Internal conversion from excited electronic states of $^{229}{\mathrm Th}$ ions

The process of internal conversion from excited electronic states is investigated theoretically for the case of the vacuum-ultraviolet nuclear transition of $^{229}{\mathrm Th}$. Due to the very low transition energy, the $^{229}{\mathrm Th}$ nucleus offers the unique possibility to open the otherwise forbidden internal conversion nuclear decay channel for thorium ions via optical laser excitation of the electronic shell. We show that this feature can be exploited to investigate the isomeric state properties via observation of internal conversion from excited electronic configurations of ${\mathrm Th}^+$ and ${\mathrm Th}^{2+}$ ions. A possible experimental realization of the proposed scenario at the nuclear laser spectroscopy facility IGISOL in Jyv\"askyl\"a, Finland is discussed.


I. INTRODUCTION
The 229 Th isotope is unique throughout the entire nuclear chart due to its first nuclear excited state lying energetically in the optical range [1]. In the language of nuclear physics, this state is an isomer, i.e., a long-lived excited nuclear state. At present the most accepted value for the energy of this level is E m = 7.8 ± 0.5 eV [1], rendering it accessible to vacuum ultra-violet (VUV) lasers. Due to the very low transition energy, the radiative decay of the state is strongly suppressed leading to an exceptionally long lifetime of the isomer. The ratio of radiative width to transition energy is presently estimated at γ γ /E m = 10 −19 . Given this very narrow width and the high robustness of nuclei to external perturbations [2], the isomeric state has been proposed for novel applications such as a nuclear frequency standard with unprecedented accuracy [2][3][4] or a nuclear laser [5]. On the other hand, these features render a direct spectroscopic search for the nuclear resonance with a narrow-band laser extremely tedious. A coarse determination of the isomer energy or a restriction of the search range would therefore be extremely helpful.
Since the nuclear transition energy lies above the first ionization threshold of Th (see Table I), the decay of the nuclear state can occur not only radiatively but also via internal conversion (IC). In the process of IC, the nuclear transition energy is transferred to a bound electron in the atom, which is then ionized. In the case of 229 Th, the 7.8 eV can be transferred to one of the valence electrons, which then leaves the atom. This decay channel is much stronger than the radiative decay, with an IC coefficient, i.e., the ratio of the IC rate to the radiative decay rate of the same transition, of α = γ IC /γ γ ∼ 10 9 [6,7].
Typically, in the process of IC the initial electronic state is the ground state of the atom. Due to the very low-lying nuclear excited state, 229 Th is a unique candidate for investigating IC of electrons from an excited electronic state. Optical or VUV lasers can be used to excite the atomic shell and maintain the outermost electrons available for IC in excited states. This opportunity exists and is relevant only for 229 Th, since for higher nuclear transition energies mostly electrons from inner shells of the ground-state electronic configuration are ionized. Practically, inner-shell vacancies are notoriously short-lived and excited electronic configurations cannot be maintained long enough for IC to occur. In this work we investigate this unique feature of 229 Th to design new possible means to detect the isomeric state via observation of IC from excited electronic configurations.
Better knowledge of the isomeric state energy and its properties is required for all nuclear quantum optics applications based on the 229 Th isomeric transition proposed so far. Several approaches have been pursued, for example atomic spectroscopy of trapped Th ions [10] and laser spectroscopy of Th nuclei grown in VUVtransparent crystal lattices [11][12][13]. Significant progress has been recently achieved by the detection of the IC electrons from the isomeric state in neutral 229 Th atoms [14]. This result is considered to be the first direct observation of the low-energy isomeric state of 229 Th. However, direct or indirect excitation of the isomeric state has not been achieved so far [15,16], raising the question of whether the isomeric energy lies somewhat higher than the region thus far considered [7,16]. An experimental confirmation of the 229m Th isomeric state energy and lifetime thus remains a very important experimental challenge.
Here we envisage a new possible approach to detect the isomeric state taking advantage of IC from excited electronic states. IC is energetically allowed in the neutral atom, but should be forbidden in Th ions, as shown by the ionization potential values tabulated in Table I. Thus, IC from the electronic ground state becomes energetically forbidden for higher degrees of ionization so that only IC from excited states remains allowed. If a 229m Th ion prepared in an excited electronic state undergoes IC at a rate fast enough compared with the spontaneous decay of the electronic excited state, ions of a higher charge state will be produced in the process. Detection of such ions can be performed with close-to-unity efficiency and direct comparison with the case of 232 Th (which does not possess any nuclear states at optical energies) will indicate the presence of the isomeric state. Based on the observation of occurring IC-induced ionization, one can estimate the isomeric energy and compare the IC rate with theoretical predictions. A schematic illustration of the IC from an excited electronic state of Th + leading to the formation of Th 2+ ions is presented in Fig. 1.  Following present speculations [7,16], in this work we analyze possible excited states as candidates for the initial IC configuration assuming that the value of E m may also lie above 7.8 eV. As upper limit we choose 20.0 eV which is the ionization potential of Th 2+ . Comparing the IC rates and the rates of radiative decay for the electronic levels, we show that this detection scheme is applicable for Th + in the case when E m lies between approx. 9.0 and 12.0 eV, while Th 2+ can be used only in the rather unlikely case that E m is higher than 19.1 eV, but less than 20.0 eV. A possible experimental verification of this scenario at the IGISOL facility in Jyväskylä, Finland is discussed towards the end of the paper.
The paper is structured as follows. We start by giving a short overview of the theoretical calculations for IC rates from ground and excited state electronic configurations in Section II. Numerical results are presented in Section III. In the following Section we discuss possible experimental approaches to reach excited electronic states relevant for the IC scenario, with the proposed experiment for isomeric state detection in Section V. The paper closes with a summary in Section VI. Atomic units (m e = = e = 1) are used throughout the paper unless otherwise mentioned.

II. THEORETICAL CONSIDERATIONS
IC is one of the few nuclear processes directly involving atomic electrons. The electronic and nuclear degrees of freedom couple electromagnetically, and the interaction Hamiltonian for nuclear transitions of electric multipolarities is determined by the Coulomb interaction between the nucleus and the converted electron, where ρ n ( r n ) is the nuclear charge density, r n ( r e ) denotes the nuclear (electronic) coordinate and the integration is performed over the whole nuclear volume. For magnetic nuclear transitions the Hamiltonian is given in second order perturbation theory by the interaction between the electronic and nuclear charge currents [19], where c is the speed of light, α is the vector of the Dirac α matrices (α x , α y , α z ) and j n ( r n ) is the nuclear current. Also in this case the integration is performed over the whole nuclear volume. The isomeric transition in 229 Th is a multipole mixture between a stronger M 1 (magnetic dipole) transition and a typically neglected E2 (electric quadrupole) transition. Indeed, for the pure radiative decay the contribution of the electric quadrupole part to the transition rate is completely negligible due to the very small nuclear transition energy. However, this might not hold true for IC rates which have a weaker dependence on the transition energy. The IC rate for a specific multipolarity is determined by the squared absolute value of the interaction Hamiltonian matrix element, usually averaged over all possible initial states and summed over all final states. The total IC rate for a transition with multipole mixing is then given by the sum of the individual M 1 and E2 IC rates. For the simplest case of a single bound electron undergoing IC in an otherwise closed-shell electronic configu-ration, the IC rate for the M 1 contribution yields and correspondingly for the E2 contribution In the equations above, j (j i ) and κ (κ i ) are the total angular and the Dirac angular momentum quantum numbers for the continuum (initial bound) electron and denotes the reduced probability of the nuclear transition from the isomeric to the ground state. The notations I e and I g represent the isomeric and ground state nuclear spin, respectively, andM (Q) is the nuclear magnetic dipole (electric quadrupole) operator [20]. The summation over the continuum partial wave Dirac quantum number κ is performed such that the selection rules for the particular multipolarity transition apply. The radial integrals R M1 εκ and R E2 εκ in Eqs. (3) and (4) are given by (7) where g βκ and f βκ are the radial wave functions of the initial (bound) and final (continuum) one-electron states, respectively. The total wave function for the electron is given by where the functions Ω κm (r) of the argumentr = r/r are the spherical spinors. The generic notation β stands for the principal quantum number n for bound electron orbitals and for the continuum electron energy ε for free electron wave-functions, respectively. The IC rate expressions in the case of Th ions are more complicated generally speaking due to possible electronic couplings between the electrons in the outer-shell orbitals with principal quantum numbers n = 5, 6, 7. These shells are not closed and one should in this case consider (i) the coupling of the angular momenta of the IC electron and the remaining two valence electrons of the initial ion, (ii) the coupling of the angular momenta of the free electron and the electronic shell of the final ion, (iii) the possibility of simultaneous population of different electronic states of the final ions.
An accurate treatment of the case of two electrons in the outer electronic shell (as in the case of Th 2+ ) leads again to the expressions (3) (for M 1 transition multipolarity) and (4) (for E2 transition multipolarity) and the corresponding selection rules for the IC electron parity. However, now the IC rate is non-zero only if the total angular momentum of the spectator electron remains unchanged before and after conversion. A further increase of the number of outer-shell electrons makes the angular momenta coupling more complex as different coupling schemes are rendered possible. For three electrons in the outer electron shell (the case of Th + ), we distinguish between two cases. First, we consider a configuration |J i M i of electrons with individual total angular momenta j 1 , j 2 , j i (the latter denoting the electron which undergoes IC) such that • j 1 , j 2 are first coupled to the momentum J 0 .
• J 0 and j i are then coupled to the total momentum J i .
Under these conditions the initial coupling to the momentum J 0 is not broken by IC and similar to the case of two electrons, we derive the expressions (3) and (4) with the corresponding restrictions on the IC electron parity. Further selection rules require the angular momenta j 1 , j 2 of the non-IC electrons and their common momentum J 0 to be conserved. The total angular momentum J f of the final two-electron configuration should therefore exactly equal J 0 . Now let us consider a configuration |J i M i of electrons with total angular momenta j 1 , j 2 , j i such that • j 2 , j i are first coupled to the momentum J 0 .
• J 0 and j 1 are then coupled to the total momentum J i .
In this case the initial coupling to the momentum J 0 is broken by IC. It leads to the expressions (3) (for M 1 transition multipolarity) and (4) (for E2 transition multipolarity) with the corresponding restrictions on the IC electron parity, but with an additional coefficient The selection rules still imply conservation on the angular momenta j 1 and j 2 . The other restrictions are contained in the 6j-symbol, namely, the four sets (j 1 , j 2 , J f ), (j 1 , J i , J 0 ), (j 1 , j 2 , J 0 ), (j i , J i , J f ) have to satisfy the triangle rule. Note that in this case the momentum J f is not restricted only to the value J 0 .

III. NUMERICAL RESULTS
We have calculated the total IC rate for a number of excited electronic configurations of Th in the charge states Th + and Th 2+ . The key ingredients in the calculation of the IC rate (3), (4) and (9) are • the reduced probabilities of the nuclear transition B ↓ (M 1) and B ↓ (E2). At the moment, only theoretical estimates are available for the nuclear transition strength and the value of B ↓ can vary over a wide range depending on the method used for its calculation [7]. Here we assume B ↓ (M 1) = 0.048 W.u. (Weisskopf units) in accordance to Ref. [21]. For the electric quadrupole transition we consider a recent preliminary estimate B ↓ (M 1) = 29 W.u. [22].
• the radial wave functions g niκi and f niκi for the bound electron.
• the radial wave functions g εκ and f εκ for the continuum electron for all κ values allowed by the selection rules.
• the electronic energy levels for the thorium ions before and after IC.
We evaluate the bound electron radial wave functions g niκi and f niκi with the grasp2K package provided by the Computational Atomic Structure Group [23]. The package consists of a number of tools for computing relativistic wave functions, energy levels, transition rates and other properties of many-electron atoms. The energy levels for many-electron heavy ions such as Th (atomic number Z = 90) calculated with grasp2K are not reliable on the degree of accuracy required here. Although the relative accuracy is better than 10 −5 , due to the very large total electronic energy, errors on the order of eV may occur and often the order of the levels is uncertain. This can considerably lower the quality of the calculated atomic level scheme. We therefore choose to adopt the initial-and final-state electronic energies from the experimental database [24]. The experimental values were obtained in laser spectroscopy laboratories and are accurate on the level of 10 −3 cm −1 , which is beyond reach for atomic structure calculations. In contrast, the electronic wave functions can be calculated with grasp2K with sufficient accuracy.
For evaluation of the radial wave functions g εκ and f εκ of the continuum state of the IC electron we use the program xphoto from the Ratip package [25]. Xphoto performs relativistic calculations for photo-ionization crosssections of multi-electron atomic systems and can provide electronic wave functions of the continuum states for all required κ values.
We have validated our calculation method and numerical results by comparing IC coefficients with theoretical values tabulated in the literature [26,27] for a number of test cases. The compilations in Refs. [26,27] use the relativistic Dirac-Hartree-Fock method for the calculation of electronic wavefunctions. In addition, Ref. [26] takes into account the effect of atomic vacancies created in the conversion process. In Fig. 2 we depict the calculated IC coefficients as a function of the nuclear transition energy together with data from Refs. [26,27]. We consider a fictitious nuclear transition of multipolarity M 1 which ionizes the 6d-and 7s-electrons in the neutral Th atom. In all cases good agreement with at least one of the tabulated values is achieved.
We now proceed with numerical results for IC from excited electronic states of Th + and Th 2+ . We take into account both the M 1 contribution and the multipole mixing of the E2 multipolarity contribution, which although negligible for IC of the thorium ground state [7], turns out to be relevant for IC from excited states. In the following tables we list the calculated total IC rates for a number of relevant cases and consider a larger interval for the isomeric energy E m . The minimum isomeric energy required for the respective IC channel to be open is labeled as E min m . In particular we focus on Th + configurations and two Th 2+ excited states that are of interest for the experimental investigation of the electron bridge process [9]. We present the IC rate as a function of the possible nuclear isomeric state energy and take into account all available final states within the considered energy range. Note that the number of digits to represent the IC rates is chosen in each case to be sufficient to depict the dependence of the result on the isomeric state energy within the framework of the calculation model. With increasing charge state, the required electronic excitation and ionization energies are also increasing. Thus, we note that due to the large ionization potential of Th 2+ , the required isomeric energy to render IC from the chosen excited electronic configuration is approx. 19 eV, which is according to present knowledge highly unlikely. We conclude that the most interesting ionization state for IC from an excited electronic state is Th + . +   Table II presents theoretical radiative lifetimes calculated with the grasp2K package for the excited electronic states of interest for the IC scheme. We note that the Auger decay of these states is energetically forbidden. The choice of the excited states is unfortunately limited by the available electronic energy data in the experimental database [24] and by the short radiative lifetimes of the excited states. Based on the values in Table II, we choose to consider two suitable initial excited electronic states: 5f 6d 2 at 30223 cm -1 with J = 15/2 and the 7s 2 7p at 31626 cm -1 with J = 1/2. The IC rates for these two configurations are presented in Tables III and IV. The excited state 5f 6d 2 has no electric-dipole decay channels; its calculated radiative lifetime is 0.4 s. IC from the  Internal conversion rates for the state 5f 6d2 at 30223 cm −1 of 229m Th + as a function of the possible isomeric transition energy. For comparison, the half-life of the electronic excited state was calculated to be 0.4 s. 5f 6d 2 state becomes possible provided that the isomeric state energy would be higher than 9.0 eV, in which case the characteristic decay time becomes considerably less than the level lifetime. Provided that the isomeric energy indeed lies higher than 7.8 eV as speculated at present, this opportunity seems to be unique, as the other states at such high excitation energies typically decay very fast (see Table II, the case for the 7s 2 7p state at 31626 cm -1 ). On the other hand, in case the isomer energy is higher than 12 eV, the excitation method would no longer be applicable since the ground state 6d 2 7s electrons could undergo IC. We are therefore focusing on the range of 9.0 eV to 12.0 eV isomeric state energy. We note here that the laser excitation of this particular electronic configuration is not straightforward due to the large difference of its total angular momentum and the angular momentum of the ground state: 15 2 − 3 2 = 6. Several possible experimental approaches are addressed in the next Section. Table III is given in Fig. 3. We depict the IC rates for the initial 5f 6d 2 state as a function of the possible nuclear isomeric state energy and take into account all available final states within the considered energy range. The step-like jumps represent openings of new channels of decay corresponding to different electronic states of the final ion Th 2+ .

B. Results for Th 2+
With increasing charge state, the required electronic excitation is also increasing in energy. The value of E m at which one can apply the same method for Th 2+ lies in the range from 19.1 eV to 20.0 eV (the Th 2+ ionization potential). Due to the relatively simple structure of the Th 2+ spectrum, there is no opportunity for a highly excited state to not decay through fast E1 transitions. However, if E m is close enough to the Th 2+ ionization threshold such that the excited level does not have to be very high in order to observe IC, one can expect moderate E1 decay rates due to their proportionality to the third power of the photon frequency. On the other hand, we can choose a level undergoing IC at a rate high enough to compete with the E1 decay.
We consider two levels of Th 2+ , namely 5f 7s at 7501 cm −1 and 6d7s at 16038 cm −1 . IC from these states becomes energetically allowed at the lowest values of E m among the other states (19.1 eV and 19.2 eV, respectively). The calculated rate of IC from the level at 7501 cm −1 is equal to 7 · 10 5 s −1 and is an order of magnitude higher than the radiative decay rate 5·10 4 s −1 calculated with grasp2K, see Table II. The rate of IC from the state at 16038 cm −1 is calculated to be 2 · 10 5 s −1 and is at least one order of magnitude less than the radiative decay of 10 7 s −1 , see Tables V and VI. It is worth noting that the difference of angular momenta between the level 5f 7s at 7501 cm −1 and the ground state equals 4 − 3 = 1 and, contrary to the case of Th + , this allows a more straightforward excitation scheme.

IV. POSSIBLE METHODS TO EXCITE THE
5f 6d2 LEVEL OF Th + AT 30223 cm −1 The experimental population of the 5f 6d 2 state of Th + at 30223 cm −1 is, at first sight, not straightforward due to the difference of the total angular momentum with respect to the ground state: 15 2 − 3 2 = 6. In addition, if one were to excite from the ground state using six resonant photon transitions, one also needs to introduce a change in parity, i.e., one of these transitions would need to have M 1 multipolarity. In the following we therefore consider other possible ways to reach the level.
Firstly, the state of interest can be reached via three E1 excitation/de-excitation steps from the 6d 2 7s excited level at 6213 cm −1 as indicated in the possible laser excitation scheme in Fig. 4. The levels at 30310 cm −1 and 42645 cm −1 can serve as intermediate stages, so that one can use two laser excitations with wavelengths of 415 nm and 811 nm, followed by one induced de-excitation at 805 nm. We note that the dynamics of laser de-excitation (from Rydberg levels) has been investigated in connection with anti-hydrogen atoms [28]. All of the wavelengths are easily produced using Ti:sapphire lasers [29], one of which is frequency doubled. The 6d 2 7s state at 6213 cm −1 has only one E1 decay channel at the ex-tremely low photon energy of 5.6 · 10 −3 eV and very low decay rate of 10 −4 s −1 , so it can be considered as metastable.
The electronic level at 6213 cm −1 has a total angular momentum 9/2 and even parity. It can be reached with four laser (de)excitations in the following manner: an excitation from the ground state to an odd parity 3/2 level at 26965 cm −1 (371 nm), a second excitation step to an even parity 5/2 state at 40644 cm −1 (731 nm), a stimulated de-excitation to an odd parity 7/2 state at 18974 cm −1 (461 nm) and finally a de-excitation to the even parity 9/2 level at 6213 cm −1 (784 nm), all of which are suitable for Ti:sapphire lasers. Nevertheless, such an excitation scheme would require 4 different lasers and though possible, subsequent optical excitation from the metastable level towards the state of interest at 30223 cm −1 is simply impractical.
Alternative means to populate the 6213 cm −1 state will require experimental investigation and thus we simply highlight them here: • Following the α-decay of a source of 233 U (which proceeds, following gamma-ray de-excitation, primarily to the ground state of 229 Th with a 2% branching to the low-energy isomeric state [30]) the 229 Th recoils may be stopped in a buffer gas cell [14,31]. Excitation of the 6213 cm −1 state may occur via collisions in the gas cell; • Excitation of the nuclear isomeric state of the ion 229 Th + through an electron bridge scheme chosen such that the final electronic level is not the ground state, but the state at 6213 cm −1 (this would work for particular values of E m ); • Thermal excitation to the metastable level via laser ablation of 229 Th from a dried thorium nitrate solution deposited on a tungsten or tantalum substrate. This has recently been realized for the study of isotope shifts and hyperfine structures of resonance lines in 229 Th + , with a low pressure argon buffer gas used to cool the ions to room temperature and quench the population of metastable states optically pumped by the laser excitation [32].
In the current proposal however population of the metastable state is a requirement.

V. PROPOSED MEASUREMENT FOR IDENTIFICATION OF THE ISOMERIC STATE
We propose to perform an experimental verification of the internal conversion from the aforementioned excited state of 229 Th + at the IGISOL facility, Jyväskylä, Finland. The study of thorium through laser spectroscopic techniques is an ongoing project at the facility, whereby an alternative to the direct detection of the isomeric decay would be the inference, through an optical measurement, of the atomic hyperfine structure. Due to different spins and magnetic moments of the ground-and isomeric state (the latter produced via the alpha decay of 233 U), such a measurement will allow the separation and identification of the two states [31,33]. A description of the current facility and the historical developments leading towards the ion guide method of radioactive ion beam production are described in [34,35] and references therein. Figure 5 illustrates the layout of the facility of specific relevance to our proposal. Following population of the metastable level at 6213 cm −1 in 229 Th (see previous section), the 229 Th + ion beam is injected into the radiofrequency (rf) quadrupole cooler-buncher [36]. Inside this device, the ions lose their residual energy through viscous collisions with low pressure (∼1 mbar) helium gas, and a weak axial field is applied to the segmented electrodes in order to guide the ions to the exit region within 1 ms. Here, the ions may be accumulated with a trapping potential and are bunched before extraction through a miniature quadrupole into a low-energy transfer line operating at 800 V before re-acceleration by the platform potential (∼30 kV) towards the experimental setups. The time structure of a typical ion bunch has been determined to be ∼10 µs.
In recent years, a method of optical pumping within the rf cooler-buncher in connection with collinear laser spectroscopy has been pioneered in Jyväskylä [37]. Generally, laser spectroscopy is performed using electronic transitions from the ground state due to reasons of population. In order to access a wider number of elements for nuclear structure interest, excitation of ground states within the cooler-buncher using laser light generated from high power pulsed tunable Ti:sapphire lasers results in optical pumping and efficient redistribution of the ground state population to a selected metastable state. Importantly, the state survives extraction from the cooler, acceleration and delivery to the collinear set-up, the general beamline layout of which is shown in Fig. 5.
In the near future, much effort will be directed towards the production of pure beams of a single isotope or even isomer. This will be realized through a multistep excitation process with typically three pulsed lasers being used to resonantly ionize a selected element from a singly-charged to a doubly-charged state while inside the cooler, effectively an extension of the optical manipulation used to populate selected metastable states. This method is similar to that proposed for the population of the 5f 6d 2 state of Th + at 30223 cm −1 from the metastable 6213 cm −1 state. When the ions are released from the cooler in bunches, their time-of-flight to the detection region (a multi-channel plate, MCP, detector located at the end of the beamline) depends on the mass-to-charge ratio, m/q. Doubly-charged ions, in this instance 229 Th 2+ which is produced via IC from the excited 30223 cm −1 state, will leave the cooler more quickly. The doubly-charged ion bunch will rapidly become spatially separated from the bunch of contaminant singlycharged ions thus allowing the latter to be electrostatically deflected from the beam path. The typical flight time from the ion beam coolerbuncher to the MCP detector shown in Fig. 5 for a mass A=100 singly-charged ion is 100 µs. This flight time scales with the square root of m/q and thus for singlycharged 229 Th + the expected flight time increases to approx. 1.5 ms. The corresponding flight time for doublycharged 229 Th 2+ is approx. 1.1 ms. This difference in the ion bunch arrival time is sufficient for a simple electrostatic deflection of one species and therefore a clear identification of the presence of the isomeric state can be made. It is expected that background doubly-charged ions may be formed inside the cooler-buncher via resonant excitation and ionization of singly-charged thorium. This process will be studied in detail using stable 232 Th + which does not possess any nuclear states at optical energies.

VI. SUMMARY AND CONCLUSIONS
The low-lying isomeric state of 229 Th opens the possibility to observe for the first time IC from excited electronic states. We have investigated how this process can be used for an estimate of the nuclear transition energy and an experimental determination of the nuclear transition strength. The scenario put forward involves laser excitation of the electronic shell in thorium ions, which subsequently opens the IC decay channel of the isomeric state. Experimentally, the decay of the nuclear excited state could be then observed via the appearance and detection of ions belonging to the next charged state. Numerical results for IC rates from excited states of 229 Th + and 229 Th 2+ ions have been presented. A possible experimental setup for the determination of the isomeric state properties using IC from excited electronic states at the IGISOL facility, Jyväskylä, Finland, has been discussed. The main challenge here appears to be the laser excitation of the electronic shell to states which live long enough to allow for IC to efficiently depopulate the isomeric state. In addition, we note that new laser spec-  troscopy data for 229 Th + and 229 Th 2+ electronic excited states above 60000 cm −1 might enlarge the choice of states that can be used for the proposed scheme, if additional longer-lived excited electronic states are found. Further experimental data at higher excitation energies is also important in the search for an electron bridge mechanism for the isomeric state and further laser spectroscopy scans are continuing at the moment.