Recent unpublished laboratory work on rovibrational line strengths and broadening coefficients which is of interest in the study of planetary atmospheres was reviewed. The molecules discussed are PH3, CH3D and CO2

Chackerian, C., Jr.

Suarez, C. B.

Tarrago, G.

Valero, F. P. J.

English

NASA Technical Reports Server

N90-26751
A
RECENT HIGH RESOLUTION LABORATORY DETERMINATIONS
OF LINE BROADENING AND INTENSITY PARAMETERS: PH3,
CH3D AND CO2
C. B. SUAREZ*, C. CHACKERIAN, JR.*, F.P.J. VALERO* AND G. TARRAGO**
*NASA-Ames Research Center, Moffett Field, CA 94035-1000
**Laboratoire d'Infrarouge, Universite de Paris-sud, Bat. 350
91405 Orsay, France
ABSTRACT
We review our recent unpublished laboratory work on
rovibrational line strengths and broadening coefficients which is of
interest in the study of planetary atmospheres. The molecules
discussed are PH3, CH3D and CO2.
Introduction
High spectral resolution infrared rovibrational observational
astronomy is a powerful tool for understanding the compositional
and dynamical structures of planetary atmospheres. At NASA Ames
we are engaged in the measurement of line intensity and broadening
coefficients (and their temperature dependence) which are required
for the proper interpretation of such astronomical observations.
Line broadening coefficients are dependent on the dynamics of
molecular collisions and therefore are temperature dependent.
Accordingly we have made some of our measurements down to
temperatures as low as 80K with specially constructed absorption
cells. On the other hand line intensities, aside from the Boltzmann
factor, are independent of temperature. Nevertheless we have
measured line intensities at low temperatures because in some cases
the very complex structure of the spectra of overlapping lines at
room temperature is greatly simplified at the lower temperatures
and becomes, therefore, more amenable to analysis. To date we have
made broadening measurements with self-, He-, and N2- collision
partners.
The spectra we analyze are recorded with a BOMEM Model
DA3.002 Fourier transform spectrometer, and the absorption cell we
used was internally coated with evaporated gold and cooled with an
Air Products Cryotip. The spectral intensity and broadening
parameters were obtained via non-linear least-squares fits (of the
spectra) wherein small corrections were applied to the theoretical
instrumental resolution.
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https://ntrs.nasa.gov/search.jsp?R=19900017435 2020-03-24T03:05:24+00:00Z
In the 5 [xm spectral region PH3 and CH3D spectra overlap and
accurate line intensities and broadening coefficients are needed to
obtain column densities for either species. PH3 spectra have been
obtained between 137 and 294 K in the 2000-2500 cm -1 spectral
region. Fig. 1 shows a portion of a strongly absorbing region of PH3's
spectrum under self-broadening conditions with a path length of
0.21 cm. So far self- and He-broadening coefficients have been
measured. Fig. 2. shows a typical result where the line broadening
parameter for a particular phospine line at 2250.107 cm -1 is plotted
versus pressure. To date, 212 line intensities recorded at room
temperature have been analyzed with programs (G. Tarrago, paper in
preparation) worked out for the frequency and intensity treatment
of the pentad, 2v2, v2 + v4, 2v4, v 1, v3. The two dipole moment
derivatives relative to the normal co-ordinates, Q1 and Q3, plus three
Herman-Wallis type elements of the dipole moment matrix
contribute significantly to the fit, and allow the measured intensities
to be reproduced to about 5%. Preliminary values, derived for the
band strengths S1 and $3, confirm the ratio of 0.28 previously
determined 1 for S1/$3, but lead to an overall strength S1 + $3 about
10 per-cent higher than the value measured by van Straten.2 The
transition moments for the v 1 and v3 bands are respectively, _1,1_ =
0.0712(23) D and <Ix3> = 0.1351(14) D.
CHaD
The v2 rovibrational band of CH3D, which is observed in the
outer planets is important for understanding the formation of the
solar system as well as determining the D to H ratio important in
cosmological theories.
At room temperature the Q branch of this band is complicated
enough so that even Doppler limited resolution is not sufficient to
provide a definitive analysis of the intensities. To circumvent this
problem we have _ made intensity measurements at temperatures
down to 80 K with an unapodized instrumental resolution of about
0.0045 cm -1. Figs. 3. and 4. show the evolution of the Q branch with
temperature. The resulting rotationless transition moment obtained
from the Q branch is compared to that obtained from the P and R
branches and work is in progress to understand a global interaction
with the v 6 band. In addition we have determined the low
75
temperature broadening parameters with N2 for this band as well as
He- and self-broadening coefficients at room temperature.
A careful study has been made of the temperature dependence
of the self-broadening coefficient, 3 y, of CO2 in the parallel band
centered at 4978 cm -1. In the temperature range 165 to 300 K the
average value of the temperature exponent (for J in the range 6 to
32) is n = 0.745 +- 7%. Here n is defined via the equation y(T) =
y(To)(To/T) n, where To is some reference temperature.
References
• A. Baldacci, V. M. Devi, K. Narahari Rao and G. Tarrago,
J. Mol Spectrosc. 81, 179 (1980).
A. J. van Straten, J. Mol. Spectrosc. 65, 202 (1977)•
C. B. Suarez and F. P. J. Valero, J. Quant. Spectrosc. Radiat.
Transfer (accepted).
o
o
(I3
o
o
I
E_
o3
o
..C
°.-.) •
"-_o
I
t._. cxl
....) o
'r-o
O
123
o
O |
i i | |
O. 0 25.0 50.0
pressure [tort)
Fig. 1. Retrieved half width (X's) for a PH 3 self-broadened line at
T = 293.5 K. o's represent fitted values. Broadening coefficient
determined from slope of the line thru the data.
I
75.0
76
...)
O3
©
.+.9
C
_-a bO
0
2320.0 232 ].0 2322.0 2323.0 2324.0 2325.0
Fig. 2. Portion of the PH 3 rovibrational spectrum between 2320 and
2325 cm 1. T=293.5;L=0.21 cm; P=14.9 torr.
I I I I ! i 1 i I i i
2199.0
! i ! ! i i
' I
2200.0
Fig. 3. Q branch of CH3D v 2 band. T=296.3 K; L=5.74 cm; P=9.35 torr.
o
I i ! i ! i ! I I I !
2198.0 2199.0
wove number" cm-1
Fig. 4. Q branch of CH3D 1/2 band. T=81 K; L=5.74 cm; P=0.79 torr.
' I
2200.0
77


Recent high resolution laboratory determinations of line broadening and intensity parameters: PH3, CH3D, and CO2

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