Diode laser spectroscopy in the ν14 band of benzene

INFRAREDPHYSICS &TECHNOLOGY ELSEVIER

Infrared Physicsand Technology37 (1996) 193 198

Diode laser spectroscopy in the Y14 band of benzene J. Waschull, B. Sumpf, Y. Heiner, H.-D. Kronfeldt Optisches Institut der Technischen Universit?it Berlin, Sekretariat PN 0-1, Hardenbergstrafle 36, 10623 Berlin, Germany

Abstract

The benzene spectrum in the v~4 band near 1023 cm -1 was investigated by means of a pulse driven diode laser spectrometer with a resolution of about 6 x 10 -4 cm -l. We give individual absolute line strengths and air-broadening coefficients of 9 lines. The air-broadening coefficients vary between 0.17 cm-t/atm and 0.23 cm-~/atm. These data we used to estimate detection limits for a diode laser benzene detection device. High resolution spectra of toluene were measured in the same wavenumber region.

1. Introduction

Tunable diode laser absorption spectroscopy (TDLAS) is a suitable tool to investigate molecules with a small number of atoms which exhibit a discrete absorption line structure. This method shows its capability for quantitative measurements of trace components of car exhausts for years. The T D L A S system presented by Riedel et al. [1] shall be quoted here exemplarily. With their system formaldehyde, ethene, and carbon monoxide were measured with a time resolution better than 2 seconds. In contrast to these gases, however, benzene (C6H6) is a heavy molecule with a very dense spectrum. Therefore it was interesting to study at first the feasibility of a benzene detection with TDLAS. The aim of this work is to place spectroscopic data at disposal which allow to estimate the device requirements for benzene detection in car exhausts.

For the investigations the v~4 band was chosen since superpositions of the benzene lines with spectra of other exhaust components like CO2 and H 2 0 are widely avoided. Unfortunately the Via band has the smallest integrated intensity of all four infrared active benzene fundamentals. Up to now there exist only a few experimental high resolution spectroscopic data concerning this band. Pliva and Johns [2] scanned the Vl4 band with a high resolution Fourier transform spectrometer. They give line positions and relative strengths of about 2800 lines. Junttila et al. [3] achieved the highest accuracy of rotational constants by investigating a section of the v14 band with sub-Doppler resolution. Theoretical calculations of absolute line strengths were carried out for all four fundamentals by DangNhu and Pliva [4]. Recently we were able to present the first results of self-broadening measurements of single lines of benzene [5]. To calculate the absorption coefficients of benzene the knowledge of experimental absolute

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194

J. Waschull et al./lnfrared Physics and Technology 37 (1996) 193-198

line intensities and proper broadening coefficients is required. Since N 2 is the dominating fraction in exhausts as well as in air, air was chosen as collision partner for convenience.

2. Experiment The benzene spectra were recorded with a self-built pulse driven two channel diode laser spectrometer whose scheme is shown in Fig. 1. The diode laser was mounted in a liquid helium flow cryostat. After mode selection by the monochromator the laser beam is divided into two parts, i.e. the light passes through a confocal 6talon with a free spectral range of 0.01 cm-m and through a Herriott cell [6], optical path length L = (472 + 2) cm, with a gas mixture system and a capacitive pressure gauge. Two HgCdTe detectors with signal amplifiers were applied for intensity detection, an oscilloscope for digitisation, and a PC for data storage. Benzene was taken from above a stock of liquid C6H6 . Laboratory air without further purification was used for the air-broadening exper-

iments. With our TDLAS system we achieve a resolution of about 6 x 10-4cm -I. To improve the signal-to-noise ratio 30 laser pulses were recorded and then linearised by using the corresponding 6talon fringes for the calculation of relative wavenumber scales. Before the accumulation the spectra were shifted against each other according to the maximum of a selected absorption line to level out temperature fluctuations of the laser. The conversion of the relative wavenumber scale into an absolute one was done by using the well known line positions of carbonylsulfide (OCS) [7]. Measurements of the empty cell providing the signal I 0 were carried out additionally and used to calculate the absorption

- I n (I/Io) = ~L,

(1)

where I is the intensity with the absorbing gas, ct is the absorption coefficient of the gas and L is the optical path length. Analytical representations of the Voigt profile as given by Whiting [8] were fitted to the spectra under investigation. We used a (1,10)evolution strategy implemented in the toolbox

Cryostat with Diode Laser Optical System ~

Optical System

with OAE- Mirror[,1 ~ ~ l j

j

-

,

I Laser Control Unit --~ Preamplifier ~ ' -

Confocal Etalon

HgCdTe-Detector

1 I

HerriottMultipass - Cell

H

--~ Preamplifier ~-

~

HgCdTe-Detector

Gas Mixture System ]

Digital .Storage Oscilloscope

Fig. 1. Scheme of the experimental set-up; OAE-off-axis elliptical mirror.

E

J. Waschull et al./lnfrared Physics and Technology 37 (1996) 193-198

OPTIMIZE [9] which allows the fit of up to nine lines simultaneously. Since the benzene spectrum is very dense sections of only some hundredths wavenumbers were chosen to facilitate a single line analysis. The regarded intervals contained 27 lines but only 9 of them could be assigned to rovibrational transitions using the data by Pliva and Johns [2]. The following analysis is based on these assigned lines.

3.

Results

3.1. Line strengths

For a single absorption line with the position ~0 the absorption coefficient at the wavenumber q can be written as ~(~) = q~(~7- ~70,Afv) Spx ,

(2)

where S is the absolute line strength, p+ the pressure of the absorber, q~ the Voigt line-shape function normalised to area 1 and A~7v the Voigt half-width. We fit this Voigt function • to our experimental data - ln(I/Io), - In

(I/Io)

= q~a.

(3)

195

The factor tr is the product of the line strength, the pressure and the path length: -

a -

In (I/Io)

(4)

- SpxL.

The values of tr were determined for different pressures of pure benzene and then drawn versus Px (see Fig. 2). Here the partial pressure was equal to the total pressure in the absorption cell. The slope of the regression divided by the cell length gives the line strength S. We compared our data with the relative line strengths given by Pliva and Johns [2] from their Fourier transform investigations of the benzene Vl4 band. Thus, we were able to convert their values into absolute line strengths with a conversion factor of (3.5 + 0 . 6 ) x 10 -6 cm-2/atm. 3.2. A i r broadening

To determine air-broadening coefficients air was added step by step to approximately 1 Torr benzene up to a total pressure of about 10 Torr within one series of measurements. After spectra accumulation the curves were fitted as described above.

12 10 "7

E O

c?

O v--

8

"7

6

o

,r--

4 1

2 / /

0

"

0

I

I

I

I

1

2

3

4

p / 10 .3 atm Fig. 2. Pressure dependence of the fit parameter tr of the benzene line (46,38) ~-- (47,39) at 1022.9907 cm -1. To estimate the line strength from these pure benzene measurements the slope of the regression has to be divided by the path length of 472 cm.

0

0

I

I

I

P

I

I

I

2

4

6

8

10

12

14

Ptot / 1 0 -3 a t m

Fig. 3. Decrease of the fit parameter a of the benzene line (46,38)*--(47,39) at 1022.9907cm -~ during the time o f the air-broadening measurements (total pressure Ptot was increased stepwise). I - p u r e benzene; D - b e n z e n e with air.

196

J. Waschull et al./Infrared Physics and Technology 37 (1996) 193-198

D u r i n g the a i r - b r o a d e n i n g m e a s u r e m e n t the value tr (see Eq. (4)) was expected to be c o n s t a n t . In the consecutive scans with increasing t o t a l pressure we o b s e r v e d a decrease o f tr d o w n to a p p r o x i m a t e l y 6 0 % o f the initial value over a time o f a b o u t 8 m i n (Fig. 3). T h a t was a t t r i b u t e d to a d e p o s i t i o n o f benzene at the cell walls. T h e tr values were used to recalculate the benzene pressures: o-x

Px = - - P , ,

(5)

ai

where tr~ a n d trx are the line fit p a r a m e t e r s for p u r e C6H 6 at the pressure p~ a n d for the b e n z e n e - a i r mixture, resp. K n o w i n g the s e l f - b r o a d e n i n g coefficients 7 ~ [5] a n d the real benzene partial pressure Px we d e t e r m i n e d the c o n t r i b u t i o n o f the air b r o a d ening A~Tair to the fitted L o r e n t z i a n line w i d t h A~7L (HWHM): AlTair= AVL

- -

Px ~)self"

regressions to calculate the values a n d limits o f a c c u r a c y n o t e d in the table.

4. Estimation

fimits

T o use the a d v a n t a g e s o f the high r e s o l u t i o n s p e c t r o s c o p y for a sensitive gas d e t e c t i o n single spectral lines are necessary. A s s h o w n in Fig. 4 this c o n d i t i o n is met only up to a total pressure o f a b o u t 10 T o r r due to the very dense s p e c t r u m o f benzene. T o assess the m i n i m u m r e q u i r e m e n t s to a d i o d e laser benzene d e t e c t i o n device we calculated a b s o r p t i o n coefficients o f benzene using a total

1.2 1.0

(6)

Since this p r o c e d u r e includes m a n y fit p a r a m e t e r s it leads to larger uncertainties for the air b r o a d e n i n g ( 1 1 - 7 0 % ) t h a n for the s e l f - b r o a d e n i n g ( 8 - 2 5 % ) . Table 1 summarises the results for the line strengths a n d the air b r o a d e n i n g coefficients. F o r c o m p a r ison we a d d o u r results o f the s e l f - b r o a d e n i n g m e a s u r e m e n t s p u b l i s h e d in Ref. [5]. T o estimate the uncertainties we used one-times G a u s s s t a n d a r d d e v i a t i o n for the line p a r a m e t e r s . A s s u m i n g n o r m a l d i s t r i b u t i o n o f the m e a s u r e m e n t errors a n d i n d e p e n d e n c e o f the fit p a r a m e t e r s these intervals were used as weights in the linear

of detection

0.8 o

"

0.6 i

0.4 0.2 0.0 1022.97

i

I

i

1022.99

I

1023.01

i

I 1023.03

i

I 1023.05

Wavenumber / cm 1

Fig. 4. Section of the % band of benzene measured with TDLAS. Bold line-1 Torr benzene; fine line-air added up to 10.6 Torr.

Table 1 Line strengths, self- and air-broadening coefficients in the % band of benzene (PP branch); line positions and quantum numbers as given by Pliva and Johns [2] Line position [cm- t] 1022.9852 1023.0904 1022.9512 1022.9963 1022.9798 1023.0987 1022.9907 1023.1053 1022.9566

Quantum number J"

K"

Self-broadening coefficient [5] 27 [cm '/atm]

42 42 43 44 45 45 47 47 48

7 9 13 20 26 28 39 41 45

0.40 + 0.10 0.52 +__0.06 0.57 _+_0.13 0.54 + 0.07 0.56 _+0.11 0.52 ___0.04 0.54 + 0.05 0.57 _ 0.07 0.55 __+0.05

Air-broadening coefficient 27 [cm- l/atm]

Line strength S at 295 K [10- 3cm- 2/atm]

0.18 + 0.05 0.18 _ 0.05 0.23 __+0.16 0.20 __+0.03 0.20 _ 0.05 0.18 + 0.03 0.19 + 0.02 0.17 ___0.05 0.17+0.09

2.6 + 0.3 1.96 + 0.15 1.7 + 0.3 3.5 + 0.3 2.6 ___0.3 2.59 _ 0.14 5.2 _+0.3 3.2 _+0.3 5.0+0.3

J. Waschull et al./lnfrared Physics and Technology 37 (1996) 193-198

pressure of 10 Torr, a mean air-broadening coefficient 2Tair of 0.18 c m - l / a t m and a line strength of 8 × 10 -3 cm-2/atm. Such line strengths appear around 1030 cm- l and 1050 cm-l. Applying a Lorentzian line-shape function the absorption coefficient in the line centre can be estimated by -

S

1

7~ ~airPtot PC6H6"

(7)

For the detection sensitivity of the diode laser system we presuppose a value of 1/10 000. Using all these parameters we found an optical path length of 45 m necessary for the detection of a benzene concentration of 1 ppm. Typical car exhaust concentrations range in the order of 25 ppm for cars without catalyst and around 3 ppm for such with catalyst [10]. The next step towards a technical application of the detection of benzene by diode laser techniques in car exhausts has to be the evaluation of the spectral regions mentioned above for the influence of interfering substances. In the wavenumber region around 1030cm-' for instance CO2 has to be considered whereas near 1050 cm -1 H 2 0 and NH3 have a remarkable influence. In contrast to the small molecules high resolution spectra of many organic substances appearing in car exhausts are unknown. Therefore, we extended our investigations on the benzene derivative toluene, C7H8, whose concentration in car exhausts is of the same order of magnitude as that of benzene (50 ppm and 2 ppm, resp.).

197

1.00

0.95

~ " ~ , ~ ~ ~ , r~'~'~

c

.9 E 0.90 F--

0.85 .

.

O.80

.

.

di~eTas~mo~es

i

1010

1020

1030

i

I

1040

1050

1060

Wavenumber / cm 1

Fig. 5. Section of the toluene spectrum measured with FTIR (resolution 0.03cm-l); 15.3 Torr toluene, path length 10cm [11]. The horizontal lines mark the modes of the laser which was used for the investigations described in this paper. The ellipse indicates the region which is given as high resolution spectrum in Fig. 6.

1.2 1.0 0.8 "C 0.6 "3"

0.4

Ill

0.2 0.0 1042.65

I

I

l

1042.75

1042.85

1042.95

Wavenumber / cm -1

5. Measurements of toluene

Toluene exhibits an absorption band centred at about 1032 cm -~ (Fig. 5). We scanned all modes of our diode laser between 1010 cm-~ and 1060 c m which are marked in Fig. 5. In contrast to benzene toluene does not show separate absorption lines. The most pronounced structure we found is shown in Fig. 6 together with the corresponding benzene spectrum. Notice, that the spectra were recorded separately at similar pressures. In car exhausts both components will occur simultaneously in comparable concentration ratios which would lead to a superposition of the spectra. This illustrates the

Fig. 6. Section of benzene and toluene spectra measured with TDLAS. Bold line--2.6 Torr benzene; fine line-2.2 Torr toluene.

problems which will occur when high resolution techniques shall be used for toluene detection. Due to the mode structure of our diode laser we cannot give, however, any statement concerning the strong and narrow toluene Q-branch peak.

6. Conclusion

We have applied a diode laser spectrometer to measure sections of the vz4 band of benzene. Line

198

J. Waschull et al./Infrared Physics and Technology 37 (1996) 193-198

profiles were fitted to small spectral regions in order to determine line strengths and line widths. The measured line strengths are the first experimental values for absolute line intensities in the v~4 band and have to be compared to calculations according to the formulas given by Pliva and Johns [2]. A mean value for the broadening coefficient and a line strength of a comparatively strong v~4 line were used to estimate detection limits for benzene in air. Measurements of toluene in the same wavenumber region showed that toluene has to be taken into consideration while using diode laser spectroscopy techniques for benzene detection in car exhausts. Since on-line detection of car exhausts is usually carried out at temperatures of about 180°C it will be interesting for further investigations to extend the line strength and broadening measurements to this temperature. To improve detection limit predictions an increasing number of interfering exhaust components has to be implemented into the calculations. For example, the high resolution spectra of ethylene and acetaldehyde which occur in exhausts in similar concentrations as benzene and toluene are unknown.

Acknowledgements We gratefully thank Prof. J. Pliva, Pennsylvania State University, USA, for sending us the complete line position and relative strength data from his F T I R investigations of the vj4 band. We thank

Dr. H. M. Heise from the Institut fiir Spectrochemie und Angewandte Spektroskopie Dortmund, Germany, for sending us Fourier transform spectra of toluene and benzene. Y. Heiner and J. Waschull acknowledge a grant from the "Deutsche Bundesstiftung Umwelt", B. Sumpf is supported by the "Deutsche Forschungsgemeinschaft".

References [1] W.J. Riedel, R. Grisar, U. Klocke, M. Knothe, H. Wolf, P. Schottka, E. Besseyand N. Pelz, Monitoring of gaseous pollutants by tunable diode lasers, in: Proceedings of the International Symposiumheld in Freiburg, Germany, 1718 October 1991 (KluwerAcademicPublishers, Dordrecht, 1992) ISBN 0-7923-1826-9. [2] J. Pliva and J.W.C. Johns, J. Mol. Spectrosc. 107 (1984) 318. [3] M.-L. Junttila, J.L. Domenech,G.T. Fraser and A.S. Pine, J. Mol. Spectrosc. 147 (1991) 513. [4] M. Dang-Nhu and J. Pliva, J. Mol. Spectrosc. 138 (1989) 423. [5l J. Waschull, B. Sumpf, Y. Heiner, V.V. Pustogov, H.-D. Kronfeldt, Self-broadening of benzene in the v~4 band, submitted to Ber. Bunsenges. Phys. Chem. [6] H.-D. Kronfeldt and J. Berger, SPIE Lens Opt. Systems Design 1780 (1992) 650. [7] A.G. Maki and J.S. Wells, Wavenumber Calibration Tables from Heterodyne Frequency Measurements, NIST special publication 821 (Gaithersburg, 1991). [8] E.E. Whiting, J. Quant. Spectrosc. Radiat. Transf. 8 (1968) 1379. [9] V. Tfirck and O. Stier, Scientific Tool OPTIMIZE 5.0, Technische Universit/it Berlin, unpublished (1994). [10] Unregulated Motor Vehicle Exhaust Components, Volkswagen AG, Research, Physico-ChemicalMetrology, Wolfsburg, Germany (1989). [I 1] H.M. Heise, private communication (1994).