Direct observation of the k 3Π state of 12C18O

Chemical Physics Letters 404 (2005) 49–52 www.elsevier.com/locate/cplett

Direct observation of the k 3P state of Jacob Baker a

a,*

, Franc¸oise Launay

12

C18O

b

Division of Environmental Health and Risk Management, School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK b 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 k P (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 k 3P (v = 3)–E1P (v = 0) interaction. This is the first direct observation of the k 3P state for 12 18 C O.
2005 Elsevier B.V. All rights reserved. 3

1. Introduction Carbon monoxide (CO) is the most abundant molecule 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 isotopomers of CO is also used to map isotopic ratios of elemental carbon and oxygen [2]. These ratios are ultimately linked to stellar origins and hence provide information 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 this occurs mainly through absorption into rotationally resolved bands followed by predissociation, radiation

*

Corresponding author. Fax: +44 121 414 3078. E-mail address: [email protected] (J. Baker).

0009-2614/$ - see front matter
2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.11.138

shielding and isotope shifts are expected to lead to isotopic fractionation [4–6]. The modelling studies of Van Dishoek and Black [6] show that the E1P–X1R+ (1–0) band is most effective in contributing to isotopic fractionation. The E1P (v = 1) Rydberg state is in fact perturbed by the k 3P valence state, which causes local rotational line shifts and enhanced predissociation rates [7–9]. The k 3P state, which is believed to be strongly predissociated by a 3P repulsive state, acts as a doorway state to enhanced dissociation [7]. Forbidden transitions directly to the k 3P 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 k 3P valence state has previously been directly observed for 12C16O and 13C16O [8,10–13]. Berden et al. [12] in 1997 showed that the vibrational labelling of 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 spectrum of 12C18O which we assign to the k 3P–X1R+ (3–0) band. This represents the first direct observation of the k 3P 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 spectrograph at the Meudon observatory. Previously, we have reported on absorption bands corresponding to forbidden transitions from the X1R+ (v = 0) ground state to the v = 1, 3, 4 and 6 vibrational levels of the k 3P 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 12C18O has previously been photographed at a maximum pressure of 0.02 Torr using the spectrograph at Meudon, as part of a study of singlet–singlet absorption transitions [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 energy side of the much stronger E1P–X1R+ (0–0) absorption band and occurring in the region of the expected k–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` lÕEnergie Atomique with a certified isotopic purity of 97.6%. Calibration was achieved by recording known atomic emission lines (Cu II, Ge II, Si II) from a windowless hollow cathode lamp as well as known lines appearing in the actual 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.

sons with the corresponding bands of 12C16O and 13 16 C O [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 k 3P state. Although, the lines were clearly identified on the photographic plate and measured with the Meudon photoelectric 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 rotational fit of the measured line positions. The 3P excited state rotational energy levels were fitted to the f2 eigenvalue of the effective 3P Hamiltonian matrix of 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 cm1 and D = 5.554 · 106 cm1 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 corresponding values determined for 13C16O [13]. Table 2 also presents 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 equilibrium molecular constants of 12C16O derived from previous VUV absorption studies (Table VI of Ref. [10]) but with the corrected vibrational assignment [12] (see Table 3). Isotopically scaled molecular parameters can then be estimated from [19] T iv0 ¼ T e  ZPE ðX1 Rþ Þi þ qxe ðv þ 1=2Þ 2

3

 q2 xe xe ðv þ 1=2Þ þ q3 xe y e ðv þ 1=2Þ ; Biv ¼ q2 Be  q3 ae ðv þ 1=2Þ;

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 k 3P (v = 3)–X1R+ (v = 0) band of Q

J

Q

0 1 2 3 4 5 6 7

92 740.50(40)b 92 737.87(40) 92 733.57(40) 92 728.15(40)

Table 2 Molecular constantsa for the k 3P (v = 3) state of 12 18 C O 12

C18Oa

OC

R

0.11 0.13 0.21 0.36

92 744.92(40)b 92 744.92(40)b

0.14 0.17

92 740.50(40)b 92 736.02(40)

0.30 0.05

R

92 723.58(40)

OC

0.05

All values are in units of cm1. Values between parentheses are estimated errors. O  C signifies observed  calculated value. b Overlapped lines. a

where ZPE (X1R+) is thep zero ffiffiffiffiffiffiffiffiffipoint energy of the ground electronic state and q ¼ l=li with l and li the reduced

Molecular parameter

12

C16O

b

T30 B3 D · 106 A (o + p + q)

92 782.67(2) 1.24405(26) 9.86(80) 30.976(12) 0.298(14)

13

C16O

12

C16O,

c

92 742.78(10) 1.1917(6) 9.0 fixed 30.95(4) 0.31(4)

13

C16O and

12

C18O

d

92 739.1(3) 1.1860(36) 9.0 fixed 30.95 fixed 0.3 fixed

All molecular constants are in units of cm1. 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 cm1. The error in T30 includes an estimate for the calibration error. a

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

to the experimentally determined values of 92 739.1(3) and 1.1860(36) cm1, respectively. Hence these isotopically 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 band assuming a rotational temperature of T = 298 K and a Gaussian linewidth of 0.6 cm1 (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 factors were taken from Kovacs [20], where the 3P state is treated as an intermediate HundÕs 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 k 3P–1P interaction. The intensity is plotted on a logarithmic scale to simulate the response of the photographic 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

Table 3 Equilibrium molecular constants for the k 3P and B1R+ states of 12 16 C O Molecular parameter

k 3Pa

B1R+b

Te xe xexe xeye Be ae ˚) re (A

90 968.34(160) 846.64 (91) 5.46(11)

86 951.32 2084.8 24.01 15.60

51

1.29817(105) 0.01531(31) 1.3761

All values are in cm1 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 deperturbed). The constants given reproduce the vibrational band origins of 12 16 C O (v = 1, 3, 4, 6) to <0.5 cm1. 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 0 1R+ state [21]. The degrees of freedom in the fit is zero and so no fitting errors are given.

masses of 12C16O and the isotopomer, i, in question. For 12 18 C O, q = 0.97584 and ZPE (X1R+)i = 1055.537 cm1 (derived from Ref. [13]). The isotopically scaled values of T30 and B3 calculated for 12C18O are 92 740.8 and 1.1864 cm1, respectively. These compare very closely

1

5 QQ

0 10

5

*

*

*

* *

**

92770

92760

92750

92740

RR

92730

92720

92710

92700

92690

Transition Energy (cm -1)

Fig. 1. A simulation of the forbidden k 3P (v = 3)–X1R+ (v = 0) absorption band of 12C18O. Only the asterisked rotational lines above the threshold represented by the dashed horizontal line were observed (see text for details).

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 lines for 12C18O. There are a few possibilities as to why these higher J lines (appearing below 92 723 cm1) were not observed. One possibility is that close to threshold non-uniformities in the sensitivity of the photographic plate may be significant. Another possibility 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 origin 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 92 675 ± 45 cm1, which places it on the 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 factors used. As mentioned above the forbidden transition gains oscillator strength through a k 3P–1P perturbation and the line strength factors used assumes that the perturbing states are widely separated from the k 3P state such that the rotational energy is insignificant compared to the energy separation of the states [20]. If the k 3P (v = 3) state is perturbed mainly by the E1P (v = 0) state then this assumption is no longer strictly 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 k 3P (v = 3) state are given in the last column of Table 2. From this we determine an E1P (J = 1, v = 0)–k 3P1 (J = 1, v = 3) energy separation of 189 cm1 and an E1P (J = 10, v = 0)– k 3P 1 (J = 10, v = 3) energy separation of 264 cm1. 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 together 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 k 3P–X1R+ (3–0) transition with a measured band origin of 92 739.1 ± 0.3 cm1. Only the two strongest branches QQ and RR were observed corresponding to transitions to the f- and e-parity levels, respectively, of the f2 (3P1) spin–orbit component of the k 3P state. A consideration of the intensity structure of this band indicates that it gains intensity through a k 3P (v = 3)–E1P (v = 0) spin–orbit interaction.

Acknowledgements We are indebted to Franc¸ois Rostas for supporting and encouraging the work. We thank Maurice Benharrous for technical assistance.

References [1] N. Neininger, M. Guelin, H. Ungerechts, R. Lucas, R. Wielebinski, Nature 395 (1998) 871. [2] P. Harjunpaa, K. Lehtinen, L.K. Haikala, Astron. Astrophys. 421 (2004) 1087. [3] A. Heikkila, L.E.B. Johansson, H. Olofsson, Astron. Astrophys. 332 (1998) 493. [4] A.E. Glassgold, P.J. Huggins, W.D. Langer, Astrophys. J. 290 (1985) 615. [5] Y.P Viala, C. Letzelter, M. Eidelsberg, F. Rostas, Astron. Astrophys. 193 (1988) 265. [6] E.F. Van Dishoeck, J.H. Black, Astrophys. J. 334 (1988) 771. [7] J. Baker, J.L. Lemaire, S. Couris, A. Vient, D. Malmasson, F. Rostas, Chem. Phys. 178 (1993) 569. [8] J. Baker, F. Launay, J. Mol. Spectrosc. 165 (1994) 75. [9] W. Ubachs, I. Velchev, P. Cacciani, J. Chem. Phys. 113 (2000) 547. [10] J. Baker, J. Mol. Spectrosc. 167 (1994) 323. [11] A. Mellinger, C.R. Vidal, J. Chem. Phys. 101 (1994) 104. [12] G. Berden, R.T. Jongma, D. Van der Zande, G. Meijer, J. Chem. Phys. 107 (1997) 8303. [13] J. Baker, F. Launay, J. Mol. Spectrosc. 203 (2000) 196. [14] M. Eidelsberg, J.-Y. Roncin, A. Le Floch, F. Launay, C. Letzelter, J. Rostas, J. Mol. Spectrosc. 121 (1987) 309. [15] M. Eidelsberg, F. Rostas, Astron. Astrophys. 235 (1990) 472. [16] F. Launay, Proceedings International Conference on Image Processing Techniques in Astronomy, Utrecht, Reidel, Dordrecht, 1975, p. 265. [17] J.M. Brown, A.J. Merer, J. Mol. Spectrosc. 74 (1979) 488. [18] G. Guelachvili, D. De Villeneuve, R. Farrenq, W. Urban, J. Verges, J. Mol. Spectrosc. 98 (1983) 64. [19] G. HerzbergSpectra of Diatomic Molecules, vol. 1, Van Nostrand, New York, 1950. [20] I. Kovacs, Rotational Structure in the Spectra of Diatomic Molecules, Hilger, London, 1969. ¨ .L. Tchang-Brillet, P.S. Julienne, J. Chem. Phys. [21] J. Baker, W.-U 102 (1995) 3956.