Quantum Cascade Laser Measurements of Line Intensities,N 2 -,O 2 - and Ar- Collisional Broadening Coefficients of N 2 O in the ν 3 Band Near 4.5 µm

Abstract

This study deals with precise measurements of absolute line intensities, N₂-, O₂- and Ar- collisional broadening coefficients of N₂O in the P-branch of the ν₃ vibrational band near 4.5 µm. Collisional broadening coefficients of N₂O-air are derived from the N₂- and O₂- broadening contributions by considering an ideal atmospheric composition. Studies are performed at room temperature for 10 rotational transitions over 2190–2202 cm^–1 spectral range using a distributed-feedback quantum cascade laser. To retrieve spectroscopic parameters for each individual transition, measured absorption line shape is simulated within Voigt and Galatry profiles. The obtained results compare well with previous experimental data available in the literature: the discrepancies being less than 4% for most of the probed transitions. The spectroscopic data reported here are very useful for the design of sensors used to monitor the abundance of N₂O in earth's atmosphere.

Keywords

Nitrous oxide absolute intensities collisional broadening infrared spectroscopy quantum cascade laser

Introduction

Nitrous oxide (N₂O), a potent greenhouse gas, is an important trace atmospheric constituent. Its tropospheric abundance has increased from approximately 273 ppb in 1750 to 325 ppb in 2012.¹ In higher altitudes, this molecule plays a crucial role in the chemistry of the troposphere and stratosphere involving ultraviolet solar photon absorption.² Soil and oceans are the main natural sources of N₂O production.^3–5 This molecule is also produced by the use of nitrogen fertilizers,^6,7 combustion processes and fossil fuels.^8–12 Since N₂O is the precursor for NO_x production that leads to ozone destruction in the higher atmosphere,^13,14 detection and concentration distribution measurements of N₂O have great environmental significance. For atmospheric sensing purposes, vertical profiles of nitrous oxide have been monitored using a ground-based mm-wave spectrometer.^15–17 In the mid-infrared (IR) spectral domain, the N₂O molecule has several strong absorption bands which have been used to measure its concentration and altitude distribution in Earth's atmosphere.^18–21 Remote sensing of the Earth’s atmosphere, however, requires accurate knowledge of the spectroscopic parameters of N₂O. For precise atmospheric monitoring, accurately measured and assigned line positions, line intensities and pressure broadening coefficients are required.

Nitrous oxide has its strongest vibrational bands in the IR spectral region.^22,23 The ν₃ band appears over 2160–2260 cm^–1 range (i.e., near 4.5 µm) and has its origin near 2224 cm^–1. The ν₁ band is relatively strong and appears near 1285 cm^–1 (i.e., 7.8 µm). The ν₂ band appears at long wavelengths near 17 µm (i.e., 589 cm^–1).^24–26 Both the 2224 cm^–1(ν₁) and 1285 cm^–1(ν₃) vibrational bands show doublet features corresponding to the P- and R-branches and their rotational structures can be resolved.

Over the years, the study of the ν₃ band of N₂O with IR spectroscopy, achieving higher and higher resolution, led to the precise knowledge of spectroscopic parameters, such as pressure-induced line shift, line intensities and collisional broadening coefficients. Using grating spectroscopy, Lowder²⁷ reported room-temperature measurements of line intensities belonging to the ν₃ fundamental and a combination band. He also reported half-width measurements of N₂-broadened lines in the R-branch (R4 to R50 lines) of the ν₃ band. Boissy et al.²⁸ combined calculations and measurements to determine line intensities at 300 K for 30 vibration rotation lines in the P-branch of ν₃ band. Lacome et al.²⁹ obtained Fourier transform infrared (FT-IR) measurements in the 4–8 µm spectral domain to measure self-, N₂- and O₂- broadening of N₂O over the temperatures range of 220–296 K. Moreover, the air-broadened values of N₂O were derived from the combination of N₂- and O₂- broadened parameters. Henry et al.³⁰ obtained N₂- and O₂- broadening parameters of N₂O for transitions in the R-branch of ν₃ band at 195 and 297 K using a vacuum grating spectrometer. Using FT-IR spectroscopy, Toth³¹ measured self-broadened line-width and frequency-shift parameters at room temperature for 136 lines of N₂O in the ν₃ band over the spectral region of 1800–2630 cm^–1 and found that the coefficients have a weak J-dependence. Margottin-Maclouet et al.³² studied N₂O pressure-induced line shift in the ν₃ band for P- and R-branches between 1800 and 3100 cm^–1 perturbed by N₂, O₂, He, Ar and Xe using FT-IR spectroscopy. Accurate pressure-shift with N₂ and air-broadening coefficients over 1800 – 4800 cm^–1 were measured at room temperature by Toth³³ for more than 500 vibration–rotation lines by means of a 0.011 cm^–1 spectral resolution FT-IR spectrometer. Moreover, the J-dependence of spectroscopic parameters was also parameterized using polynomial expressions.³³ Nemtchinov et al.³⁴ measured N₂O absolute line intensities and broadening coefficients due to N₂, O₂ and air perturbers for 41 transitions belonging to the ν₃ band in the R-branch between 2224 and 2253 cm⁻¹ at temperature values of 216, 250 and 296 K using FT-IR spectroscopy. They expressed the J-dependence using empirical fitting to extrapolate the measured broadening data to other transitions. Fukabori et al.³⁵ obtained line intensities and N₂- and O₂- broadening parameters in the ν₃ band as well as in the ν₁ + 2ν₂ and 2ν₁ combination bands of N₂O at 296 K using an FT-IR spectrometer with a 0.01 cm^–1 resolution. More recently, Loos et al.³⁶ reported measurements of air-broadening, pressure-shift, speed-dependence and line-mixing parameters for the ν₃ band of N₂O using FT-IR spectroscopy.

Until recently, however, few measurements have been performed using narrow line-width laser absorption spectroscopy to study the ν₃ band of N₂O near 4.5 µm. Margottin-Maclouet et al.³² used a tunable diode laser spectrometer to measure N₂O pressure-shifts with N₂, He and Ar perturbers for several transitions near 2200 cm^–1. To our knowledge, broadening parameters of N₂O-Ar have not been studied in the 2190–2202 cm^–1 region. The current study deals with precise measurements of absolute line intensities, N₂-, O₂- and Ar- collisional broadening coefficients in the ν₃ band of N₂O using a quantum cascade laser (QCL) system. Experiments are performed at room temperature for 10 rotational transitions (P-branch) accessible to our laser system over 2190–2202 cm^–1 spectral range. To retrieve spectroscopic parameters for each individual transition, measured absorption line shapes were fitted using Voigt and Galatry profiles. The second section describes the experimental apparatus. In the third section, we present Voigt and Galatry profiles to retrieve N₂O spectroscopic parameters. Line intensity and broadening coefficient results are presented in the fourth section and compared with literature data. Since our laser system covers only 10 transitions in the ν₃ band of N₂O, we report a parameterization of the measured broadening coefficients using empirical expressions to extend the study to other transitions in the P-branch.

Experimental

The experimental setup is schematically illustrated in Figure 1. It consists of an IR laser source producing few milliwatt (mW) power near 2185 cm^–1, a temperature control unit (TCU 200), a current source, a function generator, a stainless-steel absorption cell wedged with KBr windows, an IR sensitive detector and an oscilloscope system. The IR source is a distributed-feedback (DFB) QCL procured from a commercial supplier (Alpes Lasers ). Heat dissipation from the laser was enhanced by mounting it on an aluminum plate with recirculating water. Laser emission wavelength can be varied by changing the laser temperature or injection current. Emission frequency was measured using a wavemeter that has an absolute accuracy of ±2 × 10^–3nm and a resolution of ±0.001 nm (Bristol Instruments 721B ).³⁷ To perform wavelength modulation, a waveform generator (Stanford Research Systems DS-345 ) supplied a saw-tooth ramp of 80 Hz frequency and 0.5 V_pk–pk to the laser head. The operating conditions (laser temperature and current) used to access of N₂O transitions with our laser system are listed in Table 1. The line-center positions are obtained from HITRAN 2012 database.²³

Figure 1.

Experimental setup.

Table 1

Transitions of N₂O molecule accessible to the laser system used in the present work. The operation conditions of the laser to reach the desired transition in the P-branch are shown.

Assignment	Wavenumbers*, cm^–1	Laser Temperature, ℃	Laser Current, mA
P 25	2200.74656	–33.0	35.7
P 26	2199.73737	–28.7	58.0
P 27	2198.72140	–24.8	71.0
P 28	2197.69865	–20.4	80.3
P 29	2196.66912	–16.5	96.0
P 30	2195.63281	–13.4	116.0
P 31	2194.58973	–10.0	129.5
P 32	2193.53989	–4.0	128.0
P 33	2192.48329	+ 0.6	129.0
P 34	2191.41993	+ 4.6	133.8

Wavenumber centers are obtained from HITRAN 2012 database.²³

The IR light is split into two beams by the use of a thin beam splitter (Figure 1). One beam passes through the optical cell and the other beam is directed to a wavemeter to measure the laser emission frequency. The transmitted laser light is focused onto a photodetector using a parabolic mirror with a focal length of 4.5 cm. The photodetector (Vigo System PVMI–3TE–10.6) is thermo-electrically cooled and has an active area of 4 mm². The background signal is recorded under the same conditions but without a sample. The output from the photodetector is sent to a 1 GHz oscilloscope (Lecroy ). The stainless-steel absorption cell can be operated over a range of pressures and temperatures. Research grade (99.999 %) N₂O, O₂, N₂ and Ar were supplied by Abdullah Hashim Gases . Primary impurities include nitrogen, ethylene, ethane and carbon dioxide. Gas pressure in the absorption cell is monitored with a baratron capacitance manometer (MKS Instruments; full-scale range of 100 Torr). To analyze measured line shapes, we employed the experimental procedure described previously.^38–40 To determine line strengths and pressure broadening coefficients in the spectral range of interest, absorption was measured over a wide range of gas pressures and for fixed values of path length (11.5 cm), temperature (297 K) and mixture composition (0.15% N₂O/perturber gas). A typical scan is shown in Figure 2a for the P(25) transition belonging to the ν₃ band (0001←0000) of N₂O. Gas pressures in the range of 10–100 Torr have been used in the present work. A Germanium etalon with a free spectral range (FSR) of 0.01634 cm^–1 was used for relative frequency calibration (Figure 2b). Line intensity values were determined from integrated absorbance values which linearly depend on the product between gas pressure and optical path length. Broadening coefficients in different perturbed gases were measured from the half-width at half-maximum of the absorbance of each transition as a function of the gas pressure.

Figure 2.

(a) An example transmission signal for P(25) transition at 2200.74656 cm^–1; mixture: 0.15%N2O/O2; T = 297 K. (b) Ring interferometer signal obtained from the solid etalon used for frequency calibration.

Data Analysis

For direct absorption spectroscopy, transmitted intensity, I_t, of a monochromatic laser source through a gaseous sample is given by Beer-Lambert law

I t = I 0 exp (- α X (ν)) = I 0 exp (- SX (ν) pN rel L)

(1)

where I₀ is the transmitted intensity in the absence of absorber, α is the integrated absorption (in cm⁻¹), X(ν) is the area normalized line-shape function (in cm), S is the line strength (in cm⁻²/atm), p is the total gas pressure (in atm), N_rel is the relative concentration of the N₂O absorber (dimensionless), L is the interaction length (in cm) and ν is the frequency of the light (in cm⁻¹). Moreover, the line strength, S (in cm⁻²/atm), can be related to the molecular line strength, S' (in cm⁻¹/molecule cm⁻²), which is often tabulated in spectral databases, such as HITRAN²³ and GEISA⁴¹. The two are related as

S' = \frac{n x S}{pN rel} = 1.01325 \times 105 \frac{S}{kT} = 2.48 \times 1019 (\frac{T 0}{T}) S

(2)

where n_x is the number density of the absorber (in cm⁻³), k is the Boltzmann constant, T is the temperature of the gas and T₀ = 273 K.

Generally, the absorption line shape is modeled by a Voigt profile which is a convolution of a Gaussian and Lorentzian functions and is given as

V (x, y, A) = A \frac{y}{π} \int_{\infty}^{- \infty} \frac{exp (- t 2)}{ay 2 + (x - t) 2} d t

(3)

where the integration parameter (A), the normalized frequency (x) and collisional coefficients (y) are given as

A = \frac{200 A 0 \sqrt{\ln 2}}{γ D \sqrt{π}}, x = \sqrt{\ln 2} \frac{ν - ν 0}{γ D} and y = \sqrt{\ln 2} \frac{γ c}{γ D}

(4)

The frequency $ν 0$ (cm^–1) is the line center of the considered transition. The parameter $γ C$ (cm^–1) is the collisional width and $γ D (cm - 1)$ is defined as (1/e) Doppler half-width at half-maximum (HWHM). The Doppler full-width at half-maximum, $Δ ν D (cm - 1)$ , is related to $γ D$ by

Δ ν D = 2 \sqrt{\ln 2} γ D = 7.162 \times 10 - 7 ν 0 \sqrt{T / M}

(5)

where T = 297 K is the gas temperature and M = 44 is molecular weight of the absorbing species (N₂O).

The Voigt profile can present systematic differences with the measured line shape and can thus lead to inaccuracy in the deduced line intensities and collisional broadening data. When Doppler broadening is dominant, collisional narrowing of absorption line shapes can be significant.^42,43 When collisional narrowing/Dicke effect is considered, as in the case of Rautian and Galatry profiles, the fit of the observed line profiles is usually very good and the collisional broadening coefficients are quite independent of pressure. To incorporate the Dicke effect, the Galatry line shape, known as a soft-collision model, was incorporated in the present work to obtain spectroscopic parameters of N₂O. The Galatry line shape function is given by the following expression

G (x, y, z) = \frac{1}{\sqrt{π}} Re (\int_{0}^{\infty} \exp {ixt - yt + \frac{1}{2 z 2} [1 - zt - exp (- zt)]} d t)

(6)

where z is a normalized parameter related to the collisional narrowing coefficient β (in cm^–1atm^–1) as

z = P β / γ D

(7)

It should be noted that when the normalized parameter z = 0, the Galatry profile is reduced to the Voigt profile. To analyze experimental line shape data using Galatry profile, the retrieval program adjusts collisional width, collisional narrowing, line center and integrated area. During the fitting process, the program was initially provided with frequency position, line intensity and collisional width from the HITRAN 2012 database²³ as initial guess values.

Using Voigt or Galatry profiles, line intensities (S) and collisional broadening coefficients ( $γ i$ ) were deduced using the following equations

A = PLX N 2 O S

(8)

γ c = P [γ self . X N 2 O + (1 - X N 2 O) γ i]

(9)

where A is the integrated absorbance, P (atm) is the total gas pressure, L (cm) is the optical path length of the absorbing medium,

X N 2 O

is the mole fraction of absorbing species (N₂O),

γ C

(cm^–1) is the measured HWHM,

γ self

(cm^–1atm^–1) is the self-broadening coefficient (N₂O–N₂O) and

γ i

(cm^–1atm^–1) is the collisional broadening coefficient of N₂O with perturbing gas (i) (γ _i ; i = N₂, O₂ or Ar).

Results and discussion

Voigt and Galatry line profiles are used to determine spectroscopic parameters, line intensity (S) and broadening coefficient (γ_i), of N₂O at room temperature. Figure 3a shows a representative case where the two profiles have been used to fit the measured P(25) transition in the ν₃ band of N₂O at 297 K. The two profiles agree very well the experimental line shape. Figure 3b shows the residuals which represent percentage difference between measured and calculated values, (M − C) × 100. We can see the typical W-shape signature Voigt residual and the improvement achieved by considering the Galatry profile. This small difference is due to the Dicke narrowing effect being incorporated in the Galatry model. Such collisional narrowing effects can manifest in the peak absorption when the Voigt profile is used. At the pressure range used here (10–100 Torr), Doppler broadening is relatively small compared to collisional broadening. Therefore, the Dicke effect is quite small for the present study. The collisional narrowing parameter, β, for the studied transitions is too small to be accurately determined from our measurements. For the conditions of this work, the collisional narrowing effects can thus be neglected. Another physical effect that can be important for species with closely spaced rotational lines is “line mixing”. However, we believe that at the low pressures of current study, line mixing does not play any significant role.

Figure 3.

(a) An example of the measured absorption line shape for P(25) transition of N₂O at 2200.74656 cm^–1. Measurements are performed with 0.15% N₂O/O₂ at a total pressure of 51 Torr and at a temperature of 297 K. Measured data are simulated with Voigt and Galatry profiles. (b) The residual obtained from the two least-square fittings.

Absolute Line Intensities

Absolute line intensities for the ten transitions accessible to our laser system were determined. For precise measurements with high accuracy, each transition was measured at six pressures. A representative plot of the integrated absorbance versus the product of N₂O partial pressure and optical path length is shown in Figure 4. Integrated absorbance values obtained from the Voigt and Galatry profiles agree quite well. The line intensity (S) obtained from the slope of the linear line (Eq. 8) for a given data set routinely yielded a relative standard deviation of < 2% and a high linear correlation of 0.998 with zero-intercept.

Figure 4.

(a) Integrated absorbance versus the product of N₂O partial pressure and optical path length (PLX_N2O) for the P(25) transition; mixture: 0.15% N₂O/O₂; T = 297 K. Integrated absorbance values are derived from the Voigt and Galatry fits. The lines represent linear fits for absolute line intensity determination. (b) Percentage difference between the integrated absorbance determined from Galatry and Voigt profiles.

The resulting line intensities derived using the Voigt and Galatry models are listed in Table 2. The effect of narrowing is very small and this is clearly demonstrated in the difference of the intensities obtained from the Voigt and Galatry approaches. The errors listed in Table 2 for the measured absolute line intensities are attributed to the mole fraction of the prepared gas mixture, the optical path length, deviation from the analytical fitting, absorbance measurement and the gas temperature. Line intensities for each transition were measured using three gas mixtures (N₂O/N₂, N₂/O₂ and N₂/Ar) and the values reported in Table 2 are the average obtained from the three mixture values. Measured line intensities compare extremely well with HITRAN 2012²³ and experimental data of Toth³¹ with all deviations being less than 4%.

Table 2

Measured line intensities for the ten transitions of N₂O belonging to the P-branch of the ν₃ band near 4.5 µm. Comparison is made with HITRAN database²³ and Toth.³¹ The last three columns present the percentage differences: χ between Galatry and Voigt; δ between Galatry and Rothman et al.²³ and ɛ between Galatry and Toth.³¹

Assignment	Wavenumbers, cm^–1	Line intensities (S′), 10^–19(cm^–1/molecule cm^–2)				Difference, %
		This work (Voigt)	This work (Galatry)	Rothman et al.²³	Toth³¹	χ, %	δ, %	ɛ, %
P 25	2200.74656	6.704 ± 0.212	6.789 ± 0.212	6.693	6.698	+ 1.2	+1.4	+1.3
P 26	2199.73737	6.298 ± 0.302	6.325 ± 0.302	6.290	6.294	+0.4	+0.6	+0.5
P27	2198.72140	5.904 ± 0.342	5.984 ± 0.342	5.846	5.850	+1.4	+2.3	+2.2
P28	2197.69865	5.496 ± 0.243	5.510 ± 0.243	5.403	5.406	+0.25	+1.9	+1.9
P29	2196.66912	4.901 ± 0.10	5.022 ± 0.10	4.959	4.963	+2.4	+1.2	+1.2
P30	2195.63281	4.671 ± 0.303	4.701 ± 0.303	4.556	4.559	+0.6	+3.1	+3.0
P31	2194.58973	4.075 ± 0.433	4.175 ± 0.433	4.153	4.156	+2.4	+0.5	+0.4
P32	2193.53989	3.750 ± 0.361	3.780 ± 0.361	3.746	3.724	+0.8	+0.9	+1.5
P 33	2192.48329	3.388 ± 0.332	3.405 ± 0.332	3.375	3.377	+0.5	+0.9	+0.8
P34	2191.41993	3.070 ± 0.332	3.150 ± 0.332	3.028	3.030	+2.5	+3.9	+3.8

Foreign Gas Collisional Parameters

N₂- and O₂-broadening coefficients

To obtain foreign gas broadening coefficients of N₂O, the line shape is recorded at a range of pressures of N₂, O₂ and Ar for a fixed N₂O mole fraction of 0.15%. The broadening effect may then be derived from a plot of HWHM versus partial pressure of N₂O according to Eq. 9. Absorption spectra of N₂O were measured at a series of pressures between 10 and 100 Torr to infer collisional–broadening coefficients of N₂O with high precision. Figure 5 presents an example of the integrated absorbance versus total gas pressure for the P(25) line in the ν₃ band of N₂O. Measured half-width is clearly a linear function of the pressure. Excellent correlation coefficients of > 0.989 are obtained from the linear fits. Half-widths and linear fits obtained with Voigt and Galatry models are quite similar. To determine foreign gas broadening parameters, the small contribution of self-broadening coefficients is included (see Eq. 9.), where self-broadening coefficients are taken from HITRAN database.²³ Tables 3 and 4 list the results obtained for N₂- and O₂- broadening coefficients at 297 K for P(25)–P(34) transitions in the ν₃ band of N₂O. As in the case of the line intensity, we did not observe significant difference between the collisional broadening coefficients obtained with the Voigt and Galatry models. The resulting values are quite same and suggest negligible Dicke narrowing effect under our experimental conditions. However, the Voigt profile does result in systematically smaller values than those determined using the Galatry model.

Figure 5.

(a) Plot of the collisional half-widths versus N₂O total pressure (P) for the P(25) transition with 0.15% N₂O/O₂ at 297 K. Half-widths are derived from the Voigt and Galatry profiles. The slopes of the best-fit lines, passing through zero, represent collisional broadening coefficients γ₀. The γ₀^self data of N₂O are taken from HITRAN 2012 database. (b) The percentage difference between collisional half-widths determined from Galatry and Voigt profiles.

Table 3

Collisional-broadening coefficients of N₂O with N₂ ( $γ N 2$ ) at 297 K. Literature data measured using FT-IR spectroscopy are also shown for comparison.

Assignment	$γ N 2$ , 10^–3(cm^–1atm^–1)
	This work (Voigt)	This work (Galatry)	^aLowder²⁷	^bLacome et al.²⁹	^cHenry et al.³⁰	^dToth³³	^eNemtchinov et al.³⁴	^fFukabori et al.³⁵
P 25	75.2 ± 0.7	76.1 ± 1.2	71.1	74.43	75.7	75.4	75.7	77.21
P 26	75.0 ± 0.9	75.8 ± 0.8	71.5	75.00	75.2	75.0	75.4	76.34
P 27	74.4 ± 0.8	75.5 ± 0.6	70.4	75.00	74.8	74.5	75.0	76.03
P 28	74.3 ± 0.4	75.1 ± 1.1	68.1	75.44	74.4	74.6	74.4	76.10
P 29	74.1 ± 1.0	74.7 ± 1.2	70.4	74.97	74.0	74.4	74.2	75.65
P 30	74.0 ± 1.1	74.4 ± 1.3	68.9	76.19	73.6	74.1	74.0	75.35
P 31	73.8 ± 1.1	74.2 ± 0.6	70.4	74.64	73.3	73.9	73.7	74.64
P 32	73.6 ± 1.3	73.9 ± 1.2	70.8	75.21	72.9	74.1	73.6	74.48
P 33	73.3 ± 1.2	73.7 ± 1.2	91.9	74.50	72.6	73.3	73.2	74.49
P 34	73.1 ± 0.6	73.4 ± 0.4	69.4	70.77	72.3	73.5	73.0	74.25

Lowder²⁷: Broadening parameters belonging to the ν₃ band of N₂O at 297 K were taken from the smoothed values in R-branch. Measurements are performed using a grating monochromatic.

Lacome et al.²⁹: Broadening parameters belonging to the ν₃ band of N₂O at 295 K were taken from the R-branch lines. Measurements are performed using FT-IR spectroscopy at 5.4 × 10^–3cm^–1 resolution.

Henry et al.³⁰: Broadening parameters were taken from the smoothed values of R- branch lines in the ν₃ band of N₂O at 297 K. Measurements are performed using vacuum grating spectrometer.

Toth³³: Broadening parameters were taken from the measured values of R- branch lines in the ν₃ band of N₂O at 296 K. Measurements are performed using FT-IR spectroscopy at 0.011 cm^–1 resolution.

Nemtchinov et al.³⁴: Broadening parameters belonging to the ν₃ band of N₂O at 296 K were measured between 2224 and 2253 cm^–1. Measurements are performed using FT-IR spectroscopy at 0.002 cm^–1 resolution.

Fukabori et al.³⁵: Broadening parameters belonging to the ν₃ band of N₂O at 299 K were taken from the P- branch. Measurements are performed using FT-IR spectroscopy at 0.01 cm^–1 resolution.

Table 4

Collisional-broadening coefficients of N₂O with O₂ ( $γ O 2$ ) at 297 K. Literature data measured using FT-IR spectroscopy are also shown for comparison.

Assignment	$γ O 2$ , 10^–3(cm^–1atm^–1)
Assignment	This work (Voigt)	This work (Galatry)	Lacome et al.²⁹	Henry et al.³⁰	Nemtchinov et al.³⁴	Fukabori et al.³⁵
P 25	63.5 ± 1.8	64.1 ± 1.5	63.13	64.6	64.0	63.64
P 26	62.9 ± 0.9	63.3 ± 1.0	62.84	64.2	64.0	62.54
P 27	62.8 ± 0.6	63.1 ± 0.6	61.55	63.8	63.7	62.77
P 28	62.7 ± 1.5	63.0 ± 1.4	62.01	63.4	63.1	63.22
P 29	62.4 ± 2.0	62.9 ± 1.3	61.32	63.1	62.6	62.76
P 30	62.3 ± 1.9	62.7 ± 1.6	61.80	62.7	62.4	62.51
P 31	62.0 ± 1.1	62.4 ± 0.6	61.88	62.4	62.4	62.08
P 32	61.8 ± 2.0	62.0 ± 1.6	60.32	62.1	62.3	61.92
P 33	61.7 ± 1.6	61.9 ± 1.6	60.45	61.9	62.3	62.02
P 34	61.6 ± 0.8	61.8 ± 1.2	60.14	61.7	61.7	61.95

Comparisons of broadening coefficients with available data in the literature, measured by FT-IR spectroscopy, are also shown in Tables 3 and 4. Current work is the first study of broadening coefficients of N₂O in this wavelength range using a narrow line width tunable laser source. Values reported by Lowder²⁷ are systematically smaller than the ones measured here. Our results are in reasonable agreement with the tabulated values of Lacome et al.²⁹ and Henry et al.³⁰ Our data are also in good agreement with the FT-IR experimental work performed by Fukabori et al.³⁵ and Nemtchinov et al.³⁴

Air-Broadening Coefficients

Collisional broadening coefficients of air with N₂O may be derived from the broadening contributions of N₂ and O₂ by considering an ideal atmospheric composition and using the relation:

γ air = P O 2 γ O 2 + P N 2 γ N 2

, where

P O 2

and

P N 2

are the partial atmospheric pressures of O₂ and N₂ (0.20946 and 0.78084 atm, respectively) and the γ _i (i = O₂ and N₂) are the respective broadening coefficients determined experimentally. The calculated air-broadening coefficients are listed in Table 5 for the considered N₂O transitions. Our QCL-measured results are compared with the tabulated FT-IR spectroscopic data reported in the literature. The air-broadening coefficients of all works (Rothman et al.²³; Lacome et al.²⁹; Toth³³; Nemtchinov et al.³⁴; Loos et al.³⁶) agree with our results within 5%.

Table 5

Collisional-broadening coefficients of N₂O with air (γ_Air) at 297 K. Literature data measured using FT-IR spectroscopy are also shown for comparison.

Assignment	γ_Air, 10^–3(cm^–1atm^–1)
Assignment	This work (Voigt)	This work (Galatry)	Lacome et al.²⁹	Toth³³	Nemtchinov et al.³⁴	Rothman et al.²³	^gLoos et al.³⁶
P 25	72.74 ± 0.9	73.58 ± 1.3	72.6	72.6	73.2	72.6	–
P 26	72.46 ± 0.9	72.17 ± 0.8	72.3	72.5	73.1	72.2	73.80
P 27	71.96 ± 0.7	72.89 ± 0.6	72.0	72.0	72.6	71.9	73.43
P 28	71.86 ± 0.6	72.56 ± 1.2	71.7	71.6	72.1	71.6	73.03
P 29	71.64 ± 1.2	72.22 ± 1.2	71.5	71.6	71.8	71.3	72.81
P 30	71.54 ± 1.3	71.94 ± 1.4	71.2	71.4	71.6	71.1	72.50
P 31	71.32 ± 1.1	71.72 ± 0.6	71.0	71.1	71.3	70.9	72.28
P 32	71.12 ± 1.4	71.40 ± 1.3	70.8	70.8	71.2	70.7	72.09
P 33	70.86 ± 1.3	71.22 ± 1.3	70.7	70.8	70.9	70.5	–
P 34	70.68 ± 0.6	70.96 ± 0.6	70.5	70.3	70.6	70.3	–

Loos et al.³⁶: Broadening parameters belonging to the ν₃ band of N₂O at 294 K were taken from the P-branch. Measurements are performed using FT-IR spectroscopy.

Ar-Broadening Coefficients

This work reports, to our knowledge, the first measurements of Ar-broadening coefficients of the P-branch transitions of N₂O in the ν₃ band near 4.5 µm. Results obtained using the Galatry and Voigt profiles are compiled in Table 6. Compared to N₂, O₂ and air, Ar-broadening coefficients are much smaller. The absence of vibrational and rotational modes makes argon’s interaction with N₂O relatively weak.

Table 6

Collisional-broadening coefficients of N₂O with Ar (γ_Ar) at 297 K.

Assignment	γ_Ar, 10^–3(cm^–1atm^–1)
Assignment	This work (Voigt)	This work (Galatry)
P 25	54.85 ± 0.8	55.90 ± 1.0
P 26	55.2 ± 0.6	55.80 ± 0.8
P 27	55.04 ± 1.0	55.70 ± 0.4
P 28	55.46 ± 0.8	55.70 ± 0.7
P 29	54.82 ± 1.2	55.60 ± 1.2
P 30	53.59 ± 0.7	54.30 ± 0.4
P 31	52.85 ± 0.9	54.1 ± 0.6
P 32	52.58 ± 1.1	53.80 ± 0.7
P 33	52.90 ± 0.6	53.80 ± 0.4
P 34	52.12 ± 0.8	53.60 ± 0.7

Broadening Coefficient Parameterization

Since our tunable laser system covered only ten transitions in the ν₃ band of N₂O, the broadening coefficients measured in various foreign gases are parameterized to extend the study to other transitions in the P-branch for rotational quantum numbers (m) between 1 and 40. The obtained equation can be used to easily predict broadening coefficients in a computer simulation. In addition to the measured values of current work, the parameterization also includes FT-IR-measured N₂- and O₂- broadening data of Fukabori et al.³⁵ (Figure 6a–b) and the recent FT-IR measurements of air-broadening by Loos et al.³⁶ (Figure 6c). All data from the literature^35,36 are for the P-branch lines of the same ν₃ vibration–rotation band of N₂O. The following equation is used to fit the experimental broadening data

γ i (m) = (am + b)

(10)

Figure 6.

Parameterization of the measured N₂-, O₂-, Air- and Ar- broadening coefficients via Eq. 10 to extend data to P-branch transitions from m = 1 to 40 in the ν₃ band of N₂O.

The fitting constants, a and b, are listed in Table 7. The fits are shown in Figure 6a–d for N₂, O₂, air and Ar broadening coefficients. The fits reproduce the present experimental data to within 5% over the entire spectral range of P(1) – P(40).

Table 7

Constant parameters, a and b, determined using Eq. (10) for N₂-, O₂-, Air- and Ar- broadening coefficients.

Foreign gas	a, cm^–1atm^–1	b, cm^–1atm^–1
N₂	0.0694	0.17
O₂	0.0583	0.13
Air	0.0678	0.138
Ar	0.0457	0.264

Conclusions

Absolute intensities, N₂-, O₂- and Ar- collisional broadening coefficients of N₂O were measured at room temperature for ten transitions in the P branch of the ν₃ vibrational band. Measured absorption line shapes were fitted with Voigt and Galatry profiles to obtain spectroscopic parameters. Collisional broadening coefficients of air were derived from N₂- and O₂- broadening contributions by considering ideal atmospheric composition. The use of narrow line-width DFB QCL enabled measurements with high spectral resolution. Compared to the previously reported data with FT-IR spectroscopy, our laser-based measurements result in higher precision and higher accuracy of the deduced collisional broadening coefficients. Future work will attempt to expand our measurements to experimental conditions where line-mixing and collisional narrowing effects are more pronounced so that narrowing and mixing parameters can be retrieved by using more sophisticated line-shape models. We believe that the current measurement results are very useful for the design of laser-based diagnostic sensors aimed at monitoring the abundance of N₂O in the earth's atmosphere.

Footnotes

Acknowledgments

Research reported in this publication was supported by the King Abdullah University of Science and Technology (KAUST).

Conflict of Interest

The authors report there are no conflicts of interest.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

References

United Nations Environment Programme (UNEP). “Drawing Down N2O to Protect Climate and the Ozone Layer”. Nairobi, Kenya. 2013. http://www.unep.org/pdf/UNEPN2Oreport.pdf.

Iqbal

S.A.

Mido

Chemistry of Air & Air Pollution, New Delhi: Discovery Publishing House, 1995.

Keller

Kaplan

W.A.

Wofsy

S.C.

“Emissions of N2O, CH4, and CO2 from Tropical Forest Soils”. J. Geophys. Res 1986; 91(D11): 11791–11802.

Denman

K.L.

Brasseur

“Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change”. The 590 Physical Science Basis, Cambridge, UK: Cambridge University Press, 2007, pp. 499–587.

Chapuis-Lardy

Wrage

Metay

Chotte

J.-L.

Bernoux

“Soils, a Sink for N2O? A Review”. Global Change Biol 2007; 13(1): 1–17.

Davidson

E.A.

“The Contribution of Manure and Fertilizer Nitrogen to Atmospheric Nitrous Oxide since 1860”. Nat. Geosci 2009; 2: 659–662.

Bouwman

A.F.

Boumans

L.J.M.

Batjes

N.H.

“Emissions of N2O and NO from Fertilized Fields: Summary of Available Measurement Data”. Global Biog. Cycles 2002; 16(4): 6.1–6.13.

Pierotti

Rasmussen

R.A.

“Combustion as a Source of Nitrous Oxide in the Atmosphere”. Geophys. Res. Lett 1976; 3(5): 265–267.

Linak

W.P.

McSorley

J.A.

Hall

R.E.

Ryan

J.V.

Srivastava

R.K.

Wendt

J.O.L.

Mereb

J.B.

“Nitrous Oxide Emissions from Fossil Fuel Combustion”. J. Geophys. Res 1990; 95(D6): 7533–7541.

10.

Hao

W.M.

Wofsy

S.C.

Mcelroy

M.B.

Beer

J.M.

Togan

M.A.

“Sources of Atmospheric Nitrous Oxide from Combustion”. J. Geophys. Res 1987; 92(D3): 3098–3104.

11.

Kramlich

J.C.

Linak

W.P.

“Nitrous Oxide Behavior in the Atmosphere, and in Combustion and Industrial Systems”. Prog.Energy Combust. Sci 1994; 20(2): 149–202.

12.

Crutzen

P.J.

Mosier

A.R.

Smith

K.A.

Winiwarter

“N₂O Release from Agro-Biofuel Production Negates Global Warming Reduction by Replacing Fossil Fuels” Atmos. Chem. Phys 2008; 8: 389–395.

13.

Crutzen

Oppenheimer

“Learning About Ozone Depletion” Clim. Change 2008; 89: 143–154.

14.

Ravishankara

A.R.

Daniel

J.S.

Portmann

R.W.

“Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century”. Science 2009; 326(5949): 123–125.

15.

Parrish

deZafra

R.L.

Jaramillo

Connor

Solomon

P.M.

Barett

J.W.

“Extremely Low N2O Concentrations in the Springtime Stratosphere at McMurdo Station, Antarctica”. Nature 1988; 332: 53–55.

16.

Crewell

Cheng

de Zafra

R.L.

Trimble

“Millimeter Wave Spectroscopic Measurements over the South Pole, 1. A Study of Stratospheric Dynamics using N2O Observation”. J. Geophys. Res 1995; 100(D10): 20839–20844.

17.

Muscari

di Sarra

A.G.

de Zafra

R.L.

Lucci

Baordo

Angelini

Fiocco

“Middle Atmospheric O₃, CO, N₂O, HNO₃, and Temperature Profiles During the Warm Arctic Winter 2001–2002”. J. Geophys. Res 2007; 112(D14): D14304.

18.

Podolske

Loewenstein

“Airborne Tunable Diode Laser Spectrometer for Trace-Gas Measurement in the Lower Stratosphere”. App. Optics 1993; 32(27): 5324–5333.

19.

Gunson

M.R.

Farmer

C.B.

Norton

R.H.

Zander

Rinsland

C.P.

Shaw

J.H.

Gao

B.-C.

“Measurements of CH₄, N₂O, CO, H₂O, and O₃ in the Middle Atmosphere by the Atmospheric Trace Molecule Spectroscopy Experiment on Spacelab 3”. J. Geophys. Res 1990; 95(D9): 13867–13882.

20.

Scott

D.C.

Herman

R.L.

Webster

Ch.R.

May

R.D.

Flesch

G.J.

Moyer

E.J.

“Airborne Laser Infrared Absorption Spectrometer (ALIAS-II) for In Situ Atmospheric Measurements of N₂O, CH₄, CO, HCL, and NO₂ from Balloon or Remotely Piloted Aircraft Platforms”. Appl. Optics 1999; 38(21): 4609–4622.

21.

Michelsen

H.A.

Manney

G.L.

Gunson

M.R.

Rinsland

C.P.

Zander

“Correlations of Stratospheric Abundances of CH4 and NzO Derived from ATMOS Measurements”. Geophys. Res. Letters 1998; 25(15): 2777–2780.

22.

Sharpe

Johnson

Sams

Chu

Rhoderick

Johnson

“Gas-Phase Databases for Quantitative Infrared Spectroscopy”. Appl. Spectrosc 2004; 58(12): 1452–1461.

23.

Rothman

L.S.

Gordon

I.E.

Babikov

Barbe

Chris Benner

Bernath

P.F.

Birk

Bizzocchi

Boudon

Brown

L.R.

Campargue

Chance

Cohen

E.A.

Coudert

L.H.

Devi

V.M.

Drouin

B.J.

Fayt

Flaud

J.-M.

Gamache

R.R.

Harrison

J.J.

. “The HITRAN2012 Molecular Spectroscopic Database”. J. Quant. Spectrosc. Radiat. Transfer 2003; 130: 4–50.

24.

Weber

Marcos Sirota

Reuter

D.C.

“l-Resonance Intensity Effects and Pressure Broadening of N₂O at 17 µm”. J. Mol. Spectrosc 1996; 177(2): 211–220.

25.

Johns

J.W.C.

Weber

Sirota

J.M.

Reuter

D.C.

“Absolute Intensities in the ν2 Fundamental of N₂O at 17 µm”. J. Mol. Spectrosc 1996; 177(2): 203–210.

26.

Hartmann

J.-M.

Bouanich

J.-P.

Jucks

K.W.

Blanquet

Gh.

Walrand

Bermejo

Domenech

J.L.

Lacome

“Line-Mixing Effects in N₂O Q Branches: Model, Laboratory, and Atmospheric Spectra”. J. Chem. Phys 1999; 110(4): 1959–1968.

27.

Lowder

J.E.

“Band Intensity and Line Half-Width Measurements in N₂O Near 4.5µm”. J. Quant. Spectrosc. Radiat. Transfer 1972; 12(5): 873–880.

28.

Boissy

J.P.

Valentin

Cardinet

Ph.

Claude

M.L.

Henry

“Line Intensities of the ν3 Fundamental Band of Nitrous Oxide”. J. Mol. Spectrosc 1975; 57(3): 391–396.

29.

Lacome

Levy

Guelachvili

“Fourier Transform Measurement of Self-, N₂-, and O₂-Broadening of N2O Lines: Temperature Dependence of Linewidths”. Appl. Opt 1984; 23(3): 425–435.

30.

Henry

Margottin-Maclou

“N₂- and O₂-Broadening Parameters in the ν3 Band of ¹⁴N₂¹⁶O”. J. Mol. Spectrosc 1985; 111(2): 291–300.

31.

Toth

R.A.

“Line Strengths (900–3600 cm⁻¹), Self-Broadened Linewidths, and Frequency Shifts (1800–2360 cm⁻¹) of N₂O”. Appl. Opt 1993; 32(36): 7326–7365.

32.

Margottin-Maclou

Rachet

Henry

Valentin

“Pressure-Induced Line Shifts in the ν3 Band of Nitrous Oxide Perturbed by N₂, O₂, He, Ar and Xe”. J. Quant. Spectrosc. Radiat. Transfer 1996; 56(1): 1–16.

33.

Toth

R.A.

“N₂- and Air Broadened-Linewidths and Frequency-Shifts of N₂O”. J. Quant. Spectrosc. Radiat. Transfer 2000; 66(3): 285–304.

34.

Nemtchinov

Sun

Varanasi

“Measurements of Line Intensities and Line Widths in the ν3-Fundamental Band of Nitrous Oxide at Atmospheric Temperatures”. J. Quant. Spectrosc. Radiat. Transfer 2004; 83(3–4): 267–284.

35.

Fukabori

Aoki

Watanabe

“Measurements of the Line Strengths, N₂-, and O₂-Broadened Half-Widths in the ν3, ν1 + 2ν2 and 2ν2 Bands of 14N₂16O at Room Temperature”. Mol. Physics 2004; 102(8): 811–818.

36.

Loos

Birk

Wagner

“Pressure Broadening, -Shift, Speed Dependence and Line Mixing in the ν3 Rovibrational Band of N₂O”. J. Quant. Spectrosc. Radiat. Transfer 2015; 151: 300–309.

37.

Bristol Instruments, Inc. www.bristol-inst.com.

38.

Owen

Es-sebbar

Et.

Farooq

“Measurements of NH₃ Linestrengths and Collisional Broadening Coefficients in N₂, O₂, CO₂, and H₂O near 1103.46 cm⁻¹”. J. Quant. Spectrosc. Radiat. Transfer 2013; 121: 56–68.

39.

Sajid

M.B.

Es-sebbar

Et.

Farooq

“Measurements of Linestrengths, N₂-, Ar-, He- and Self-Broadening Coefficients of Acetylene in the ν4 + ν5 Combination Band using a cw Quantum Cascade Laser”. J. Quant. Spectrosc. Radiat. Transfer 2014; 148: 1–12.

40.

Es-sebbar

Et.

Farooq

“Intensities, Broadening and Narrowing Parameters in the ν3 Band of Methane”. J. Quant. Spectrosc. Radiat. Transfer 2014; 149: 241–252.

41.

N. Jacquinet-Husson, L. Crepeau, R. Armante, C. Boutammine, A. Chédin, N.A. Scott et al. The 2009 edition of the GEISA spectroscopic database, J. Quant. Spectrosc. Radiat. Transfer. 112(15): 2395–2445.

42.

Varghese

Ph.L.

Hanson

R.K.

“Collisional Narrowing Effects on Spectral Line Shapes Measured at High Resolution”. Appl. Optics 1984; 23(14): 2376–2385.

43.

D’Eu

J.-F.

Lemoine

Rohart

“Infrared HCN Lineshapes as a Test of Galatry and Speed Dependent Voigt Profiles”. J. Mol. Spectrosc 2002; 212(1): 96–110.

Quantum Cascade Laser Measurements of Line Intensities,N 2 -,O 2 - and Ar- Collisional Broadening Coefficients of N 2 O in the ν 3 Band Near 4.5 µm

Abstract

Keywords

Introduction

Experimental

Data Analysis

Results and discussion

Absolute Line Intensities

Foreign Gas Collisional Parameters

N2- and O2-broadening coefficients

Air-Broadening Coefficients

Ar-Broadening Coefficients

Broadening Coefficient Parameterization

Conclusions

Footnotes

Acknowledgments

Conflict of Interest

Funding

References

N₂- and O₂-broadening coefficients