Author's Accepted Manuscript

Infrared cross-sections and integrated bandintensities of propylene: Temperature-depen-dent studies

Et-touhami Es-sebbar, Majed Alrefae, AamirFarooq

PII: S0022-4073(13)00395-6DOI: http://dx.doi.org/10.1016/j.jqsrt.2013.09.019Reference: JQSRT4486

To appear in: Journal of Quantitative Spectroscopy & Radiative Transfer

Received date: 28 April 2013Revised date: 17 September 2013Accepted date: 18 September 2013

Cite this article as: Et-touhami Es-sebbar, Majed Alrefae, Aamir Farooq,Infrared cross-sections and integrated band intensities of propylene:Temperature-dependent studies, Journal of Quantitative Spectroscopy & RadiativeTransfer, http://dx.doi.org/10.1016/j.jqsrt.2013.09.019

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Infrared cross-sections and integrated band intensities of propylene:

Temperature-dependent studies

Et-touhami Es-sebbar, Majed Alrefae, Aamir Farooq*

Clean Combustion Research Center, Division of Physical Sciences and Engineering, King Abdullah

University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia

*Corresponding author: email: aamir.farooq@kaust.edu.sa, Tel.: +96628082704

Abstract

Propylene, a by-product of biomass burning, thermal cracking of hydrocarbons and incomplete

combustion of fossil fuels, is a ubiquitous molecule found in the environment and atmosphere. Accurate

infrared (IR) cross-sections and integrated band intensities of propylene are essential for quantitative

measurements and atmospheric modeling. We measured absolute IR cross-sections of propylene using

Fourier Transform Infrared (FTIR) Spectroscopy over the wavenumber range of 400 – 6500 cm-1 and at

gas temperatures between 296 and 460 K. We recorded these spectra at spectral resolutions ranging from

0.08 to 0.5 cm-1 and measured the integrated band intensities for a number of vibrational bands in certain

spectral regions. We then compared the integrated band intensities measured at room temperature with

values derived from the National Institute of Standards and Technology (NIST) and the Pacific Northwest

National Laboratory (PNNL) databases. Our results agreed well with the results reported in the two

databases with a maximum deviation of about 4%. The peak cross-sections for the primary bands

decreased by about 20 to 54 % when the temperature increased from 296 to 460 K. Moreover, we

determined the integrated band intensities as a function of temperature for certain features in various

spectral regions; we found no significant temperature dependence over the range of temperatures

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considered here. We also studied the effect of temperature on absorption cross-section using a Difference

Frequency Generation (DFG) laser system. We compared the DFG results with those obtained from the

FTIR study at certain wavenumbers over the 2850 – 2975 cm-1 range and found a reasonable agreement

with less than 10% discrepancy.

Keywords: Propylene; IR cross-section; Integrated band intensity; FTIR, DFG spectroscopy

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1. Introduction

Propylene, also called propene (CH2=CH–CH3; C3H6), is an unsaturated hydrocarbon with a

double bond and the second alkene after ethylene. Found in the environment and atmosphere,

this molecule is ubiquitous and thus its quantification is very important [1-2]. The primary

sources of propylene production include biomass burning [3-9], thermal cracking of

hydrocarbons [10-11] and incomplete combustion of fossil fuels [12-13]. It is present in

motor vehicle exhaust as a result of fuel-rich combustion of hydrocarbon fuels [14-15].

Propylene is widely used by a number of chemical industries in the preparation of alkylates as

additives to gasoline [16] and in the production of polypropylene [17], acrylonitrile [18] and

propylene oxide [19].

Recently, propylene has been detected in the interstellar medium and the Taurus Molecular

Cloud (TMC-1) and it has been suggested that propylene may play an important role in

interstellar chemistry [20-22]. The gas phase chemistry and the propylene production

mechanism in cold interstellar clouds were studied by Herbst et al. [22]. Moreover,

Marcelino et al. [20] reported a relative propylene abundance of 4 x10-9 (i.e., a column

density relative to H2 of 4x1013/cm2) near TMC-1. Detection of propylene in interstellar

environment is difficult and requires good knowledge of the spectral characteristics and

temperature-dependent absolute absorption cross-section data.

Furthermore, propylene plays a key role in the pyrolysis and oxidation chemistry of larger

hydrocarbons. Understanding propylene formation and decay is important for predicting the

concentrations of combustion emissions, soot and unburnt hydrocarbons. Propylene is one of

the primary intermediate species formed during the oxidation of large alkanes and its

concentration can be relatively large in many combustion environments [23]. Temperature-

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dependent cross-sections are required to make quantitative time-history measurements in

combustion systems. Such measurements will prove very useful in the improvement and

validation of chemical kinetic mechanisms of large alkanes.

A number of spectroscopic experimental investigations on propylene, primarily using

infrared spectroscopy, have previously been reported [24-26]. Silvi et al. [24] reported

infrared spectra over 200 – 4000 cm-1 at a resolution between 0.5 and 1 cm-1 and assigned

vibrational bands to determine the force constant parameters. Durig et al. [25] measured the

far-infrared spectrum of propylene and described methyl torsional fundamental bands as well

as two hot bands. Lafferty et al. [26] measured higher resolution (0.005 cm-1) IR jet spectra

over 911 – 930 cm-1 to resolve the fine structure of the ν18 and ν19 bands in an attempt to

determine the rotational constants and to characterize the torsional splitting of propylene.

Detailed IR spectra of propylene at room temperature can be found in the National Institute

of Standards and Technology (NIST) database [27] and at 278, 298 and 323 K in the Pacific

Northwest National Laboratory (PNNL) database [28]. Transmission spectra taken from

NIST at room temperature are a composite of five combination burdens in a 25% C3H6/N2

mixture and the optical path length is 5 cm [27]. The spectrum from PNNL at room

temperature is an average of 10 concentration – optical path length burdens [28]. At other

temperature values, the spectrum is a composite of 6 combination burdens [28]. The two

databases report spectra at low to moderate spectral resolutions, 2 cm-1 and 0.112 cm-1,

respectively. However, relatively high-resolution IR absorption cross-sections of propylene

and its dependence on gas temperature have not been reported in the literature or other

commonly used spectral databases like HITRAN and GEISA [29-30].

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Here, we used Fourier Transform Infrared Spectroscopy (FTIR) to measure the IR cross-

sections of propylene and their temperature dependence over the wave number range of 400

– 6500 cm-1. We recorded absorption spectra at spectral resolutions between 0.08 and 0.5 cm-

1 and at gas temperatures ranging from 296 to 460 K. We obtained integrated band intensities

for various features of propylene and compared with the values calculated from the NIST and

PNNL databases [27-28]. In addition, we conducted a complementary study on the effect of

temperature on cross-sections by absorption spectroscopy using Difference Frequency

Generation (DFG) laser system operating near 3.3 – 3.5 μm. We compared the DFG results

with the measured FTIR spectra for certain wavenumbers over 2850 – 2975 cm-1.

2. Experimental Methods 

2.1. FTIR Setup

Spectra were measured over 400 – 6500 cm-1 using a Fourier Transform Infrared (FTIR)

spectrometer manufactured by Bruker (VERTEX 80V). The experimental setup was

described previously for the investigation of 1-butene IR cross-sections and their temperature

dependence [31]. The configuration and settings of the FTIR for the measurements reported

here are listed in Table 1. IR light was passed through an absorption cell, which was

constructed from stainless steel and KBr windows, with an optical path-length of 10 cm. The

cell temperature was set using a heating jacket connected to a controller. The temperature

was measured continuously with five type-K (Omega) thermocouples along the length of the

absorption cell. The uncertainty on the temperature measurements was less than 0.5%. The

beam spot size at the interferometer Jacquinot stop was set either 1.5 or 2.5 mm. The smaller

aperture (A) allowed for higher resolution at lower wavenumbers, but for some

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measurements the aperture was opened to 2.5 mm (reducing the resolution) in order to obtain

sufficient signal strength with the room-temperature DLaTGS detector. Based on maximum

interferometer mirror displacement, this FTIR spectrometer has a spectral resolution up to

0.06 cm-1, and operates over a large wavenumber range (from 400 to 6500 cm-1). The

measurements presented here were collected at spectral resolutions ranging from 0.08 to 0.5

cm-1, which are chosen to resolve various features of propylene with sufficient accuracy. The

interferograms were corrected with the Mertz phase function. The boxcar apodized

interferograms were zero-filled by a factor of 2 to produce the final spectrum. Additional

spectra were zero-filled by a factor of 4 to resolve the principal features of C3H6. To calibrate

the FTIR wavenumber scale and to assign the observed features with corresponding line

positions, spectra of CO and CH4 gases were recorded and compared with the corresponding

lines in the HITRAN database [29].

To study the effect of temperature, spectra between 400 and 1024 cm-1 were recorded at a

spectral resolution of 0.08 cm-1 (A = 2.5 mm). At higher wavenumbers, the desired resolution

(0.08 cm-1) is difficult to achieve. In order to measure spectra from 1024 to 6500 cm-1 at a

resolution of 0.08 cm-1, an aperture of 1 mm is needed for a collimating mirror focal length

of 100 mm. Such a small aperture leads to very low signal-to-noise ratio (SNR) and the

baseline deteriorates. To provide better overall SNR, an aperture of 1.5 mm was used which

resulted in spectral resolution of 0.09 cm-1 for 1024 – 3200 cm-1 range and 0.18 cm-1 for 3200

– 6500 cm-1.

Spectra measurements were made at seven temperatures (296, 318, 343, 362, 389, 424 and

460 K) with the 10 cm optical cell. At each temperature, a series of spectra were recorded

over 400 – 3200 cm-1 at various pressures to determine the integrated band intensity form the

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linear dependence of the absorbance area with gas pressure. Before and after each

measurement, a background measurement of 120 co-added scans was carried out while the

cell was evacuated. The background signals were used to determine the uncertainty due to

the drift of the intensity signal. This is particularly important for shorter and higher

wavenumber regions where absorption is relatively weak. At high temperatures, self-

emission from the gas gets modulated by the interferometer of the FTIR instrument and

subsequently falls on the IR detector to induce an offset to the measured signal. The emission

varies as a function of temperature, pressure and frequency, where it can be more significant

at low frequencies (less than 1000 cm-1). Since our measurements were carried out at

relatively low temperatures (< 460 K), self-emission was negligible.

Highly pure (99.99%) propylene gas and research grade nitrogen (99.999%), supplied by

Abdullah Hashim Industrial Gases & Equipment Co. Ltd., were used for the experiments. A

10% propylene in nitrogen mixture was prepared manometrically for the measurements

carried out in dilute propylene. The pressure in the gas handling system was monitored using

three Baratron capacitance gauges (20, 100, and 1000 Torr full-scale ranges). The absorption

cell was evacuated to about 1 x 10-4 Torr using a turbo-molecular pump (Turbo-v 81-M

Varian) backed by a rotary mechanical pump (Varian DS 102). The overall leak rate during

the measurements is very low, estimated to be about 6 x 10-4 Torr/min. For the room-

temperature measurements (296 K), the entire FTIR compartment was evacuated to pressures

less than 0.1 mbar to avoid interference absorption by ambient CO2 and H2O. In the elevated

temperature measurements, N2 gas was used to purge the FTIR spectrometer.

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2.2. Difference Frequency Generation (DFG) Laser System

Two tunable mid-IR Difference Frequency Generation (DFG) laser systems (Novawave IRIS

1000 [32]) were used to measure IR cross-sections as a function of temperature. One laser

system was tuned over 2907 – 2985 cm-1, while the second one covered the 2833 – 2907 cm-1

wavenumber range. To produce mid-IR light, two pump lasers operating at 1064 nm and

1074 nm were combined with near-IR distributed feedback (DFB) diode lasers (~ 40 mW

power) in a periodically poled lithium niobate (PPLN) crystal. The DFB laser diodes used for

the DFG system were user-replaceable, which allowed different frequencies to be accessed.

The spectral linewidth of the resulting mid-IR light was less than 3 MHz and the output

power was about 0.6 mW. The temperature of the PPLN crystal and the temperature and

current of the DFB laser must be chosen appropriately at each frequency to satisfy the phase-

matching condition and hence maximizing the output power. The schematic of the

experimental setup is shown in Fig. 1. We used a common-mode rejection (CMR) scheme to

account for the laser intensity variation. Without CMR, the laser intensity varies over time

and can result in errors up to 20%. The laser intensity was measured using photo-detectors

supplied by Vigo Systems (PVMI-3TE-10.6, 2mm x 2mm active area). As in the case for

FTIR measurements, the gas temperature along the absorption cell was monitored using K-

type thermocouples. Before each measurement, the absorption cell was evacuated to less than

10-3 mbar. Two MKS Baratron capacitance manometers (20 and 1000 Torr range) were used

for measuring the gas pressure. Measurements of the propylene cross-sections were

performed at seven wavenumbers between 2850 and 2975 cm-1. The absolute value of the

mid-IR wavelength was measured using a Spectrum Analyzer (Bristol Instruments 721 B).

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3. Data Analysis

Absorption of mid-infrared radiation is caused by vibrational and rotational excitation of

molecules. The absorption is influenced by the absorption cross-section, optical path-length

and the partial pressure of the absorbing molecules. The absorbance, Aν, can be written as:

Aν=ln (I0/I)ν=εν.L.P, (1)

where I0 is the light intensity at wavenumber ν passing through the empty cell (background);

I is the light intensity passing through the propylene sample; εν is the absorption coefficient

at wavenumber ν in cm-1/bar; L is the optical path length in cm; and P is the pressure of the

gas in bar. The absorption coefficient is converted to cross-section (in units of cm2/molecule)

using:

σν= (εν /N). (T(K)/273.15), (2)

where N=2.6857x1019/cm3 is the Loschmidt constant given at standard temperature/pressure

(273.15 K/1013.25 mbar).

Integrated absorbance (∫Aν dν ) was calculated for a number of propylene bands at different

gas pressures. For all bands of sufficient intensity, we determined the integrated band

intensity by plotting the integrated absorbance as a function of the product of the optical

path-length and gas pressure (L.P).

∫Aν dν = L P∫ εν dν = L P Sν , (3)

The linear dependence of the integrated absorbance with L.P was verified and the absolute

integrated band intensity (Sν), given in atm-1.cm-2, was deduced from the slope of the linear

fit.

The integrated IR cross-sections of propylene were compared with quantitative results taken

from NIST and PNNL databases [27-28]. Absorbance spectra taken from PNNL were first

10  

converted to cross-section values by using equations (1) and (2) for an optical path length of

100 cm and a pressure of 1.013x10-3 mbar. In the case of NIST spectra, the transmissions

were converted to absorption cross-sections for propylene’s partial pressure of 199.98 mbar

diluted in N2, a total pressure of 799.93 mbar and an optical path length of 5 cm. The

obtained cross-sections were integrated over the studied spectral region for comparison with

measured band intensities.

A common-mode rejection (CMR) scheme was used for direct absorption measurements by

the DFG laser system. A slightly modified form of Beer-Lambert’s law was used for the

absorbance:

Acmr=ln ((I0/I0,ref) (Iref /I))ν, (4)

where Iref and I0,ref are the reference signals measured by the detector before the absorption

cell, I0 and I are the transmitted intensities without and with the gas sample as measured by

the second detector (see Fig. 1).

4. Uncertainty Analysis

All potential error sources in the measurement of cross-sections and integrated band

intensities were considered and quantitatively evaluated. According to equations (1) and (2),

uncertainty in cross-section measurements results primarily from uncertainties in measured

absorbance (δA/A), the optical path-length (δL/L), the gas pressure (δPC3H6/PC3H6) and the gas

temperature (δT/T). Thus, the uncertainty in C3H6 cross-section measurements can be

estimated using the following expression:

(δσC3H6/σC3H6)= (δA/A) + (δL/L)+ (δPC3H6/PC3H6)+ (δT/T), (5)

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The uncertainty on the optical path-length is less than 0.5% arising from the expansion of the

cell as the temperature increases. The measured pressure has an uncertainty of less than 2%,

which results from the uncertainty in the Baratron measurement and the pressure variation

due to the adsorption of C3H6 on the cell walls. The uncertainty in temperature is about 0.3%

arising from the statistical error of the readings given by the five thermocouples and from the

temperature gradient over the length of the optical cell. Due to relatively small absorption

over 400 – 480 cm-1, 1100 – 1350 cm-1, 2000 – 2500 cm-1 and 3500 – 6500 cm-1 spectral

regions, the error on the absorbance (δA/A) can be higher. This problem was somewhat

constrained by performing measurements at higher pressures. The uncertainty due to the drift

of the intensity signal was evaluated by measuring the background before and after sample

measurements. The combination of all error sources yielded an overall uncertainty of about

5% in the measured absorption cross-sections in the 400 – 3500 cm-1 range and 8%

uncertainty in the higher wavenumber range of 3500 – 6500 cm-1. Similar analysis was

conducted to estimate the measurement uncertainty for data collected using the DFG laser

system and the uncertainty was found to be less than 5%.

 

5. Results and Discussion

5.1. IR cross-sections at room temperature and vibrational assignments

Fig. 2 shows the IR cross-sections of propylene (C3H6) measured at room temperature (296

K). For clarity, the recorded spectra over 400 – 6500 cm-1 are divided into different regions.

Spectra in the relatively weak absorption regions (400 – 480 cm-1 and 3500 – 6500 cm-1)

were recorded using pure gas, whereas all other measurements were carried out using a 10%

C3H6/N2 mixture. Using two different aperture sizes, measurements between 400 and 3200

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cm-1 are recorded at spectral resolution of 0.09 cm-1, whereas the resolution is 0.18 cm-1 for

3200 – 6500 cm-1. Propylene exhibits strong absorption bands in the region extending from

the mid to long IR. The band assignments are labeled in Fig. 2 and a description of the types

of vibrations and assignments is presented in Table 2. The vibrational assignments of various

propylene features are based on the previous study of Silvi et al. [24].

Propylene has 21 vibrational modes and most of the vibrational bands with their

corresponding frequencies are identified here [24]. The relatively strong band centered at

426.02 cm-1 corresponds to the CCC deformation mode (ν14). Over the 500 – 1100 cm-1

spectral region, ν18 (991.05 cm-1), ν19 (912.62 cm-1) and ν20 (576.27 cm-1) involve CH

bending, CH2 twisting and wagging, and CH3 rocking, respectively. We identified two

relatively weak bands, ν10 (1297.86 cm-1) and ν11 (1170.04 cm-1), in our spectra but their

intensities were quite small. The main spectral features over 1300 – 2000 cm-1 include CH3

symmetric deformation (ν9; 1377.94 cm-1), CH3 antisymetric bending (ν16; 1442.71 cm-1),

and C=C stretch (ν6; 1653.18 cm-1). In the spectral region between 2500 and 3500 cm-1, the

spectra consist of many partially overlapping spectral features that make the assignment of

the vibrational structures challenging. The CH3 asymmetric stretch corresponding to ν15

(2954.30 cm-1) is quite complex due to two overlapping peaks centered at 2950.54 and

2931.07 cm-1. The ν1 band of the CH2 asymmetric stretch consists of two main peaks at

3090.89 cm-1 and at 3091.60 cm-1. We note that the ν2, ν4, ν8, ν12 and ν13 features of

propylene are either missing or not easy to identify in our spectra due to their relatively weak

absorption strengths and the overlapping transitions from neighboring bands. Except ν4, these

bands ( ν2, ν8, ν12, ν13) were also missing in the gas phase IR spectra reported by Silvi et al.

[24]. These features, however, were detected in the liquid phase at low temperature [24].

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5.2. Integrated band intensities and comparison with literature data We compared our measured spectra with spectra taken from NIST and PNNL databases [27-

28]. The spectral resolution in these databases is 2 and 0.112 cm-1, respectively, whereas the

resolution of our measurements is 0.09 cm-1. Due to this difference, direct comparisons of

measured cross-sections with the two datasets were not performed. Instead, integrated band

intensities for six absorption regions were compared. These regions were (400 – 480 cm-1),

(525 – 680 cm-1), (800 – 1060 cm-1), (1600 – 1700 cm-1), (1340 – 1550 cm-1) and (2500 –

3200 cm-1). The integrated absorbance (base-e) for the measured spectra is plotted as a

function of the product of the partial pressure of C3H6 and the optical path-length in Fig. 3.

Sharpe et al. [28] and Chu et al. [33] recorded spectra over a range of burden measurements

to increase the accuracy of the measured cross-sections and integrated intensities. For the

weaker ν14 absorption band over 400 – 480 cm-1 spectral region, good SNR was derived from

burden measurements at eight pressure values in pure C3H6 to get reasonable absorbance

values (Fig. 3). For other strong absorption features, burden measurements were performed

in a 10% C3H6/N2 gas mixture for fifteen pressure values. In all cases, absorption bands do

not approach saturation (see Fig. 3). The resulting integrated absorbance is clearly a linear

function of the product of pressure and optical path length with zero intercept. Only small

scatter around the calculated regression fits is observed. For all the selected spectral regions,

high correlation coefficients of � 0.99 were obtained. On the basis of this assessment, the

authors believe it is reasonable to apply Beer-Lambert law in our analysis method rather than

the approach reported by Chu et al [33] for intensity bands that approach saturation. Note that

spectra are recorded in pure C3H6 for the 400 – 480 cm-1 region due to weak absorbance. This

may have stronger effect on the linewidths due to larger self-broadening. The integrated

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intensities for various regions are calculated from the slopes of the linear fits and are given in

Table 3 together with uncertainties. Values derived from NIST and PNNL databases are also

shown. Since NIST and PNNL spectra are averaged for multiple burdens, the uncertainties in

the integrated band intensities derived from the integration procedure are also included. The

differences in the integrated intensities between our measurements and literature are also

presented in Table 3. Our results agreed very well with the PNNL data and we observed a

maximum difference of 2.7%. There was also reasonable agreement between our

measurements and NIST data. A maximum difference of 3.6 % was observed for the 2500 –

3200 cm−1 wavenumber region. In comparing integrated intensities between our

measurements and the literature data, it was important to use a consistent integration method.

Here, spectral integration was carried out by drawing a straight line between peaks of the two

endpoints for the selected wavenumber interval and then integrating the area above this line.

5.3. The effect of gas temperature on IR spectra and band intensities

The influence of gas temperature on IR cross-sections of C3H6 was investigated by

measuring spectra at seven different temperatures: 296, 318, 343, 362, 389, 424 and 460 K.

Similar to the room temperature (296 K) measurements, elevated temperature spectra were

recorded as a function of gas pressure. Fig. 4 shows an example of the influence of

temperature (296 and 460 K) on the cross-sections for selected stronger bands over 520 – 680

cm-1 (Fig. 4a), 800 – 1050 cm-1 (Fig. 4b) and 2800 – 3200 cm-1 (Fig. 4c). The resolution for

the measured high-temperature spectra is 0.08 cm-1, except region between 2800 and 3200

cm-1 in which spectra were recorded at 0.25 cm-1 (A = 2.5 mm). Results show that the

absorption bands exhibit a decrease in the peak cross-section with increasing gas

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temperature. This behavior is more pronounced for the relatively narrow features, i.e. ν20,

ν19, ν18, ν15 and ν5 and less observed in the case of ν1 band which is relatively broader. An

example is shown in the insert of Fig. 3a of the influence of the temperature on the ν20

maxima. On the other hand, the continuum of each band increases with increasing gas

temperature as illustrated in Fig. 4c for the three overlapped bands ν1, ν5 and ν15 over 2500 –

3200 cm-1. Moreover, crossover points were observed in which IR cross-sections remain

unchanged with increasing temperature. A similar dependence of IR cross-sections on

temperature was observed recently for 1–butene molecule [15]. Temperature dependencies of

the infrared spectra are largely demonstrated and discussed in the literature for various

species from low to high temperature values [34-38]. This effect is due to a change in the

vibrational and rotational population distribution towards higher rotational energy levels as

the temperature increases leading to a broadening of the bands and a decrease in the peak

cross-section. This is generally expected in the case of cold bands originating from the

vibrational ground state.

 

We calculated the integrated band intensities at different temperatures for three selected

wavenumber intervals: 2500 – 3200 cm-1, 800 – 1060 cm-1 and 525 – 680 cm-1. The

absorption cross-sections for many features of C3H6 in these spectral regions are relatively

large. The results are presented in Table 4 for seven temperature values ranging from 296 to

460 K. No significant temperature dependence was observed in the three spectral regions

considered. The difference in the integrated band intensities between room temperature (296

K) and elevated temperatures is about 1.8 – 4% for various spectral regions considered here.

It is difficult to predict quantitatively whether the integrated band intensities indeed show

16  

slight temperature dependence or if the observed variation is just a reflection of the

measurement uncertainties. While ν14 band is isolated from other features, it is important to

mention that the integrated band over 2500 – 3200 cm-1 range contains three interference

features (ν1, ν5 and ν15). Likewise, the integrated band area for the ν19 feature in the

800 −1060 cm-1 range interferes with two smaller (ν17, ν18) bands. It has been reported in the

literature [39-42] that the integrated intensities of fundamental vibrational bands for various

species can exhibit small temperature dependence because of the overlap from other bands.

Moreover, similar behavior can be expected for overtone, combination and hot bands in

which small temperature dependence ( � 4.7 – 7 %) was observed for different molecules:

CS2, N2O, OCS [41], HNO3 [43] and (CH3)2CO [44]. Further, spectra taken from PNNL

database for C3H6 molecule at 278, 298 and 323 K were integrated over the same three

spectral ranges studied in the present work. The resulting integrated intensities at 323 K vary

by about 8% compared to the 298 K values for 2500 – 3200 cm-1 and 800 – 1060 cm-1

regions and more than 15% for the 525 – 680 cm-1 spectral region. In another work [45],

spectra were measured over 2800 – 3200 cm-1 at for 298, 524 and 775 K and the integrated

band intensities at different temperatures were within 2% of each other.

In addition, a series of measurements were also performed at a relatively low resolution of 0.6

cm-1 (data not shown). Spectral resolution did not influence the integrated band intensities, and a

maximum deviation of less than 2% was observed between the measurements carried out at

lower and higher spectral resolutions.

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5.4. Comparison between FTIR and DFG Results

We compared the IR absorption cross-sections of propylene obtained by FTIR with

measurements made by a Difference Frequency Generation (DFG) laser system at specific

wavenumbers over the operating range of the DFG (2850 – 2975 cm-1). At each individual

wavenumber, a series of measurements were performed at four pressures and the cross-

sections obtained at the various pressures were averaged with standard deviation of less than

5% of the mean value. Results are shown in Fig. 5 at three temperatures. In general, good

agreement was found between the FTIR and DFG measurements with a maximum difference

of about 10%. The DFG system has a narrower linewidth (~ 0.0001 cm-1) and can be applied

to in-situ propylene measurements in systems requiring high time resolution. This work will

be extended in future to use DFG system to measure IR cross-sections of propylene in a

shock tube at higher temperatures over the range of 800 – 1200 K.

6. Summary

This work presents new IR absorption cross-sections for propylene at spectral resolutions

between 0.08 and 0.5 cm-1 using Fourier Transform Infrared Spectroscopy. The

measurements were performed in the 400 – 6500 cm-1 wavenumber region with varying gas

temperatures from 296 K to 460 K. The estimated uncertainty in our cross-section

measurements was about 5% except at higher wavenumbers (3500 – 6500 cm-1) where the

uncertainty was about 8% due to the relatively weak absorption. Integrated IR band

intensities for a number of vibrational bands are also reported and compared with values

derived from PNNL and NIST databases at room temperature. Our results agree very well

with the two databases for all the bands considered here with a maximum difference of about

18  

4%. Moreover, we studied the influence of temperature on the IR cross-sections of

propylene. We observed that as the temperature increases, the absorption bands of all

structures exhibit a decrease in the peak cross-sections whereas the continuum absorption

increases. We also found that the integrated band intensities of 2500 – 3200 cm-1, 800 – 1060

cm-1 and 525 – 680 cm-1 spectral regions are insensitive to gas temperature between 296 and

460 K with a maximum variation of 4 %. A complementary study on the effect of

temperature on cross-sections was conducted by absorption spectroscopy using Difference

Frequency Generation (DFG) for certain wavenumbers in the 2850 to 2975 cm-1 range.

Reasonable agreement was achieved with a maximum difference of about 10 %. The use of

the DFG system can open up several opportunities for the development and application of

novel in-situ detection strategies for propylene and other fuels. The complete dataset

comprising the absorption cross-sections at different temperatures is available in electronic

format from the Chemical Kinetics & Laser Diagnostics Laboratory website at

http://kinetics.kaust.edu.sa/.

Acknowledgements

We are grateful for the financial support provided by King Abdullah University of Science of

Technology (KAUST) and the Clean Combustion Research Center (CCRC) at KAUST.

19  

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23  

 

Table 1: Configuration of FTIR spectrometer for propylene spectra measurements.

FTIR Spectrometer Configuration

Resolution 0.08 – 0.5 cm-1

Beamsplitter Potassium Bromide (KBr)

FTIR input aperture

Focal length of the collimator mirror

1.5 or 2.5 mm

100 mm

Detector DLaTGS

Light Source Globar (Mid-Infrared)

Optical path length (Reflex cell) 10 cm

FT phase correction, zerofill Mertz, 2x and 4x zero-filling

FT apodization function Boxcar

Pressure gauges Baratrons (20, 100 and 1000 Torr): ±0.05 % accuracy

24  

Table 2: Vibrational assignments of various IR bands of propylene measured at 296 K. Spectra are

measured at 0.09 cm-1 resolution. The peak positions in our measurements correspond to the maximum

values of the cross-sections (m: medium, w: weak, vw: very weak, s: strong). The vibrational assignments

are based on the study of Silvi et al. [24].

* Peak positions determined in liquid phase at low temperatures [24].

~ The peak position at 1451.04 cm-1 of ν7 band corresponds to the maximum cross-section in the current work. In [24], the

position at 1458.5 cm-1 corresponds to the minimum absorption of ν7 band.

Band Peak position, cm-1 (Silvi et al. [24])

Peak position, cm-1 (this work)

Vibrational assignment

ν1 (a') 3091.0 3091.62 (s) CH2 asymmetric stretch

ν2 (a') ---- ---- CH stretch

ν3 (a') 2991.0 2991.03 (w) CH2 symmetric stretch

ν4 (a') 2973.0 ---- CH3 asymmetric stretch

ν5 (a') 2931.9 2931.46 (s) CH3 symmetric stretch

ν6 (a') 1652.8 1653.18 (m) C=C stretch

ν7 (a') 1458.5~ 1451.04 ~ (w) CH3 asymmetric deformation

ν8 (a') 1414* ---- CH2 scissor

ν9 (a') 1378.0 1377.94 (m) CH3 symmetric deformation

ν10 (a') 1298.0 1297.86 (vw) CCH bend

ν11 (a') 1170 1170.04 (vw) C-C stretch

ν12 (a') 934.5 * --- CH2 rock

ν13 (a') 919 * --- CH3 rock

ν14 (a') 428 426.02 (m) CCC deformation

ν15 (a'') 2952.8 2954.30 (s) CH3 asymmetric stretch

ν16 (a'') 1442.5 1442.71 (s) CH3 antisymetric in plane bending

ν17 (a'') 1044.7 1045.04 (w) CH3 rocking + CH out of plane bending

ν18 (a'') 990.0 991.05(s) CH2 twisting+CH out of plane bending

ν19 (a'') 912 912.62 (s) CH2 wagging

ν20 (a'') 575.2 576.27 (s) CH out of plane bending +CH2 twisting+CH3 rocking

25  

Table 3: Integrated band intensities of C3H6 at 296 K for six regions. Spectra are measured at a spectral

resolution of 0.09 cm-1. Data are compared with NIST [27] and PNNL [28] databases. No data available

from PNNL and NIST for ν14 band over 400 – 480 cm-1. %δ = 100×(Sthis work−SPNNL or NIST)/SPNNL or NIST.

Wavenumber range,

cm-1

S band (296 K),

cm-2 atm-1

(This work)

Integrated band

intensity x10-18,

cm/molecule

(This work)

Integrated band

intensity x10-18,

cm/molecule

(PNNL)

Integrated band

intensity x10-18,

cm/molecule

(NIST)

%δ

with

PNNL

%δ

with

NIST

2500 – 3200 411.02±10.0 16.58±0.4 16.82±0.2 17.17±0.6 - 1.7 - 3.6

1340 – 1550 74.77±2.5 3.01±0.1 3.03±0.05 3.10±0.2 - 0.7 - 2.9

1600 – 1700 44.51±2.5 1.79±0.1 1.80±0.04 1.76±0.1 - 0.6 + 1.7

800 – 1060 213.56±5.0 8.62±0.2 8.86±0.1 8.38±0.3 - 2.7 + 2.9

525 – 680 35.03±2.4 1.41±0.1 1.39±0.05 1.38±0.1 + 1.4 + 2.2

400 – 480 2.65±0.1 0.11±0.004 -- -- -- --

26  

Table 4: Effect of gas temperature on the integrated band intensities of propylene for various

wavenumber regions. The measured values are determined from spectra recorded at a spectral resolution

of 0.08 cm-1, except the range 2500-3200 in which measurements are done at 0.5 cm-1.

Temperature,

K

Integrated band intensity,

x 10-18, cm/molecule

2500 – 3200 cm-1

Integrated band intensity,

x 10-18, cm/molecule

800 – 1060 cm-1

Integrated band intensity,

x 10-18, cm/molecule

525 – 680 cm-1

296 16.58±0.4 8.62±0.2 1.41±0.1

318 16.41±0.5 8.48±0.3 1.37±0.2

343 16.32±0.5 8.47± 0.3 1.37± 0.2

362 16.38±0.5 8.47±0.3 1.39±0.2

389 16.36±0.5 8.56±0.3 1.38±0.2

424 16.28±0.5 8.68±0.3 1.36±0.2

460 16.27± 0.5 8.85±0.3 1.42±0.2

 

Figur

reject

re 1. Experim

tion.

mental arranggement for th

27 

he DFG absoorption measuurements usin

ng common mode

28  

 

Figure 2. IR cross-section measurements of propylene at 296 K. The main features are identified and

summarized in Table 2 with peak positions. The spectral resolution is 0.09 cm-1 for 400 – 3200 cm-1 and is

0.18 cm-1 for 3200 – 6500 cm-1 spectral region. Due to very weak absorbance, spectra in 400 – 480 cm-1

and 3500 – 6500 cm-1 wavenumber regions are recorded in pure gas at atmospheric pressure. All other

spectra are obtained in 10% C3H6/N2 for a total pressure of 200 Torr.

 

3500 4000 4500 5000 5500 6000 65000.0

1.0x10-21

2.0x10-21

3.0x10-21

4.0x10-21

2500 2600 2700 2800 2900 3000 3100 3200 3300 3400 35000.0

5.0x10-20

1.0x10-19

1.5x10-19

2.0x10-192000 2100 2200 2300 2400 2500

0.0

1.0x10-21

2.0x10-21

3.0x10-21

4.0x10-21

wavenumber, cm-1

(f)

(e)

(d)

ν15ν5

ν1

C3H

6 cro

ss-s

ectio

n, c

m2 /m

olec

ule

 

1300 1400 1500 1600 1700 1800 1900 20000.0

3.0x10-20

6.0x10-20

9.0x10-20

1.2x10-19

500 600 700 800 900 1000 1100 1200 13000.0

3.0x10-19

6.0x10-19

9.0x10-19

1.2x10-18

1.5x10-18400 420 440 460 480

0.0

1.5x10-21

3.0x10-21

4.5x10-21

6.0x10-21

(c)

(b)

ν16

ν9

ν19

ν14

(a)

ν18ν20

ν6

C3H

6 cr

oss

sect

ion,

cm

2 /mol

ecul

e

ν17

29  

 

 

Figure 3. Linear dependence of the integrated absorbance (base-e) of C3H6 as a function of the

product of C3H6 partial pressure and optical path-length for various wavenumber regions. All spectra

are recorded in a mixture of 10% C3H6/N2, except 400 – 480 cm-1 region where measurements are

recorded in pure C3H6 due to low absorbance.

 

 

 

0 2 4 6 8 100

10

20

300.00 0.25 0.50 0.75 1.000

102030400.00 0.25 0.50 0.75 1.000

200

400

400-480 cm-1

P . L, atm.cm

1600-1700 cm-1

525-680 cm-1

2500-3200 cm-1

800-1060 cm-1

1340-1550 cm-1

C3H

6 inte

grat

ed A

bsor

banc

e, c

m-1

30  

Figure 4. IR cross-section measurements of propylene at 296 and 460 K. The spectral resolution is

0.08 cm-1 in regions of figures (a) and (b) and 0.5 cm-1 for (c). Spectra are recorded in pure C3H6 at 20

Torr.

2800 2900 3000 3100 32000.0

5.0x10-20

1.0x10-19

1.5x10-19

2.0x10-19800 850 900 950 1000 1050

0.03.0x10-19

6.0x10-19

9.0x10-19

1.2x10-18540 560 580 600 620 640 660 680

0.05.0x10-20

1.0x10-19

1.5x10-19

2.0x10-19

ν5

ν15

ν1

(c)

(b)

296 K 460 K

Wavenumber, cm-1

(a)

C3H

6 cro

ss-s

ectio

n, c

m2 /m

olec

ule

575.5 576.0 576.5 577.00.0

5.0x10-20

1.0x10-19

1.5x10-19

2.0x10-19

ν18

ν19

ν20

ν20

31  

 

Figure 5. Comparison of FTIR and DFG cross-sections of propylene over the 2850 – 2975 cm-1 range

at three gas temperatures: (a) 296 K, (b) 343 K and (c) 383 K. The symbols show the results obtained

by the DFG system (linewidth ~ 0.0001 cm-1) while the lines are FTIR measurements (spectral

resolution of 0.5 cm-1). Measurements are performed in pure C3H6 at 20 Torr.

 

2850 2875 2900 2925 2950 29750.0

5.0x10-20

1.0x10-19

1.5x10-19

2.0x10-19

2850 2875 2900 2925 2950 29750.0

5.0x10-20

1.0x10-19

1.5x10-19

2.0x10-192850 2875 2900 2925 2950 2975

0.05.0x10-20

1.0x10-19

1.5x10-19

2.0x10-19

2950.87 cm-1

Wavenumber, cm-1

C3H

6 cro

ss-s

ectio

n, c

m2 /m

olec

ule

2931

.45

cm-1

2931

.69

cm-1

2918

.38

cm-1

2915

.67

cm-1

2867

.91

cm-1

2854

.83

cm-1

(c) 389 K

(b) 343 K

(a) 296 K

32  

Highlights

• Temperature dependence of IR absorption cross-section and integrated band of C3H6. • The peak cross-section decreases by 20 to 54% with increasing temperature to 460 K. • Integrated bands agree with PNNL and NIST and are independent on temperature. • Reasonable agreement was achieved between FTIR and DFG spectroscopy. • Accurate IR cross-sections and integrated bands can be used for atmospheric study.