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International Journal of Spectroscopy
VolumeΒ 2012Β (2012), Article IDΒ 474639, 5 pages
http://dx.doi.org/10.1155/2012/474639
Research Article

Integrated Band Intensities of Ethylene (12𝐂2𝐇4) by Fourier Transform Infrared Spectroscopy

Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616

Received 22 June 2012; Accepted 22 July 2012

Academic Editor: KarolΒ Jackowski

Copyright Β© 2012 G. B. Lebron and T. L. Tan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The integrated band intensities of ethylene (12C2H4) in the 640–3260 cmβˆ’1 region were determined by Fourier transform infrared (FTIR) spectroscopy. The infrared absorbance spectra of the 𝜈7 and 𝜈10, 𝜈12, 𝜈7+𝜈8, 𝜈6+𝜈10, 𝑣11, and 𝜈9 and 𝜈2+𝜈12 bands of ethylene recorded at a resolution of 0.5 cmβˆ’1 were measured at an ambient temperature of 296 K at various vapor pressures ranging from 3Γ—10βˆ’5 to 1Γ—10βˆ’3 atm to obtain respective Beer-Lambert's law plots. The measured integrated band intensities in cmβˆ’1/cm atm were 𝑆(𝜈9and𝜈2+𝜈12)=112.20Β±0.24, 𝑆(𝜈11)=55.35Β±0.14, 𝑆(𝜈12)=41.22Β±0.30, and 𝑆(𝜈7and𝜈10)=328.66Β±16.55. In addition, the measured infrared band intensities of the 𝜈7+𝜈8 and 𝜈6+𝜈10 combination bands of ethylene are reported for the first time: 𝑆(𝜈7+𝜈8)=21.701Β±0.028 cmβˆ’1/cm atm and 𝑆(𝜈6+𝜈10)=2.568Β±0.025 cmβˆ’1/cm atm.

1. Introduction

Ethylene (12C2H4), the simplest of all alkenes, is an important hydrocarbon. Terrestrially, it is a known tropospheric pollutant that affects the ozone concentration in the atmosphere [1]. It is produced by plants [2, 3], the incomplete combustion of fossil fuels [4], forest fires [5–7], volcanic emissions, and natural gas [4, 8]. As such, the measurement of ethylene concentration in the atmosphere is of great interest and importance. Beyond Earth, ethylene has been detected as a trace component of the atmospheres of the outer planets Jupiter, Saturn, Neptune [9–12], and the satellite Titan [13–18]. Ethylene has also been observed in circumstellar clouds IRC+10216 [19] and IRL618 [20]. The 10 μm band system of 12C2H4 which contains the strong 𝜈7 band as well as the 𝜈10 and 𝜈4 bands is of particular interest to spectroscopists and astrophysicists as the region is being used for C2H4 remote measurements in the infrared range [21, 22].

Given the integrated band intensities of ethylene, it is possible to determine spectroscopically the concentration and distribution of ethylene in the atmosphere at any given temperature or pressure. Recent measurements of vibrational band intensities of 12C2H4 include the 𝜈7, 𝜈10, and 𝜈12 bands [21–24] while Bach et al. [25] measured relative line intensities of the 𝜈11, 𝜈2 + 𝜈12 and 𝜈9 bands. New data on the intensities and line positions in the 𝜈12 band of 12C2H4 as reported by Rotger et al. [21] are included in the 2009 edition of the GEISA database [26] and in the HITRAN 2008 molecular spectroscopic database [27]. Also included in the HITRAN database [28] are intensities of the 𝜈7 [22], 𝜈9, and 𝜈11 bands [29] of ethylene, and the calculated intensity ratio of the 𝜈7 band to the 𝜈10 band of ethylene [30]. In this paper, we report the improved results of our laboratory measurements of the integrated band intensities of 12C2H4 between 640 cmβˆ’1 and 3260 cmβˆ’1 by Fourier transform infrared (FTIR) spectroscopy covering the following ethylene bands: 𝜈7 and 𝜈10, 𝜈12, 𝜈7 + 𝜈8, 𝜈6 + 𝜈10, 𝜈11, and 𝜈9 and 𝜈2 + 𝜈12. To our knowledge, the measured integrated band intensities of 𝜈7 + 𝜈8 and 𝜈6 + 𝜈10 are reported for the first time. This work is part of our FTIR studies of the ethylene molecule and its isotopomers [31–34].

2. Experimental Details

The ethylene gas samples used in the experiments with a stated purity of 99.99% were purchased from Sigma-Aldrich in USA. All unapodized spectra were recorded at a resolution of 0.5 cmβˆ’1 in the 640–3260 cmβˆ’1 region using a Bruker IFS 125 HR Michelson Fourier transform spectrometer located at the Spectroscopy Laboratory of the National Institute of Education, Nanyang Technological University in Singapore. A globar infrared source together with a high-sensitivity liquid nitrogen cooled Hg-Cd-Te detector and KBr beamsplitter were used for all recordings. The spectra were collected at an ambient temperature of 296 K and at about 30 different vapor pressures in the 3 Γ— 10βˆ’5 to 1 Γ— 10βˆ’3 atm (0.03–1.00 mb) range as measured by a capacitance pressure gauge. An absorption length of 0.80 m was achieved by adjusting for four passes in the multiple-pass absorption cell with a 20-cm base path. A total of 100 scans were coadded to produce each spectrum of high signal-to-noise ratio. A background spectrum of the evacuated cell was recorded at the same resolution. The ratio of the 12C2H4 spectrum to the background spectrum yielded a transmittance spectrum with relatively smooth baseline. The transmittance spectrum was then converted to absorbance spectrum (ln(𝐼0/𝐼)) and the area for each band in the region was determined at different pressures using the OPUS 6 software.

3. Results and Discussion

The vibrational bands of pure ethylene vapor in the 640–3260 cmβˆ’1 wavenumber region are shown in the survey absorbance spectrum in Figure 1. The spectrum was recorded at a resolution of 0.5 cmβˆ’1 at a 4.56 Γ— 10βˆ’4 atm vapour pressure and at an ambient temperature of 296 K. Figure 1 also shows the overlapping of the 𝜈10 and 𝜈7 bands at the far right and of the 𝜈9 and 𝜈2 + 𝜈12 bands at the opposite end of the spectrum.

474639.fig.001
Figure 1: A survey FTIR absorbance spectrum of C2H4 collected at 0.5 cmβˆ’1 resolution at an ambient temperature of 296 K and at 4.56 Γ— 10βˆ’4 atm vapor pressure in the 640–3260 cmβˆ’1 region.

The integrated band intensity, 𝑆(𝜈) is given by the equation [35, 36]: ξ€œπ‘†(𝜈)=band1π‘˜(𝜈)π‘‘πœˆ=ξ€œπ‘π‘™band𝐼ln0πΌπ‘‘πœˆ,(1) where 𝑝 is the vapor pressure in the cell, 𝑙 is the path length, and 𝐼0 and 𝐼 are the incident and transmitted intensities of light at frequency 𝜈. To find ∫bandln(𝐼0/𝐼)π‘‘πœˆ, the absorbance area of each band in the ethylene spectrum was determined for different pressures. The band area (cmβˆ’1) was then plotted against pressure (atm) and the integrated band intensities 𝑆(𝜈) in cmβˆ’1/cm atm were determined from the gradient of each plot. The experimental gas path length (𝑙) used in the calculation was 0.80 m. The procedure used in the determination of 𝑆(𝜈) is similar to that of Kagann and Maki [36]. In Figure 2, the Beer-Lambert’s law plots are shown for the 𝜈9 and 𝜈2 + 𝜈12, 𝜈11, 𝜈6 + 𝜈10, and 𝜈7 + 𝜈8 bands of ethylene. Straight lines passing through the origin were accurately fitted with correlation coefficient 𝑅2 ranging from 0.985 to 0.990.

fig2
Figure 2: Beer-Lambert’s law plots of C2H4 bands that were measured in the 640–3260 cmβˆ’1 region: (a) combined 𝜈9 and 𝜈2 + 𝜈12, (b) 𝜈11, (c) 𝜈6 + 𝜈10, and (d) 𝜈7 + 𝜈8.

Table 1 presents the results of the present work together with those of previous works [21, 22, 29, 35] for comparison. Previously reported integrated band intensities of the 𝜈9 and 𝜈11 bands of 12C2H4 were from Golike et al. [35]: 𝑆(𝜈9) = 101Β±3 cmβˆ’1/cm atm at band center 𝜈0 = 3105.5 cmβˆ’1 and 𝑆(𝜈11) = 54.9Β±0.3 cmβˆ’1/cm atm at band center 𝜈0 = 2985.5 cmβˆ’1. A more recent study [29] included in the HITRAN database [28] that involves 148 selected measurements in the 3 μm region of 12C2H4 gives the integrated band intensities of the 𝜈9 and 𝜈11 bands as 𝑆(𝜈9) = 82.52Β±0.86 cmβˆ’1/cm atm at 𝜈0 = 3104.885 cmβˆ’1 and𝑆(𝜈11) = 65.87Β±0.64 cmβˆ’1/cm atm at 𝜈0 = 3012.436 cmβˆ’1. In our work, since the 𝜈9 band closely overlaps with the 𝜈2 + 𝜈12 combination band, we could only determine the combined band intensity of S(𝜈9 and 𝜈2 + 𝜈12) = 112.20Β±0.24 cmβˆ’1/cm atm. The ratio of S(𝜈9 and 𝜈2 + 𝜈12) to 𝑆(𝜈11) calculated as 2.0 was found to be close to the value of 1.8 determined by Bach et al. [25] in their FTIR rotational analysis. For the isolated 𝜈11 band, 𝑆(𝜈11) = 55.35Β±0.14 cmβˆ’1/cm atm at 𝜈0 = 2989 cmβˆ’1 was determined in our work. These values, 𝑆(𝜈11) and 𝜈0, closely agree with those obtained by Golike et al. [35] but differ with those reported by Dang-Nhu et al. [29] (see Table 1). The discrepancy in the 𝑆(𝜈11) values obtained may be explained by the different band center used by Dang-Nhu et al. in their study. Their band center 𝜈0 = 3012.436 cmβˆ’1 for the 𝜈11 band of 12C2H4 is a few wavenumbers off from what Golike et al. [35] and our work had used which are close to what is reported in recent literature (~2989 cmβˆ’1) [25, 37]. For the 𝜈12 band, our result of 𝑆(𝜈12) = 41.22Β±0.30 cmβˆ’1/cm atm agrees well with the latest reported value of 𝑆(𝜈12) = 41.16Β±0.50 cmβˆ’1/cm atm [21] in the updated GEISA [26] and HITRAN [27] databases. For the first time, the integrated band intensities of the weak 𝜈6 + 𝜈10 and strong 𝜈7 + 𝜈8 combination bands have been measured. Moreover, high accuracy is shown in the values of S(𝜈6 + 𝜈10) = 2.568Β±0.025 cmβˆ’1/cm atm and S(𝜈7 + 𝜈8) = 21.701Β±0.028 cmβˆ’1/cm atm. The areas of the overlapping bands, 𝜈10 and 𝜈7, and 𝜈9 and 𝜈2 + 𝜈12, were not separated and the integrated intensities of these bands are reported as combined (see Table 1). As expected, the strong 𝜈7 band has the largest band intensity in the region with very small contribution from the overlapping but weak 𝜈10 band, 𝑆(𝜈10) = 1.16Β±0.47 cmβˆ’1/cm atm [22]. Overall, our results which show good agreement with those from previous studies are more accurately determined.

tab1
Table 1: The integrated band intensities 𝑆(𝜈) of 12C2H4 at 296 K in the 640–3260 cmβˆ’1 region.

Acknowledgment

The authors thank the National Institute of Education, Singapore for the financial support extended to the project through research grants RS 3/08 TTL and RI 9/09 TTL.

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