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The effect of ultraviolet radiation on water vapour absorption between 5 and 50cm−1
Infrmd Prmted
P/II.F.
Vol. 24.
in &eat
THE
No.
5, pp.
431
441.
1984
0020-0891/X4
Britain
S3.00 f0.00
Pergamon Press Ltd
EFFECT VAPOUR
OF ULTRAVIOLET RADIATION ON WATER ABSORPTION BETWEEN 5 AND 50cm-’
P. DIAS-LALCACA, N. J. C. PACKHAM and H. A. GEBBIE Department of Electrical Engineering, Imperial College, London SW7 2BT, England (Received
24 Junuary
1984; accepted in revised form 26 February
1984)
Abstract-Both field measurements and laboratory experiments suggested that absorption of UV radiation affected the near-millimetre wave absorption by water vapour. Experiments with different flux values and at different temperatures have confirmed this and pumping by UV radiation has been shown to give near-millimetre wave emission.
INTRODUCTION
The background to this investigation is given in Refs (1) and (2), the latter of which appears in this issue of Infrared Physics (p. 429). In Ref. (2) it was suggested that the short-wavelength photons in sunlight appear to produce spectral anomalies in atmospheric absorption of near-millimetre waves and that the mercury lamp sources used for laboratory measurements could be responsible for these also showing similar spectral features. There was a need for systematic experiments to verify this laboratory effect and here we give an account of a preliminary study. The fact that medium-pressure mercury-vapour discharge lamps, which are commonly used as continuum sources of near-millimetre wave radiation, also give a high flux of UV radiation is both a complication and a benefit. Designing experiments which depend on removing the UV component by filters which have an acceptably low insertion loss in the near-millimetre wave region is not easy, so in the present work the alternative of deliberately exploiting the UV component has been chosen. The dominant consideration in making spectral measurements in the 5-50 cm-’ range is that the low brightness of continuum sources make integration times very long, so optimum use must be made of the available power. If several independent variables such as the short-wavelength flux, sample temperature and water vapour saturation value are to be explored along with spectral distribution, an extended investigation would be needed. We have, therefore, sought to establish the main facts by the simplified study described here. It has been assumed, on the basis of the experiments reported in Ref. (3), that the short-wavelength flux responsible for the effects is the near-UV region. At a later stage it will be necessary to measure the actual spectral distribution of the short-wavelength component responsible for the phenomenon. EXPERIMENTS
AND
RESULTS
To determine the effect of high values of the short-wavelength flux (SWF) an untuned resonator ce11(4)was initially used. It is shown diagrammatically in Fig. 1A. Either one lamp or two could provide both the measuring radiation and short wavelengths, with nearly all of both interacting with the vapour sample. The cell could be pumped down to about 1 torr to provide a reference spectrum. The advantage of this kind of cell was in providing maximum values of flux, but with the hardware available to us no filtering of the radiation was practicable and there was no independent control of sample temperature. Figure 2B shows the spectra recorded in the untuned resonator with one (-) and two lamps (---). Comparing these with the lower line of Fig. 2A, which is a prediction for approximately the same amount of water and temperature, it is seen that both the observed spectra show considerably more structure than is predicted. Furthermore, the spectrum taken with the lower SWF is all below the 100% line corresponding to sample absorption, but the spectrum taken with the higher value of SWF is above the 100% line for a good part of the wavenumber range, indicating emission from the water sample under these conditions. Examination of the features marked by 437
438
P. DIAS-LALCACA rc al.
250 W Lamp
Lamp
Scale Golay
Sample Parh
Derector
Volume Length
= 2
= 08
m
B Thermal
lnsulat~on
t
\
MelKlex
M e II ” e x
Wlndow
Wlndow \
125
Sample
Path
W Lamp
volume
Length
=I2
lives
~15 m
I
Scale From
Refrigerator
Fig. 1. The two optical arrangements used for the experiments, as described in the text. A, an untuned resonator cell, which allows most of the radiation from the mercury lamp sources to interact with the water vapour sample. B, a triple pass cell with a jacketed chamber for temperature control.
aa’, bb’ etc. shows that there is a coincidence in wavenumber value between absorption minima in the low SWF spectrum and emission maxima in the high SWF spectrum. This last result highlights the need, when seeking wavenumber correlations between such spectra representing different conditions, to use coincidence of turning points as the criterion for agreement rather than relying on absorption minima alone. To make measurements at lower temperatures and with some UV filtering a triple pass cell, as shown in Fig. IB, was used. With this the SWF values were very much lower since the source solid angle over which there was a useful emission was smaller. In addition the polyethylene terpthalate (Melinex) filter (thickness 0.2 mm) cut-off all wavelengths less than about 300 nm. It was to be expected, however, from the free energy values of the metastable polymers”) that either reducing the temperature or increasing the SWF would increase their steady-state concentration so that the changes in going from the untuned resonator cell to the triple pass cell would, to some extent, compensate each other. This appears to have occurred. The spectra in Fig. 2B show good correlation in features with those shown in Figs 3B and 3C, which is important in demonstrating that these cannot be an artefact associated with resonances in the untuned resonator cell which might have failed to be completely cancelled by taking a ratio. The spectrum in Fig. 3B shows mostly absorption, which might be expected from the much lower
UV radiation
439
on water vapour
SWF as compared with that used in the untuned resonator, but this result and that in the next paragraph show that quantitative comment on the relative importance of temperature and SWF value is beyond the scope of the present study. Figure 3C represents the lowest temperature we have investigated and it may be noted that, even with the water amount being less than one-tenth of that used in the other measurements, there is good correlation with them in the wavenumber positions if the turning-point criterion is used. Indeed it is essential for comparing Fig. 3C with the other spectra since it appears that, for the temperature and SWF conditions to which it refers, the sample is emitting over most of the region investigated.
A. PredIctIon 10
02
V
c I
I
lb
6. Untuned
resonator Temp
= 320
lxlo~” (240-400 rh
-
+ IOK
photon
set-’
““I
20%
Temp
-3lOflOK
3 4 x IO” l240-400
photon
set-’
nml
r h 30%
0’
I
I
I
I
I
IO
20
30
40
50
WAVENUMBER
(cm-‘)
Fig. 2. In A the lower line is a predicted spectrum for water vapour at 300 K amounting to 10pm of precipitable water broadened by I atm of nitrogen; the upper line is for 0.3 pm of precipitable water. Both spectra have been convolved with an instrument function appropriate to 0.66 cm ~’ resolution. In B the solid line represents measurements made in the untuned resonator when the radiation from one lamp was used and the dashed line when both lamps were used. The water amount was about 10 pm precipitable in both spectra with atmospheric broadening. The spectral resolution is 0.5 cm-’ and the total integration time in deriving each ratio was 30 hr. Detector noise at this integration time gives an uncertainty in the ratio value of about 27: of full scale (Trans = 1).
P. DIAS-LBl_C.A<.A<‘I Ul.
440
COMPARISON
OF
LABORATORY
AND
ATMOSPHERIC
SPECTRA
Figure 3A is an atmospheric spectrum recorded at Jungfratjoch with approximately the same resolution as the other spectra in Fig. 3 and plotted on the same wavenumber scale. To make a meaningful comparison of features it is necessary to allow for the atmospheric spectrum showing absorption, which inevitably is missing from the laboratory spectra. First, the water amounr is at
Temp
= 280 2 10 K
4 x 10li (320
-400
photon
w-1
nml
-
‘g
is
I
Temp = 230 + IOK 4 x 1o’7 photon set -1
05
1320-400
nmi
_L_
0
5
10
15
20
25
WAVENUMHER
30
35
40
..--1
I 45
50
(cm-‘)
Fig. 3. A shows a spectrum of solar radiation modified by atmospheric absorption recorded at Jungfraujoch. ‘2,S.htThe resolution was 0.25 cm ’ and this value was matched in B and C. B is derived from measurements made in the triple pass cell of saturated water vapour amounting to IOpm precipitable without atmospheric broadening, The total integration time was 60 hr and detector noise is about 2”,, of full scale. C represents saturated water vapour amounting to 0.6/cm precipitable without atmospheric broadening. The Integration time was 30 hr and detector noise is about 3”,,.
UV radiation on water
441
WpOUr
least 50 times greater than that relevant to Fig. 3B, which means that the pure rotation lines are very much stronger. Figure 3A also shows magnetic dipole oxygen rotation lines, which are absent in the laboratory spectrum. When allowance is made for these differences, it is found that nearly all the wavenumber turning points of Fig. 3A can be seen in the UV-induced laboratory spectra. Thus the conclusion of Ref. (2) that the effect of UV radiation from mercury lamps is causing the same effect as solar radiation is supported. It may be noted that the finding of absorption minima coinciding with emission maxima had been recognized in different members of the series of atmospheric spectra from which Fig. 3A was taken. When this was first observed in 1957 it was a considerable mystery, but this can now be explained in terms of possible variation in the solar UV ffux reaching the absorbing water vapour. CONCLUSIONS The present experiments show the following. (1) There is no doub& that short-wavelength radiation, probably of near-UV wavelength, has a major effect on the absorption by water vapour in the wavenumber range 5-50 cm-‘. (2) The induced effect has a highly reproducible spectral structure as far as wavenumber minima and maxima are concerned but either absorption or emission can be seen depending on sample temperature and SWF value. It is very likely that the fractional saturation value (relative humidity) also has a role but in the present experiments we had insufficient evidence about it as an independent parameter to comment. (3) The UV-induced spectra show excellent agreement in wavenumber turning points with examples of atmospheric spectra observed in full sunlight, thus supporting the proposed connection between solar UV radiation and “anomalous” atmospheric absorption. The present preliminary study has given valuable pointers to the design of a more comprehensive investigation. The greatest needs we have identified are: (a) to have independent control of the radiation being used for measurement and that used for excitation; (b) to be able to vary temperature and relative humidity continuously SO as to determine how absorption and emission intensity vary with these variables; (c) to be able to vary the SWF continuously, particularly to study the translation from absorption to emission; (d) to extend the wavenumber coverage so that a spectral pattern of the photoninduced phenomenon can be established, from which theoretical models, to account for it, can be formulated. AcknoM;ledgemenrs--This
PD.-L.
work was supported by the Procurement Executive of the Ministry of Defence.
gratefully acknowledges support from S.E.R.C, by a research studentshj~.
REFERENCES I. Gebbie H. A., 2. 3. 4. 5. 6.
Gebbie Hoppel Gebbie Gebbie Debbie
Nature 296, 422 (1982). H. A., hfrared P?ry.~ 24, 429 (I 984). W. A. and Dinger _I. E., .F. atmos. Sci. 30, 331 (1973). H. A. and Llewellyn-Jones D. T., Inf. J. it$wred milfime!er H. A., Phys. Ret. 107, 1194 (1957). H. A. and Burroughs W. J., Naiure 217, 1241 (1968).
Waves
2, 197 (1981).
P/II.F.
Vol. 24.
in &eat
THE
No.
5, pp.
431
441.
1984
0020-0891/X4
Britain
S3.00 f0.00
Pergamon Press Ltd
EFFECT VAPOUR
OF ULTRAVIOLET RADIATION ON WATER ABSORPTION BETWEEN 5 AND 50cm-’
P. DIAS-LALCACA, N. J. C. PACKHAM and H. A. GEBBIE Department of Electrical Engineering, Imperial College, London SW7 2BT, England (Received
24 Junuary
1984; accepted in revised form 26 February
1984)
Abstract-Both field measurements and laboratory experiments suggested that absorption of UV radiation affected the near-millimetre wave absorption by water vapour. Experiments with different flux values and at different temperatures have confirmed this and pumping by UV radiation has been shown to give near-millimetre wave emission.
INTRODUCTION
The background to this investigation is given in Refs (1) and (2), the latter of which appears in this issue of Infrared Physics (p. 429). In Ref. (2) it was suggested that the short-wavelength photons in sunlight appear to produce spectral anomalies in atmospheric absorption of near-millimetre waves and that the mercury lamp sources used for laboratory measurements could be responsible for these also showing similar spectral features. There was a need for systematic experiments to verify this laboratory effect and here we give an account of a preliminary study. The fact that medium-pressure mercury-vapour discharge lamps, which are commonly used as continuum sources of near-millimetre wave radiation, also give a high flux of UV radiation is both a complication and a benefit. Designing experiments which depend on removing the UV component by filters which have an acceptably low insertion loss in the near-millimetre wave region is not easy, so in the present work the alternative of deliberately exploiting the UV component has been chosen. The dominant consideration in making spectral measurements in the 5-50 cm-’ range is that the low brightness of continuum sources make integration times very long, so optimum use must be made of the available power. If several independent variables such as the short-wavelength flux, sample temperature and water vapour saturation value are to be explored along with spectral distribution, an extended investigation would be needed. We have, therefore, sought to establish the main facts by the simplified study described here. It has been assumed, on the basis of the experiments reported in Ref. (3), that the short-wavelength flux responsible for the effects is the near-UV region. At a later stage it will be necessary to measure the actual spectral distribution of the short-wavelength component responsible for the phenomenon. EXPERIMENTS
AND
RESULTS
To determine the effect of high values of the short-wavelength flux (SWF) an untuned resonator ce11(4)was initially used. It is shown diagrammatically in Fig. 1A. Either one lamp or two could provide both the measuring radiation and short wavelengths, with nearly all of both interacting with the vapour sample. The cell could be pumped down to about 1 torr to provide a reference spectrum. The advantage of this kind of cell was in providing maximum values of flux, but with the hardware available to us no filtering of the radiation was practicable and there was no independent control of sample temperature. Figure 2B shows the spectra recorded in the untuned resonator with one (-) and two lamps (---). Comparing these with the lower line of Fig. 2A, which is a prediction for approximately the same amount of water and temperature, it is seen that both the observed spectra show considerably more structure than is predicted. Furthermore, the spectrum taken with the lower SWF is all below the 100% line corresponding to sample absorption, but the spectrum taken with the higher value of SWF is above the 100% line for a good part of the wavenumber range, indicating emission from the water sample under these conditions. Examination of the features marked by 437
438
P. DIAS-LALCACA rc al.
250 W Lamp
Lamp
Scale Golay
Sample Parh
Derector
Volume Length
= 2
= 08
m
B Thermal
lnsulat~on
t
\
MelKlex
M e II ” e x
Wlndow
Wlndow \
125
Sample
Path
W Lamp
volume
Length
=I2
lives
~15 m
I
Scale From
Refrigerator
Fig. 1. The two optical arrangements used for the experiments, as described in the text. A, an untuned resonator cell, which allows most of the radiation from the mercury lamp sources to interact with the water vapour sample. B, a triple pass cell with a jacketed chamber for temperature control.
aa’, bb’ etc. shows that there is a coincidence in wavenumber value between absorption minima in the low SWF spectrum and emission maxima in the high SWF spectrum. This last result highlights the need, when seeking wavenumber correlations between such spectra representing different conditions, to use coincidence of turning points as the criterion for agreement rather than relying on absorption minima alone. To make measurements at lower temperatures and with some UV filtering a triple pass cell, as shown in Fig. IB, was used. With this the SWF values were very much lower since the source solid angle over which there was a useful emission was smaller. In addition the polyethylene terpthalate (Melinex) filter (thickness 0.2 mm) cut-off all wavelengths less than about 300 nm. It was to be expected, however, from the free energy values of the metastable polymers”) that either reducing the temperature or increasing the SWF would increase their steady-state concentration so that the changes in going from the untuned resonator cell to the triple pass cell would, to some extent, compensate each other. This appears to have occurred. The spectra in Fig. 2B show good correlation in features with those shown in Figs 3B and 3C, which is important in demonstrating that these cannot be an artefact associated with resonances in the untuned resonator cell which might have failed to be completely cancelled by taking a ratio. The spectrum in Fig. 3B shows mostly absorption, which might be expected from the much lower
UV radiation
439
on water vapour
SWF as compared with that used in the untuned resonator, but this result and that in the next paragraph show that quantitative comment on the relative importance of temperature and SWF value is beyond the scope of the present study. Figure 3C represents the lowest temperature we have investigated and it may be noted that, even with the water amount being less than one-tenth of that used in the other measurements, there is good correlation with them in the wavenumber positions if the turning-point criterion is used. Indeed it is essential for comparing Fig. 3C with the other spectra since it appears that, for the temperature and SWF conditions to which it refers, the sample is emitting over most of the region investigated.
A. PredIctIon 10
02
V
c I
I
lb
6. Untuned
resonator Temp
= 320
lxlo~” (240-400 rh
-
+ IOK
photon
set-’
““I
20%
Temp
-3lOflOK
3 4 x IO” l240-400
photon
set-’
nml
r h 30%
0’
I
I
I
I
I
IO
20
30
40
50
WAVENUMBER
(cm-‘)
Fig. 2. In A the lower line is a predicted spectrum for water vapour at 300 K amounting to 10pm of precipitable water broadened by I atm of nitrogen; the upper line is for 0.3 pm of precipitable water. Both spectra have been convolved with an instrument function appropriate to 0.66 cm ~’ resolution. In B the solid line represents measurements made in the untuned resonator when the radiation from one lamp was used and the dashed line when both lamps were used. The water amount was about 10 pm precipitable in both spectra with atmospheric broadening. The spectral resolution is 0.5 cm-’ and the total integration time in deriving each ratio was 30 hr. Detector noise at this integration time gives an uncertainty in the ratio value of about 27: of full scale (Trans = 1).
P. DIAS-LBl_C.A<.A<‘I Ul.
440
COMPARISON
OF
LABORATORY
AND
ATMOSPHERIC
SPECTRA
Figure 3A is an atmospheric spectrum recorded at Jungfratjoch with approximately the same resolution as the other spectra in Fig. 3 and plotted on the same wavenumber scale. To make a meaningful comparison of features it is necessary to allow for the atmospheric spectrum showing absorption, which inevitably is missing from the laboratory spectra. First, the water amounr is at
Temp
= 280 2 10 K
4 x 10li (320
-400
photon
w-1
nml
-
‘g
is
I
Temp = 230 + IOK 4 x 1o’7 photon set -1
05
1320-400
nmi
_L_
0
5
10
15
20
25
WAVENUMHER
30
35
40
..--1
I 45
50
(cm-‘)
Fig. 3. A shows a spectrum of solar radiation modified by atmospheric absorption recorded at Jungfraujoch. ‘2,S.htThe resolution was 0.25 cm ’ and this value was matched in B and C. B is derived from measurements made in the triple pass cell of saturated water vapour amounting to IOpm precipitable without atmospheric broadening, The total integration time was 60 hr and detector noise is about 2”,, of full scale. C represents saturated water vapour amounting to 0.6/cm precipitable without atmospheric broadening. The Integration time was 30 hr and detector noise is about 3”,,.
UV radiation on water
441
WpOUr
least 50 times greater than that relevant to Fig. 3B, which means that the pure rotation lines are very much stronger. Figure 3A also shows magnetic dipole oxygen rotation lines, which are absent in the laboratory spectrum. When allowance is made for these differences, it is found that nearly all the wavenumber turning points of Fig. 3A can be seen in the UV-induced laboratory spectra. Thus the conclusion of Ref. (2) that the effect of UV radiation from mercury lamps is causing the same effect as solar radiation is supported. It may be noted that the finding of absorption minima coinciding with emission maxima had been recognized in different members of the series of atmospheric spectra from which Fig. 3A was taken. When this was first observed in 1957 it was a considerable mystery, but this can now be explained in terms of possible variation in the solar UV ffux reaching the absorbing water vapour. CONCLUSIONS The present experiments show the following. (1) There is no doub& that short-wavelength radiation, probably of near-UV wavelength, has a major effect on the absorption by water vapour in the wavenumber range 5-50 cm-‘. (2) The induced effect has a highly reproducible spectral structure as far as wavenumber minima and maxima are concerned but either absorption or emission can be seen depending on sample temperature and SWF value. It is very likely that the fractional saturation value (relative humidity) also has a role but in the present experiments we had insufficient evidence about it as an independent parameter to comment. (3) The UV-induced spectra show excellent agreement in wavenumber turning points with examples of atmospheric spectra observed in full sunlight, thus supporting the proposed connection between solar UV radiation and “anomalous” atmospheric absorption. The present preliminary study has given valuable pointers to the design of a more comprehensive investigation. The greatest needs we have identified are: (a) to have independent control of the radiation being used for measurement and that used for excitation; (b) to be able to vary temperature and relative humidity continuously SO as to determine how absorption and emission intensity vary with these variables; (c) to be able to vary the SWF continuously, particularly to study the translation from absorption to emission; (d) to extend the wavenumber coverage so that a spectral pattern of the photoninduced phenomenon can be established, from which theoretical models, to account for it, can be formulated. AcknoM;ledgemenrs--This
PD.-L.
work was supported by the Procurement Executive of the Ministry of Defence.
gratefully acknowledges support from S.E.R.C, by a research studentshj~.
REFERENCES I. Gebbie H. A., 2. 3. 4. 5. 6.
Gebbie Hoppel Gebbie Gebbie Debbie
Nature 296, 422 (1982). H. A., hfrared P?ry.~ 24, 429 (I 984). W. A. and Dinger _I. E., .F. atmos. Sci. 30, 331 (1973). H. A. and Llewellyn-Jones D. T., Inf. J. it$wred milfime!er H. A., Phys. Ret. 107, 1194 (1957). H. A. and Burroughs W. J., Naiure 217, 1241 (1968).
Waves
2, 197 (1981).