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

Abstract

The application of mass-analyzed threshold ionization (MATI) for the state-selective preparation of 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.

Figures (6)

• Figure 1: Principle of Mass-Analyzed Threshold Ionization (MATI). a) Energy-level scheme for the photoexciation with Rydberg series converging to three ionic levels $\ket{\alpha}, \ket{\beta}, \ket{\gamma}$. The laser is slightly red-detuned from the $\ket{\beta}$ ionization limit. b) Separation of prompt and MATI ions for dc and pulsed MATI. The grey arrow indicates the molecular beam and the dark blue circle the excitation region. The composition of the ion clouds is indicated by color, $\ket{\alpha}$ (dotted, blue) vs $\ket{\beta}$ (dotted, red), and the relative position of the neutral molecules (dotted, black) is shown. c) Trajectories for pulsed MATI of the prompt (blue) and MATI (red) ions. See text for details.
• Figure 2: a) Stark map for the ($n=181$, $m=0$) manifold. Only every 20th Stark component is shown for clarity. Various field-ionization rates are indicated for the individual components by dots. The red line indicates the classical saddle-point ionization threshold. b) Survival fraction of Rydberg states as a function of the electric field. For all shown calculations, the Rydberg constant $R_\infty$ is used, without the reduced mass correction.
• Figure 3: The energy ratio $\kappa$ (indicated by the contour lines) in the $\epsilon$-$\tau$ parameter space with a constant $\alpha=128$. For points (a)-(e), diagrams show the relative positions of the prompt (blue) and MATI (orange) ions at the moment the main pulse is applied. The dashed lines indicate when the prompt ions have left the separation region. For the $\epsilon>0$ and $\epsilon<0$ cases, the laser is drawn in different locations so that the relative positions of the prompt and MATI ions can be exaggerated for the purposes of illustration. See text for details.
• Figure 4: Nominal trajectories at $0\%$ energy offset are shown in (a), where a ray that enters along the incident beam's axis leaves the bender exactly at the center. Each color denotes a specific offset from the incident beam's axis. In (b), trajectories for incident particles at differing energies (denoted by the colors) are shown, with each color corresponding to the location of an axis tick in (c), which shows typical energy-selectivity characteristics of a quadrupole bender. Depending on the criterion used to define the acceptance of the quadrupole bender, broadly similar characteristics are obtained.
• Figure 5: MATI based ion injection system. Upper panel: electrode cross-sections and ray fan plot showing ion trajectories in the tangential plane of the apparatus. The radial position of the ions is always taken with respect to the beam axis, which is taken to be circular within the quadrupole bender to smoothly transition between the two perpendicular sections of the apparatus. Lower panel: Potential energy of the MATI ions along the instrument axis.