0022 4073 83 $3 00 + 00 Pergamon Pres Ltd
MEASUREMENTS OF NITROGEN-BROADENED LINE WIDTHS OF ACETYLENE AT LOW TEMPERATURESJP. VARANASI Laboratory
for Planetary
Atmospheres Stony
Research, State University Brook, NY 11794, U.S.A.
of New York at Stony
Brook,
and L. P. GIVER Astrophysical
Experiments
Branch,
NASA
(Received
and F. P. J. VALERO Ames Research U.S.A. 29 Murch
Center,
Moffett
Field.
CA 94035,
1983)
Abstract-Spectral transmittance measurements have been performed on N,-broadened lines of CZHL in the 7.53 pm region at 153 and 200K with a spectral resolution of O.O6cm-‘. The line
widths have been determined as functions of temperature and rotational quantum number using line-by-line comparison of theoretical and experimental values of spectral transmittance.
INTRODUCTION
Acetylene
is a trace constituent
in the N,-dominant
atmospheres
of Earth
and Titan.’
4 The
spectra relayed back to Earth during their encounters with Titan by the IRIS (Infrared Radiometer-Infrared Spectrometer) instruments on board Voyagers 1 and 2 is the central Q-branch of the v,-fundamental vibration. It has only recently been recognized that the rapidly increasing concentrations of this trace constituent in the terrestrial atmosphere might, by virtue of the choice location as well as the strength of its fundamental band at 13.7 pm, pose a potential planetary-climatic hazard in the not too distant future. The current estimate’ is that the concentration of C,H, in the atmosphere would by the year 2030 reach 1.5 times its value in 1980. The brightness temperaures around 13.7 pm, as recorded by Voyagers 1 and 2, are as low as 130 K on Titan.The corresponding temperatures in the terrestrial atmosphere are 200 and 300 K. Thus, data on N,-broadened line widths and on their dependence on temperature would be among the spectroscopic parameters needed in thermal i.r. studies of these planetary atmospheres. No data are available on either the intensities or the half-widths of any of the rotational lines inthe v,-region; the only reported data on any lines appear in the abstracts of oral presentations by one of US.‘.~ In view of the foregoing important considerations, we have performed spectral transmittance measurements at 153 and 200 K on several N,-broadened lines in the 7.53 pm region. The choice of lines in this spectral region for the purpose of determining line widths was a natural one. Since we employed a Michelson interferometer for observing the i.r. spectrum of CzHz with our primary goal as the measurement of the intensities of the rotational lines in the strong fundamental band at 13.7 pm, the weaker combination bands around 7.53 /lrn were already a part of the recorded signal (interferogram) that was obtained Gng an i.r. filter with a bandpass that included this spectral region. Furthermore, accurate data on band strength7,x and line positions’ exist for the parallel bands in this region. prominent
spectral
feature
at 13.7 pm in the emission
tsupported, in part, by the Planetary Atmospheres Branch of the Earth and Planetary Exploration Division of NASA under Grant-in-Aid No. NCR 33-015-139 (to P.V.) and performed at NASA-Ames Research Center during the first author’s sabbatical leave of absence (1981-82) from the State University of New York at Stony Brook. Stony Brook. NY 11794. U.S.A. so5
106
V\K\\,\SI
P. EXPERIMENT.AI
<‘I t/i.
PROC‘EDI‘RF
The spectroscopic apparatus employed in the present studies consisted of a Nicolet Fourier transform spectrometer with ;I KBr beam splitter. an i.r. glower, a HgC‘dTc (photoconductive) detector operated at liquid nitrogen temperature, and a computer with a magnetic tape recorder accessary that was interfaced with the interferometer~~ spectrometer. The low temperature cell was 4.9 cm long and was sealed using ZnS (Irtran 2) windows mounted in specially designed Kovar-ceramic cup seals. The cryogenic refrigeration system and the vacuum shroud around the absorption cell have been described previously bq Varanasi ct LII.“’” Dilute mixtures of C,HI and N, were prepared in lecture bottles using instrument grade C,H, and ultra-high-purity grade NI at pressures between IO and 30 atm. The gases were allowed to mix for a couple of days before spectra were taken. Compressibility corrections to the high pressures of N, were applied using the van der Waals equation of state. In determining the mixing ratio, allowance for “C”CH, was made assuming the species to be present in its natural abundance. Temperatures of the absorption cell were measured using gold-chrome1 thermocouples. Pressures of the gas mixture in the cell were monitored using an MKS Baratron guage for pressures below I00 torr and a Wallace-Tiernan absolute pressure guage for higher pressureb.
Our experimental results arc presented as spectral transmittance data measured at 0.03 cm ’ intervals. The apodized spectral resolution was 0.06 cm ‘. Signal-to-noise ratio was 200: I or better for most of the recorded spectra. (‘OMPIIT.ATION
OF
SPLIC TR.4L
I‘RAhSMIl-TANC’E
The apparent spectral transmittance at wave number 1’(cm ‘) through an absorption cell of length L (cm) containing an absorbing gas that exerts a partial pressure of p (atm) is
T,”
zz
F
.I
” ’ ..l(\“)exp(~,pL)dr”. ’
if is instrumental function. In present the HappCenzel apodization the limits the amplitude the oscillatory function A(v) is negligible. Under assumption that absorption due overlapping vibration-rotation is additive. spectral absorption k, is
1 .Y,,,f’c \’
\,,,,).
.f’(~ -- r,,J is the collision-broadened protile of a line situated at \I,,,(cm ‘) with an absolute intensity of S,,, (cm ’ -atm ‘). rotational line belonging to a The absolute intensity, S,,,. of an individual vibration-rotation band may be related to the absolute intensity of the band. S,. by
L ,s,
,y,IV,,j;,,[I - exp( - hCV,,,,XI‘)J N,.
[I - exp( ~ IICY,,‘liT)]
(1)
where, R, is the nuclear statistical weight factor. N,,, is the Boltzmann population of the nz th rotational level in the lower vibrational level of population IV,,, Y,,,,and Y, arc the wave numbers of the no th line and of the band center, respectively, and j;,, is proportional to the square of the matrix element of the rotational transition. The rotational quantum number ttt is. as usual, J in the P- and Q-branches and J + I in the K-branch. Several parallel bands of the type Y, + 1’. that are located in the 7.53 /urn region have been thoroughly analysed by Palmer ot t/l.” In addition to the (v, + 11~) band. there appear
Measurements
of nitrogen-broadened
line widths
of acetylene
at low temperatures
507
four transitions from the val (612.18 cm-‘) level, three from the vsl (730.33 cm-‘) level, the vq+ v5 band belonging to the isotope ‘ZC’3CH, and the “forbidden” (vq + vJ* band. Only the strongest of the bands, namely, the (v, + vs)Oband, is observable at low temperatures (Figs. l-4). Before line strengths can be computed using Eq. (1) for the lines belonging to this band, its absolute intensity has to be determined either experimentally or be deduced from the combined intensity of all of the bands appearing in this spectral region. The latter is the quantity measured by the authors of Refs. 7 and 8. Under the assumption that the vibrational transition moments of the “hot bands” are the same as for the transition from the ground vibrational level, the ratio of the intensity of a “hot band” to that of the (v, + v5) band would be given by the product of the ratio of their central wavenumbers and the Boltzmann population of the vq’or v5’level relative to the ground vibrational level. It follows, then, that the absolute intensity of the v, + v5band is 65% of the “total regional strength” for which we adapted the value given in Ref. 7t. The intensity factors f, may be obtained from the formulae given by Penneri for the rotational transitions in parallel bands of linear molecules. The nuclear statistical weight factor is 3 for odd values of J and 1 for even values of J. RESULTS
AND
DISCUSSION
The solid curves in Figs. l-4 represent spectral transmittance computed using the algorithm developed by Pierluissi et al. l5 for Voigt lines. The appropriate collisionbroadened half-widths were developed from the self-broadened line widths measured by Varanasi and Bangaru16 and the N,-broadened line widths reported by Varanasi.6 the line-width data of Refs. 6 and 16 were revised until the excellent agreement shown in Figs. 224 between experimental and theoretical spectral transmittance was obtained. Our results are as follows: ~~“(7) = 0.125 (1 + 0.0843~~ - 0.0069~~~+ 0.000131m3) for self-broadened
lines, and
ymo= 0.114(1 - 0.0063~1 - 0.0013m2 + 0.000024~~) for N,-broadened lines. The units of ymoare cm _ ‘-atm - ‘. The variation of the line widths with temperature suggested by the earlier measurements6~‘6 in the 1.515 pm band seems to hold within the experimental errors in Figs. 2-4 of this paper.
1250
I
I
I
I280
:310
I343
I 1370
14x
I\‘AVENU~:ELH
Fig.
I. The i.r. spectrum of C,H, in the 7.53 pm region at 200 K; total pressure = 300 torr, 15 = 4.9 cm, mixing ratio (C,H,/N,) = 0.009, spectral resolution = 0.06 cm ‘.
tThis result is consistent with the line-strength measurements in the R-branch of the v4 + v5 band performed recently by Podolski et al. using a tunable diode laser. The band strength derived from the line intensity data (at 296 K) of Ref. 13 is 63 + 2cm-2-atm-‘.
-
I
I 1354
I
I 1358
WAVENUMBER
of experimental (squares) and theoretical (solid curve) spectral transmittances Fig. 7. Compartson for N,-broadened lines in the 7.53 pm region of C2H, at 153 K; total pressure z 400 torr. mixing ratio = 0.009. L = 4.9 cm, spectral resolution = 0.06 cm ’
Fig. 3. Comparison of experimental (squares) and theoretical (solid curve) spectral transmittances for Nz-broadened lines in the 7.53 pm region of C?H2 at 200 K; total pressure = 300 torr, mixing ratio = 0.009. L = 4.9 cm. spectral resolution = 0.06 cm ‘.
Fig. 4. Comparison of experimental (squares) and theoretical (solid curve) spectral transmittances for N-broadened lines in the 7.53 urn region of C,HZ at 200 K: total pressure = 300 torr, mixing ratio = 0.009. 1. = 4.9 cm. spectral resolution = 0.06 cm ‘.
Measurements
of nitrogen-broadened
line widths
of acetylene
at low temperatures
SO9
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