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High-resolution spectroscopy of SO2 using a frequency-doubled, continuous-wave dye laser
Volume 33, number 3
OPTICS COMMUNICATIONS
June 1980
HIGH-RESOLUTION SPECTROSCOPY OF SO 2 USING A FREQUENCY-DOUBLED, CONTINUOUS-WAVE DYE L A S E R B.R. MARX, K.P. BIRCH, R.C. FELTON, B.W. J O L L I F F E , W.R.C. ROWLEY and P.T. WOODS Division of Quantum Metrology, National Physical Laboratory, Teddington, Middlesex TW11 OLW, UK Received 19 February 1980
The SO2 molecule is of considerable interest in the context of atmospheric pollution, and in many laser monitoring techniques the ultraviolet absorption band at 300 nm is used to determine SO2 concentrations in the atmosphere. Recent laboratory experiments with a resolution of 2 X 10 -3 nm showed that variations could occur in absorption cross-section measurements made with different laser bandwidths due to unresolved fine structure. We have investigated absorption spectra with a line width of 3 × 10 -6 nm, using a frequency-doubled continuous-wave dye laser, and have confirmed the existence of fine structure in the absorption even when collisionallybroadened with an atmosphere of nitrogen. These measurements provide a data base from which valid absorption cross sections may be calculated for all monitoring laser bandwidths. We estimate the pressure broadening coefficient for nitrogen in this wavelength region as 83 ± 38 kHz Pa -l (11 + 5 MHz torr-1 ). The temperature dependence of the absorption cross-section was also investigated.
1. Introduction Interest in remote atmospheric pollution monitoring has grown in recent years and m a n y laser-based techniques have been developed. Differential absorption Lidar (DIAL) systems are used to monitor chemical species [1 ] and involve sequentially tuning a laser into coincidence with an absorption band o f the pollutant under study and then to an adjacent region o f low absorption. The differential absorption, proportional to pollutant concentration, is then measured. In the particular case o f SO 2 the absorption band around 300 nm is most often used to probe the pollutant. This peak is described b y Clements as " G " in his letter band system [2], and designated b y Hamada and Merer [3] as a hybrid band o f the 1 A 2 - 1 A 1 and 1B11 A 1 transitions. The absorption data usually quoted for this band is that published by Thompson, Hoell and Wade [4]. Their spectrum was obtained for pure SO~ using a pulsed dye laser with the relatively low nominal resolution o f 2 × 10 - 2 nm. As an atmospheric pollutant, however, the SO 2 absorption is pressure-broadened and the bandwidth o f a probe laser m a y be significantly different, so that Thompson, Hoell and Wade's results
may not be applicable. More recent experiments [5] have investigated the SO 2 spectrum broadened b y an • atmosphere of nitrogen using a pulsed dye laser of 2 X 10 - 3 nm bandwidth. These demonstrated that fine structure existed over this absorption peak which could lead to anomalies in the measurement o f pollution concentrations. However, an earlier experiment using a narrow-band continuous-wave (cw) frequency-doubled dye laser and an optoacoustic cell [6], with a nominal resolution o f better than 10 - 3 nm had failed to see more highly resolved structure than that measured at the resolution o f 2 X 10 - 2 nm in [4]. To investigate these anomalies, the experiments reported here use a cw dye laser with a bandwidth o f 3 X 10 - 6 nm, sufficient to resolve completely any fine structure and to provide a scan over the whole SO 2 absorption region around 300 nm.
2. Experimental
procedure
The light source for the experiment was a frequencydoubled, single-mode dye laser (Coherent Radiation, CR599-21), pumped by an argon-ion laser. The dye laser had a linewidth of several megahertz in the visible 287
Volume 33, number 3
OPTICS COMMUNICATIONS Chopper
BS
I I
!
BS
I I
In~er ferometer L
>
Fig. 1. Schematic arrangement of the experimental apparatus: FDC, oven containing frequency doubling crystal; V, visible blocking falters; PMT, photomultiplier tube; BS1 and 2, wedged fused silica beam-splitters. and could be scanned continuously over 30 GHz at 600 nm. The experimental arrangement is shown schematically in fig. 1. The laser light passed through a wedged fused silica beam splitter (BS1). The beam reflected from the front surface supplied light to a wavemeter [7] for measurement of the laser wavelength. The internally reflected beam supplied light to a F a b r y Perot interferometer which produced calibration markers every 1.5 GHz. The main laser beam passed through a chopper into a 30 mm long frequencydoubling crystal of ammonium dihydrogen arsenate (ADA) contained in an oven. This crystal was 90 ° phase matched and temperature tuned around 70°C to give maximum uv output at the central wavelength of each 30 GHz scan. A maximum uv power of 3 #W was obtained, for an input power of 100 mW. However, the uv power was reduced by a factor of about 103 for the experiment to prevent detector saturation. The uv output was then split into a reference and a probe beam. The latter passed through the SO 2 cell, which was 98 mm in length with quartz windows. Both beams were directed through visible blocking filters onto photomultipliers (EMI 9789QB), and the resultant signals were synchronously detected at the chopping frequency and ratioed to minimize the effects of changes in laser intensity. The linearity of the combined photomultiplier, amplification and ratiometric systems was checked against changes in laser intensity and found to be better than 3% over our total operating range. Gas pressures were initially measured using a calibrated Baratron gauge Model 221. However, problems were encountered due to slow gas interdiffusion when 288
June 1980
filling the cell with a mixture of SO 2 and nitrogen from separate cylinders. This effect remained even when filling the cell from a larger pre-mixing reservoir. The exact SO 2 pressure in the cell was therefore uncertain. A procedure was thus adopted which normalized the results to those of previous measurements [5] at a particular wavelength. The wavelength selected was Xvac = 299.4 nm, because the nitrogen-broadened spectrum in this region of low absorption is relatively flat and direct comparison could be made with the pulsed-laser results in which the SO 2 pressure was well known and a gas mixing problem was not observed. The ratioed absorption signal and the signal obtained from the Fabry-Perot interferometer were recorded on a two-pen chart recorder, the x-axis of which was driven by the scan electronics of the dye laser. Thus intensity-normalized absorption spectra with wavelength markers were obtained. Absolute wavelength calibration was achieved with readings from the wavemeter at the beginning and end of each 30 GHz scan. The programme computing the wavelength [7] had to be modified to cope with intensity fluctuations of the dye laser. The reduction in accuracy caused by this modification resulted in a wavelength uncertainty for a single scan measurement of 5 parts in 107, (1.5 X 10 -4 nm at 300 nm). This allowed accurate matching and piecing together of the different scans, so that the whole wavelength range of interest was covered. The laser linewidth in the uv has 3 X 10 -6 nm. The resolution however, determined largely by the time constants of the phase -sensitive-detection systems, was measured to be 2 X t0 -4 nm. This resolution, being better than the Doppler width of the non-pressurebroadened SO 2 lines at this wavelength, should be sufficient to resolve all detail, in both the broadened and unbroadened spectra. To confirm this experimentally, absorption measurements were made using the cell containing a low pressure of SO 2 alone. In this situation it was demonstrated that an increase in resolution by a factor of three did not decrease detectably the width of the spectral features scanned.
2. Results The value of a, the absorption coefficient in units of cm -1 atmosphere -1 , was determined at each wavelength using the equation,
Volume 33, number 3
OPTICS COMMUNICATIONS
(-'i/Io) ct = pathlength X pressure ' where I i and I o are the input and transmitted intensities respectively. We have used a as it is a parameter commonly used by research workers in this field. However, we prefer to express the results in units o f absorption cross-section (o in metre 2) since o depends on pressure rather than number density. The numerical relationship between o ( i n m 2) and a (cm -1 atm - 1 ) is [5] o =a×
1 . 3 6 × 10 --26× T ,
where T is the absolute temperature. The absorption cross-section was measured for various pressures o f SO 2 with and without an atmosphere of nitrogen gas. The value o f I i was determined by using a reference cell matched to the absorption cell but either evacuated or containing nitrogen as appropriate. The wavelength dependence o f tr over the absorption peak around 300 nm for SO 2 with 1 atmosphere (1.01 X 105 Pa) o f nitrogen is shown in fig. 2. Several values for each peak and trough were obtained, the results averaged, and a straight line interpolation o f the absorption coefficient was made between them. The peak and trough heights o f the absorption cross section are accurate relatively to within a standard deviation o f +0.25 X 10 -23 m 2. The absolute uncer-
16
--
40 ¢,
June 1980
tainty of the whole cross-sectioned data is dominated by a pressure calibration uncertainty, which is estimated to be +2.5%. The SO 2 pressure was between 0.3 and 0.4 kPa for all the scans, and from our studies o f the absorption of various pressures o f SO 2 without nitrogen broadening we deduce that self broadening is small compared with our experimental resolution over this pressure range. It can be seen from fig. 2 that there is considerable structure over this region even in conditions which simulate SO 2 pollution in the atmosphere. There is a variation o f almost 35% in the region ~'vac= 300.08 nm to Xyac = 300.14 nm. By comparison, the lower resolution studies [4] show a single peak with a variation of only 4% across the wavelength region given above. Our experiments thus confirm the existence o f fine structure in this absorption region and it is unclear why the previous cw laser experiment [6] failed to observe it. Fig. 3 shows the effect o f an atmosphere o f nitrogen on part o f the SO 2 absorption spectrum, the vibration-rotational structure being smoothed b y pressure broadening. The extent o f the broadening is shown quantitatively in fig. 4. This shows the peak-to-trough ratio of the absorption coefficient as a function o f nitrogen pressure for constant SO 2 pressure for the peaks labelled in fig. 3 as 1 , 2 and 3. It is difficult to obtain a meaningful pressure-broadening coefficient from such a complex spectrum, and ambiguities arise from choosing an absorption value for the background quasi-continuum o f lines which do not respond to
i 3s 3
0 14~
I I I I II Ill I! t ~ t VI
b ~12
• , I o
b 10-
I
,,.
'
i'"
It
'(
[
yl )lll~i"i
l i II
I u
V
r
tr
~ "l'
1 '
LIlt
T
alone
~30~
I/
I
SO2 ÷ 1 Atmosphere of nitrogen
i i
I
300 000 299917
I~
040 -060
300.100 300L017
,l&O ~.160
-
3001200 Vocuurn wavelength (nrn) 300J'117 Air wavelength (nrn)
Fig. 2. Wavelength dependence of the absorption cross-section of SO2 at 25 °C over the absorption peak around 300 rim. The SO2 pressure was around 0.3 kPa with 1.01 X l0 s Pa of nitrogen buffer gas. The absolute accuracy of wavelength was +0.0003 rim, and the resolution was approximately ±2X 10 --4 nm.
300085
300 090
300 093
300002
300007
300 010
Vacuum wavelength (nm) Air wavelength (nm)
Fig. 3. High resolution absorption spectrum of SO2 around 30 300 nm showing the effects of atmospheric pressure broadening. The value of the absorption cross-section at each of the features marked is given below in units of m 2 X 10 -23 : 1, a = 22.7; 2, a = 17.9; 3, a -- 24.4; 4, a = 9.30; 5, a = 13.2; 6, o = 10.6; 7, tr = 12.7; 8, o = 14.8. 289
Volume 33, number 3
o3t
OPTICS COMMUNICATIONS
June 1980
I Error bar 20
'l "7 o
[
10
I
o-
25
Pressure of Nitrogen in kPa
Fig. 4. Peak-to-trough ratio of the absorption cross-section of
SO2 as a function of nitrogen pressure. SO2 pressure 0.3 kPa throughout, e peak 1 (X = 300.085 nm), + peak 2 (X = 300.090 nm), X peak 3 (k = 300.093 nm). pressure broadening. However, by fitting curves to the data for the three labelled peaks, taking into account only those peaks either side of them, and assuming the same broadening coefficient for all lines, we estimate the value to be 83 _+38 kHz Pa -1 (11 + 5 MHz torr-1). We also investigated the region of low absorption around the wavelength of 299.4 nm (i.e. between peaks G and H in Clement's notation [4]). This is the wavelength region often used for the low absorption part of the DIAL measurement. In this region the average absorption cross-section is approximately 3.2 X 10 .23 m 2, and the fine structure on the pure SO 2 spectrum is proportionately smaller. The effect of the nitrogen broadening on this part of the spectrum is qualitatively the same as on the peak. The absorption is flattened by atmospheric broadening, so that the remaining structure has a negligible effect on the differential absorption which is much more sensitive to changes in the larger, peak absorption. The temperature dependence of the atmospherically broadened SO 2 absorption was also investigated. The sample cell was heated and the temperature of the cell wall at the centre of the absorption region measured with a thermocouple probe. Scans were made in the region of the wavelength kvac = 300.04 nm at various temperatures, and the value of the absorption crosssection was compared with that at room temperature. Room temperature scans were taken before and after each scan at an elevated temperature to observe the effect of any actual decrease in SO 2 number density due to the heating process. The two ambient results 290
50
75 100 TernperQture in °C
125
150
Fig. 5. Percentage decrease, with respect to ambient temperature, of the absorption cross-section, a, for SO2 in one atmosphere of nitrogen at X = 300.04 nm. were then averaged to yield the percentage decrease in absorptio n at the elevated temperature. The average diminution of SO 2 concentration for each heating and cooling cycle was 3%. The percentage decrease in cross section at each temperature compared with the value at ambient temperature ( 2 5 - 2 5 °C) is shown in fig. 5. These values are derived from an average of peak and 'trough decreases in the nitrogen-broadened spectrum. No significant difference was found between peak and trough decreases up to 150°C in the 2 × 1 0 - 2 n m region scanned. The temperature dependence of the cross section indicates that there may be a contribution to the absorption from higher levels of the ground state.
3. Summary and conclusions We have confirmed the existence of considerable fine structure in the absorption of SO 2 around 300 nm even when the molecular spectrum is broadened with an atmosphere of nitrogen. This would indicate a need for careful evaluation of laser bandwidth and laser wavelength if valid pollution concentration measurements are to be made. Since our results are not laserbandwidth limited, the numerical data obtained in this experiment may be used to perform an integration of the results over the appropriate laser bandwidth used for other atmospheric SO 2 measurements, and thus yield absorption cross-sections approximately valid under field conditions. The digital data and the conditions under which accurate integration may be performed will be the subjects of a later paper. We also
Volume 33, number 3
OPTICS COMMUNICATIONS
provide values for the corrections to this data necessary when making plume-temperature atmospheric measurements, and results for the pressure-broadening of SO 2 by a nitrogen atmosphere.
References [1] R.L. Byer and M. Garbuny, Appl. Optics 12 (1973) 1496. [2] J.H. Clements, Phys. Rev. 47 (1935) 224.
June 1980
[3] Y. Hamada and A.J. Merer, Can. J. Phys. 53 (1975) 2555. [4] R.T. Thompson, J.M. Hoell and W.R. Wade, J. Appl. Phys. 46 (1975) 3040. [5] P.T. Woods, B.W. JoUiffe and B.R. Marx, Optics Comm. 33 (1980) 281. [6] K.P. Koch and W. Lahmann, Appl. Phys. Lett. 35 (1978) 289. [7] W.R.C. Rowley, K.C. Shotton and P.T. Woods, Laser Spectroscopy III, eds. J.L. Hall and J.L. Carlsten (Berlin, Springer-Verlag 1977) p. 425.
291
OPTICS COMMUNICATIONS
June 1980
HIGH-RESOLUTION SPECTROSCOPY OF SO 2 USING A FREQUENCY-DOUBLED, CONTINUOUS-WAVE DYE L A S E R B.R. MARX, K.P. BIRCH, R.C. FELTON, B.W. J O L L I F F E , W.R.C. ROWLEY and P.T. WOODS Division of Quantum Metrology, National Physical Laboratory, Teddington, Middlesex TW11 OLW, UK Received 19 February 1980
The SO2 molecule is of considerable interest in the context of atmospheric pollution, and in many laser monitoring techniques the ultraviolet absorption band at 300 nm is used to determine SO2 concentrations in the atmosphere. Recent laboratory experiments with a resolution of 2 X 10 -3 nm showed that variations could occur in absorption cross-section measurements made with different laser bandwidths due to unresolved fine structure. We have investigated absorption spectra with a line width of 3 × 10 -6 nm, using a frequency-doubled continuous-wave dye laser, and have confirmed the existence of fine structure in the absorption even when collisionallybroadened with an atmosphere of nitrogen. These measurements provide a data base from which valid absorption cross sections may be calculated for all monitoring laser bandwidths. We estimate the pressure broadening coefficient for nitrogen in this wavelength region as 83 ± 38 kHz Pa -l (11 + 5 MHz torr-1 ). The temperature dependence of the absorption cross-section was also investigated.
1. Introduction Interest in remote atmospheric pollution monitoring has grown in recent years and m a n y laser-based techniques have been developed. Differential absorption Lidar (DIAL) systems are used to monitor chemical species [1 ] and involve sequentially tuning a laser into coincidence with an absorption band o f the pollutant under study and then to an adjacent region o f low absorption. The differential absorption, proportional to pollutant concentration, is then measured. In the particular case o f SO 2 the absorption band around 300 nm is most often used to probe the pollutant. This peak is described b y Clements as " G " in his letter band system [2], and designated b y Hamada and Merer [3] as a hybrid band o f the 1 A 2 - 1 A 1 and 1B11 A 1 transitions. The absorption data usually quoted for this band is that published by Thompson, Hoell and Wade [4]. Their spectrum was obtained for pure SO~ using a pulsed dye laser with the relatively low nominal resolution o f 2 × 10 - 2 nm. As an atmospheric pollutant, however, the SO 2 absorption is pressure-broadened and the bandwidth o f a probe laser m a y be significantly different, so that Thompson, Hoell and Wade's results
may not be applicable. More recent experiments [5] have investigated the SO 2 spectrum broadened b y an • atmosphere of nitrogen using a pulsed dye laser of 2 X 10 - 3 nm bandwidth. These demonstrated that fine structure existed over this absorption peak which could lead to anomalies in the measurement o f pollution concentrations. However, an earlier experiment using a narrow-band continuous-wave (cw) frequency-doubled dye laser and an optoacoustic cell [6], with a nominal resolution o f better than 10 - 3 nm had failed to see more highly resolved structure than that measured at the resolution o f 2 X 10 - 2 nm in [4]. To investigate these anomalies, the experiments reported here use a cw dye laser with a bandwidth o f 3 X 10 - 6 nm, sufficient to resolve completely any fine structure and to provide a scan over the whole SO 2 absorption region around 300 nm.
2. Experimental
procedure
The light source for the experiment was a frequencydoubled, single-mode dye laser (Coherent Radiation, CR599-21), pumped by an argon-ion laser. The dye laser had a linewidth of several megahertz in the visible 287
Volume 33, number 3
OPTICS COMMUNICATIONS Chopper
BS
I I
!
BS
I I
In~er ferometer L
>
Fig. 1. Schematic arrangement of the experimental apparatus: FDC, oven containing frequency doubling crystal; V, visible blocking falters; PMT, photomultiplier tube; BS1 and 2, wedged fused silica beam-splitters. and could be scanned continuously over 30 GHz at 600 nm. The experimental arrangement is shown schematically in fig. 1. The laser light passed through a wedged fused silica beam splitter (BS1). The beam reflected from the front surface supplied light to a wavemeter [7] for measurement of the laser wavelength. The internally reflected beam supplied light to a F a b r y Perot interferometer which produced calibration markers every 1.5 GHz. The main laser beam passed through a chopper into a 30 mm long frequencydoubling crystal of ammonium dihydrogen arsenate (ADA) contained in an oven. This crystal was 90 ° phase matched and temperature tuned around 70°C to give maximum uv output at the central wavelength of each 30 GHz scan. A maximum uv power of 3 #W was obtained, for an input power of 100 mW. However, the uv power was reduced by a factor of about 103 for the experiment to prevent detector saturation. The uv output was then split into a reference and a probe beam. The latter passed through the SO 2 cell, which was 98 mm in length with quartz windows. Both beams were directed through visible blocking filters onto photomultipliers (EMI 9789QB), and the resultant signals were synchronously detected at the chopping frequency and ratioed to minimize the effects of changes in laser intensity. The linearity of the combined photomultiplier, amplification and ratiometric systems was checked against changes in laser intensity and found to be better than 3% over our total operating range. Gas pressures were initially measured using a calibrated Baratron gauge Model 221. However, problems were encountered due to slow gas interdiffusion when 288
June 1980
filling the cell with a mixture of SO 2 and nitrogen from separate cylinders. This effect remained even when filling the cell from a larger pre-mixing reservoir. The exact SO 2 pressure in the cell was therefore uncertain. A procedure was thus adopted which normalized the results to those of previous measurements [5] at a particular wavelength. The wavelength selected was Xvac = 299.4 nm, because the nitrogen-broadened spectrum in this region of low absorption is relatively flat and direct comparison could be made with the pulsed-laser results in which the SO 2 pressure was well known and a gas mixing problem was not observed. The ratioed absorption signal and the signal obtained from the Fabry-Perot interferometer were recorded on a two-pen chart recorder, the x-axis of which was driven by the scan electronics of the dye laser. Thus intensity-normalized absorption spectra with wavelength markers were obtained. Absolute wavelength calibration was achieved with readings from the wavemeter at the beginning and end of each 30 GHz scan. The programme computing the wavelength [7] had to be modified to cope with intensity fluctuations of the dye laser. The reduction in accuracy caused by this modification resulted in a wavelength uncertainty for a single scan measurement of 5 parts in 107, (1.5 X 10 -4 nm at 300 nm). This allowed accurate matching and piecing together of the different scans, so that the whole wavelength range of interest was covered. The laser linewidth in the uv has 3 X 10 -6 nm. The resolution however, determined largely by the time constants of the phase -sensitive-detection systems, was measured to be 2 X t0 -4 nm. This resolution, being better than the Doppler width of the non-pressurebroadened SO 2 lines at this wavelength, should be sufficient to resolve all detail, in both the broadened and unbroadened spectra. To confirm this experimentally, absorption measurements were made using the cell containing a low pressure of SO 2 alone. In this situation it was demonstrated that an increase in resolution by a factor of three did not decrease detectably the width of the spectral features scanned.
2. Results The value of a, the absorption coefficient in units of cm -1 atmosphere -1 , was determined at each wavelength using the equation,
Volume 33, number 3
OPTICS COMMUNICATIONS
(-'i/Io) ct = pathlength X pressure ' where I i and I o are the input and transmitted intensities respectively. We have used a as it is a parameter commonly used by research workers in this field. However, we prefer to express the results in units o f absorption cross-section (o in metre 2) since o depends on pressure rather than number density. The numerical relationship between o ( i n m 2) and a (cm -1 atm - 1 ) is [5] o =a×
1 . 3 6 × 10 --26× T ,
where T is the absolute temperature. The absorption cross-section was measured for various pressures o f SO 2 with and without an atmosphere of nitrogen gas. The value o f I i was determined by using a reference cell matched to the absorption cell but either evacuated or containing nitrogen as appropriate. The wavelength dependence o f tr over the absorption peak around 300 nm for SO 2 with 1 atmosphere (1.01 X 105 Pa) o f nitrogen is shown in fig. 2. Several values for each peak and trough were obtained, the results averaged, and a straight line interpolation o f the absorption coefficient was made between them. The peak and trough heights o f the absorption cross section are accurate relatively to within a standard deviation o f +0.25 X 10 -23 m 2. The absolute uncer-
16
--
40 ¢,
June 1980
tainty of the whole cross-sectioned data is dominated by a pressure calibration uncertainty, which is estimated to be +2.5%. The SO 2 pressure was between 0.3 and 0.4 kPa for all the scans, and from our studies o f the absorption of various pressures o f SO 2 without nitrogen broadening we deduce that self broadening is small compared with our experimental resolution over this pressure range. It can be seen from fig. 2 that there is considerable structure over this region even in conditions which simulate SO 2 pollution in the atmosphere. There is a variation o f almost 35% in the region ~'vac= 300.08 nm to Xyac = 300.14 nm. By comparison, the lower resolution studies [4] show a single peak with a variation of only 4% across the wavelength region given above. Our experiments thus confirm the existence o f fine structure in this absorption region and it is unclear why the previous cw laser experiment [6] failed to observe it. Fig. 3 shows the effect o f an atmosphere o f nitrogen on part o f the SO 2 absorption spectrum, the vibration-rotational structure being smoothed b y pressure broadening. The extent o f the broadening is shown quantitatively in fig. 4. This shows the peak-to-trough ratio of the absorption coefficient as a function o f nitrogen pressure for constant SO 2 pressure for the peaks labelled in fig. 3 as 1 , 2 and 3. It is difficult to obtain a meaningful pressure-broadening coefficient from such a complex spectrum, and ambiguities arise from choosing an absorption value for the background quasi-continuum o f lines which do not respond to
i 3s 3
0 14~
I I I I II Ill I! t ~ t VI
b ~12
• , I o
b 10-
I
,,.
'
i'"
It
'(
[
yl )lll~i"i
l i II
I u
V
r
tr
~ "l'
1 '
LIlt
T
alone
~30~
I/
I
SO2 ÷ 1 Atmosphere of nitrogen
i i
I
300 000 299917
I~
040 -060
300.100 300L017
,l&O ~.160
-
3001200 Vocuurn wavelength (nrn) 300J'117 Air wavelength (nrn)
Fig. 2. Wavelength dependence of the absorption cross-section of SO2 at 25 °C over the absorption peak around 300 rim. The SO2 pressure was around 0.3 kPa with 1.01 X l0 s Pa of nitrogen buffer gas. The absolute accuracy of wavelength was +0.0003 rim, and the resolution was approximately ±2X 10 --4 nm.
300085
300 090
300 093
300002
300007
300 010
Vacuum wavelength (nm) Air wavelength (nm)
Fig. 3. High resolution absorption spectrum of SO2 around 30 300 nm showing the effects of atmospheric pressure broadening. The value of the absorption cross-section at each of the features marked is given below in units of m 2 X 10 -23 : 1, a = 22.7; 2, a = 17.9; 3, a -- 24.4; 4, a = 9.30; 5, a = 13.2; 6, o = 10.6; 7, tr = 12.7; 8, o = 14.8. 289
Volume 33, number 3
o3t
OPTICS COMMUNICATIONS
June 1980
I Error bar 20
'l "7 o
[
10
I
o-
25
Pressure of Nitrogen in kPa
Fig. 4. Peak-to-trough ratio of the absorption cross-section of
SO2 as a function of nitrogen pressure. SO2 pressure 0.3 kPa throughout, e peak 1 (X = 300.085 nm), + peak 2 (X = 300.090 nm), X peak 3 (k = 300.093 nm). pressure broadening. However, by fitting curves to the data for the three labelled peaks, taking into account only those peaks either side of them, and assuming the same broadening coefficient for all lines, we estimate the value to be 83 _+38 kHz Pa -1 (11 + 5 MHz torr-1). We also investigated the region of low absorption around the wavelength of 299.4 nm (i.e. between peaks G and H in Clement's notation [4]). This is the wavelength region often used for the low absorption part of the DIAL measurement. In this region the average absorption cross-section is approximately 3.2 X 10 .23 m 2, and the fine structure on the pure SO 2 spectrum is proportionately smaller. The effect of the nitrogen broadening on this part of the spectrum is qualitatively the same as on the peak. The absorption is flattened by atmospheric broadening, so that the remaining structure has a negligible effect on the differential absorption which is much more sensitive to changes in the larger, peak absorption. The temperature dependence of the atmospherically broadened SO 2 absorption was also investigated. The sample cell was heated and the temperature of the cell wall at the centre of the absorption region measured with a thermocouple probe. Scans were made in the region of the wavelength kvac = 300.04 nm at various temperatures, and the value of the absorption crosssection was compared with that at room temperature. Room temperature scans were taken before and after each scan at an elevated temperature to observe the effect of any actual decrease in SO 2 number density due to the heating process. The two ambient results 290
50
75 100 TernperQture in °C
125
150
Fig. 5. Percentage decrease, with respect to ambient temperature, of the absorption cross-section, a, for SO2 in one atmosphere of nitrogen at X = 300.04 nm. were then averaged to yield the percentage decrease in absorptio n at the elevated temperature. The average diminution of SO 2 concentration for each heating and cooling cycle was 3%. The percentage decrease in cross section at each temperature compared with the value at ambient temperature ( 2 5 - 2 5 °C) is shown in fig. 5. These values are derived from an average of peak and 'trough decreases in the nitrogen-broadened spectrum. No significant difference was found between peak and trough decreases up to 150°C in the 2 × 1 0 - 2 n m region scanned. The temperature dependence of the cross section indicates that there may be a contribution to the absorption from higher levels of the ground state.
3. Summary and conclusions We have confirmed the existence of considerable fine structure in the absorption of SO 2 around 300 nm even when the molecular spectrum is broadened with an atmosphere of nitrogen. This would indicate a need for careful evaluation of laser bandwidth and laser wavelength if valid pollution concentration measurements are to be made. Since our results are not laserbandwidth limited, the numerical data obtained in this experiment may be used to perform an integration of the results over the appropriate laser bandwidth used for other atmospheric SO 2 measurements, and thus yield absorption cross-sections approximately valid under field conditions. The digital data and the conditions under which accurate integration may be performed will be the subjects of a later paper. We also
Volume 33, number 3
OPTICS COMMUNICATIONS
provide values for the corrections to this data necessary when making plume-temperature atmospheric measurements, and results for the pressure-broadening of SO 2 by a nitrogen atmosphere.
References [1] R.L. Byer and M. Garbuny, Appl. Optics 12 (1973) 1496. [2] J.H. Clements, Phys. Rev. 47 (1935) 224.
June 1980
[3] Y. Hamada and A.J. Merer, Can. J. Phys. 53 (1975) 2555. [4] R.T. Thompson, J.M. Hoell and W.R. Wade, J. Appl. Phys. 46 (1975) 3040. [5] P.T. Woods, B.W. JoUiffe and B.R. Marx, Optics Comm. 33 (1980) 281. [6] K.P. Koch and W. Lahmann, Appl. Phys. Lett. 35 (1978) 289. [7] W.R.C. Rowley, K.C. Shotton and P.T. Woods, Laser Spectroscopy III, eds. J.L. Hall and J.L. Carlsten (Berlin, Springer-Verlag 1977) p. 425.
291