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Chemical Physics Letters 404 (2005) 49–52

Direct observation of the k3P state of 12C18O

Jacob Baker a,*, Francoise Launay b

a Division of Environmental Health and Risk Management, School of Geography, Earth and Environmental Sciences,

University of Birmingham, Edgbaston, Birmingham B15 2TT, UKb Observatoire de Paris, Section de Meudon, LERMA, UMR 8112 du CNRS, 92195 Meudon Cedex, France

Received 27 October 2004; in final form 27 October 2004

Abstract

A weak rotationally resolved absorption band of 12C18O has been identified from photographic VUV spectra and assigned to the

k3P (v = 3) X1R+ (v = 0) forbidden transition. The experimentally determined band origin and upper state rotational constant are

in close agreement to that derived from isotopic scaling. A consideration of the intensity structure of the band suggests that the band

gains its intensity mainly through a k3P (v = 3)–E1P (v = 0) interaction. This is the first direct observation of the k3P state for12C18O.

� 2005 Elsevier B.V. All rights reserved.

1. Introduction

Carbon monoxide (CO) is the most abundant mole-

cule after molecular hydrogen in interstellar space and

is widely used as a tracer for mapping out the density

and flow of molecules in the interstellar medium [1].

Measurements of column densities of the different iso-

topomers of CO is also used to map isotopic ratios of

elemental carbon and oxygen [2]. These ratios are ulti-

mately linked to stellar origins and hence provide infor-mation on the stellar sources and evolution of the

interstellar medium [3]. To be used reliably as a tracer,

it is necessary to understand all the processes governing

the CO abundance, including processes giving rise to

isotopic fractionation.

Photodestruction of CO, which occurs in the 90–

112 nm VUV region, is one of the most important

processes governing the CO abundance and since thisoccurs mainly through absorption into rotationally

resolved bands followed by predissociation, radiation

0009-2614/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2004.11.138

* Corresponding author. Fax: +44 121 414 3078.

E-mail address: j.baker@bham.ac.uk (J. Baker).

shielding and isotope shifts are expected to lead to isoto-

pic fractionation [4–6]. The modelling studies of VanDishoek and Black [6] show that the E1P–X1R+ (1–0)

band is most effective in contributing to isotopic frac-

tionation. The E1P (v = 1) Rydberg state is in fact per-

turbed by the k3P valence state, which causes local

rotational line shifts and enhanced predissociation rates

[7–9]. The k3P state, which is believed to be strongly

predissociated by a 3P repulsive state, acts as a doorway

state to enhanced dissociation [7]. Forbidden transitionsdirectly to the k3P state from the X1R+ ground state

may also become a significant dissociation source in

the inner regions of dense interstellar clouds where

self-shielding effects diminish the importance of the

stronger absorption bands.

The k3P valence state has previously been directly

observed for 12C16O and 13C16O [8,10–13]. Berden

et al. [12] in 1997 showed that the vibrational labellingof this state in prior studies was incorrect and needed

to be incremented by one unit. In this article we report

on a weak band appearing in the VUV absorption spec-

trum of 12C18O which we assign to the k3P–X1R+ (3–0)

band. This represents the first direct observation of the

k3P state for 12C18O.

50 J. Baker, F. Launay / Chemical Physics Letters 404 (2005) 49–52

2. Experimental

This study made use of the 10.68 m VUV spectro-

graph at the Meudon observatory. Previously, we have

reported on absorption bands corresponding to forbid-

den transitions from the X1R+ (v = 0) ground state tothe v = 1, 3, 4 and 6 vibrational levels of the k3P state

of the normal isotopomer of carbon monoxide 12C16O,

recorded over the pressure range of 0.1–1.5 Torr [8,10].

The strongest of the reported absorption bands is the

k–X (3–0) band. This band has also been identified for

the 13C16O isotope recorded at a pressure of 0.02 Torr

[13].

The absorption spectrum of the rarer isotope 12C18Ohas previously been photographed at a maximum pres-

sure of 0.02 Torr using the spectrograph at Meudon,

as part of a study of singlet–singlet absorption transi-

tions [14,15]. A careful examination of a photographic

plate containing the absorption spectrum of 12C18O at

0.02 Torr revealed seven new faint lines to the lower en-

ergy side of the much stronger E1P–X1R+ (0–0) absorp-

tion band and occurring in the region of the expectedk–X (3–0) band at �107.8 nm.

The experimental details of the spectrograph have

been described in detail elsewhere [8,14]. The 12C18O

gas was purchased from the Commissariat a l�EnergieAtomique with a certified isotopic purity of 97.6%. Cal-

ibration was achieved by recording known atomic emis-

sion lines (Cu II, Ge II, Si II) from a windowless hollow

cathode lamp as well as known lines appearing in the ac-tual 12C18O absorption spectrum. The photographic

plates used to record the spectra have an approximately

logarithmic response to the transmitted light between

the threshold and saturation regions of the plate.

3. Results and discussion

Table 1 gives the line positions and assignments for

the faint absorption band of 12C18O observed at

�107.8 nm. Initial assignments were based on compari-

Table 1

Line positions for the k3P (v = 3)–X1R+ (v = 0) band of 12C18Oa

J QQ O � C RR O � C

0

1 92740.50(40)b 0.11 92744.92(40)b �0.142 92737.87(40) 0.13 92744.92(40)b 0.17

3 92733.57(40) �0.214 92728.15(40) �0.36 92740.50(40)b 0.30

5 92736.02(40) 0.05

6

7 92723.58(40) �0.05a All values are in units of cm�1. Values between parentheses are

estimated errors. O � C signifies observed � calculated value.b Overlapped lines.

sons with the corresponding bands of 12C16O and13C16O [8,13]. Only the two strongest rotational

branches QQ and RR were observed, which correspond

to transitions to the f and e parity levels, respectively,

of the f2 (3P1) spin–orbit component of the k3P state.

Although, the lines were clearly identified on the photo-graphic plate and measured with the Meudon photoelec-

tric comparator [16] they did not appear clearly on

positive prints and so we are unable to show the actual

photographic spectrum here.

Table 2 (last column) gives the results of a rota-

tional fit of the measured line positions. The 3P ex-

cited state rotational energy levels were fitted to the

f2 eigenvalue of the effective 3P Hamiltonian matrixof Brown and Merer [17] while the X1R+ (v = 0)

ground state rotational energy levels were fitted to

the expression F(J) = BJ(J + 1) � D(J(J + 1))2 with

B = 1.8309808 cm�1 and D = 5.554 · 10�6 cm�1 derived

from Ref. [18]. The band origin, T30, and upper state

rotational constant, B3, were varied in the fit, while

other molecular constant were fixed to the correspond-

ing values determined for 13C16O [13]. Table 2 alsopresents the molecular constants for the corresponding

bands of 12C16O and 13C16O for comparison.

To check this k–X (3–0) assignment for 12C18O, the

band origin T30 and upper state rotational constant B3

were estimated via isotopic scaling of the k state equilib-

rium molecular constants of 12C16O derived from previ-

ous VUV absorption studies (Table VI of Ref. [10])

but with the corrected vibrational assignment [12] (seeTable 3). Isotopically scaled molecular parameters can

then be estimated from [19]

T iv0¼ T e � ZPE ðX1RþÞi þ qxeðvþ 1=2Þ� q2xexeðvþ 1=2Þ2 þ q3xeyeðvþ 1=2Þ3;

Biv ¼ q2Be � q3aeðvþ 1=2Þ;

where ZPE (X1R+) is the zero point energy of the ground

electronic state and q ¼ffiffiffiffiffiffiffiffiffil=li

pwith l and li the reduced

Table 2

Molecular constantsa for the k3P (v = 3) state of 12C16O, 13C16O and12C18O

Molecular parameter 12C16O b 13C16O c 12C18O d

T30 92782.67(2) 92742.78(10) 92739.1(3)

B3 1.24405(26) 1.1917(6) 1.1860(36)

D · 106 9.86(80) 9.0 fixed 9.0 fixed

A 30.976(12) 30.95(4) 30.95 fixed

(o + p + q) 0.298(14) 0.31(4) 0.3 fixed

a All molecular constants are in units of cm�1. The numbers in

parenthesis are errors to one standard deviation, in the least significant

figure.b Ref. [8].c Ref. [13].d This work. The standard deviation of the fit was r = 0.22 cm�1.

The error in T30 includes an estimate for the calibration error.

Table 3

Equilibrium molecular constants for the k3P and B1R+ states of12C16O

Molecular parameter k3Pa B1R+b

Te 90968.34(160) 86951.32

xe 846.64 (91) 2084.8

xexe 5.46(11) �24.01xeye �15.60Be 1.29817(105)

ae 0.01531(31)

re (A) 1.3761

All values are in cm�1 except re.a Rederived from Ref. [10] using corrected vibrational numbering.

The vibrational constants were derived from v = 1, 3, 4, 6, levels which

do not show any obvious vibrational perturbation shifts, while the

rotational constants were derived from v = 1, 3, 4 (these level either do

not show any obvious rotational perturbations or have been deper-

turbed). The constants given reproduce the vibrational band origins of12C16O (v = 1, 3, 4, 6) to <0.5 cm�1. The numbers in parenthesis are

errors to one standard deviation and are relatively large due to the

limited degrees of freedom in the fit (one) and large correlations

between the fitted parameters.b Derived from v = 0 to 3 term values [14,21]. The negative values

for the anharmonicity constants xexe and xeye arise from the strong

perturbation of the B1R+ state with the D 01R+ state [21]. The degrees of

freedom in the fit is zero and so no fitting errors are given.

J. Baker, F. Launay / Chemical Physics Letters 404 (2005) 49–52 51

masses of 12C16O and the isotopomer, i, in question. For12C18O, q = 0.97584 and ZPE (X1R+)i = 1055.537 cm�1

(derived from Ref. [13]). The isotopically scaled values

of T30 and B3 calculated for 12C18O are 92740.8 and

1.1864 cm�1, respectively. These compare very closely

* *

* *

* *

05

1

9273092740927509276092770

Transition

* *

* *

* *

9273092740927509276092770

Fig. 1. A simulation of the forbidden k3P (v = 3)–X1R+ (v = 0) absorption b

represented by the dashed horizontal line were observed (see text for details

to the experimentally determined values of 92739.1(3)

and 1.1860(36) cm�1, respectively. Hence these isotopi-

cally scaling calculations confirm the 12C18O k–X (3–0)

band assignment.

Finally, we consider the intensity structure of the

band. Fig. 1 is a simulation of the absorption bandassuming a rotational temperature of T = 298 K and

a Gaussian linewidth of 0.6 cm�1 (FWHM). The line

positions were calculated from upper and lower state

term energy differences, where the term energies were

determined as described above using the upper state

molecular constants given in Table 2 and the known

ground state constants [18]. 3P–1R+ linestrength fac-

tors were taken from Kovacs [20], where the 3P stateis treated as an intermediate Hund�s coupling case (a/b).

In these linestrength formulae there is an adjustable

parameter r = D/E, where D and E correspond, in

the present case, to the effective transition moments

arising from mixing of 1R+ and 1P character into

the 3P state, respectively. For the corresponding band

of both 12C16O and 13C16O a value of r � �0.1 was

determined and is used here. This implies that the k–X (3–0) band gains it intensity mainly through a

k3P–1P interaction. The intensity is plotted on a log-

arithmic scale to simulate the response of the photo-

graphic plate above the threshold.

Only a few rotational lines were clearly visible on

the photographic plate and these are labelled with

asterisks in Fig. 1. It is clear that the absorption band

*

R

Q5

92690927009271092720

Energy (cm )-1

*

RR

QQ

10

92690927009271092720

and of 12C18O. Only the asterisked rotational lines above the threshold

).

52 J. Baker, F. Launay / Chemical Physics Letters 404 (2005) 49–52

was close to the detection threshold region and lines

below the dashed horizontal line were below threshold.

The spectrum recorded for 13C16O at 0.02 Torr was

similar but higher J lines were also observed [13].

The intensity simulation of Fig. 1 suggests that we

may have expected to observe a few higher J linesfor 12C18O. There are a few possibilities as to why

these higher J lines (appearing below 92723 cm�1)

were not observed. One possibility is that close to

threshold non-uniformities in the sensitivity of the

photographic plate may be significant. Another possi-

bility is an underlying diffuse band that helps to raise

the blue (higher transition energy) side of the band

above the threshold. The diffuse B1R+–X1R+ (3–0)band would be expected to occur in this energy region

and has been observed for 12C16O but at much higher

pressures [21]. An estimate of the B–X (3–0) band ori-

gin for 12C18O can be obtained via isotopic scaling

(vide supra) of the equilibrium molecular constants

of the B state of 12C16O derived from Refs. [14,21]

(see Table 3 (last column)). The B–X (3–0) band origin

so determined is 92675 ± 45 cm�1, which places it onthe red (lower transition energy) side rather than the

blue side of the k–X (3–0) band, so this diffuse band

does not explain our observation and is probably

too weak to have any effect. The uncertainty in the

given band origin is estimated from the uncertainty

in the experimental band origin of the diffuse B–X

(3–0) band of 12C16O [21].

One other possibility lies with the line strength fac-tors used. As mentioned above the forbidden transition

gains oscillator strength through a k3P–1P perturba-

tion and the line strength factors used assumes that

the perturbing states are widely separated from the

k3P state such that the rotational energy is insignifi-

cant compared to the energy separation of the states

[20]. If the k3P (v = 3) state is perturbed mainly by

the E1P (v = 0) state then this assumption is no longerstrictly valid. The molecular constants of the E1P(v = 0) state of 12C18O are well known and can be used

to determine rovibronic term energy values [15], while

the molecular constants of the k3P (v = 3) state are gi-

ven in the last column of Table 2. From this we deter-

mine an E1P (J = 1, v = 0)–k3P1 (J = 1, v = 3) energy

separation of 189 cm�1 and an E1P (J = 10, v = 0)–

k3P 1 (J = 10, v = 3) energy separation of 264 cm�1.This implies that the higher J line strengths will be

weaker relative to the lower J line strengths as a result

of this increasing energy separation with J. This to-

gether with non-uniformities in the sensitivity of the

plate close to threshold provides an explanation of

the observations.

4. Conclusion

A faint absorption band of 12C18O has been observed

and assigned to the forbidden k3P–X1R+ (3–0) transition

with ameasured band origin of 92739.1 ± 0.3 cm�1. Only

the two strongest branches QQ and RRwere observed cor-responding to transitions to the f- and e-parity levels,

respectively, of the f2 (3P1) spin–orbit component of the

k3P state. A consideration of the intensity structure of

this band indicates that it gains intensity through a k3P(v = 3)–E1P (v = 0) spin–orbit interaction.

Acknowledgements

We are indebted to Francois Rostas for supporting

and encouraging the work. We thank Maurice Benhar-

rous for technical assistance.

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