2+1 RESONANTLY ENHANCED MULTIPHOTON IONIZATION OF CO VIA THE E1-PI-X1-SIGMA+ TRANSITION - FROM MEASURED ION SIGNALS TO QUANTITATIVE POPULATION-DISTRIBUTIONS

The 2+1 resonantly enhanced multiphoton ionization (REMPI) spectrum of the CO E1II-X1SIGMA+ (0,0) transition is used to determine ground state rotational populations with a detection sensitivity of approximately 3 X 10(6) molecules per quantum state per cm3. Low rotational states of CO are ionized to CO+; however, high rotational states form both C+ and CO+. This effect is shown to be both branch dependent and J dependent. In order to extract reliable ground state populations, both the C+ and CO+ channels must be measured. When the C+ channel is not accounted for, high rotational states are systematically undercounted. New rotational constants are determined for the C12O16 E1II state; Bf0 is 1.9526 cm-1 and Be0 is 1.9645 cm-1. The large lambda doubling (q = 0.0119 cm-1) of the E state is attributed to a perturbation by the nearby C 1-SIGMA+ state.