# Integrated Band Intensities of Ethylene (C212H4) by Fourier Transform Infrared Spectroscopy

Authors: G. B. Lebron; T. L. Tan
Journal: International Journal of Spectroscopy
 (2012)

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## 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.


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## Body

## 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 of12C2H4 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 × 10−4atm vapor pressure in the 640–3260 cm−1 region.The integrated band intensity,S(ν) is given by the equation [35, 36]:
(1)S(ν)=∫bandk(ν)dν=1pl∫bandlnI0Idν,
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.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.
(a)
ν
9 and ν2 + ν12
(b)
ν
11
(c)
ν
6 + ν10
(d)
ν
7 + ν8Table1 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 intensitiesS(ν) of 12C2H4 at 296 K in the 640–3260 cm−1 region.
Band assignment
Band centreν0 (cm−1)
Band region(cm−1)
S
(
ν
) (cm−1/cm atm)
Previous works
Present work
ν
9 and ν2 + ν12
3105 and 3079
3034–3260
S
(
ν
9
)
=
100
±
3
a
112.20
±
0.24
S
(
ν
9
)
=
82
.
52
±
0
.
86
b
ν
11
2989
2920–3034
56
.
0
±
0
.
3
a
55.35
±
0.14
65
.
87
±
0
.
64
b
ν
6
+
ν
10
2048
1990–2100
—
2.568
±
0.025
ν
7
+
ν
8
1889
1820–1950
—
21.701
±
0.028
ν
12
1443
1340–1540
41
.
16
±
0
.
50
c
41.22
±
0.30
ν
7 and ν10
949 and 826
640–1200
S
(
ν
7
)
=
321
.
69
±
0
.
36
d
329
±
16
S
(
ν
10
)
=
1
.
16
±
0
.
47
d
aSee [35]. Value adjusted for 296 K. ν9 band center ν0=3105.5 cm−1 and ν11 band center ν0=2989.5 cm−1.
bSee [29]. ν9 band center ν0=3104.885 cm−1 and ν11 band center ν0=3012.436 cm−1. Included in HITRAN database [28].
cSee [21]. Value adjusted for 296 K. Included in GEISA and HITRAN databases [26, 27].
dSee [22]. Value converted to cm−1/cm atm. Included in HITRAN database [28].



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*Source: 474639-2012-08-28.xml*