The cross section as well as the branching ratios for the dissociative recombination of ground-state CH ϩ ions with electrons have been measured using the heavy-ion storage-ring technique and two-dimensional fragment imaging. Although the absolute value of the cross section at thermal energies is found to be in very good agreement with the theory, several unpredicted narrow resonances are also present in the data. These structures are interpreted as due to an indirect recombination process via core-excited Rydberg states. The branching-ratio measurement shows that at low electron energy the 2 2 ⌸ state, producing carbon fragments C͑ 1 D͒, is the most important dissociative state, although transitions during the dissociation to other dissociative potential curves are also present. Anisotropy in the angular distribution of the dissociating fragments is visible for some of the final states. Dissociative recombination of ions in the metastable excited a 3 ⌸ state is also observed, and the lifetime as well as the excitation energy of this state are deduced from the imaging data.

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The characteristics and the performance of a thermal lens detector, which uses a double-beam absorption scheme, were studied in a capillary electrophoresis system with various types of toxic pollutants, e.g., pesticides. The setup of the detector system was miniaturized using the smallest diverging path lengths between the cell and the pinhole (4 mm). The probe laser beam (He:Ne laser, 633 nm) and the excitation beam (Ar + ion laser, 364, 457, 488, and 514 nm) with a crossed setup were directed by mirrors into two microscope objectives that focused the beam to a 5-m waist inside the capillary. The detection volume was on the order of 75 nl when a 75-m capillary was employed. The change in intensity of the probe beam was detected by a photodiode behind a pinhole, which was protected with different band-pass interference filters. The excitation laser can be used in the multiline order. Micellar electrokinetic methods are used for pesticide separation. The performance of the detector in capillary electrophoresis was assessed with various types of capillaries and compared with a conventional absorption detector. The limit of detection is at least one order of magnitude better than it is with the absorption detector. © 1997 Society of Photo-Optical Instrumentation Engineers. [S1083-3668 (97)01003-4]