Measurement of the photoabsorption cross section of CO in the 1060-1550 A region using synchrotron radiation is described. The oscillator strengths for the transitions from CO (Chi1Sigma+ to various excited states are obtained from these data. Fluorescence from excited CO was observed in the 1150 to 3000 A and 3000 to 8000 A regions. The quantum yields for the production of fluroescence from the Alpha(1)P and B(1)Sigma(+) states are unity because their excitation energies are below the dissociation limit. The C(1)Sigma(+) , v = O level has significant fluorescence quantum yields both in the UV and visible region, but the yields for the E(1)Pi, v = O and C(1)Sigma(+), v  = 1 levels are very small. The C(1)Sigma(+), v = 1 level is presumably predissociated. The cross sections for the production of fluroescence from the a'(3)Sigma(+), d(3)Delta sub 1, e(3)Sigma(-) yields a(3)Pi, and B(1)Sigma(+), C(1)Sigma(+) yields A(1)Pi transitions upon excitation from the X(1)Sigma(+) were measured

Guest, J. A.

Lee, L. C.

English

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QUANTITATIVE ABSORPTION AND FLUORESCENCE STUDL
OF CO FROM 1060 TO 1550 A
LC Lee and J A Gues t
Molecular Physics Laboratory
SRI International
Menlo Park, California 94025
ABSTRACT
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i
The photoabsorption cross s-ction of CO has been measured in the 1060-
1550 A region using synchrotron radiation. The oscillator strengths for the
transitions from CO(%1E+) to various excited states are obtained from these
data. Fluorescence from excited CO was observed in the 1150 -3000 A and 3000-
8000 A regions. The quantum yields for the production of fluorescence from
the A111 and B1 E+ states are unity because their excitation energies are
below the dissociation limit. The C 1 E + , v - 0 level has significant
fluorescence quantum yields both in the UV and visible region, but the yields
for the E III, v - 0 and C 1E+, v - 1 levels are very small. The C I E+, v - 1
level is presumably predissociated. The cross sections for the production of
fluorescence from the a ' 3E+, d3Ai , e3E + a3H, and BI E+, C 1 E + + A1II
transitions upon excitation from the R 1 E+ have also been measured.
(NASA-CR-169292) QUANTITATIVE ABSOSPTICN
	 $82-32088
AND FLUORESCENCE STUDY Of CC FECO 10bC 10
1550 A (SRI International CozF., Menlo Park,
Calif.) 15 p HC A02/ME A; 1
	 CSCL 20H
	 Unclas
G3/72 23740
MP 80-151
02/06/80
1
1. INTRODUCTION
Because carbon monoxide is abundant in interstellar space,.the knowledge
of its photoexcitation cross sections is important for determining Its role in
interstellar p►&otochemistry.
Although the photoabsorption cross section of CO in the 1060-1550 A
region has been repeatedly measured (Watanabe, Zelikoff, and Inn 1953, Slanger
and Black 1969, Meyer and Samson 1970), the results are not consistent. For
example, at 1470 A, the cross section measured by Slanger and Black (1969) is
two orders of magnitude higher than that measured by Watanabe, Zelikoff, and
Inn (1953) and Meyer and Samson (1970). This great variation is caused mainly
by the different light source bandwidths and gas pressures used for the
measurements. Since CO absorption spectra have very narrow rotational
lines, the measured cross sections depend on the bandwidth of the excitation
sources. The oscillator strength, which is proportional to the integrated
absorption cross section and is independent of the light source bandwidth,
should be a more meaningful quantity for comparison when the bandwidth of
excitation source is limited. However, the oscillator strengths for some CO
transitions given by various measurements and theoretical calculations vary
over an order of magnitude (Carlson et al, 1978). We attempt here to
reconcile previous inconsistencies through the measurement of average
absorption cross sections at low pressures, using continuum synchrotron
radiation as a background source.
Fluorescence from the triplet systems by photoexcitation of CO from its
ground state has been reported (Becker and Welge 1965, Slanger 1968). Direct
excitation from the CO(%1 8+) ground state to the triplet states is optically
forbidden, but some rotational tr_.:.sitions are allowed through the mixture of
2
the triplet states with the A 1 H state (Rrupenie 1966, Field 1971). However,
the strength for excitation of the CO(XIE) into the d 
3 
a i state at 1470 A
reported by Becker and Welge (1965) is much smaller than that reported by
Slanger (1968). Also, the cross sections for the photoexcitation oL'
CO(X1 E+) into the triplet states have not been measured previously. We
report here the cross sections for CO visible fluorescence at low pressure.
2. EXPERIMENTAL
The experimental setup has been described in a previous paper (Lee,
1980). In brief, synchrotron radiation produced by the electron storage ring
of the University of Wisconsin was dispersed by a 1-m vacuum monochromator and
used as the light source. The absorption cross section was measured by light
attenuation when CO was introduced into the 40-cm gas cell. The photon flux
of synchrotron radiation was measured by a calibrated photodiode or a combina-
tion of sodium salicylate and a photomultiplier. The CO molecular fluores-
cence was observed 7 cm from the monochromator slit at right angles to the
synchrotron radiation by a CsTe solar blind photomultiplier tube (EMI G26H215)
in the 1150-3000 A region and by a visible photomultiplier tube (EMI 9558B)
for wavelengths longer than 3000 A. The measurements were performed at a
temperature of 20° C.
CO gas obtained from Liquid Carbonic with a minimum purity of 99.5% was
used without further purification. The cell was continuously pumped by a
sorption pump and a constant pressure maintained by a slow flow of fresh CO
gas. This flow system conveniently prevented the accumulation of impurities
in the cell. Since the flow rate was very slow, the cross sections measured
in this system are found to be no different from those for a static system.
3. RESULTS AND DISCUSSION
3.1	 Photoabsorption cross section
The photon flux, I(A 0),	 through the g
by
I(A0) - J 10g(A)e-°
where the integration is over the monochromator bandwidth, I o
 is the photon
flux without gas in the cell, g(A) is the monochromator slit function
Ug(A)dA - 1 when it is integrated over the monochromator bandwidth AA),
n is the gas concentration, I is the length of the gas cell, and a(A) is
the photoabsorption cross section.
If the molecular abs3rption band is much broader than AA, then I(A0)
will depend on the gas concentration by Beer's law, and the photoabsorption
cross section is proportional to Xn(I 0/I). However, as noted earlier, CO
has very sharp structure in the region studied. Several rotational lines are
covered in a minimum monochromator bandwidth of 0.3 A.
	 Therefore,
In(I0/I) is not linearly dependent on the gas pressure, as shown in
Figure 1. Since the cross sections for the rotational lines are very large,
the portion of photon fluxes resonant with the rotational lines will be
totally absorbed at low gas pressure. Thus I(A0 )	 represents only the
nonresonant photon flux when the gas pressure becomes high. As shown in
Figure 1, In(Io/I) is quickly saturated at pressures higher than 2 mtorr.
This pressure dependence is consistent with the earlier measurements of
Slanger and Black (1969), where they used the CO(A ln + %l E+) emission from a
rare-gas resonance lamp as an excitation source.
4
For low gas pressure, we have a nO M 1 - 1 - na(X)1, and
in(I0/I) - nR j g(X)a M da
The cross section averaged over the monochromator bandwidth defined as
a(ao )	 f g(a) a(a) dX
can be obtained from Rn(I0/I).	 The result for the average cross section in
the 1060-1550 A region is shown in Figure 2, where the monochromator band-
width is 0.3 A. The uncertainties for the data are estimated to be within
10% of the given values. The positions for the various electronic states and
their vibrational levels (Krupenie 1966, Tilford and Simmons 1972) are
indicated in Figure 2.
Our measured average cross sections are about two orders of magnitude
higher than the cross sections reported by Watanabe et al (1953) and Myer and
Samson (1970). The pressures of 0.05-30 torr used in the measurements of Myer
and Samson (1970) are well beyond the linear absorption region (see Figure 1),
so their measured cross sections will be considerably lower than the true
average cross section measured in the linear region. The cross section at
1470 A measured at low gas pressure by Slanger and Black (1969) is about a
factor of 4 higher than the present value. Since Slanger and Black (1969)
used the CO(A 1II + X 1 r + ) emission band as light source, their measured cross
section will be close to the rotational line cross section, which is of course
larger than the average cross section reported here.
It is more meaningful to compare the vibrational oscillator strength,
which is proportional to Ja(v)dv - fa(v)dv integrated over the molecular
5
band and is independent of monochromator bandwidth. The oscillator strengths
for the v` - 0 + v" - 0 vibrational bands of the A, B, C, and E + X
transitions are listed in Table 1. The oscillator strength is calculated from
the measured peak absorption cross section, a (10 18 cm), the wavelength,
A (A), and the molecular vibrational half-bandwidth, AX (A), by the
equation f - 113 oAX/A 2 , which is derived from
f - uhc2vB/vet - (uc2 /we 2 )Iadv (Herzberg 1967). The oscillator strengths
obtained from the lifetime measurements by Carlson et al (1978) are also
listed in Table 1 for comparison. The agreement for the A + X and C + X
transitions is sur-risingly good. However, the present value for the B + X
transition is only about one-third of the value of Carlson et al. The reason
for this disagreement is not clear. The oscillator strengths for the A + X
transition obtained from various experimental measurements and theoretical
calculations are generally consistent within a factor of 2, but the values for
the B + X and C + X transitions show great discrepancies, varying from
0.29 x 10-2 to 5.3 x 10-2 and from 0.15 x 10 -2 to 1.63 x 10-2,
respectively.
In the wavelength region from 1060 to 1550 A, absorption continua under
the discrete states are very small. We estimate that the absorption cross
section for a dissociative continuum is smaller than 8 x 1 20 cm20	 . At
higher gas pressures additional absorption bands show up at the positions of
13CO isotope absorption. The doublet features for some of the absorption
bands shown in the absorption spectrum given by Myer and Samson (1970) result
from such an isotopic effect.
6
3.2
	
Fluorescence cross section
For photon wavelengths longer than the dissociation threshold of
1116.1 d (Krupenie 1966), the excited CO decays only through emission, so the
fluorescence quantum yields for the A I R and B1 E+ states must be equal to
unity. However, for energies above the dissociation limit, the excited
molecules may decay through either emission or dissociation. Since the C 1 E +
and E I n states absorb light at energies above the first dissociation limit
of CO, the quantum yield for fluorescence from these states may be less than
unity because of possible dissociation channels.
We find that the C 1 E+ , v - 0 state has significant fluorescence
intensity in the 1150-3000 A region, but the emissions from the C I E+, v 1
and E 1R, v - 0 states are very weak. Similarly, only the C I E+, v 0
state fluoresces at a > 3000 A.	 It is apparent that the C I E+, v	 1 state
is predissociated. Although emission from the E, v' - 0 + X, v" - 0 transi-
tion has been observed (Hopfield and Birge 1927, Simmons and Tilford 1974),
this fluorescence cross section may be small. Simmons and Tilf ord (1974) have
observed that accidental predissociation occurs at the R(30) and P(32) lines
of the E, v' - 0 + X, v" - 0 band, but the intermediate state that
predissociates the E state has not been identified.
The cross sections for the production of fluorescence from the
a ,3E+ , d3A i , e 3 E- i a 3 n transitions in the a > 3000 A region are shown in
Figure 3 as a function of excitation wavelength. The fluorescence cross sec-
tion is obtained from the fluorescence radiation rate and put into absolute
scale by normalization to the OH(A 2E+
 + X2 R)fluorescence rate from H2O
photodissociation, for which the cross section is known (Lee, 1980). However,
in this normalization the photomultiplier response was assumed as constant.
Since the CO triplet emission systems have wavelengths spread widely in the
7
visible and infrared region (Krupenie 1966), it is difficult to include the
photomultiplier response in the absolute normalization. Actually, the photo-
multiplier response for the OH(A2E+ + X2 H)band at 3100 A is higher than
that for the CO triplet systems, so the fluorescence cross sections shown in
Figure 3 present only the values of low limit.
In general, the cross sections for the production of the triplet systems
are smaller than 6 x 10 20 cm2 . The triplet systems have significant
absorption cross sections only when they are perturbed by the A 1 n state.
For example, the d 3 A i v - 4 and e 3E v - 1 levels have significant
absorption cross sections because they are perturbed by the A 1 R v - 0
level. Because of the small cross section, the fluorescence radiation rate
for every excitation band is linearly dependent on the gas pressure up to
100 mtorr. This linear pressure dependence has also been reported by Slanger
(1968) for 1470 A excitation.
The yields for fluorescence from the C + A and B + A transitions
decrease with increasing excitation energy. The yield is more than 2O% for
the B 1 E
+, 
v - 0 level and 10% for the v - 1 level, but for the C 1 E
+, 
v - 0
level it decreases to less than 2%. The E 1II, v - 0 and C 1 E
+, 
v - 1 levels
exhibit insignificant fluorescence both in the W and visible region. These
results imply that the states at energies higher than the C 1 E+ , v - 1 state
(a < 1060 A) are probably predissociated.
f
4
Y
3
4
d
8
4. CONCLUDING REMARKS
The average absorption cross sections reported here are much larger than
the published data. However, the oscillator strengths derived from these
absorption cross sections agree with those from lifetime measurements (Carlson
at al 1918). The validity of our results is further strengthened by the fact
that the oscillator strengths for the NO molecule, which were measured in the
same experiment (Guest and Lee, 1980), also agree well with other experiments.
Except for the C I E+ , v - 1 and E IH, v - 0 states, the CO excited 	
i
states are not dissociated by the photon energies in the region considered
here. This result indicates that CO is not significantly dissociated by
	 j
interstellar UV radiation, which is most intense in the wavelength region we
studied. In contrast, the NO molecule has very high photodissociation yields
(Guest and Lee 1980) which leads to a high dissociation rate by interstellar
UV radiation. These results may be useful for explaining the observation that
CO is very abundant in the interstellar medium, whereas NO is found only in
dark clouds that do no*_ expose NO to UV radiation.
9
ACKNOWLEDGMENTS
The authors are grateful to the staff of the Synchrotron Radiation Center
(SRC) of the University of Wisconsin, particularly E. M. Rowe and R. Otte. We
thank Dr. C. G. Olson for the use of the MacPherson Vacuum Monochromator. We
also thank Dr. T. G. Slanger, Dr. G. Black, Dr. D. L. Huestis and Dr. J. R.
Peterson for helpful discussions. J. A. Giest thanks Prof. R. N. Zare for
financial support under NSF Grant PHY 79-08694. The SRC is supported by the
National Science Foundation under Grant Yo. DMR-77-21888.
This work is supported by the National Aeronautics and Space
Administration/Astrophysics Division under Contract No. NASW-3270.
'F	 10
References
Becker K H and Welge K H 1965, Z. Naturforsch 20s 1692-9.
Carlson T A Duric N Erman P and Larsson M 1978 Z Physik A287 123-36.
?ield R W 1971 Spectroscopy and Perturbation Analysis in Excited States of CO
and CS, Thesis of Harvard University.
Guest J A and Lee L C 1980 to be published.
Herzberg G 1967 "Molecular Spectra and Molecular Structure" Van Nostrand, p.
381-3.
Hopfield J J and Birge R T 1927 Phys. Rev. 29 9220
Krupenie P H 1966 Nat. Stad Ref. Data Ser. NSRDS-NBS 5.
Lee L C 1980 J. Chem. Phys. 72 4334-40.
Myer J A and Samsom J A R 1970 J. Chem. Phys. 52 266-71.
Simmons J D and Tilford S G 1974 J. Mol. Spectrosc. 49 167-8.
Slanger T G 1968 J. Chem. Phys. 48 586-96.
Slanger T G and Black G 19692. Chem. Phys, 51 4534-8.
Tilford S G and Simmons J D 1972 J. Phys. Chem. Ref. Data 1 147-88.
Watanabe K Zelikofi M and Inn E C Y 1953 Geophys. Res. Pavers (U. S.) No. 21.
11
r1GURE CAPTIONS
Fig. 1 The logarithm of the ratio of the incident photon flux, Io , to the
attenuated photon flux, I, at gas pressures up to 100 mtorr for
photon wavelengths of 1076, 1368, 1393, and 1479 A.
Fig. 2 The photoabsorption cross section of CO in the 1060-1550 A region.
The wavelength positions of various states (Rrupenie 1966) are 	
t
S
indicated.
Fig, 3 The cross sections for fluorescence in the a > 3000 A region upon
excitation of the CO(X 1E+) ground state molecule in the 1060-
1550 A region. The wavelength positions of various states (Krupenie
1966) are indicated. The fluorescence cross sections for the triple:
states of a > 1240 A are small, and their magnitudes are indicated
by the right-hand scale.
12
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Quantitative absorption and fluorescence study of CO from 1060 to 1550 A

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