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Chemical Physics Letters 415 (2005) 296–301

Intensity borrowing in high J levels of the c3P–X1R+ (0–0)transition of carbon monoxide

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 11 June 2005; in final form 2 September 2005Available online 28 September 2005

Abstract

Several weak new absorption lines in the room temperature spectrum of carbon monoxide have been identified in the 108.9 – 109.0 nmwavelength range. The lines emerge from the long wavelength side of the C1R+–X1R+ (0–0) band and appear to form a single rotationalbranch. Their most probable assignments are to high J lines (J > 40) of the OP branch of the c 3P–X1R+ (0–0) band rather than to lines ofthe k3P–X1R+ (2–0) band. The appearance of these lines may be explained by intensity borrowing from the C–X (0–0) transition as aresult of mixing between the f1e spin-parity component of the c3P state and the C1R+ state, which are predicted to cross in this energyregion.� 2005 Elsevier B.V. All rights reserved.

1. Introduction

We have previously investigated the c3P–X1R+ (0–0)absorption band of carbon monoxide [1] and reported rota-tional lines up to J = 25. This band, first identified by Til-ford in 1969 [2], has an origin �160 cm�1 higher in energythan that of the strong C1R+–X1R+ (0–0) band [3,4] and isslightly blue-degraded. The c3P state, which is isoconfigu-rational to the E1P state, is Rydberg in character with anelectronic configuration consisting of a 2R+ ionic core anda 3pp Rydberg electron. Recently an updated set of molec-ular constants has been determined for the c3P state basedupon a reanalysis of the high-resolution near-infrared c3P–b3R+ (0–0) band and incorporating data from the c–X (0–0) band [5,6].

The c3P (v = 0) state is perturbed by the k3P (v = 2) va-lence state, which lies at lower energy [7]. Here, we notethat Berden et al. [8] showed that the vibrational labellingof the k3P state in studies prior to 1997 need to be incre-mented by one unit. To date, ground state VUV absorption

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

doi:10.1016/j.cplett.2005.09.015

* Corresponding author. Fax: +44 121 414 3078.E-mail address: j.baker@bham.ac.uk (J. Baker).

to the v = 1, 3, 4 and 6 vibrational levels of the k3P statehave been identified and analysed [9,10]. These bands arehighly red degraded due to the k3P state having a signifi-cantly smaller rotational constant than that of the groundstate [8,9,11]. Although ground state VUV absorption tothe v = 2 level of the k3P state has not yet been identi-fied, the k3P (v = 2) state has been observed via excitationfrom the a3P and a 0 3R+ states in two previous laser basedstudies [8,12] and rotational levels up to J = 5 have beenmeasured. From these studies, the origin of the k–X (2–0)band would be expected at �91960 cm�1, i.e., about40 cm�1 above the C–X (0–0) band origin. Consequently,the k–X (2–0) band should be overlapped by the much moreintense C–X (0–0) band, but might be expected to be seenemerging from the long wavelength side of this band, simi-lar to that observed for the k–X (6-0) band which emergesfrom the long wavelength side of the E–X (1–0) band [9].

This Letter is a follow-up study to Baker et al. [1]. Here,we report on several weak absorption lines appearing in theroom temperature VUV spectrum of normal isotopic com-position carbon monoxide (99.997% purity). The absorp-tion spectra were photographed using the 10.68 m VUVspectrograph at the Meudon observatory (see [1] for the

Fig. 2. The absorption spectrum of carbon monoxide showing the newabsorption lines on an expanded scale. Two simulations of the k–X (2–0)band with (a) r = 0.1 and (b) r = 20 are shown below the spectrum. Thebranches and their J numbering are also indicated. A simulation of the c–X (0–0) band is shown above the spectrum. The error bars associated withthe line position measurements are indicated above the c–X (0–0)simulation. See text for further information.

J. Baker, F. Launay / Chemical Physics Letters 415 (2005) 296–301 297

experimental details). The photographic plates haveapproximately a logarithmic response to the transmittedlight between the threshold and saturation regions of theplate. This provides for a wide signal detection range en-abling both allowed and forbidden transitions to be jointlyrecorded.

2. Results and discussion

Fig. 1 shows the absorption spectrum of carbon monox-ide over the transition energy range of 92300–91700 cm�1.The figure shows the spin forbidden c3P–X 1R+ (0–0) andspin allowed C1R+–X1R+ (0–0) bands. The latter is totallysaturated about its band centre. Also observed were somenew absorption lines emerging from the low energy sideof the C1R+–X1R+ (0–0) band. Although the lines wereclearly identified on the photographic plates they wereweak and somewhat difficult to reproduce in figures. Theregular separation of these new absorption lines and theapparent regularity of intensity, which falls off rapidly withdistance from the C–X (0–0) band, indicate that these linesbelong to a single rotational branch. Fig. 2 shows thesenew lines on an expanded scale, while Table 1 lists theirmeasured positions. The assignments are considered below.

In this region, we might expect to observe lines from thek–X (2–0) band. From the laser based studies of Mellingerand Vidal [12] and Berden et al. [8], both the origin andshape of the k–X (2–0) band can be predicted and,although the low J transitions would be expected to be to-tally overlapped and hence hidden by the C–X (0–0) band,the higher J transitions should appear to emerge from the

Fig. 1. The room temperature VUV absorption spectrum of natural isotopic composition carbon monoxide recorded in the wavenumber range 92300–91700 cm�1 at a pressure of 1.5 Torr. At this pressure the C–X (0–0) band is saturated and only lines at the wings of the band are resolved. The positionsof the new lines are indicated. Also shown are simulations of the various bands of carbon monoxide known or expected in this energy region. Thesimulations assumed that T = 298 K and the lines were Gaussian with a linewidth of 0.4 cm�1 (FWHM). Values of r = 20 and 2 were used for the c–X (0–0) and k–X (2–0) simulations, respectively. See text for further details.

Table 1Assignments and positions for the new absorption lines of 12C16Oa

OP(J) Line position (cm�1) o–c (cm�1)

44 91797.26 (50)b 0.2745 91789.99 (50) �0.4746 91783.28 (50) �0.5047 91776.39 (50) �0.5248 91769.40 (50) �0.4449 91762.88 (50) 0.3250 91755.58 (50) 0.54

a Numbers in parenthesis are estimated errors in the least significantfigures. o–c signifies observed – calculated value. See text for furtherdetails.b Indicates overlapped line.

298 J. Baker, F. Launay / Chemical Physics Letters 415 (2005) 296–301

long wavelength side of the C–X (0–0) band. However, thek–X (2–0) band would be expected to have several observa-ble rotational branches rather than one and a much greaterspacing between successive branch lines due to the largedifference in the k (v = 2) and X (v = 0) state rotationalconstants. The k–X (6–0) band, for example, representsan analogous case. Its low J transitions are overlappedby the E–X (1–0) band [9] but three rotational branchesare observed to emerge on the long wavelength side, withincreasing line separations with J in the range 25–35 cm�1. In the present case, we seem to have a singlerotational branch with energy separations of about 6.5–7.3 cm�1, with no apparent increase in energy separationwith J, which indicates the upper and lower (X1R+) stateshave similar rotational energy spacings, i.e., rotational con-stants. However, it seems possible that the observed linescould be a coincidental overlap of branches that give theappearance of a single branch. If this was the case all thebranches would also have to fall off in intensity to the samedegree with J, as the lines do not extend far from the C–X(0–0) band.

A simulation of the k3P (v = 2)–X1R+ (v = 0) band is gi-ven in Fig. 1. Line strength factors for a 3P–1R+ transitionswere taken from Kovacs [13]. These factors contain aparameter r = D/E, where D and E correspond to the tran-sition moments arising from mixing of 1R+ and 1P charac-ter into the upper 3P state, respectively [13]. The k–X (2–0)band has not previously been identified and a value of r = 2was used, which assumes that the k3P (v = 2) state hassome 1R+ and 1P character. The known k–X (3,0) and(4,0) bands have r � �0.1 and 3.5, respectively [9,14].The molecular constants for the k3P (v = 2) state were ta-ken from [5] which are based on a deperturbation analysisof the term values derived from Mellinger and Vidal [12].The ground X1R+ state molecular constants were takenfrom [11]. For all simulations room temperature(T = 298 K) and Gaussian linewidths of 0.4 cm�1 (FWHM)were assumed.

The simulated k–X (2–0) line positions and intensitiesdid not match the new lines observed in this study. Varyingthe parameter and even varying the k (v = 2) state molecu-lar constants (consistent with the previous work for this

state [5,12]) proved unsuccessful. A coincidental overlapof three rotational branches would be required and in sum-mary we were unable to assign these new lines to the k–X(2–0) band – see Fig. 2 for some comparisons.

Fig. 1 also shows room temperature simulations of theC–X (0–0) and c–X (0–0) bands. The dominant absorptionfeature is the C1R+–X1R+ (0–0) parallel band, with strongP and R rotational branches. This band was simulatedusing standard line strength factors [15] and the knownmolecular constants for the ground X1R+ [11] and upperC1R+ Rydberg states [4]. Also included are simulationsof the C–X (0–0) bands of the isotopomers of carbon mon-oxide in their natural abundance [3,4,16].

The c3P� 1R+ X (0–0) band has been the subject of aprevious study and simulation [1]. The simulation inFig. 1 is similar to that given in [1] except the more recentlydetermined molecular constants for the upper state wereused [5]. The previous study by Baker et al. [1] indicatedthat this band gains its intensity mainly via mixing of thec3P (v = 0) state with a 1R+ state, most likely the C1R+

(v = 0) state. The c–X (0–0) band essentially consists ofsix rotational branches, S-, R-, P- and O-type branches(DN = ±2, ±1) and two overlapping Q-type branches(DN = 0), which form the band head. The S(J) and R(J)type branches can be followed up to J = 21 and 25, respec-tively. The simplified structure of this band is due to thefact that the c3P state is essentially of Hund�s case bcharacter.

As described above the new lines appear as a single rota-tional branch both in terms of the regularity of the linespacing and the line intensities. The fact that no otherbranches are observed suggest that this branch is a Pbranch and the origin of the band is to higher energy andoverlapped with the C–X (0–0) band. The fact that the linespacing of successive lines of the branch are approximatelyconstant suggest that the rotational constant of the lowerstate, the X1R+ (v = 0) ground state, is similar to that ofthe upper state. This provides a case that the upper stateis in fact the c3P (v = 0) state and that these new linesare part of the OP(J) rotational branch of the c–X (0–0)band.

To test this hypothesis a molecular fit of the new lineswas carried out. The interacting two state c 3P (v = 0)–k3P (v = 2) Hamiltonian given in [1,5] was used to repre-sent the c3P (v = 0) state levels. All molecular constantswere fixed to those values recently determined in [5], exceptan additional rotational distortion term, H Æ (J Æ (J + 1))3

was included and fitted for the c3P (v = 0) state. The bestfit of the new data is given in Table 1 where the last columngives the difference between the observed and calculatedline positions. A fitted value of H = �(1.2 ± 0.1) · 10�9

cm�1 was determined. The calculated line positions agreewith the measured line positions to within about the exper-imental resolution. Fig. 2 shows a simulation of these highJ OP branch lines and they compare well with the observa-tion. Previously, the highest rotational level observed forthe c3P state was J = 26 (corresponding to the upper level

J. Baker, F. Launay / Chemical Physics Letters 415 (2005) 296–301 299

of the RR(25) line [1]) and so there is quite an extrapolationto these new lines, which correspond to transitions to theJ = 43–49 rotational levels of the f1e spin-parity compo-nent. Hence, we cannot rule out the possibility that this Jnumbering is out by one or two units. It seems surprisingthat such high J transitions are observable for the OPbranch lines but not for the other branches, e.g., theSR(J) and RR(J) branches can only be followed up toJ = 21 and 25, respectively [1]. The latter two branches cor-respond to transitions to the f3e and f2e spin-parity compo-nents of the c3P (v = 0) state, respectively.

Fig. 3 shows the reduced term values of the k3P (v = 2),c 3P (v = 0) and C1R+ (v = 0) states, derived from theirknown molecular constants ([1,4,5,12] and this work).The C1R+ (v = 0) state has an energy origin below bothof the other states and is predicted to cross the k3P(v = 2) state at low J and the c3P (v = 0) state at higherJ. Parity selection rules [17] mean that the C1R+ (v = 0)state can potentially interact with the e-parity componentsof the k3P and c3P states. The C1R+ (v = 0) and k3P(v = 2) states have a nearest coincidence at J = 4, wherethey are separated by �2.6 cm�1, but also have near coin-cidences at J = 3 (3.5 cm�1), J = 7 (5.1 cm�1) and J = 8and 10 (5.7 cm�1). Despite these near coincidences thereare no reported perturbations in the C1R+ (v = 0) state[4] indicating that the interaction between the C1R+

(v = 0) and k3P (v = 2) states are not significant. Thismay help to explain why the k–X (2–0) band is notobserved.

Fig. 4 shows the energy crossing region between theC1R+ (v = 0) and c3P (v = 0) states. The C1R+ (v = 0)state is predicted to cross the f1e spin-parity componentof the c3P (v = 0) state between J = 40 and 50. The nearestcoincidence is predicted to be at J = 44 with an energy sep-

C 1Σ+ (v=0)

10 20 30 4090500

91000

91500

92000

92500

93000

93500

J(

Red

uced

Ter

m V

alue

s: T

– 1

.6×

J(J+

1) /

cm

-1

C 1 + (v=0)

10 20 30 4090500

91000

91500

92000

92500

93000

93500

Fig. 3. Reduced term value plot showing the k3P (v = 2) (h), c 3P

aration of �1.3 cm�1, while the energy separations atJ = 39 and 49 are predicted to be �23 and 25 cm�1, respec-tively. These c3P (v = 0) state rotational levels correspondto the upper levels of the new lines assigned in Table 1.Hence, the appearance of the OP (J = 43–49) branch linesmay be explained by this crossing. High J lines of theSR(J) and RR(J) branches of the c–X (0–0) band are notobserved [1] but correspond to transitions to the f3e andf2e spin-parity components, which are expected to crossthe C1R+ (v = 0) state at much higher J. A previous studyindicated that there is an interaction between the c3P(v = 0) and C1R+ (v = 0) states [1]. This interaction wouldbe expected to be strongest in the crossing region and hence�intensity borrowing� from the C–X (0–0) band throughinterstate mixing could help to explain the appearance ofthese high J lines.

Having observed high J lines in the OP(J) branch itmight be expected to observe the corresponding lines inthe QR(J) branch, which have the same spin-parity upperstate. Identification of these lines would enable the J levelnumbering to be confirmed. These lines (J = 38–48) wouldbe expected in the 92122–92130 cm�1 transition energy re-gion, and the branch is predicted to have a band head atQR(47). This region occurs in the low J region of the RRand SR branches of the c–X band and in the tail of the Rbranch of the C–X band. An examination of this regiondid not identify clearly any additional lines that might beattributable to high J transitions in the QR branch. Poten-tial reasons for this are (a) overlap with the more intenseRR(J = 9–11) and SR(J = 4–5) lines and (b) unresolvablelines due to the formation of the band head. It is worthyto note that the high J lines of the OP(J) branch occur ina region where there are essentially no overlapping stronglines. In addition, there appears to be a diffuse absorption

c3Π (v=0)

k3Π (v=2)

50

J+1) with J labelling

c3 (v=0)

k3 (v=2)

50

(v = 0) (s), and C1R+ (v = 0) (·), states (e-parity levels only).

91750

91800

91850

91900

91950

92000

92050

92100

92150

92200

c3Π (f1)

C 1Σ+

40

c3Π (f2)

c3Π (f3)

503091750

91800

91850

91900

91950

92000

92050

92100

92150

92200

J(J+1) with J labelling

Red

uced

Ter

m V

alue

s: T

– 1

.955

× J(

J+1)

/ c

m-1

40 5030

Fig. 4. Reduced term value plot showing the crossing between the c3P (v = 0) (s) spin substates and the C1R+ (v = 0) (·) state (e-parity levels only). Notethat the reduced term value scale, T – 1.955 · J(J + 1), is different to that of Fig. 3, and was chosen to show more clearly the differences between the energylevels of these two states.

300 J. Baker, F. Launay / Chemical Physics Letters 415 (2005) 296–301

background in this region, associated with the P branch tailof the C–X (0–0) band. This diffuse absorption backgroundmay have enhanced the sensitivity of the photographicplate to these new weak lines, by taking the lines abovethe plate detection threshold and into its logarithmic re-sponse region.

In summary, the available evidence suggests that thenew lines observed emerging from the long wavelength sideof the strong C–X (0–0) band are high J lines of the OP(J)branch of the c–X (0–0) band. The branch lines gain inten-sity by intensity borrowing from the C–X (0–0) band as a

k3Π (v=2)

k3Π (v=3)

k3Π (v=4)

10 20 3091500

92000

92500

93000

93500

k3 (v=2)

k3 (v=3)

k3 (v=4)

J(J+1)

Red

uced

Ter

m V

alue

s: T

– 1

.6×

J(J+

1) /

cm

-1

Fig. 5. Reduced term value plot showing the v = 2, 3 and 4 vibrational levels ok3P (v = 3) and k3P (v = 4) states are predicted to �cross� the c3P (v = 0) sta

result of crossing of high J rotational levels between theC1R+ (v = 0) state and the f1e spin-parity component ofthe c3P (v = 0) state. It is noted that this interaction maybe expected to give rise to small line shifts in the corre-sponding levels in both states. However, Ubachs et al. [4]did not report any line shifts from their high temperature2 + 1 REMPI spectrum of the C–X (0–0) band where levelsup to J � 50 were observed.

Finally, it is necessary to consider an additional compli-cation. It is known that the c3P (v = 0) state is perturbedby the k3P (v = 2) state at low J [1,5]. However, due to

c3Π (v=0)

40 50

c3 (v=0)

with J labelling

f the k3P state (h) and the c 3P (v = 0) state (s) (e-parity levels only). Thete – see text for further details.

J. Baker, F. Launay / Chemical Physics Letters 415 (2005) 296–301 301

the large difference in the rotational constants between thec3P Rydberg and k3P valence states, higher vibrationallevels of the k3P state will also be expected to perturbthe c3P (v = 0) state but at higher J. Fig. 5 shows a reducedterm value plot that includes the v = 3 and 4 vibrationallevels of the k3P state. The term values for these statesin the �crossing� regions (in the diabatic sense [17]) representextrapolations to higher J of molecular constants based onlower J data [9] and potential c 3P state interactions wereneglected. Given these limitations the v = 3 and 4 levelsof the k3P state are predicted to �cross� the f1e componentof the c3P (v = 0) state in the J = 31–37 and J = 45–50 re-gions, respectively. Hence, the assigned OP(J) lines mightbe expected to be significantly shifted as a result of a per-turbation with the k3P (v = 4) state. However, the ob-served lines do not appear to show any strong line shifts.This might indicate that either the interaction betweenthe states is small or that the extrapolations are inaccurate.At present there is insufficient available information toinvestigate these potential higher k(v)–c(0) interactionsfurther.

3. Conclusion

Several weak absorption lines within the room tempera-ture VUV spectrum of carbon monoxide have been identi-fied in the 108.9–109 nm wavelength region. The linesemerge from the long wavelength side of the C1R+–X1R+

(0–0) absorption band and appear to form a single rota-tional branch. Their most probable assignments are to highJ lines of the OP branch of the c 3P–X1R+ (0–0) band. Areasonable fit of the new data was obtained with the twostate c 3P (v = 0)–k3P (v = 2) Hamiltonian and the molec-ular constants given in [1,5], with the addition of a rota-tional distortion term, H Æ (J Æ (J + 1))3. A fitted value ofH = �(1.2 ± 0.1) · 10�9 cm�1 was determined, and the cal-culated line positions agreed with the measured line posi-

tions to within about the experimental resolution.However, since the upper levels were only observed inone branch, and there was a large gap to the known lowerJ levels, it is possible that the J assignment is out by 1 or 2.The appearance of these lines may be explained by intensityborrowing from the C–X (0–0) band as a result of mixingbetween the f1e spin component of the c3P state and theC1R+ state which are predicted to cross in this energy re-gion. Higher vibrational levels of the k3P state are also ex-pected to �cross� the c3P state and may perturb these levels.However, there is a lack of data on the high J levels of thesestates to make accurate predictions of their likely effects.

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Cacciani, J. Mol. Spectrosc. 174 (1995) 388.[5] J. Baker, J. Mol. Spectrosc. (in press).[6] I. Dabrowski, M. Vervloet, D.-C. Wang, Can. J. Phys. 65 (1987) 1171.[7] J. Baker, J.L. Lemaire, S. Couris, A. Vient, D. Malmasson, F. Rostas,

Chem. Phys. 178 (1993) 569.[8] G. Berden, R.T. Jongma, D. Van der Zande, G. Meijer, J. Chem.

Phys. 107 (1997) 8303.[9] J. Baker, F. Launay, J. Mol. Spectrosc. 165 (1994) 75.[10] J. Baker, J. Mol. Spectrosc. 167 (1994) 323.[11] G. Guelachvili, D. De Villeneuve, R. Farrenq, W. Urban, J. Verges, J.

Mol. Spectrosc. 98 (1983) 64.[12] A. Mellinger, C.R. Vidal, J. Chem. Phys. 101 (1994) 104.[13] I. Kovacs, Rotational Structure in the Spectra of Diatomic Mole-

cules, Hilger, London, 1969.[14] J. Baker, F. Launay, J. Mol. Spectrosc. 203 (2000) 196.[15] G. Herzberg, Molecular Spectra and Molecular Structure. I. Spectra

of Diatomic Molecules, Van Nostrand, New York, 1950.[16] M. Eidelsberg, J.J. Benayoun, Y. Viala, F. Rostas, P.L. Smith, K.

Yoshino, G. Stark, C.A Shettle, Astron. Astrophys. 265 (1992) 839.[17] H. Lefebvre-Brion, R.W. Field, Perturbations in the Spectra of

Diatomic Molecules, Academic Press, Orlando, 1986.