Pulsed Photoacoustic Spectroscopy of I 2 and NO 2

Int J Thermophys (2012) 33:2055 2061 DOI 10.1007/s10765-012-1323-9 Pulsed Photoacoustic Spectroscopy of I 2 and NO 2 Fahem Yehya Anil K. Chaudhary Received: 30 January 2012 / Accepted: 26 September 2012 / Published online: 7 October 2012 Springer Science+Business Media New York 2012 Abstract A novel approach based on the use of λ = 532 nm pulsed radiation obtained by the second-harmonic generation from the Q-switched Nd: YAG laser and its application for excitation of acoustic modes in a specially designed multi-resonant high-q factor photoacoustic (PA) cell is reported. The cell is employed for time-domain spectroscopic studies of gaseous NO 2 and I 2 molecules. The time-resolved spectra of NO 2 and I 2 were recorded up to 8000 Hz and 26 000 Hz, respectively. It has been demonstrated for the first time that excited cavity modes which correspond to different transition lines of the I 2 molecules show different behaviors under different pressure and input laser energy. The lowest limit of detection attained with the proposed PA system for NO 2 buffered in realistic air was 14.6 ppbv. Keywords I 2 Longitudinal mode NO 2 Photoacoustic Pulsed laser Radial modes 1 Introduction Both NO 2 and I 2 are related to the ozone cycle of the atmosphere. For example, NO 2, one of the greenhouse pollutants, emitted from automobile exhaust, industrial boilers, electrical power generators, etc., play a very important role in the troposphere [1,2]. Achieving low limit detection (near ppt level) for NO 2 is an important objective in environmental pollution monitoring. F. Yehya A. K. Chaudhary (B) Advanced Centre of Research in High Energy Materials, University of Hyderabad, Hyderabad 500 046, India e-mail: anilphys@yahoo.com

2056 Int J Thermophys (2012) 33:2055 2061 There is a growing interest in laser-based analytical techniques for remote trace detection of nitrogen dioxide (NO 2 ). The photoacoustic (PA) technique has advanced to the stage at which sensing the presence of NO 2 in the atmosphere (due to its broad absorption spectrum) has become a realistic option. Likewise, the collision energy transfer from laser-excited levels is very efficient because of the long natural lifetime of the NO 2 excited levels [2 5]. It is only possible due to the presence of a strong absorption band of NO 2 at 532 nm for which the lifetime of the excited levels is longer [6,7]. Therefore, the collision energy transfer with the buffer gas leads to efficient heating of the mixture. As a result, the PA detection of NO 2 using green laser excitation is potentially a very suitable method. Likewise, iodine (I 2 ), one of the important molecules that play an essential role in ozone chemistry, features a strong absorption band in the visible range and becomes ionized. The ionized I* is hyperactive in nature as it easily reacts with O 3 to produce IO. In addition, I 2 is also one of the important constituents of the bio-system controlling the metabolic rate in the body [9,10]. In this paper, we discuss recording of the PA time-domain spectra for NO 2 and I 2 as well as the saturating behavior of radial and longitudinal modes in a resonant PA cavity. The strong signal from the radial mode and a good signal-to-noise ratio (SNR) were used to achieve a lower detection limit for NO 2 gas. The same technique was extended to measure the characteristic signature of I 2 in the vapor phase. Only two strong peaks were observed in the NO 2 spectrum within the 8000 Hz range, while more than five absorption peaks between 2000 Hz and 26 000 Hz frequency were observed for the first time in the case of I 2. The pressure-dependent study of these peaks provides interesting information about the energy exchange between the excited modes. 2 Experimental Apparatus and Procedures Figure 1 shows a schematic experimental layout for recording of PA spectra of NO 2 gas and I 2 vapor. The second-harmonic radiation (i.e., 532 nm) of 7 ns pulses obtained from a Q-switched Nd: YAG laser was used to excite gaseous samples. The sealed PA cell (internal diameter of 9.0 cm, an average length of 15 cm, and made of stainless steel [7]) was used here to record the spectrum of NO 2. A time-dependent PA signal is detected with the pre-polarized microphone (placed at the center of the PA cell) with a responsivity of 50 mv Pa 1 (20 Hz to 70 khz, BSWA, China). The output PA signal from the microphone is fed to the preamplifier connected to the 200 MHz oscilloscope (Tektronix, USA). The USB/GPIB interface was used for data acquisition through a boxcar integrator (Stanford Instruments Inc., USA). Data analysis was performed using a data acquisition program made in LabView. The quality factor Q of the PA cell is defined as Q = w 0 w, (1) where w 0 is the resonance frequency and w is a full width at half maximum intensity.

Int J Thermophys (2012) 33:2055 2061 2057 Fig. 1 Experimental setup used in this study 3 Results This section is divided into two parts: the first discusses the time-domain PA spectrum and the low detection level of NO 2. On the other hand, the second part is concerned with a time domain PA spectrum of the I 2 molecule as well as the study of the pressuredependent behavior of different excited modes. 3.1 Time Domain PA Spectrum of NO 2 The PA spectrum of the NO 2 gas buffered in realistic air recorded at 532 nm is shown in Fig. 2. The first radial mode appears at 4270 Hz together with the second longitudinal mode at 2100 Hz. 3.1.1 Energy Dependence of PA Signal Below the saturation level, the relationship between the amplitude of the PA signal and incident laser power is linear as expected from theory. The experimental values for PA signals obtained for the second-order longitudinal (002) and first-order radial (010) modes are plotted in Fig. 2; the presence of saturation is self-evident. The strong growth in the strength of the PA signal of the radial mode (in comparison to other modes) is mainly due to a lower viscosity and lower losses. 3.1.2 Minimum Limit of Detection (S min ) of PAS System at 532nm The strength of the recorded background signal in air at 4.3 mj (input energy of the laser) is of the order of 1.3 µv. Figure 2 shows that the maximum strength of the PA signal of 2857 ppmv of NO 2 is 254 mv. Therefore, the SNR is 19.5 10 4, so that the minimum detection limit (S min )forno 2 gas in our PA cell is

2058 Int J Thermophys (2012) 33:2055 2061 Fig. 2 PA spectrum of NO 2 buffered in air recorded at varying incident laser energies 3.1.3 Q factor S min = 2857 ppm = 14.6 ppbv 19.5 104 The Lorentz fit of the experimental data for the first radial mode (Q is about 94) at 4270 Hz (maximum intensity) is presented in Fig. 3. 3.2 Time Domain PA Spectrum of I 2 The I 2 vapors were collected using a specially designed heating system. A special type of needle valve was used to control the flow of I 2 vapor at the inlet. We have tried to record the time domain PA spectrum of I 2 at room temperature and gradually increase the temperature to 50 C and found that the I 2 vapors at 50 C give better results. The PA spectrum of I 2 at 220 Torr pressure and at 25 mj input energy of the laser is displayed in Fig. 4. The inset shows the time-domain PA signal (in s), while the main frame features the frequency domain spectrum of I 2 obtained by a fast Fourier transform (FFT). The spectrum extending from 2000 Hz to 26 000 Hz shows the presence of multimode excited lines of I 2 molecules due to multiple transitions between B-X bands. The strongest peak of I 2 is found at 14 650 Hz. 3.2.1 Pressure-Dependent Behavior of Iodine (I 2 ) Vapor The presence of multiple lines in the time domain spectrum of I 2 molecules is due to strong absorption of this gas in the visible region. Therefore, it is quite interesting to study the pressure-dependent behavior of different excited modes of I 2. This helps

Int J Thermophys (2012) 33:2055 2061 2059 Fig. 3 Lorentz fit of the experimental data for the first radial mode (4.27 khz) Fig. 4 PA spectrum of I 2 at 220 Torr to understand their dynamics at different pressures. Figure 5a c shows an interesting rise and decay behavior of three different excited modes of I 2 molecules as a function of different pressures. Figure 5a represents the pressure dependence at 2 khz. The strength of the PA signal begins to decrease at pressures exceeding 350 Torr while Fig. 5b and c shows the growth and decay behavior of modes at 4 khz and 14.65 khz, respectively. One observes that the rise for a 4 khz mode occurs parallel to a decay at 14.65 khz due to the exchange of energy among these modes. The different behaviors of the excited modes with pressure is reflected by the effect of multi-transitions in

2060 Int J Thermophys (2012) 33:2055 2061 Fig. 5 Behavior of some modes: (a) 2 khz, (b) 4 khz, and (c) 14.6 khz as a function of pressure

Int J Thermophys (2012) 33:2055 2061 2061 re-vibrational levels in X-B bands of I 2 molecules at 532 nm for eigenmodes of the PA cell [9 11]. 4 Conclusions A highly sensitive resonant PA cell was successfully designed, fabricated, and used to record time-resolved spectra of NO 2 and I 2. The saturation behavior of the PA signal for NO 2 at 6.1 mj level of input laser energy was also demonstrated and compared to different types of excited radial and longitudinal modes. The estimated value of the Q factor at the resonance frequency of 4270 Hz is 94 and the limit of detection (S min ) for NO 2 gas is 14.6 ppbv. In addition, a time-domain PA spectrum of I 2 vapor extends to 26 000 Hz. The exchange of energy between different excited modes of I 2 molecules with respect to pressure is clearly demonstrated between 4 khz and 14.65 khz frequencies. Acknowledgments We are grateful to the referees for their useful suggestions and enlightened comments. The partial financial assistance by DBT Project No. BT/PR11226/MED/32/58/2008, DST (SERC) No. LOP-13, and DRDO, Ministry of Defense Govt. of India is gratefully acknowledged. References 1. T.E. Graedel, P.J. Crutzen, Sci. Am. 28, 261 (1989) 2. M.W. Sigrist (ed.), Air monitoring by laser photoacoustic spectroscopy, in Air Monitoring by Spectroscopic Techniques, in the Chemical Analysis Series, vol. 127 (Wiley, New York, 1994) 3. M.A. Gondal, M.A. Dastageer, J. Environ. Sci. Health A 45, 1406 (2010) 4. V. Slezak, Appl. Phys. B 73, 751 (2001) 5. A. Miklos, P. Hess, Rev. Sci. Instrum. 72, 1937 (2001) 6. V. Slezak, G. Santiagob, A.L. Peuriot, Opt. Lasers Eng. 40, 33 (2003) 7. F. Yehya, A.K. Chaudhary, J. Mod. Phys. 2, 200 (2011) 8. F. Yehy, A.K. Chaudhary, Appl. Phys. B 106, 953 (2012) 9. P. Venkateswarlu, G. Chakrapani, M.C. George, Y.V. Rao, C. Okafor, Pramana-J. Phys. 29, 261 (1987) 10. P. Luc, J. Mol. Spectrosc. 40, 41 (1980) 11. J. Ye, L. Robertsson, S. Picard, L.-S. Ma, J.L. Hall, IEEE Trans. Instrum. Meas. 48, 544 (1999)