International Journal of Spectroscopy

Volume 2012 (2012), Article ID 474639, 5 pages

doi:10.1155/2012/474639

Research Article

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

G. B. Lebron and T. L. Tan

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: Karol Jackowski

Copyright © 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 cm⁻¹ region were determined by Fourier transform infrared (FTIR) spectroscopy. The infrared absorbance spectra of the and , , , , , and and bands of ethylene recorded at a resolution of 0.5 cm⁻¹ were measured at an ambient temperature of 296 K at various vapor pressures ranging from to atm to obtain respective Beer-Lambert's law plots. The measured integrated band intensities in cm⁻¹/cm atm were , , , and . In addition, the measured infrared band intensities of the and combination bands of ethylene are reported for the first time:  cm⁻¹/cm atm and  cm⁻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 band as well as the and 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 , , and bands [21–24] while Bach et al. [25] measured relative line intensities of the , + and bands. New data on the intensities and line positions in the 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 [22], , and bands [29] of ethylene, and the calculated intensity ratio of the band to the 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 cm⁻¹ and 3260 cm⁻¹ by Fourier transform infrared (FTIR) spectroscopy covering the following ethylene bands: and , , + , + , , and and + . To our knowledge, the measured integrated band intensities of + and + 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 cm⁻¹ in the 640–3260 cm⁻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 × 10⁻⁵ to 1 × 10−3 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 () 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 cm⁻¹ wavenumber region are shown in the survey absorbance spectrum in Figure 1. The spectrum was recorded at a resolution of 0.5 cm⁻¹ at a 4.56 × 10⁻⁴ atm vapour pressure and at an ambient temperature of 296 K. Figure 1 also shows the overlapping of the and bands at the far right and of the and + bands at the opposite end of the spectrum.

Figure 1: A survey FTIR absorbance spectrum of C₂H₄ collected at 0.5 cm⁻¹ resolution at an ambient temperature of 296 K and at 4.56 × 10−4 atm vapor pressure in the 640–3260 cm⁻¹ region.

The integrated band intensity, is given by the equation [35, 36]:

where is the vapor pressure in the cell, is the path length, and and are the incident and transmitted intensities of light at frequency . To find , the absorbance area of each band in the ethylene spectrum was determined for different pressures. The band area (cm⁻¹) was then plotted against pressure (atm) and the integrated band intensities in cm⁻¹/cm atm were determined from the gradient of each plot. The experimental gas path length () used in the calculation was 0.80 m. The procedure used in the determination of is similar to that of Kagann and Maki [36]. In Figure 2, the Beer-Lambert’s law plots are shown for the and + , , + , and + bands of ethylene. Straight lines passing through the origin were accurately fitted with correlation coefficient ranging from 0.985 to 0.990.

(a) and +

(b)

(c) +

(d) +

Figure 2: Beer-Lambert’s law plots of C₂H₄ bands that were measured in the 640–3260 cm⁻¹ region: (a) combined and + , (b) , (c) + , and (d) + .

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 and bands of ¹²C₂H₄ were from Golike et al. [35]: =  cm⁻¹/cm atm at band center = 3105.5 cm⁻¹ and =  cm⁻¹/cm atm at band center = 2985.5 cm⁻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 and bands as =  cm⁻¹/cm atm at = 3104.885 cm⁻¹ and =  cm⁻¹/cm atm at = 3012.436 cm⁻¹. In our work, since the band closely overlaps with the + combination band, we could only determine the combined band intensity of S( and + ) =  cm⁻¹/cm atm. The ratio of S( and + ) to 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 band, =  cm⁻¹/cm atm at = 2989 cm⁻¹ was determined in our work. These values, and , 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 values obtained may be explained by the different band center used by Dang-Nhu et al. in their study. Their band center = 3012.436 cm⁻¹ for the 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 cm⁻¹) [25, 37]. For the band, our result of =  cm⁻¹/cm atm agrees well with the latest reported value of =  cm⁻¹/cm atm [21] in the updated GEISA [26] and HITRAN [27] databases. For the first time, the integrated band intensities of the weak + and strong + combination bands have been measured. Moreover, high accuracy is shown in the values of S( + ) =  cm⁻¹/cm atm and S( + ) =  cm⁻¹/cm atm. The areas of the overlapping bands, and , and and + , were not separated and the integrated intensities of these bands are reported as combined (see Table 1). As expected, the strong band has the largest band intensity in the region with very small contribution from the overlapping but weak band, =  cm⁻¹/cm atm [22]. Overall, our results which show good agreement with those from previous studies are more accurately determined.

Table 1: The integrated band intensities of ¹²C₂H₄ at 296 K in the 640–3260 cm⁻¹ region.

Band assignment
Band centre (cm⁻¹)
Band region

(cm⁻¹)
(cm⁻¹/cm atm)

Previous works
Present work

and +
3105 and 3079
3034–3260

2989
2920–3034

2048
1990–2100
—

1889
1820–1950
—

1443
1340–1540

and
949 and 826
640–1200

See [35]. Value adjusted for 296 K. band center  cm⁻¹ and band center  cm⁻¹.

See [29]. band center  cm⁻¹ and band center  cm⁻¹. Included in HITRAN database [28].

See [21]. Value adjusted for 296 K. Included in GEISA and HITRAN databases [26, 27].

See [22]. Value converted to cm⁻¹/cm atm. Included in HITRAN database [28].

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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