ChemicalPhysics 181 (1994) 217-245 North-Holland Predissociation rates in carbon monoxide: dependence on rotational state, parity and isotope K.S.E. Eikema, W. Hogervorst and W. Ubachs Laser Centre, Department of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HVAmsterdam, The Netherlands Received 30 August 1993 A high-resolution spectroscopic study of carbon monoxide has been performed using a narrow-band and tunable extreme ultraviolet laser source in the wavelength range 91-97 nm. For three isotopes 12C’60, r3Cr60 and “C’*O the (4px)L IfI, u=O, (4po)K’Z+, v=O and (3sa)W’lT, u=O states were investigated. The ‘II, u=2, (3so)W’D, v=2 and (3dx)L”D, v=l states were studied for r2Cr60 and r3Cr60. The ‘II, v=O state at 109564.58 cm-r, the (6~0) IX+, v=O state at 109173.68 cm-‘, the (Spa) IX+, v=O stateat 107174.44cm-‘, andthe (4do) ‘Z+, v=O stateat 105676.30cm-r were excitedonlyforthe mainisotope r2Cr60. Previously unknown states are found at 107365.87 cm-’ (‘Z’) for r2CL60and at 103203.07 cm-’ (‘II) for ‘3C’80. Indirect evidence is found for a perturber state at 107708 f 2 cm-r for r2CL60, while spectroscopic information on the E ‘4 v= 5 state of r3C’s0 is deduced from a deperturbation analysis. A rotational-state and isotope-dependent predissociation rate is found for almost all states. A parity dependence is observed for the (4px)L IfI, v= 0 and ( 3~0) W *lI, v=O states, where the predissocia- tion rate of the l-L component varies proportional to J(J+ 1). This effect is attributed to a coupling with the repulsive part ofthe D’ rE+ state. Several examples of accidental predissociations have been found, particularly in the zyxwvutsrqponmlkjihgfedcbaZYXWVU (4do) ‘C+, v=O state. 1. Introduction Considerable progress in the extreme-ultraviolet (XUV ) spectroscopy of molecular hydrogen [ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG 1 ] and carbon monoxide [ 2-91 has been reported in recent years. Of these two most widespread molecular spe- cies in the interstellar medium CO is readily observ- able by radioastronomy and for this reason it is com- monly used as a tracer molecule in searches for and detailed analyses of interstellar clouds. Absorption and radiative transfer in the XUV spectral range and XUV-induced photodissociation processes are known to largely influence the chemical dynamics of the in- terstellar medium [ 10-l 31. In particular photodis- sociation at short wavelengths, manifesting itself through predissociation of bound Rydberg states, is the dominant loss process of CO in dense molecular clouds. As photodestruction takes place via discrete line absorption rather than through continuum ab- sorption [ 21, accurate spectroscopic information on the highly excited states of CO, including the absorp- tion and dissociation rates, is required. For the inte- rior of dense clouds the Prasad-Tarafdar model in- volving cosmic-ray-induced XUV photons may play a role [ 13 1. Here the precise determination of coin- cidences between H2 emission lines and CO absorp- tion lines in the XUV range is requested. A long- standing problem is that of the C/CO ratio in the interstellar medium, which is found to be much larger than predicted by steady-state models [ 141. The rel- ative over-abundance of carbon atoms may be ex- plained assuming additional photodestruction pro- cesses of CO [ 151 or by revision of the photodissociation rates. A motivation for the contin- uing efforts devoted to spectroscopic studies of the high-lying states of CO, like the present one, is the desire to find answers to the aforementioned questions. Isotope effects are found to be of major impor- tance. Because the purely rotational transitions of the main **Ci60 isotopomer are often saturated micro- wave spectra of the less-abundant species, such as 13C160, ‘*C’*O and 13C180,are also recorded in ra- dioastronomical searches. In fact these rare isoto- pomers are primarily used to map cloud density dis- tributions. The ratio 12C/13C or 160/180 is found to 0301-0104/94/S 07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDZ 030 1-O 104(93)E0400-P