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

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 𝜈₇ and 𝜈₁₀, 𝜈₁₂, 𝜈₇+𝜈₈, 𝜈₆+𝜈₁₀, 𝑣₁₁, and 𝜈₉ and 𝜈₂+𝜈₁₂ 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 3×10⁻⁵ to 1×10⁻³ atm to obtain respective Beer-Lambert's law plots. The measured integrated band intensities in cm⁻¹/cm atm were 𝑆(𝜈₉and𝜈₂+𝜈₁₂)=112.20±0.24, 𝑆(𝜈₁₁)=55.35±0.14, 𝑆(𝜈₁₂)=41.22±0.30, and 𝑆(𝜈₇and𝜈₁₀)=328.66±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)=21.701±0.028 cm⁻¹/cm atm and 𝑆(𝜈₆+𝜈₁₀)=2.568±0.025 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 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 cm⁻¹ and 3260 cm⁻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 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 × 10⁻³atm (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 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 𝜈₁₀ and 𝜈₇ bands at the far right and of the 𝜈₉ and 𝜈₂ + 𝜈₁₂ 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, 𝑆(𝜈) is given by the equation [35, 36]: 𝑆(𝜈)=_band𝑘(𝜈)𝑑𝜈=1𝑝𝑙_bandln𝐼⁰𝐼𝑑𝜈,(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)𝑑𝜈, 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 𝑆(𝜈) in cm⁻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.

(a) 𝜈9 and 𝜈2 + 𝜈12

(b) 𝜈11

(c) 𝜈6 + 𝜈10

(d) 𝜈7 + 𝜈8

Figure 2 

Beer-Lambert’s law plots of C2H4 bands that were measured in the 640–3260 cm−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) = 101±3 cm⁻¹/cm atm at band center 𝜈₀ = 3105.5 cm⁻¹ and 𝑆(𝜈₁₁) = 54.9±0.3 cm⁻¹/cm atm at band center 𝜈₀ = 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 𝜈₉ and 𝜈₁₁ bands as 𝑆(𝜈₉) = 82.52±0.86 cm⁻¹/cm atm at 𝜈₀ = 3104.885 cm⁻¹ and𝑆(𝜈₁₁) = 65.87±0.64 cm⁻¹/cm atm at 𝜈₀ = 3012.436 cm⁻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) = 112.20±0.24 cm⁻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) = 55.35±0.14 cm⁻¹/cm atm at 𝜈₀ = 2989 cm⁻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 cm⁻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 cm⁻¹) [25, 37]. For the 𝜈₁₂ band, our result of 𝑆(𝜈₁₂) = 41.22±0.30 cm⁻¹/cm atm agrees well with the latest reported value of 𝑆(𝜈₁₂) = 41.16±0.50 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 𝜈₆ + 𝜈₁₀ and strong 𝜈₇ + 𝜈₈ combination bands have been measured. Moreover, high accuracy is shown in the values of S(𝜈₆ + 𝜈₁₀) = 2.568±0.025 cm⁻¹/cm atm and S(𝜈₇ + 𝜈₈) = 21.701±0.028 cm⁻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) = 1.16±0.47 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 𝑆(𝜈) of ¹²C₂H₄ at 296 K in the 640–3260 cm⁻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.