Pressure-shift measurements of the oxygen A-band byFourier-transform spectroscopy

Christian Hill,a,* David A. Newnham,b and John M. Browna

a Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UKb Space Science and Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK

Received 25 September 2002; in revised form 21 December 2002

Abstract

The pressure shifts, d, of the (0–0) band of the b1Rþg –X

3R�g electronic transition of oxygen (the so-called �A-band�) have been

measured at room temperature using high-resolution Fourier transform spectroscopy. The rotational dependence of d has beenmeasured and characterised; the average value of the shift over the whole band is �0:009� 0:001cm�1 atm�1. No significant var-iation in d is found for different O2 mixing ratios in N2, and the results presented here are therefore appropriate for high-resolutionatmospheric applications using the O2 A-band.

� 2003 Elsevier Science (USA). All rights reserved.

1. Introduction

Despite being formally forbidden by spectroscopic

selection rules, the b1Rþg –X

3R�g electronic transition of

oxygen plays a significant role in atmospheric studies,

e.g. [1–3]. It is often important in these studies to know

the exact line-centre position of the component rota-tional transitions as a function of pressure. The results

presented here focus on the (0–0) vibrational band of the

b–X system, the so-called A-band of oxygen. Since the

half-width of these lines at 1 atm is typically 0:05cm�1, ashift of up to 0:01cm�1 may be significant and is com-parable to the atmospheric wind-induced Doppler shift

(measured by Balthasar et al. [4] to be a maximum of

�0.008 cm�1). In the atmosphere, the cumulative effectof pressure shifts at different altitudes is known to be

one reason for the observed asymmetry of absorption

lines [5].

Several studies of pressure shifts in the A-band have

been published, as briefly summarised in Table 1. It is

clear that there is still considerable disagreement be-

tween these studies (even for comparatively recent

measurements) and the uncertainties quoted for d are ashigh as 50%. In particular, the most commonly used

database for line parameters in atmospheric science, the

HITRAN list [6], currently recommends values for d inthis band which are up to half the magnitude of those

found in a recent study by Phillips and Hamilton [7].

The relative magnitude of the self-shift ðdself ) and air-shift ðdair) parameters is also undecided (see Table 1). Asnoted by Brown and Plymate [8], further work is nec-

essary to obtain a consensus about the A-band pressure

shift, and this need motivates the present study.

2. Experimental

All spectra were recorded at the Rutherford Appleton

Laboratory with a Bruker IFS 120HR high-resolutionFourier-transform spectrometer and a multi-pass optical

absorption cell in a White configuration [9]. A halogen

lamp (Phillips 7158, 150W) provided the spectrometer

light source through a 0.5mm aperture; all optical

windows were made of CaF2. The stainless-steel cell was

adjusted to give a total path length of 16.137m. Optical

filters (Corion S10-767-F) were chosen to isolate the

band pass and maximise the signal-to-noise ratio withina practical acquisition time. Interferograms were

co-added and processed using the Bruker acquisition

Journal of Molecular Spectroscopy 219 (2003) 65–69

www.elsevier.com/locate/jms

*Corresponding author. Fax. +44-0-1865-275410.

E-mail addresses: hill@physchem.ox.ac.uk (C. Hill), jmb@phy-

schem.ox.ac.uk (J.M. Brown).

0022-2852/03/$ - see front matter � 2003 Elsevier Science (USA). All rights reserved.doi:10.1016/S0022-2852(03)00017-1

processor and OPUS software [10]. Since line-centre

positions were of primary interest in this study, no nu-

merical apodisation (i.e., a box-car function) was ap-

plied in the Fourier transformation of the data, to

minimise line broadening. At the pressures used, most

lines were well-separated and ringing effects (side-lobe

artefacts introduced by truncating the interferogram

when the instrument lineshape is broader than the mo-lecular lineshape) were not significant. A typical spec-

trum is shown in Fig. 1, presented as transmittance as a

function of wavenumber.

In addition to air (Air Products Controlair, 99.6%

stated purity), several oxygen (BOC O2, 99.5% purity)

and nitrogen (BOC 99.998% N2 þAr, oxygen-free)mixtures were studied so that, in total, four different

mixing ratios of oxygen in nitrogen were used: xO2 ¼0:21, 0.50, 0.75, and 1.00 (i.e., pure oxygen). For each ofthese oxygen concentrations, spectra were taken at five

total pressures (100, 250, 500, 750, and 1000mbar), as

measured by a calibrated 1000 Torr full-scale Baratron

capacitance gauge (MKS model 390). The spectrometer

resolution was 0:005cm�1 for measurements at 100 and250mbar, and 0:01cm�1 at higher pressures. The spec-trometer and optical absorption cell were housed in anair-conditioned room and thermistor sensors inside the

cell monitored the gas temperature throughout the

acquisition time. All measurements were made at

295� 0:5K.To ensure safe handling of pure oxygen and oxygen-

enriched mixtures, gases were vented to the atmosphere

and the cell purged with nitrogen before being evacuated

through oil-filled rotary pumps.

To obtain the centre position of each unblended line,the smoothed first derivative of 50cm�1 regions of eachspectrum was calculated by the application of a 24-point

Savitzky–Golay filter with a highest conserved moment

of 4 [11]. The line centre was interpolated as the inter-

section of the centre of each first-derivative lineshape

with a linear first-derivative baseline as shown in Fig. 2.

As a check, the data were also inverted to form optical

depth spectra and the lines fitted to a model based on theVoigt lineshape. As there was a negligible difference in

the retrieved line-centre positions between these two

methods, the fitting procedure was only used for blen-

ded lines where the underlying curvature of nearby (<2half-widths) lines affects the determined line-centre po-

sition.

Negligible calibration drift was measured over the

course of each set of measurements at the same O2mixing ratio, as determined by scheduling the acquisi-

Table 1

Comparison of rotationally averaged pressure shifts in the oxygen A-band

Reference dself (cm�1 atm�1) dair (cm�1 atm�1) N 00 dependence?

This study �0:009� 0:001 �0:009� 0:001 Yes

Phillips and Hamilton [7] �0:010� 0:002 �0:015� 0:003 Yes

Brown and Plymate [8] �0:0051� 0:0015 �0:0052� 0:0015 Yes

Adiks and Dianov-Klokov [12,13] — �0:0078� 0:0020 No

Galkin [14,15] — �0:0040� 0:0026 Yes

Cheah et al. [16] �0:0011� 0:0025 — Not reported

Fig. 1. Typical spectrum of the O2 A-band. xO2 ¼ 0:75 in N2, ptot ¼ 750mbar, T ¼ 295K, 16.137m path length, and 0:01cm�1 resolution. Inset:comparison of the PP(17) line at 100mbar (grey) and 750mbar (black). The vertical scale has been adjusted for clarity.

66 C. Hill et al. / Journal of Molecular Spectroscopy 219 (2003) 65–69

tion of low-pressure spectra in between higher-pressure

measurements. Comparison of spectra taken several

hours apart at 100mbar showed a difference in mea-

sured line-centre positions of much less (typically<0:0001cm�1) than the pressure shift to be measured,and the good straight line regression shown in Fig. 3

also testifies to the calibration stability. Of course, this

method provides no absolute calibration of the wave-

number scale, and consequently line positions are not

reported in this study.

The pressure shifts, d (reported in cm�1 atm�1), areobtained by linear least-squares regression of the re-trieved line-centre positions against the total pressure at

which they were measured, as shown in Fig. 3 for two

oxygen mixing ratios (pure O2: xO2 ¼ 1:00, and air:xO2 ¼ 0:21). As can be seen from this figure, a good

linear relation is obtained, with a slope (within experi-

mental error) independent of xO2 . The regression wasonly attempted when line-centre positions were obtained

for all five pressures considered.

3. Results and discussion

The retrieved pressure-shift parameters for 37 lines in

the oxygen A-band are given in Table 2, with errors at�2r, and plotted in Fig. 4 as d (the average oxygenpressure shift in cm�1 atm�1) against line-centre wave-number. As can be seen from this figure, our values for dlie between those retrieved in two recent studies [7,8].

The large uncertainty in the measurements of Brown

and Plymate [8] was noted by those authors, whose

primary objectives were measurements of line intensities

and broadening parameters, and who concluded theirpaper with an exhortation to other groups to improve

on the accuracy to which the shifts are measured. The

present study, covering a wider range of pressures and at

higher resolution does just that and we find values for dwhich lie closest to those of Phillips and Hamilton [7],

who recorded spectra at three pressures and lower

(0:05cm�1) resolution. Quantitatively, the average ratioof our values of d to those of Phillips and Hamilton is0.89 (standard deviation 0.07); comparison with Brown

and Plymate�s results gives a ratio of 1.89 (standarddeviation 0.47).

Fig. 4 also shows a characteristic dependence of d onthe ground-state rotational quantum number N 00, whichis reported in all three studies. However, Phillips and

Hamilton [7] found the same pressure shift for each

Fig. 3. Linear regression of the retrieved centre position of the PP(15)

O2 line, for two oxygen concentrations. Note that the slope of this

regression is very similar for both values of xO2 , indicating that the self-shift and nitrogen-shift parameters are equal to within our experi-

mental error. Error bars are at �1r standard deviation.

Fig. 2. Determination of the line-centre position from the smoothed first-derivative lineshape, as described in the text. Linear interpolation to the

1� f baseline is found to be adequate.

C. Hill et al. / Journal of Molecular Spectroscopy 219 (2003) 65–69 67

J-component of a given DN transition, whereas ourvalues of d show a small variation with J (particularly

for the DN ¼ �1 branches), which can also be seen at adifferent magnitude in the results of Brown and Plymate

[8]. Further qualitative agreement with the results of

that study is illustrated in Fig. 5, a plot of d against theupper-state quantum number, J 0: P-branch lines are

pressure shifted to a greater extent than R-branch lines

(at least up to J 0 � 15), but the magnitude of d increasesmore slowly with J 0 for the P branch than for the R

branch.

The magnitude of d is found to be independent of O2mixing ratio in nitrogen within the experimental error,

in agreement with Brown and Plymate [8] and in marked

contrast to the reported results of Phillips and Hamilton

[7] who find N2-induced shifts a factor of 2 greater than

the self-shift.

Table 2

Oxygen pressure shifts for lines in the b–X (0–0) band

Line DNDJðN 00Þ m (cm�1) d (cm�1 atm�1) �2r error(cm�1 atm�1)

PP(21) 13041.1251 )0.0089 0.0012PQ(21) 13042.9487 )0.0093 0.0015PP(19) 13050.4813 )0.0091 0.0010PQ(19) 13052.3233 )0.0093 0.0011PP(17) 13059.4670 )0.0099 0.0007PQ(17) 13061.3278 )0.0095 0.0008PP(15) 13068.0823 )0.0097 0.0006PQ(15) 13069.9624 )0.0095 0.0006PP(13) 13076.3278 )0.0098 0.0005PQ(13) 13078.2280 )0.0095 0.0005PP(11) 13084.2040 )0.0097 0.0005PQ(11) 13086.1257 )0.0095 0.0005PP(9) 13091.7109 )0.0098 0.0005PQ(9) 13093.6563 )0.0097 0.0006PP(7) 13098.8488 )0.0099 0.0005PQ(7) 13100.8223 )0.0094 0.0006PP(5) 13105.6175 )0.0091 0.0005PQ(5) 13107.6291 )0.0091 0.0007PP(3) 13112.0166 )0.0087 0.0009PQ(3) 13114.1009 )0.0085 0.0011PP(1) 13118.0456 )0.0064 0.0015RQ(1) 13128.2695 )0.0066 0.0012RR(3) 13131.4920 )0.0066 0.0012RQ(3) 13133.4416 )0.0074 0.0010RR(5) 13136.2176 )0.0087 0.0008RQ(5) 13138.2053 )0.0079 0.0007RR(7) 13140.5679 )0.0086 0.0009RQ(7) 13142.5838 )0.0081 0.0008RR(9) 13144.5413 )0.0086 0.0008RQ(9) 13146.5810 )0.0087 0.0008RR(11) 13148.1357 )0.0098 0.0010RQ(11) 13150.1971 )0.0092 0.0009RR(13) 13151.3491 )0.0091 0.0010RQ(13) 13153.4309 )0.0089 0.0011RR(15) 13154.1793 )0.0083 0.0012RQ(15) 13156.2807 )0.0102 0.0012RR(17) 13156.6237 )0.0107 0.0018

Fig. 5. Retrieved O2 pressure shifts plotted against upper-state quan-

tum number, J 0 for each branch in the A-band (error bars at �2r). Inagreement with previous results [8], jdj is larger for the DN ¼ �1, thanfor the DN ¼ þ1 branches (labelled P(N) and R(N) respectively), but

increases less rapidly with J 0.

Fig. 4. Retrieved O2 pressure shifts for the O2 A-band, with error bars at �2r. Values of d for this study and that of [8] are averaged over xO2 ; thosefrom [7] are the oxygen self-shift.

68 C. Hill et al. / Journal of Molecular Spectroscopy 219 (2003) 65–69

4. Conclusion

The rotationally averaged pressure shift, d, of therotational lines of the O2 b–X (0–0) band is found to be

�0:009� 0:001cm�1 atm�1, in broad agreement withPhillips and Hamilton [7] and approximately twice as

large as that reported by Brown and Plymate [8] and

listed in the current HITRAN database. Nitrogen in-

duces a line-shift of the same amount as oxygen itself(the self-shift), within the experimental error. Since our

measurements were taken as part of a dedicated study of

line shifts, at higher spectral resolution, and with smaller

experimental error than previous studies we believe our

results can be taken as the most accurate measurements

of d to date.The studies published to date on the oxygen A-band

show a striking agreement in their prediction of thevariation of the pressure shift with rotational quantum

number. There is a distinctive and reproducible depen-

dence of d on N 00 which has no immediately obviousexplanation. The theory of pressure shifts in the spectra

of O2 is not well-developed, and it is clear that more

work is needed in this area to explain such phenomena.

Atmospheric applications demand spectroscopic pa-

rameters to be known to a high degree of accuracy and itis hoped that the results reported here will help to re-

solve some of the uncertainties connected with the

magnitude and nature of the O2 A-band pressure shift.

Acknowledgments

The research described in this paper was funded by theUKs Natural Environment Research Council (NERC)

through the Core-Strategic Measurements for Atmo-

spheric Science (COSMAS) Grant Ref. GST022698awarded to JMB at the University of Oxford. The

authors thank NERC for access to the Molecular Spec-

troscopy Facility at the Rutherford Appleton Labora-

tory (RAL) and Mr. R.G. Williams at RAL for

providing technical support.

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