The Rovibrational Intensities of Five Absorption Bands of (12)C(16)O2 Between 5218 and 5349/cm

33 Pages·2002·1 MB·English

by  Giver, Lawrence P.

The Rovibrational Intensities of Five Absorption
Bands of 12C1602 between 5218 and 5349 cm -1
Lawrence P. Giver
Atmospheric Physics Branch, N 245-4
NASA Ames Research Center
Moffett Field, CA 94035-1000.
Linda R. Brown
Jet Propulsion Laboratory; California Institute of Technology
480_) Oak Grove Drive; Pasadena, CA 91109.
Charles Chackerian, Jr.
SETI Institute
Mountain View, CA 94043
and
Richard S. Freedman
Space Physics Research Institue
Sunnyvale, CA 94087
Number of pages: 28
Number of figures: 5
Number of tables 9
\
2
Corresponding Author:
Lawrence P. Giver
Atrnospheric Physics Branch; SGP, N 245-4.
NASA - Ames Research Center
Moffett Field, CA 94035-1000
Phone: [phone]
E-mail: [email protected]
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ABSTRACT
Absolute line intensities, band intensities, and Herman-Wallis parameters were
measured for the (0112h+-(00°0)i perpendicular band of 12C1602 centered at 5315 cm -j,
along with the three nearby associated hot bands: (10°2)Ii_---(0110)i at 5248 cm q,
(0222)i6--(0110)i at 5291 cm -I, and (10°2h+--(0110)i at 5349 cm -l. The nearby parallel
hot band (30Ol)I+-(I0('0hi at 5218 cm -I was also included in this study.
The rotationless band intensities at 296 K are respectively
Band S°v cm-1/(molecule/cm 2)
(0112)i_-(00°0)i (47.6 ± 0.4)xlO -24
(10°2)ii6--(0110)i (1.45 ± O.04)xlO "24
(0222)i_-(0110)I (3.60 :L:O.05)xlO -24
(10°2)i_ -(0110)i (0.556 ± 0.027)x10 "24
(3 0°1)i(-(1000)II (2.279 ± 0.031)xlO "24
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INTRODUCTION
Modeling speclra of the near-infrared emission windows found on the nightside of
Venus was undertaker, by Pollack eta[. [1] in an effort to improve the determination of
the composition and cloud structure of the lower atmosphere of Venus. Since C02 is the
most abundant gas in Venus' dense, hot atmosphere, weak overtone-combination bands
and hot bands of CO2 are ubiquitous throughout Venus' near-infrared spectrum. The
intensities of many of these bands that are significant absorbers in Venus' atmosphere
have not been measured; modeling Venus' spectrum must rely on calculated intensities for
these bands. To impreve this modeling work, Giver and Chackerian [2] made laboratory
measurements of the intensity and Herman-Wallis parameters of the very weak
(3110)iv<----(00°0 ) perpendicular band of CO2 at 4416 cm -1, which is prominent in Venus'
emission window between 4040 and 4550 cm -l. Giver et aL [3] subsequently measured
intensity parameters ;3f two bands of the (40°1)+-(00°0) pentad to help improve
reliability of the modeling of the Venus emission window centered at 7830 cm -1. Before
these measurements, only calculated intensity parameters were available for simulating
these CO2 bands in atmospheric models.
The modeling of Pollack et al. [1] did not obtain a good fit to the 7830 cm -1Venus
emission window. As mentioned by Giver et aL [3], the very weak and perturbed
(2112)n +-(00o0) perpendicular band at 7901 cm -1 contributes some absorption on one
side of the 7830 cm -l Venus emission window. They measured the intensities of some Q-
and R-branch lines of this band, but because of the perturbation and the lack of
measurable P- branch lines, they did not obtain band intensity parameters. Rothman et al.
[4] revised the intensity parameters of most of the unmeasured CO2 bands for the 1992
HITRAN compilation using the Direct Numerical Diagonalization calculations of Wattson
and Rothman [5], but they did not do that for this band or the other perturbed bands.
Thus, the modeling cMculations depended on the original McClatchey et al. [6] HITRAN
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intensity estimate for this band, which has not been updated yet by either measurements
or calculations.
The 1992 HlTRAN values for the entire sequence of (n112)+--(00°0)
perpendicular bands and some of their associated hot bands are compared in Table 1 to
the 1986 HITRAN vaues of Rothman [7], which were unchanged from the McClatchey
et al. [6] estimates. There is a striking reduction for the calculated intensity of the
(01L2)I+-(00°0)I band at 5315.7 cm -1 from the 1986 to the 1992 HITRAN tabulation.
However, it was recognized that the DND calculated intensities for this sequence of
perpendicular bands may have substantial uncertainties, since none of the measured band
intensities used by Wattson and Rothman [5] to determine the dipole-moment surface for
CO2 have 2V 3 in the u._per level; therefore, measurement of the intensity is necessary for
some of these bands.
The strongest of these bands, (0112)i+---(0000)i at 5315.7 cm -l, was readily
measureable in two spectra that we obtained at the Kitt Peak solar Fourier Transform
Spectrometer in 1993. Giver et al. [8] reported a preliminary measurement of the
intensity of this band; the 1996 HITRAN [9] value for this band, which is based on this
preliminary measurement, is also presented in Table 1. The most recent version of
HITRAN described by Rothman et al. [10], released in December, 2000, has no changes
for CO2 from the prior 1996 version.
In the future, measured intensities of some of these bands could be included in
DND calculations described by Wattson and Rothman [5] to improve the dipole-moment
surface, and thereby irr prove the calculated values of higher overtone-combination bands,
especially the (21t2)j¢--(00°0)i band at 7901 cm -I and other nearby bands that are
significant in Venus near-infrared emission windows. We therefore decided to obtain
spectra at several more path length and pressure conditions in order to measure the
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(01 t2)i+---(00°0)i band at 5315.7 cm -I as accurately as possible. This article reports our
final intensity measurements from these spectra for the (0112)i<--(00°0)i band at 5315.7
cm -l and the three rela'ed nearby perpendicular hot bands listed in Table 1.
These four bands are similar to the v2 fundamental and the 3 nearby hot bands
arising from the v: level. The intensity of these bands, (0ll0)i_---(0000)I,
(0220)i_---(01 t0)b (10°0)I_--(0110)i, and (10°0)ii_-(0110)I have been measured very well
by Johns and Vander Auwera [11]. The four bands in the 5300 cm -t region have similar
vibrational assignmentt;, with 2v3 added to the upper level of each band.
An additional un_related hot band, (30 °1)i6---(1000)II appears in this spectral region
at 5217.7 cm -1. This is one of the 8 hot bands arising from the (10°0)i and (10°0)ii levels
associated with the (20)1)<---(00°0) triad parallel bands near 5000 cm -1. Since some of its
lines overlapped the region of the 5248 cm -1band, we included it in this study.
EXPERIMENTAL DETAILS
The first two spectra of CO2 covering the 3800 to 8400 cm -l region were obtained
at Kitt Peak, AZ National Solar Observatory with the McMath FTS equipped with a
quartz beamsplitter and InSb detectors. An additional 5 spectra of CO2 and one empty
cell spectrum were subsequently obtained with the same apparatus; all spectra had
resolution of 0.0102 cm -1. One more spectrum with resolution of 0.0116 cm -1 was
obtained using a CaF2 beamsplitter; this spectrum at the lowest pressure was only used
for line position measurements. All these spectra utilized a 6-meter base path White cell
[12] and research grade CO2, which had a stated minimum purity of 99.995%. The CO2
pressures were measuled with a 100-Torr MKS Baratron manometer with digitized
readout. In addition, a 2.4-m single-pass cell was placed in series with the White cell for
low pressure CO 2-(I band line position calibration and instrument lineshape
determination. The exp,:rimental conditions of all 9 spectra are given in Table 2. The Kitt
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Peakinterferograms,aereobtainedwith 1.3hoursintegrationtimeandtransformedwith
theweak"Brault" apodization(discussedby Spenceret al., [13]).
The region of the Q branch of the (0112)i_---(00°0)i band is shown in Figure 1, and
similarly the region of the Q branch of the strongest hot band, (0222)i6--(0110)i, is shown
in Figure 2. As seen in these figures, there are some substantial water vapor lines in this
spectral region formed in the air path between the White cell and the FTS despite purging
with dry N2. These lines had to be included in the fitting procedure. For the set of
spectra which included an empty cell spectrum, the water lines could be fitted directly on
the empty cell spectrun; those fits were then applied to the CO2 spectra since the water
spectrum remained consistent for the entire set and was independent of the White cell
path length and CO2 pessure. Surprisingly, only a few CO2 lines could not be measured
because of the severe water vapor contamination.
Following the procedures described in Giver et al. [3], CO 2 line intensities were
determined using non-linear least-squares fitting of the spectra. Line profiles were
computed using the lalzoratory conditions for each spectrum, the instrumental profile, and
the self-broadening coefficient for each line as given in 1992 HITRAN [4]. However,
before accepting these broadening coefficients, we fit the self-broadening coefficients
along with the intensities for 15 of the more isolated lines using the spectrum obtained
with 80 torr, our highe,;t pressure. These measured broadening coefficients on the 80 torr
spectrum averaged only 2% higher than the Rothman et al. [4] 1992 HITRAN values,
with a standard deviation of 4-2%. These measurements on the 5315 cm -1 band are not
significantly different from the 1992 HITRAN values, which were averaged from
measurements on 3 other bands, and therefore our measurements support the 1992
revision of the 1986 HI FRAN [7] CO2 self-broadening coefficients.
The linesof theseCO 2 bands were then fitted individually wherever possible on
all the spectra, holding tlle parameters of all nearby lines fixed at their approximate values.
During the fit of each spectral line, the position and intensity were adjusted for the
calculated spectrum until the sum of the squares of the differences between the observed
and calculated line profiles was minimized. The intensities were measured in units of
cm-1/(molecule/cm 2) at the temperature of each spectrum, using the total measured
pressure without correction for isotopic abundance. These line intensities for each
spectrum were then standardized to T = 296 K so the measurements from different
spectra could be aver.iged. The averaged line intensities, Sobs, are listed in Tables 3
through 7 for each of tile bands, and presented in Figure 3 for the (0112)_6-(00°0) ground-
state band. These values are consistent with the HITRAN definition for intensities in a
gas with a standard mixture of its isotopomers. Line intensities appropriate for a gas of
the pure 12C1602 isotopomer can be obtained by dividing these tabulated values of Sobs
by the isotopic fractior, f=0.9842.
LINE POSITIONS
The CO 2 line positions determined from the individual fits were then calibrated on
each spectrum using the CO 2-0 band [14]. However, these calibrated position
measurements cannot be simply averaged for each line, since the spectra were obtained
with different pressures. The small pressure shifts should be taken into account so that
the line positions at zero pressure can be determined as accurately as possible. This was
done for the best deterrained lines of the ground state band; a linear fit was performed for
the calibrated measurements of each line to determine the zero-pressure line position and
the pressure shift coefficient. For these lines the pressure shifts were in the range of
-0.004 to -0.009 cm -11atm. This represents a maximum shift of-0.001 cm -1 in our
measured positions for the 80 Torr spectrum. Because there is considerable scatter, we
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haveelectednotto reporttheempiricalshifts. However,wenotethatthemagnitudeof
theseshiftsismorethanafactoroftwo largerthantheshiftsgivenin 1996HITRAN [9]
for the(00°I)I+--(00°C)]fundamentalbandat2348cm-1. Thisis in agreemenwt ith the
expectationthatpressureshiftvaluesincreaselinearlywith increasingwavenumber.
Forthelinesofthe(0112)i_---(0000b)aI ndwith insufficientmeasurementfsorthis
procedure,anestimateJpressureshift,basedonresultsfromnearbylines,wasappliedto
thecalibratedpositionmeasurements.Thezero-pressureline positionsforthisbandare
listedin Table3to0.C0001cm-l; in all casesthesepositionsagreewith 1996HITRAN
[9] positionswithin the0.001cm-1HITRAN maximumuncertaintyestimatefor these
lines. TheyaresimilarlyclosetothemeasuredpositionsofArcas et al. [15].
The lines of the hot bands, as seen in Figures 1 and 2, are much weaker and
therefore the position measurements had less precision. For these lines it was necessary
to apply estimated pre_;sure shifts to the average of the calibrated measured positions to
determine the zero-pressure line positions; the correction was at most 0.0007 cm -l.
Because the line position measurements for the hot bands have more uncertainty than the
measurements for the stronger ground-state band, the zero-pressure positions are given in
Tables 4 through 7 to only 0.0001 cm -l. For Tables 4, 5 and 7, the positions agree with
the HITRAN values within the combined HITRAN uncertainties and our tabulated
uncertainties for almosl all lines. The weakest band we measured, (10°2)i<--(0110)i, does
not appear on the 1992 and 1996 HITRAN tabulations [4, 9], because it was presumed to
be too weak for the H[TRAN intensity cutoff, although it was on the 1986 version of
HITRAN [7].
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DETERMINATION OF BAND INTENSITY PARAMETERS
The rotationless transition moment squared and Herman-Wallis intensity
parameters for each baad were obtained from the measured line intensities standardized to
T = 296 K via the theo3-etical expression for the individual line intensities,
tt t! ,e)levl2
Sj,, = {8rc310-36/[3hcgvQvr(T)]}{vf exp(-E hc/kT)}L(J F(m), (1)
where the line intensily is in units of cm-]/(molecule/cm2), the rovibrational partition
function from Gray and Young [16] for 12C1602 is Qvr(296 K)=286.14, the square of the
rotationless transition tnoment [Rvi2 has units of Debye 2, J" is the lower state rotational
quantum number, and f=0.9842 is the isotopic fraction for 12C1602. The lower state
rotational energy levels E"(cm -1) were adopted from the HITRAN tabulations [9], and the
line positions V(cm -1) were determined from our spectral fitting procedures. T is the
Kelvin temperature and k, h and c have their usual definitions. The degeneracy factor
gv=2 for bands when I:oth the upper and lower states have non-zero vibrational angular
momentum (g>0), sirtce all rotational levels are permitted. This is the case for the
(0222)i_-(0110)I band at 5291 cm-1; for the other bands gv=l. The H6nl-London
linestrength factors L(J ",g) for perpendicular bands and the Herman-Wallis factors F(m)
were adopted from Rothman et al. [4], where m = -J", J", and J"+l in the P, Q, and R
branches respectively.
Solving Eq. 1 fcr IRvr2F(m), we defined the reduced line intensity as
[Rvl2F(m) = Sred(m),
]Rv[2F(m) = {3hc 1036/8g3vfL(J",g) } {Sj..gvQvr(T)exp(E"hc/kT) }. (2)
The Herman-Wallis fac-or is
(3)
F(m) = [1 + Aim + A2m 2 + A3m3] 2
for the P and R branches, and