www.elsevier.com/locate/cplett

Chemical Physics Letters 408 (2005) 312–316

The diffuse v = 4 and 5 vibrational levels of the B 1R+ Rydbergstate of carbon monoxide

Jacob Baker *

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

University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom

Received 1 April 2005; in final form 14 April 2005

Available online 13 May 2005

Abstract

Two new diffuse bands in the VUV absorption spectrum of carbon monoxide at 105.6 and 103.0 nm have been identified. Their

spectral characteristics are consistent with B 1R+ (v = 4, 5) X 1R+ (v = 0) 12C16O assignments, with band origins at

94700 ± 120 cm�1 and 97050 ± 150 cm�1 and predissociation linewidths of �350 and 450 cm�1, respectively. These upper state

vibrational levels occur in the �avoided crossing region� of the 3sr, B Rydberg state and the repulsive part of the D 0 1R+ valence

state potential energy surface.

� 2005 Elsevier B.V. All rights reserved.

1. Introduction

The B 1R+ state is the lowest energy singlet Rydberg

state of CO with an excited electron in the 3sr Rydbergorbital and an X 2R+ CO+ ion core. The first four vibra-

tional levels of this state are known and ground state

transitions to these levels are characterised by decreasing

oscillator strength, decreasing rotational resolution and

increasing valence character. The first two vibrational

levels of this state are relatively long lived and give rise

to known fluorescence to the A 1P valence and X 1R+

ground states [1–3]. Discontinuous weakening in emis-sion from rotational levels in both these vibrational lev-

els are observed and have been used to determine the

CO dissociation limit of 89592 ± 15 cm�1 (referenced

to the X 1R+ (v = 0) level of 12C16O) [3]. The mechanism

and state causing this predissociation is currently un-

known but it is believed to be due to weak second-order

coupling [3,4]. A strong homogeneous interaction with

the D 0 1R+ valence state explains a second emission

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

doi:10.1016/j.cplett.2005.04.046

* Fax: +44 121 414 3078.

E-mail address: j.baker@bham.ac.uk.

weakening in the v = 1 level of the B state and the

increasing valence character and decreasing lifetimes

with increasing v [4]. The B–X (2–0) absorption band

in fact shows noticeable broadening of rotational linewidths (1–2 cm�1 FWHM) while the B–X (3–0) band

is diffuse [3,5].

Tchang-Brillet et al. [4] developed a two channel close

coupling model of the B 1R+–D 0 1R+ state interaction,

where two diabatic potentials (RKR and purely repul-

sive for the B and D 0 states, respectively) and an interac-

tion parameter were optimised to best reproduce and

explain the v = 0–2, B state experimental data. Fig. 1shows these two diabatic potentials as well as the corre-

sponding adiabatic potentials. The same model was

found to successfully reproduce in terms of position

and shape the B–X (3–0) absorption band that was iden-

tified a few years afterwards [5]. Since the original study

by Tchang-Brillet et al. [4] there have been a number of

similar theoretical studies predicting higher quasi-

resonance levels of the B–D 0 state interaction [6–8]. Re-cently, Eidelsberg et al. [9] have reported rotational cold

absorption and two-photon ionisation spectra for a

number of isotopomers of CO in the 90–100 nm region

R (Å)

V(c

m-1

)

D'

B

BD'1

BD'2

0

1

2

3

diss. limit

85000

90000

95000

100000

105000

0.8 1 1.2 1.4 1.6 1.

4

5

1.8 2

Fig. 1. Model diabatic potentials (solid curves) used in the represen-

tation of the B–D 0 homogeneous interaction (from [4]). Also shown are

the corresponding adiabatic potentials (dashed curves) labelled BD 01

and BD 02. The – Æ– curve represents the deperturbed D 0 RKR potential

which has a barrier to dissociation at about 1.98 A [4] and the solid line

indicates the dissociation limit. The energy origin is taken at the

minimum of the ground state potential.

J. Baker / Chemical Physics Letters 408 (2005) 312–316 313

and have attributed some features in this region to higher

quasi-resonances involving the B state. As discussed by

them there are some discrepancies between their attribu-

tions and the theoretical studies.The present Letter, which is a follow-up study to Ba-

ker et al. [5], identifies two new absorption bands consis-

tent with ground state transitions to the v = 4 and 5

vibrational levels of the B state. The absorption spectra

were photographed using the 10.68 m VUV spectro-

graph at the Meudon observatory (see [5,10] for the

experimental details).

2. Results and discussion

Fig. 2 shows the absorption spectrum of natural iso-

topic composition CO over the 101–110 nm wavelength

range. The CO pressures used inside the spectrograph

were relatively high leading to saturation of most of

the known absorption bands, where the VUV radiationis completely absorbed. The transitions from the ground

state to the first four vibrational levels of the 3pr C 1R+

Rydberg state and the first three vibrational levels of the

3pp E1P Rydberg state are indicated in the figure. Also

indicated are the known B–X (2–0) and B–X (3–0)

absorption bands. The B–X (3–0) band appears to the

lower energy side of the E–X (0–0) band, which partially

overlaps it, and is overlapped by the forbidden k 3P–X(3–0) band [10,11].

According to the two state B 1R+–D 0 1R+ interaction

model of Tchang-Brillet and co-workers [5,7] the B

(v = 3) level is the last level below the adiabatic potential

barrier (see Fig. 1). Li et al. [12] obtained the same result

for their B–D 0 adiabatic potential, computed using

ab-initio multireference techniques. Hence, in a purely

adiabatic sense there would be no observable higher

vibrational levels of the B state (BD 01 in Fig. 1). How-

ever, quasi-resonances can occur above the adiabatic

barrier, in the diabatic crossing region, as a result of a

breakdown in the Born–Oppenheimer approximation.

In this region, higher B–X (v 0–0) absorption bands havebeen predicted to be both weaker and broader than the

B–X (3–0) band [6,7].

Predicted band origins of the B–X (4–0) and B–X

(5–0) absorption bands from two separate theoretical

studies are indicated in Fig. 2. Both studies used essen-

tially the same two state diabatic model of Tchang-Brillet

et al. [4] but one study used a close-coupling method to

obtain the quasi-resonance levels while the other studyused a discrete-variable representation approach (see

Table VI of [6]). Now looking at the absorption spec-

trum of CO in Fig. 2, two diffuse absorption bands

can be seen close to these predicted values. The first dif-

fuse absorption band lies between and overlaps the C–X

(1–0) and E–X (1–0) absorption bands. The diffuse nat-

ure of the band and its position being close to its pre-

dicted position leads to a B–X (4–0) band assignment.A 1R+–1R+ band simulation [13] was attempted of this

diffuse feature using the known rotational constants of

the X 1R+ (v = 0) ground state [14] and assuming upper

state rotational constants of B4 = 1.77 cm�1, estimated

from a quadratic extrapolation of the reported rota-

tional constants of the v = 0–3 vibrational levels of the

B state [3,5], and D4 = 9.4 · 10�6 cm�1 from [5]. The

rotational line shapes were assumed to be Lorentzian.In addition, the rotational line widths and the vibronic

transition moments (lev) were assumed to be J indepen-

dent (although in reality this is unlikely to be the case

[5]). From simulations based on these assumptions a

band origin of 94700 ± 120 cm�1 was estimated with

an average predissociation width of C4 � 350 cm�1, cor-

responding to an upper state lifetime of �15 fs.

The second new diffuse absorption feature underliesthe E–X (2–0) band and is assigned to the B–X (5–0)

absorption band. A simple simulation of this band was

also attempted where the upper state rotational con-

stants of B5 = 1.69 cm�1 and D4 = 9.4 · 10�6 cm�1 were

assumed. From these simulations a band origin of

97050 ± 150 cm�1 was estimated with an average predis-

sociation width of C5 � 450 cm�1, corresponding to an

upper state lifetime of �12 fs. The absorption band liesin the energy region predicted by the theoretical studies

although the predictions underestimate the band origin

by 400–600 cm�1.

An indication of the deviation of the B 1R+ state from

Rydberg character can be obtained by comparing the

measured vibrational energies, Gv (B) with those of the

X 2R+ CO+ ion core, Gv (ion) [15], as shown in Fig. 3,

where the level shift dGv = Gv (B) � Gv (ion) is plotted.The v = 4 level of the B state appears to continue the

trend of increasing level shift dGv with v, while the

Fig. 2. Room temperature absorption spectra of CO (natural isotopic composition; 99.997% purity) in the 91000–99000 cm�1 energy region (101–

110 nm wavelength region) for (a) 0.5 Torr and (b) 1.5 Torr total pressure. These spectra are densitometer recordings of photographic plate spectra.

Downward arrows along the top of (b) correspond to close coupling (dashed) and discrete variable (dotted) model predictions of B–X (4–0) and B–X

(5–0) band origins [4,6]. The solid lines under the spectra are based on band simulations and help to identify the diffuse bands. See text for further

details.

V

Gv(c

m1 )

-1000

-800

-600

-400

-200

0

0 1 2 3 4

δ−

5 6

Fig. 3. Vibrational energy levels shifts, dGv = Gv (B) � Gv (ion), of the

B 1R+ state of 12C16O. Experimental data for the B state were taken

from [3–5] and this Letter. The X 2R+ state vibrational energies of CO+

were derived from [15]. All vibrational energies are referenced to their

respective state potential energy minimum.

314 J. Baker / Chemical Physics Letters 408 (2005) 312–316

v = 5 level shows somewhat of a discontinuity. This may

be an indication that the D 0 state diabatic potential

crosses between the v = 4 and v = 5 outer turning points

of the B state diabatic potential, i.e., between

94700 ± 120 cm�1 and 97050 ± 150 cm�1 above the

X 1R+ (v = 0) ground vibronic state. Interestingly, the

B and D 0 diabatic potentials used in the close coupling

model of Tchang Brillet et al. cross at about

95726 cm�1 above the ground vibronic state [4,16]

which is consistent with the above. A similar observa-

tion has been made for the v = 3 and v = 4 levels ofthe 3pr C 1R+ Rydberg state [4].

Estimations of the integrated absorption cross-sec-

tions of the B–X (4–0) and B–X (5–0) bands have been

made from densitometer recordings of the photographic

spectra by comparing the intensity of these bands to that

of the B–X (3–0) band. The following relative absorp-

tion cross-sections have been determined: r (4–0)/r(3–0) � 0.47 ± 0.1 and r (5–0)/r (4–0) � 1.54 ± 0.2.These values can be compared to r (4–0)/r (3–

0) � 0.27 and r (5–0)/r (4–0) � 0.60 which were esti-

mated from Fig. 2 of Andric et al. [7] which shows the

calculated transitions cross-sections from the X 1R+

(v = 0, J = 0) state. Table 1 summarises the results of this

study and includes the known data for the v 0 = 0–3 lev-

els. The last two columns of Table 1 gives the computed

relative absorption cross-sections from Tchang-Brilletand co-workers diabatic [4,17] and coupled B–D 0 state

[4,5,7] models, respectively. As indicated in the Table

the experimental band intensities are enhanced as a re-

sult of mixing with the D 0 state which has an electronic

transition moment (D 0–X) about 50 times greater than

that of the B state [4].

Table 1

Summary of experimental data for the B–X (v 0–0) bands of 12C16Oa

v 0 Tv00 (cm�1) rb (10�18 cm2 nm) Cc (cm�1) sd r ðv–0Þ

r ð0–0Þ expt:r ðv0–0Þr ð0–0Þ B-RKRe r ðv0–0Þ

r ð0–0Þ B–D0modelf

0 86916.2 5.29 0.0002 30 ns 1.0 1.0 1.0

1 88998.3 0.78 0.0002 30 ns 0.15 0.020 0.15

0.02 300 ps

2 90988.1 0.04 0.7–2.0 3–8 ps 0.0076 3.9 · 10�5 0.011

3 92792 ± 40 �0.006 �90 59 fs 0.0011 1.3 · 10�5 0.0011

4 94700 ± 120 �0.003 �350 15 fs 0.0005 3.8 · 10�7 0.0003

5 97050 ± 150 �0.004 �450 12 fs 0.0008 3.6 · 10�7 0.0002

a Experimental data: v 0 = 0–2 from [3,4,18]; v 0 = 3 from [5]; v 0 = 4, 5 this Letter.b Integrated band cross-section.c Linewidths; for v 0 = 1, the values correspond to below and above the first emission weakening, respectively.d Upper state lifetime.e Calculated relative ground state absorption cross-sections to the diabatic RKR B state potential [4,17].f Calculated relative absorption cross-sections using the coupled B–D0 state model [4,5,7].

J. Baker / Chemical Physics Letters 408 (2005) 312–316 315

Recently, Eidelsberg et al. [9] have reinvestigated the

photoabsorption spectrum of CO at shorter wavelengths

in the 90–100 nm region and have attributed some va-

lence-like features in this region to higher quasi-reso-

nances (v = 6, 7 and 9) involving the B state. This

energy region occurs above the predicted B–D 0 diabatic

crossing where the BD 02 adiabatic state is predicted (see

Fig. 1). This region of the spectrum is dominated bytransitions to higher Rydberg states that include the

3d, 4s and 4p Rydberg states converging to the X 2R+

CO+ ion core [18]. The absorption feature attributed

to the B (v = 6) level was found to strongly interact with

the 3dr F 1R+ Rydberg state and had a deperturbed

band origin at 99868.4 cm�1. The relative intensities of

transitions to these two interacting states was explained

by interference between two nearly equal amplitudebands [19]. Eidelsberg and Rostas [18] report the inte-

grated absorption cross-section for the 3dr, F 1R+–

X 1R+ (0–0) band, although it was not recognised at that

time that the feature was associated with two different

upper states. From Eidelsberg et al.�s [9] new interpreta-

tion of this feature an integrated absorption cross-sec-

tion of r (B–X (6–0)) � 3.5 · 10�18 cm2 nm can be

estimated, i.e., r (6–0)/r (0–0) � 0.7 and the B–X (6–0) �deperturbed� band has an intensity similar to the

B–X (0–0) origin band. The cross-section of the reported

band does not follow the trend shown in Table 1 for the

lower vibrational levels. Since the energy of the absorp-

tion feature occurs in the region where the v = 0 level of

the BD 02 adiabatic potential may be expected, if the ob-

served absorption feature is associated with the B state,

it may be more appropriate to use an adiabatic nomen-clature, i.e., BD 02 (v = 0) rather than B (v = 6) to label

the upper state.

3. Conclusion

Two new diffuse bands have been observed in the

vacuum ultraviolet absorption spectrum of carbon

monoxide in the 101–107 nm region. Their spectral char-

acteristics are consistent with transitions from the X 1R+

(v = 0) ground state of 12C16O to the v 0 = 4 and 5 vibra-

tional levels of the B 1R+ Rydberg state, with band ori-

gins at 94700 ± 120 cm�1 and 97050 ± 150 cm�1,

respectively. These upper state vibrational levels occur

in the avoided crossing region of the B 1R+ Rydberg

and D 0 1R+ valence states, between the correspondingBD 01 and BD 02 adiabatic states. Reasonable agreement

is found between the observed bands and those pre-

dicted from computational studies of quasi-resonances

arising from the B–D 0 state interaction. The predissoci-

ation linewidths of the v 0 = 4 and 5 levels are estimated

as �350 and 450 cm�1, respectively, corresponding to

predissociation lifetimes of the order of 15 and 12 fs,

respectively.

Acknowledgements

The author is grateful to Francoise Launay and Mau-

rice Benharrous for photographing the spectra, Dr.

Claudina Cossart for use of a densitometer at Orsay

and Dr. Michele Eidelsberg for help with the densitom-eter recordings. The author is also grateful to Dr. Franc-

ois Rostas for supporting and encouraging the work.

References

[1] A.-G. Angstrom, T.-R. Thalen, Nova Acta Reg. Soc. Sc. Upsal.,

Ser III IX (1875).

[2] R.T. Birge, Phys. Rev. 28 (1926) 1157.

[3] M. Eidelsberg, J.-Y. Roncin, A. Le Floch, F. Launay, C.

Letzelter, J. Rostas, J. Mol. Spectrosc. 121 (1987) 309.

[4] W-U.L. Tchang-Brillet, P.S. Julienne, J.-M. Robbe, C. Letzelter,

F. Rostas, J. Chem. Phys. 96 (1992) 6735.

[5] J. Baker, W-U.L. Tchang-Brillet, P.S. Julienne, J. Chem. Phys.

102 (1995) 3956.

[6] M. Monnerville, J.M. Robbe, J. Chem. Phys. 101 (1994) 7580.

[7] L. Andric, T.P. Grozdanov, R. McCarrol, W-U.L. Tchang-

Brillet, J. Phys. B 32 (1999) 4729.

316 J. Baker / Chemical Physics Letters 408 (2005) 312–316

[8] L. Ancic, F. Bouakline, T.P. Grozdanov, R. McCarroll, Astron.

Astrophys. 421 (2004) 381.

[9] M. Eidelsberg, F. Launay, K. Ito, T. Matsui, P.C. Hinnen, E.

Reinhold, W. Ubachs, K.P. Huber, J. Chem. Phys. 121 (2004)

292.

[10] J. Baker, F. Launay, J. Mol. Spectrosc. 165 (1994) 75.

[11] G. Berden, R.T. Jongma, D. Van der Zande, G. Meijer, J. Chem.

Phys. 107 (1997) 8303.

[12] Y. Li, R.J. Buenker, G. Hirsch, Theor. Chem. Acc. 100 (1998)

112.

[13] G. Herzberg, Molecular Spectra and Molecular Structure. I.

Spectra of Diatomic Molecules, Van Nostrand, New York, 1950.

[14] G. Guelachvili, D. De Villeneuve, R. Farrenq, W. Urban, J.

Verges, J. Mol. Spectrosc. 98 (1983) 64.

[15] R. Kepa, A. Kocan, M. Ostrowska-Kopec, I. Piotrowskia-

Domagala, M. Zachwieja, J. Mol. Spectrosc. 228 (2004) 66;

R. Kepa, A. Kocan, M. Ostrowska-Kopec, I. Piotrowskia-

Domagala, M. Zachwieja, J. Mol. Spectrosc. 203 (2005) 102.

[16] R.J. LeRoy, Chemical Physics Research Report No. CP-425,

University of Waterloo, Waterloo, 1992.

[17] R.J. LeRoy, Chemical Physics Research Report No. CP-655,

University of Waterloo, Waterloo, 2002.

[18] M. Eidelsberg, F. Rostas, Astron. Astrophys. 235 (1990) 472.

[19] C. Jungen, J. Chem. Phys. 53 (1970) 4168.