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.
💡 Analysis
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.
📄 Content
Precision spectroscopy of molecular ions has emerged as a tool for testing fundamental physics and for developing molecular ion clocks. Applications include determining fundamental constants [1][2][3][4][5][6][7], searching for their possible variation [8], and probing fundamental symmetries [9].
Preparing ions in a single, well-defined quantum state is essential, both to raise signal-to-noise ratios in experiments with large ion clouds and to enable measurements with a single ion. Because molecules possess rotational and vibrational degrees of freedom, the population spreads over many levels, and so state-selective preparation methods are essential compared to atoms.
Electron bombardment is a common route to ion production, but offers little state selectivity [10]. It is often applied to thermal molecular samples, where the population is already distributed over many rovibrational levels. For heteronuclear diatomic and polyatomic ions, a non-zero electric dipole moment permits fluorescence on experimental timescales, and as a result, electronically or vibrationally excited populations tend to relax into the vibronic ground state. Nevertheless, black-body radiation at typical trap temperatures redistributes the population among rotational levels, even for light ions with large rotational constants [11].
Resonance-enhanced multiphoton ionization (REMPI) uses a resonant transition to excite a chosen rovibrational level of the neutral precursor. Because ionization accesses the continuum, selectivity in the ionic states is governed primarily by energy conservation. With a suitable resonance and an appropriate REMPI scheme, the lowest rovibrational levels of the ion can be loaded [12,13]. Compared to electron bombardment, the higher selectivity of REMPI arises from the much narrower energy spread of the laser compared to the electron beam. In addition, propensity rules associated with the autoionizing Rydberg states accessed in REMPI can bias the resulting distribution over ionic rovibrational levels [14].
Without a state-selective source of ions, quantum-logic spectroscopy (QLS) offers a practical alternative [15][16][17][18][19]. A single trapped molecular ion can be initialized in a chosen rovibrational state, repeatedly interrogated, and reused, thereby reducing the load on ion production by avoiding destructive detection. Starting from an unselective source, logic operations with a co-trapped atomic ion can be used to identify the target state and then isolate it for further study [20].
An alternative strategy is to develop ion-production methods that are both highly selective and capable of high repetition rates. Merkt et al. pointed out in 1993 that the spectroscopic scheme underlying zero-kinetic energy photoelectron spectroscopy (ZEKE PES) [21] and mass-analyzed threshold ionization (MATI) [22], delayed pulsedfield ionization (PFI) of high-n Rydberg states, can be used to prepare ions in selected rovibrational levels [23]. In their work, hydrogen molecular ions were selectively prepared in different rotational levels of the electronic ground state with v + = 2. An extension to polyatomic molecules with a focus on infrared spectroscopy was reported later by Jacovella and coworkers [24]. The state selected ions were not spatially separated and remained in the supersonic beam.
Compared to electron bombardment and REMPI, the MATI method allows selective preparation of ionic states with arbitrary rovibronic excitation. Exotic states, inaccessible from the ground state, can be prepared using a multiphoton, stepwise resonant excitation scheme, by maximizing the overall Franck-Condon factors. The highest vibrational levels of the electronic ground state and the rovibrational levels of the first excited electronic state of H + 2 , HD + and D + 2 can be prepared in such a way [25,26]. These excited states can be beneficial in precision measurements, because they offer an enhanced sensitivity for fundamental constants, such as the proton-to-electron mass ratio [27], or peculiar effects like ortho-para mixing [28][29][30].
In this paper, we review the MATI mechanism and discuss additional requirements to efficiently extract only the state-selectively prepared ions from the supersonic beam and inject them into a radio-frequency ion trap, which offers long interaction times and good control of environmental conditions. Based on designs for EBIT sources, we analyze a system containing a dc quadrupole bender and ion lenses to deflects the ion packet out of the supersonic beam and focuses it into the trap [31][32][33][34][35][36][37][38]. The results presented in this paper are general to ions of any charge-to-mass ratio.
Like ZEKE, MATI spectroscopy relies on the electric field ionization of long lived highn Rydberg states. An electric field F (in V/cm) can lower the ionization threshold by ≈ 4.8 √ F cm -1 [39]. Figure 1a shows the laser excitation from a ground or intermediate state into a region above the low
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