pressure-broadening
parameters is known to follow a power law [35, 36]:
T 
 (T )   o  o 
T 
n
(5.20)
where 0 is the pressure broadening coefficient at a reference temperature T0 and n is the
constant exponent of the temperature ratio. Previous experiments in the FIR region have
adopted this formula to characterise the data [37, 38]. The data obtained in experiment 5
was therefore fitted to equation 5.20 using a Levenberg-Marquet non-linear least squares
fitting routine, programmed and operating in Microcal Origin. These results are shown in
figure 5.48. A final value of 300K=3.31 (0.097) was obtained from the K=9 1st
derivative lineshape data. In the same fit n=1.25 (0.31).
5.0
4.8
4.6
4.4
4.2
(T)
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
220
Model: (T)=(To) x (300/T)n
(To) = 3.31848 0.09735
n = 1.24566  0.3129
230
240
250
260
270
280
290
300
310
temperature (K)
Figure 5.48: To illustrate the power law temperature dependence of the
pressure-broadening coefficient for CHF3 in air.
217
320
CHAPTER FIVE CHF3 PRESSURE BROADENING
5.6 Conclusions
As mentioned at the start of this chapter, it is important to know the amount of
CHF3 that is present in the terrestrial atmosphere in order to estimate its GWP. This
information can be acquired from spectroscopic remote sensing techniques, so the
pressure broadening coefficients that were obtained from this experiment are required to
obtain reliable results. Consequently, a number of preliminary studies and experimental
procedures were followed to ensure that the final air value was as accurate as possible.
The instrumental, foreign gas, saturation, and wall collision broadening were eliminated
from the spectra. From these pressure-broadening studies the air-pressure-broadening
coefficient of CHF3, air, has been determined for the first time. The weak K dependence
of the pressure broadening, predicted by quantum mechanics [26]:

K2
J ( J  1)
(5.21)
was not observed. This K dependence has been observed previously in NH3 using
microwave spectroscopy at much lower J values [39]. Given the high J transition studied
here the K dependence would be weak and probably lie within the scatter of
experimental values obtained at each K. A air value of 3.22 (0.037) MHz Torr-1 was
calculated, averaged over all the observed K components of the J=3433 rotational
transition. It was also possible to measure the self-broadening coefficient. For the
J=3433 rotational transition self = 20.96 (0.28) MHz Torr-1. From this data it was
possible to rationalise the intercepts of the linear fits in the air broadening.
The temperature dependence of the pressure broadening was illustrated using a
single K component of the J=3433 rotational transition. K=9 is the most intense
transition in this region of the spectrum, and it may therefore be possible in the future to
resolve it independently with high resolution remote sensing. The temperature
dependence of air is given by:
 300 
 (T )  3.32

 T 
1.25
MHzTorr 1
(5.23)
This dependence has been established for the first time in CHF3. The relationship is
important when the effects of stratospheric temperature are considered in the CHF3
remote sensing spectra.
218
CHAPTER FIVE CHF3 PRESSURE BROADENING
No CHF3 pressure broadening data have been recorded in the literature. However,
Cazzolli et al have used a very similar TuFIR spectrometer to this one, to measure the N2
and O2 pressure broadening coefficients for CHF2Cl, between 60 and 700GHz, at
pressure up to 200mT [40]. They investigated the J-dependence of the pressure
broadening coefficient, and found that the broadening coefficients decreased rapidly
beyond J=28. Assuming that air=0.8N2+0.2O2, their results suggest that the air pressure
broadening coefficient of CHF2Cl is 3.65MHz Torr-1 at 360GHz, (J=2822627126), and
3.28MHz Torr-1 at 686GHz, (J=5325152151). These values correlate very closely with
the mean air value obtained in this experiment, (3.22MHz Torr-1). As the molecules have
significantly different symmetries, and slightly different masses, it is hardly surprising
that these values should differ slightly.
219