HIGH-RESOLUTION PHOTOABSORPTION CROSS-SECTIONS OF E1-PI-X1-SIGMA+ VIBRATIONAL BANDS OF (CO)-C-12 AND (CO)-C-13

Photodissociation following absorption of extreme-ultraviolet photons is an important factor in determining the abundance and isotropic fractionation of CO in diffuse and translucent interstellar clouds. The principal channel for destruction of (CO)-C-13 in such clouds begins with absorption in the (1, 0) vibrational band of the E1-PI-X 1-SIGMA+ system; similarly, absorption in the (0, 0) band begins a significant destruction channel for (CO)-C-12. Reliable modeling of the CO fractionation process depends critically upon the accuracy of the photoabsorption cross section for these bands. We have measured the cross sections for the relevant isotropic species and for the (1, 0) band of (CO)-C-12. Our apparatus had a resolving power approximately 20 times larger than those used in previous measurements. Our results, which are uncertain by about 10%, are for the most part larger than previous measurements.