Photoacoustic spectroscopy of O3 with a 450-MHz tunable waveguide CO2 laser

JOURNAL

OF MOLECULAR

SPECTROSCOPY

152,420-433

( 1992)

Photoacoustic Spectroscopy of O3 with a 450-MHz Tunable Waveguide CO2 Laser’ MADSHAMMERICH

NOBURUSOKABE~AND

Physics Laboratory, H. C. Orsted Institute, Universitetsparken 5. DK-2100 Copenhagen 0, Denmark

THORVALD PEDERSEN Chemistry Laboratory 5. H. C. Orsted Institute, Universitetsparken 5. DK-2100 Copenhagen 0, Denmark

ARI ~LAFSSON Science Institute, Unrversity of Iceland, Dunhaga 3, 107 Reykjavik, Iceland

AND JES HENNINGSEN Danish Institute yf Fundamental Metrology, Bldg. 307, Lundtoftevej 100, DK-2800 Lyngby. Denmark

Photoacousticabsorption signatures have been obtained for ozone in 450-MHz tuning windows of a waveguide CO2 laser. Out of 42 observed absorption lines, 3 I are assigned to the V, and the rq bands, and 7 to various hot bands of I60 I60 160. Two lines are assigned to the vj band of the isotopomers I60 I60 “0 and I60 “0 160, which were present in their natural abundance. Precise collision broadening measurements are reported for two lines of the ‘60’60160 uj band. D 1992 Academic Press. Inc. 1. INTRODUCTION

Measurement of the ozone concentration in the earth’s atmosphere has attracted growing interest in recent years. Ozone in the stratosphere shields the surface of the earth from ultraviolet solar radiation, and the possible destruction of the ozone layer by fluorocarbons and other man made chemicals is considered a potential health hazard. Ozone in the upper atmosphere contributes to the greenhouse effect, while tropospheric ozone is important in photochemistry and is considered harmful to vegetation. From the point of view of trace gas detection, infrared lasers yield a combination of high selectivity, sensitivity, and accuracy, provided reliable information is available on the frequency, strength, and width of suitable absorption lines for the molecule in question. The first work on ozone with a low pressure CO2 laser was reported in Ref. ( 1 ), while Ref. (2) studied its infrared spectrum with a CO2 waveguide laser tunable over +450 MHz. Several authors (3-7) have reported high resolution Fourier transform spectra of ozone in the 9-pm wavelength region, and both experimental (2, 8-13) and theoretical (14-16) studies have been published on pressure broadening coefficients. I Supported by the Danish Science Research Council under grant no. I l-7777. 2 On leave from Department of Applied Physics. Osaka City University. Sumiyoshi

0022-2852/92

$3.00

Copyright ic lYY2 by Academr

420 Press. Inc.

All rights of reproductmn m sny form rexcrvcd.

558. Osaka, Japan.

PHOTOACOUSTIC

SPECTROSCOPY OF O3

421

FIG. 1. Experimental setup with waveguide laser WGL, power supply PS, sweep generator SG, pulse generator PG. beamsplitter BS, mirrors M, main cell MC, reference cell RC, detectors D, and lock-in amplifiers LA.

Recently, the existing information has been reviewed and published as a comprehensive atlas of spectral parameters, covering the range from 0 to 3400 cm-’ ( 17). The purpose of the present work is to record and identify absorption lines within 450-MHz spectral windows centered at CO* laser frequencies in the 9- and IO-pm bands, and on the basis of this to identify the most suitable absorption lines for performing photoacoustic monitoring of ozone. As a further aim, we study the possibility for determining high accuracy collision broadening parameters by using laser photoacoustics with a COZ waveguide laser as radiation source. Section 2 describes the experiments, Section 3 deals with the analysis of the photoacoustic spectra, Section 4 contains results of pressure broadening studies, and Section 5 contains the conclusion.

0

0 0.04

10

I

’

6

!

i

4

0 i,

‘:

I

6 2

-

/ I

-200

;

\

’

‘.

-100

Frequency

0

100

200

(MHz)

FIG. 2. The top frame shows the CO2 laser power before and after the photoacoustic cell (Detectors D I and D2, respectively). The center frame shows the absorption coefficient as deduced from the transmission data and Beers law, and the bottom frame shows the photoacoustic amplitude.

422

SOKABE ET AL. 9R36

Offset

(MH5)

FIG. 3. Photoacoustic signatures for ozone, measured on nominally 1% ozone in nitrogen at 10 mBar and 296 K.

2. EXPERIMENTAL

DETAILS

A schematic diagram of the experimental setup is shown in Fig. I. The CO? laser is electrically pulsed at a repetition rate of 730 Hz, and delivers pulses with a peak power up to about 30 W and a duration of about 10 psec. It can be tuned single line and single mode over the entire free spectral range of 450 MHz in about 80 lines. Details of its construction as well as the conditions required for single line and single mode operation have been reported previously (18, 19). The 16-cm long photoacoustic cell is an open organpipe resonator, provided with a Briiel and Kjazr type 4165 microphone at the center. The resonator is mounted in a 30-cm long Teflon cell, connected to a reservoir, a gas manifold, and a turbomolecular pump system, and the pressure is measured by a Vacutec capacitance pressure gauge. For the purpose of laser control and data analysis, a permanently sealed reference photoacoustic cell containing CO2 at a pressure of 5 mBar is inserted in series with the main cell. The raw data consist of phase and amplitude of the photoacoustic

PHOTOACOUSTIC

SPECTROSCOPY OF 0,

Offset

(MHz)

FIG. 3-Continued

423

SOKABE ET AL.

424

011 .

0.001 ABSORPTION

0.01 COEFFlCl

0.1 ENT( l/cm )

FIG 4. Photoacoustic response normalized to the laser power at the center of the cell, vs peak absorption coefficient as deduced from Beers law and transmission data. Different data points correspond to different lines and concentrations.

response as a function of laser tuning in each of the accessible CO* lines. The amplitude is normalized with respect to the laser power at the exit of the cell, or, for strongly absorbing lines, to the power at the center of the cell as given by the geometric mean of the power at the entrance and the exit. A linearized frequency axis is generated by an automatic algorithm. Data are recorded by scanning the length of the laser resonator over three free spectra1 ranges by a piezoelectric transducer. As a first step, a second-order transformation is generated which transforms the piezo voltage axis into a linear displacement axis by requiring periodicity of spectral features of the laser power profile and the reference photoacoustic signal. Zero offset is defined by the center of the CO2 absorption in the reference cell, and the span is determined from the free spectral range, corrected for 4% frequency pulling. Finally, residual non-linearities are removed by an additional second-order transformation which leaves zero offset unaffected, while forcing the span to be symmetric. Offsets determined by this procedure have been compared with offsets reported from Lamb dip spectroscopy for a large number of lines in ethylene, and agreement was found to within about t_5 MHz (20). Data are presented as graphs showing the normalized photoacoustic amplitude as a function of frequency, and all data recording and processing is performed by an AT personal computer. The spectrometer is identical to that used in previous work on ethylene, and further information about technical details, as well as about the algorithms used in the data processing, are found in Ref. (20). Ozone was obtained from an electrical discharge through dried oxygen. The discharge products were passed through a glass trap containing silica gel cooled by dry ice to 195 K. Ozone adsorbed to silica gel was then distilled in vacuum to get ozone droplets in a second trap, maintained at 77 K by liquid nitrogen. After having distilled an appropriate amount, the two traps were disconnected, the ozone containing trap was evacuated to 10e6 mBar with a turbomolecular pump, and the ozone was allowed to sublimate into the empty cell and reservoir, resulting in a final pressure of about 2

PHOTOACOUSTIC

425

SPECTROSCOPY OF Or TABLE I

Data for All Observed Lines co2

wavenumber

offset

line

cm-’

MHz

PA mV/W

AUL MHz

Assignment .I”

I<.”

K,”

J’

(lOO)-

VI

Atlas K,’

K,’

offset

s

note

(000)

9R36

1087.9483

-183

0 43

25

I.1

3

11

14

2

12

-186

2.16C22

9R32

1085.7655

-32

0 II

:34

2s

2

24

25

1

25

-24

9.25E.23

9R28

1083.4788

168

0 24

:tn

39

9

31

40

8

32

153

1.91E22

9R24

1081.0874

-12.5

0 24

23

I!1

4

16

19

3

17

-132

2.133-22

9R20

1078.5906

-1x3

1141

24

1x

i

11

19

6

14

-210

3 89E22

9R18

1077.3025

176

0.04n

:x7

:3ti

2

34

35

3

33

171

1 OlE22

9R14

1074.6463

16i

0.064

30

31

5

2i

31

4

28

lil

1 OlE22

9R06

1069 0141

228

0.16

38

Xi

3

25

26

2

24

198

107622

a

21

9

13

22

8

14

252

4 80622

a

1.2lE22

9R04

1067 5391

-224

OOGi

29

.4-l

I

43

43

2

42

-324

9PO6

1058.9488

-50

026

23

Iti

IO

6

17

9

9

-57

3.68E-22

9P16

1050.4413

-248

0.11

30

28

9

8

20

-258

1.673-22

1928

b

Nofe. The second column gives the wavenumbers of the CO2 laser line centers, and the following three columns contain the results of the fit to Voigt profiles. The offset and the line strength S given in columns 12 and 13 are in units of MHz and cm-’ /mol cm-*, and are taken from Ref. ( 17). (a) Two lines contribute to the photoacoustic signal, whereas the fit assumes a single line. (b) Line center outside the tuning window. (c) Overlapping lines from vgand 2~ + “3.

mBar. As a next step, nitrogen was admitted into the system, so as to produce a 1% mixture of ozone in nitrogen, and the mixture was allowed to homogenize for 30 min. Then the reservoir was sealed from the cell, the cell was evacuated, and finally filled to the typical operating pressure of 10 mBar. 3. PHOTOACOUSTIC

SPECTRA

A typical recording of the CO2 power level before and after the photoacoustic cell is shown in Fig. 2 (a) together with the photoacoustic amplitude (b), normalized to the laser power at the exit of the cell. The photoacoustic spectra of ozone obtained in the 9R, 9P, and 10R branches of the COz laser are shown in Fig. 3. All spectra refer to 1% ozone in nitrogen at a total pressure of 10 mBar, and at a temperature of 296 K. A total of 42 ozone lines are observed, in 30 different CO? windows. 3.1. Photoacoustic Calibration The photoacoustic response depends in a complicated way on the acoustical properties of the cell, as well as on the absorption coefficient of the molecule and on molecular dynamic effects (21). For sufficiently strong lines, however, it is possible to calibrate the response by determining the absorption coefficient directly from the

SOKABE

426

ET AL.

TABLE I-Continued co2

wavenumber

offset

line

cm-’

MHz

PA mV/W

AVL MHz

.I”

K.“.

K,”

(001) -

“3

9R04

1067.5391

Atlas

Assignment J’

Ka’

K,’

s

o&t

(000)

-56

0.052

20

62

1

61

63

1

62

87

0.034

32

65

2

64

66

2

65

105

4.47E23 4.00E22

-39

9.13E23

206

0.27

50

8

42

51

8

43

204

20

23

1

23

24

1

24

-183

3.30E20 l.llE22

9PO8

1057.3002

-177

19.5

9PlO

1055.6251

-203

0.085

18

29

1

29

29

1

28

-219

9P12

1053.9235

-113

11.8

28

16

4

12

17

4

13

120

3.11E20

9P14

1052.1956

-68

1.94

25

23

11

13

24

11

14

-72

3.35E21

9P30

1037.4342

-12

4.12

28

16

8

8

16

8

9

-18

7.20E21

9P34

1033.4881

-141

0.25

21

16

1

15

16

1

16

-150

2.80E22

9P36

1031.4775

-41

0 35

21

36

8

28

36

8

29

-45

3.43E22

9P40

1027.3822

12

4.10

26

12

9

3

11

9

2

12

4.92E21

21

49

13

37

48

13

36

-69

5.73E23

3

58

84

6.853-23 5.71622

lOR22

977.2139

-71

0.094

lORl0

969.1395

85

0.050

15

62

3

59

61

47

52

4

48

53

4

49

-339

9R12

1073.2785

-279

0.085

9P18

1048.6609

444

13.8

33

9

5

5

10

5

6

381

1.86620

9P28

1039.3694

-481

13.3

27

16

5

11

16

5

12

-528

5.12621

9P34

1033.4881

347

0.82

24

24

10

14

24

10

15

315

2.03E21

9P36

1031.4775

-538

8.5

26

10

6

4

9

6

3

-648

1.30620

9P38

1029.4421

-259

120

44

14

0

14

13

0

13

-285

4.03E20

240

0.32

29

51

7 _

45

50

7

44

249

4.32E22

lOR32

983.2522

note

partially depleted power profile at the exit of the cell. Fig. 2 (c) shows the line profile as derived in this way, and Fig. 4 shows the measured peak photoacoustic response plotted against the peak absorption coefficient as determined by this procedure for a number of different lines. Assuming the contribution to the excitation of the acoustic resonator from an element dz at position z in the resonator to be proportional to the laser power at that location and to the amplitude of the pressure oscillations in the fundamental mode, the unnormalized acoustic signal will be proportional to the integral

s diL

u-

d

cyli,eXp(

-aZ)Sill

(1)

where Zinis the laser intensity at the entrance of the cell, LYis the absorption coefficient, and the acoustic resonator of length L is assumed to be centered in a cell of length L + 2d. Normalizing the acoustic response with respect to the intensity

PHOTOACOUSTIC

427

SPECTROSCOPY OF 0,

TABLE l-Continued co2

wavenumber

offset

cm-’

MHz

line

“2 + “3 +

9P20

u, +

MHz

J”

K.”

K,”

J’

Atlas K,’

Kc’

&et

s

-197

0.31

17

30

3

27

31

3

28

-204

5.53E22

-39

0.19

20

33

3

31

34

3

32

-36

4.093-22

-178

1 00

27

13

1

13

14

1

14

-180

1.34E21

(101)

“i

lOR24

978.4723

234

0083

lOR.32

983.2522

-151

0 090

2v3 +

Assignment

27

-

(100)

40

10

30

39

10

29

225

8.02623

23

6

18

22

6

17

150

7.94E23

“3

(002) -

(001)

10R28

980.9132

-78

0.087

40

33

8

25

32

8

24

-93

6.25E23

lOR22

977.2139

-71

0.094

21

39

12

28

38

12

27

-63

2.073-23

2

34

126

2.16E23

9

14

3

1.61E23

18-l&16

0 9P16

Y3

isotope

1050.4413

(001) + 136

0017

45

35

2

16-18-16 isotope

lOR32

note

(Oil)-(010)

1035.4737

*3 -

mV/W

Avr

“2

1046.8543

9P32

PA

983.2522

33

(001) -11

0010

23

9

(000) 36

-

15

c

(000) 22

unidentified 9R28

1083.4788

40

0 050

31

9R20

1078.5906

42

0.073

32

(2) in the middle of the cell, IOU1 being the laser intensity at the exit of the cell, results in the expression

unorm

-

(3)

This expression is shown as the solid line in Fig. 4. It is seen that if the normalized photoacoustic response is below about 10 mV/ W we can assume a linear dependence on CY,with a calibration constant of 0.0033 cm-’ /( mV/ W). For larger photoacoustic signals, deviations from linearity are expected, but in our experiments this occurs for a few of the strongest lines only.

428

SOKABE ET AL.

LINESTRENGTH

0

vc

0

v3

.

VI +v3

-

vj

.

v2+v3

-

!I2

l

i’V3-v3

0

Y

0

v3

7

118-16-161 (16-18-161

3

(cti1/mol.cm2)

FIG. 5. Measured peak photoacoustic signal vs line strength as given in Ref. (17). Thin solid line is predicted photoacoustic signal, using the calibration constant determined from Fig. 4.

3.2. Line Projiles The absorption coefficient for an unsaturated transition from state i to state j is given by

-200

-100

Offset

0

100

200

(MHZ)

FIG. 6. Spectra at different pressures for the Q( 16, 8, 9) + ( 16, 8, 8) transition. The amplitudes are individually scaled.

PHOTOACOUSTIC

SPECTROSCOPY OF OI

429

30 20 10 PRESSURE ( mBar ) FIG. 7. Pressure broadening data for the vj( 16, 8. 9) + (16, 8, 8) and (24, 11, 14) + (23, 11. 13) transitions. The correlation parameter for a least square fit is 0.998 for both transitions.

a(u)

=

~~+Lijl*

g”(v) 2

ECii

(4)

where AN is the population difference between the levels connected by the transition, u is the applied frequency, and pUis the transition dipole moment. The area normalized lineshape function gv ( v), denoted the Voigt profile, is given by g,(u) = -

1

-

ReW( x + @)

(5)

Au,

W(z)

= exp( -z’)erfc(

-iz)

x=f-$iz

16) (7)

The variables x and y represent the detuning from the line center vo and the degree of homogeneous broadening, both expressed in normalized units, AU, and Au, are the HWHM Doppler and Lorentz width, respectively, and erfc is the complementary error function. The expression for LYmay be written as (y(V) = SNogv(v),

(9)

where S represents the line strength, and No is the number density of absorbing molecules. In the homogeneously broadened limit this reduces to

SOKABE ET AL.

430

V3 24,ll,l4 -23,11,13

.

-80 0

I

I

IO

20

1 30

I 40

PRESSURE ( mBar ) FIG. 8. Pressure shift for the vg( 16, 8, 9) + (16, 8, 8) and (24, 11, 14) + (23, I I, 13) transitions. The correlation parameter for a least square fit is 0.556 for the former and 0.701 for the latter transition.

a(u) = slv,

42 n- (u -

1

uo)’ + (Av~)~ .

(10)

In the data bases AFGL (22, 23) and Geisa (24), as well as in the ozone atlas ( 17), the line intensities S are expressed in units of cm-‘/mol cmM2, and in the homogeneously broadened limit, the absorption coefficient in cm-’ at the wavenumber u is then obtained from Eq. ( 10) by inserting the number density NOin units of mol/cm3 and the homogeneous HWHM linewidth Avr in cm-‘. At our operating pressure of 10 mBar, we have mixed broadening, and the rigorous expression for the lineshape function, given in Eq. (5), must be used. The observed spectra are analyzed by fitting the complex amplitude, the Lorentz width, and the offset for the appropriate number of Voigt profiles, adding a frequencyindependent term to account for the background. The observed photoacoustic signals as given in column 4 of Table I are the peak amplitudes resulting from the fits. The Lorentz widths given in column 5 are the values determined by the fitting procedure. Since the data refer to a single low pressure only, and since in many cases only part of the line profile is visible, these values should be taken as guidelines only. A more accurate study of the Lorentz broadening was carried out for two selected lines, as reported in Section 4. 3.3. Assignments All results are summarized in Table I. The offsets given in column 3 are determined with an uncertainty of about +5 MHz for lines which are centered inside the tuning windows (20)) whereas significantly larger errors are expected for lines outside the tuning windows. The assignments to the various vibration bands, as well as the rotational assignments, are performed by comparing with parameters from the atlas of ozone lines ( 17). Offsets from the atlas are quoted in column 12, and line intensities in column 13.

PHOTOACOUSTIC

SPECTROSCOPY

OF O3

431

Assignments are based on offset as well as on line strength. For all lines inside the tuning windows, the measured peak photoacoustic signal is plotted in Fig. 5 vs the line intensity as given in Ref. ( 17), and the data points are seen to cluster around a straight line with slope 1 as required. The scatter is believed to reflect mainly the dayto-day uncertainty in establishing the nominal 1% ozone concentration used in our experiments. When using the linestrength S in Fig. 5, it is implicitly assumed that all lines have the same line profile. Strictly speaking, this is not the case, since the Doppler width as well as the homogeneous linewidth will vary from line to line. However, the effect of this is smaller than that of variations in concentration, and since moreover the homogeneous width is not known with sufficient accuracy for all the lines, we have decided to neglect variations in broadening in Fig. 5. Using the calibration constant 0.0033 cm-‘/(mV/W) derived from Fig. 4, we can convert the line strengths given in Ref. ( 17) to photoacoustic signals, and assuming for all lines a homogeneous HWHM width of 23 MHz at 10 mBar, using results of Section 4, and a Doppler HWHM width of 28 MHz, we arrive at the straight line drawn in Fig. 5. Thus, our measured line strengths seem to be systematically too large by a factor of 3. This points toward a systematic error in our establishment of the 1% concentration, but we have not yet been able to identify the source of the discrepancy. The uncertainty quoted for the offsets in Ref. ( 17) is 15 MHz. For 28 lines inside our tuning windows, excluding two lines in 9 R20 and 9 R04, the standard deviation between our measured offsets and those reported in Ref. ( 17) is 9 MHz. 4. PRESSURE

BROADENlNG

The homogeneous width obtained by fitting Voigt profiles to the observed photoacaustic spectra at a pressure of 10 mBar and a temperature of 296 K are summarized in Table I. More extensive pressure broadening measurements were performed at pressures between 5 and 35 mBar on two selected lines from the v3 band, (24, 11, 14) + (23, 11, 13) at offset -68 MHz in 9Pl4, and (16, 8, 9) f (16, 8, 8) at offset -12 MHz in 9P30. Spectra at several pressures is shown for one of the lines in Fig. 6, and the measured homogeneous width (HWHM) for the two lines is shown as a function of pressure in Fig. 7. The broadening coefficients resulting from a least square fit are 2.25 and 2.3 1 MHz/mBar, corresponding to 0.076 and 0.078 cm-‘/atm-’ , respectively. In all of these measurements the ozone concentration was 0.5% or less, and the broadening coefficients therefore correspond to essentially pure nitrogen broadening. A large body of information concerning nitrogen broadening of ozone has been reported in the literature. The result of measurements by FIIR and diode laser spectroscopy for a large number of lines have been published by Ref. (13), and Fig. 5a of this reference also gives a good summary of previously published data and the agreement with the theoretical calculations of Ref. ( 15). Experimentally as well as theoretically, the broadening rate is found to increase by about 7% as J decreases from 25 to 5, the J dependence being stronger for J below 5, owing to the quasi-resonant nature of the collisions, and leveling off at J higher than 25. At J values corresponding to the lines we have studied, the experimental broadening rate reported in Ref. ( 13) is about 0.077 cm-‘/atm-’ . However, since the overall J-dependence is superimposed with irregular excursions of up to lo%, depending on K, and Kt,, a comparison at a level of accuracy matching our experimental uncertainty would require data for the same transitions, and unfortunately this is not available. Despite this, our results confirm the overall

432

SOKABEET

AL.

conclusion of Ref. ( 13) that the theoretical calculations lead to broadening rates which are about 10% smaller than what is found experimentally. Figures 8 (a) and (b) show the fitted line center positions for the two transitions as a function of pressure, and a least square fit yields pressure shifts of -0.24 MHz/ mBar or -0.0080 cm-‘/atm-’ for (24, 11, 14) + (23, 11, 13) and +0.048 MHz/ mBar or +0.0016 cm-‘/atm-’ for (16, 8, 9) + ( 16, 8, 8). For the lines reported in Ref. (I_?), the pressure shifts range between +0.003 and -0.008 cm-‘/atm-r , a vast majority having having shifts between zero and -0.003 cm-‘/atme . 5. CONCLUSION

A total of 42 ozone absorption lines belonging to the vl band, the u3 band, and various hot bands have been observed in 450-MHz tuning windows around 30 individual COz laser lines, and 33 of these lines have been assigned to specific vibrationrotation transitions. Nitrogen collision broadening rates and pressure shifts have been determined for two u3 lines, and the results for the broadening rates are consistent with previously published experimental data for this band, but are about 10% higher than predicted by theory. For spectrally resolved COZ laser photoacoustic monitoring with high sensitivity under conditions of COZ interference, strong absorption lines are required with line center within the 450-MHz tuning windows, but preferably well separated from the potential COZ interference at zero offset. From the observed spectral signatures, several such lines have been identified, the most promising candidates being in 9PO8 at -177 MHz, and in 9P12 at -113 MHz. Isotopomers of ozone are present in their natural terrestrial abundance of 0.002 for the symmetric I60 “0 160 and 0.004 for the asymmetric I80 I60 160. One v3 line has been identified for each, both being relatively strong, and well separated from lines of I60 I60 r60. The line of the asymmetric isotopomer has an offset of 136 MHz and is well suited for monitoring, whereas the - 11 MHz offset for the line of the symmetric isotopomer will lead to problems for samples containing COZ . ACKNOWLEDGMENTS One of the authors (NS) expresses his gratitude to the staff of the Physics Laboratory at the H. C. Orsted Institute for giving him the chance to stay at the Institute and to do the present work. He is also much indebted to the technicians at the local workshop.

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