State-selected preparation of diatomic molecular ions for precision measurements in radio-frequency traps

The application of mass-analyzed threshold ionization (MATI) for the state-selective preparation of diatomic molecular ions is presented. Based on photoexcitation of long-lived high-n Rydberg states, molecular ions are prepared in a single rovibronic level by pulsed-field ionization. We present a theoretical analysis and a recipe for obtaining an optimal energy ratio between such selected ions and molecular ions in unwanted rovibronic states, created by direct photoionization. It is shown that the second-order chromatic aberration of a dc quadrupole bender can be used to isolate the state-selectively prepared molecular ions. The phase-space properties of ions prepared by MATI are ideally suited for axial injection into a linear radio-frequency trap. A modified approach for carrying out MATI within such an ion trap is also described.