2+1 resonantly enhanced multiphoton ionization of CO via the E 1Π–X 1Σ+ transition: From measured ion signals to quantitative population distributions

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Abstract
The 2+1 resonantly enhanced multiphoton ionization (REMPI) spectrum of the CO E 1Π–X 1Σ+ (0,0) transition is used to determine ground state rotational populations with a detection sensitivity of approximately 3×106 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 E 1Π 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 1Σ+ state.