Introduction

Unveiling the intricate electronic structure of atoms, including understanding how their attributes define the macroscopic chemical properties, was one of the greatest scientific achievements of the last century. Still even today, the pursuit of knowledge persists, aiming to push towards the limits of these systems’ existence - for example, in superheavy elements - and to comprehend how their properties evolve as we edge closer to these boundaries1. The electron affinity (EA) is one of these fundamental attributes. It is defined as the energy released as an electron is attached to a neutral atom and, therefore, is strongly related to how the atom forms chemical bonds by sharing electrons2,3. The binding of such an additional electron is governed by complex electron-electron correlations4. Hence, EAs are powerful benchmarks for state-of-the-art atomic model frameworks, especially those reliant on many-body quantum methods5,6. Despite its fundamental importance, the EAs of several elements – especially those that are rare, heavy, and radioactive – are experimentally unknown2.

State-of-the-art methods for high-precision EA measurements include Photodetachment Microscopy7,8, Velocity Map Imaging (VMI)9, and Laser Photodetachment Threshold (LPT) spectroscopy10. Their common measurement principle involves exposing negatively-charged ions to laser photons, resulting in the detachment of the extra electron from the subsequently neutral atom. In photodetachment microscopy and VMI, an anion beam is intersected by a perpendicularly propagating laser beam and, upon successful photodetachment, the outgoing electron is detected. To access the EA, the former method takes advantage of quantum interference effects, visualized as spatial electron patterns at the detector surface11. In VMI, the electron’s velocity is mapped onto a position-sensitive detector, allowing the determination of its kinetic energy E**e and, thus, of the EA according to E**A = E − E**e, where E denotes the photon energy.

To overcome limitations of the crossed-beams configuration in terms of sensitivity, LPT spectroscopy employs a scheme in which laser and anion beams are collinearly overlapped. This enables a longer anion-laser interaction time, while simultaneously compressing the anions’ velocity spread such that Doppler broadening is reduced, leading to improved spectroscopic resolution12. By observing the number of neutralized atoms as a function of the photon energy around the element’s EA, the energy above which the anion is neutralized can be identified. However, the closer the threshold is approached, the more the photodetachment cross section diminishes13. As a result, even an LPT measurement requires access to substantial sample sizes, despite its advantage of extended laser-ion interaction time.

Enhancing the sensitivity of the LPT technique further would finally enable photodetachment studies for samples that are challenging to produce or isolate. For instance, measurements of photodetachment cross sections in molecular anions are required to understand their role and abundance in the interstellar medium14,15,16. Moreover, experimental determinations of the EA of molecules and atoms (including isotope-resolved measurements) could provide stringent benchmarks for theoretical methods in atomic many-body calculations, with implications across atomic and nuclear physics17, as well as in quantum chemistry. The latter are, among others, valuable for predicting the structure and laser-coolability of (negatively-charged) molecules in contexts such as antimatter research18,19,20,21,22 and radioactive molecules[23](https://www.nature.com/articles/s41467-025-64581-x#ref-CR23 “Gaul, K., Garcia Ruiz, R. F. & Berger, R. Stopping mass-selected alkaline-earth metal monofluoride beams of high energy via formation of unusually stable anions https://arxiv.org/abs/2403.09320

(2024). 2403.09320.“), which have recently emerged as promising probes for physics beyond the Standard Model of particle physics24,25. However, experimental data remain scarce, especially for samples that exist on Earth only in trace amounts or must be produced artificially - despite their relevance to several fields, including radiopharmaceuticals research26,27 and fundamental atomic theory. For example, since the atomic shell stabilization due to relativistic effects grows with increasing atomic number, EA measurements of actinide and superheavy elements would challenge the predictive power of fully-relativistic many-body quantum models4,5,6. They would also provide critical tests of the limits of periodicity in the table of elements5,28,29, with oganesson (Z = 118) as the extreme case – predicted to be the first noble gas with a positive EA30, and therefore capable of forming a stable anion.

As a proof of concept, the GANDALPH collaboration recently succeeded in accessing the EA of artificially produced 128I (ref. 31) and, in a subsequent milestone, measured for the first time the EA of a radioactive element, astatine3. This was achieved by employing high-efficiency, low-background neutral particle detectors32 and a beam of astatine anions of ≈ 600 fA (3.75 ⋅ 106 particles per second), produced artificially by the radioactive ion beam (RIB) facility ISOLDE33 at the European Organization for Nuclear Research (CERN). The successful EA determination, despite such a low anion intensity, underscores the potential of the LPT technique for analyzing rare samples. However, even with the collinear anion-laser overlap used in conventional LPT approaches, such as employed by GANDALPH, each anion is exposed to the laser for less than a few microseconds before being discarded, resulting in a negligible fraction of anions undergoing photodetachment. Thus, such a single-pass approach cannot efficiently utilize the most scarcely produced samples, e.g., the actinides and superheavy elements, where production yields are at or even below just a few anions per second.

A pathway to higher efficiency is to increase the probability for anion-laser interaction by confining the anions in an ion trap. While this approach has been employed in radiofrequency14,15,34,[35](https://www.nature.com/articles/s41467-025-64581-x#ref-CR35 “Nötzold, M., Wild, R., Lochmann, C. & Wester, R. Spectroscopy and ion thermometry of

$${{{{{\rm{C}}}}}_{2}}^{-}$$

C

2

−

using laser-cooling transitions. Phys. Rev. A 106, 023111 (2022).“) and Penning traps36,37,38 for decades, it has limitations. The quasi-stationary motion of stored anions results in a lack of control over the generated neutral atom, leading to a low detection efficiency and hence a persistent reliance on large anion ensembles. In our ambition to overcome this limitation, we draw inspiration from a recent LPT measurement conducted in a storage ring. At the Double Electrostatic Ion Ring ExpEriment (DESIREE)39,40, a beam of oxygen anions was repeatedly probed by a laser along a section of the revolving anions’ trajectories41. The resulting neutral atoms maintained their momentum and left the storage ring along a well-defined path towards a detector, resulting in a high detection efficiency compared to previous studies in ion traps. This approach allowed the use of low-power narrow-bandwidth continuous wave (cw) lasers, enabling the highest precision in an EA measurement to date. However, the anion beam intensity used was several orders of magnitude higher than what is available for exotic species and, therefore, not compatible with experimental requirements of RIB facilities, the only places where short-lived radioactive species can be produced.

Here, we present a novel technique for the precise determination of EAs, tailored to effectively utilize rare samples produced in RIB facilities, while also leveraging the ion-beam storage concept pioneered at DESIREE for these studies. Our approach involves LPT experiments within an electrostatic ion beam trap, also referred to as a Multi-Reflection Time-of-Flight (MR-ToF) device42,43. Within these traps, anions are confined between a pair of electrostatic mirrors, separated by a field-free drift region where they travel at a constant velocity. For the present experiment, they are collinearly illuminated by a spectroscopy laser. This method is an extension of the Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy (MIRACLS) technique44,45, which enhances experimental sensitivity by exposing confined ions to lasers for longer durations. It allows for repeated laser probing of rare species while maintaining the high resolution offered by the collinear geometry12. Originally developed for fluorescence-based laser spectroscopy, the MIRACLS scheme is also particularly advantageous for LPT due to the efficient inertial guiding of neutralized atoms to an externally positioned detector, thus greatly increasing geometrical detection efficiency compared to other trap-based LPT methods. In the following, we demonstrate that our new approach achieves accurate EA determinations with competitive precision employing orders of magnitude fewer anions than conventional techniques, paving the way towards EA measurements at an one-atom-at-a-time sensitivity. The small footprint of MR-ToF devices, coupled with their reduced operational and maintenance requirements compared to storage rings, and their widespread use at RIB facilities as high-resolution mass spectrometers and mass separators46,47,48,49,50,[51](https://www.nature.com/articles/s41467-025-64581-x#ref-CR51 “Virtanen, V. A. et al. High-resolution multi-reflection time-of-flight mass spectrometer for exotic nuclei at IGISOL https://arxiv.org/abs/2508.10048

(2025). 2508.10048.“) make our approach particularly compelling for LPT in rare-isotope research.

Results

Development of the experimental technique

To showcase the innovative nature of our new technique, we chose chlorine (Cl) for an EA measurement with improved experimental sensitivity. Cl− stands out as the most tightly bound anion of any element in the periodic table, much like helium’s unique position among neutral atoms. This strong binding allows Cl− to be easily produced in specialized sources with the added simplicity of not exhibiting any excited state in its anion. Hence, Cl’s EA is well investigated both theoretically13,17,52 and experimentally37,53,54 and thus serves as a suitable case for benchmarking novel methods. Our primary objective is to quantify the number of anions required, comparing our method’s sensitivity with that obtained with conventional approaches, while maintaining the highest level of experimental precision.

The measurement is performed with MIRACLS’ low-energy MR-ToF setup44,55,56,57,58,59,60 located at ISOLDE/CERN, which has been modified for the present photodetachment measurements. A schematic layout of the experimental setup is shown in Fig. 1a. A continuous beam of chlorine anions containing both stable isotopes of chlorine, 35Cl and 37Cl, is produced by a negative surface ion source61. The beam is delivered to a Paul trap, where the anions are captured, accumulated, and cooled via collisions with room-temperature helium buffer gas. Subsequently, the anions are released from the trap in low-emittance bunches and sent towards the MR-ToF device. In this process, the anion bunches pass through two high-voltage switching elements: a pulsed drift tube, which adjusts the anions’ kinetic energy, and a deflector, which isolates the 35Cl− isotope by time-of-flight selection, ensuring that the measurement pertains solely to the electron affinity of 35Cl−.

Fig. 1: Schematic of the experimental setup.

a A continuous beam of Cl− anions produced by an ion source is accumulated in a He-filled Paul trap, from where anions are sent downstream in low-emittance bunches. After kinetic energy adjustment and isotope selection by time-of-flight, 35Cl− anions are stored in an MR-ToF device, where they can be illuminated by a collinear laser beam. Atoms produced by neutralization processes leave the MR-ToF instrument and are registered by a detector. b The cumulative signal of neutral particles impinging on the detector grows with storage time. By measuring the difference (yellow band) between the cumulated neutral atom counts per cycle with (red circles) and without (green squares) laser illumination, the laser-induced photodetachment signal is determined. Error bars represent 1σ uncertainty, and lines are guiding the eye.

Once the 35Cl− anions arrive at the center of the MR-ToF’s central drift tube (CDT), its electric potential is switched to capture the anion bunch62. As a result, the anions - propagating with a kinetic energy of approximately 1.4 keV - are repeatedly reflected between two electrostatic mirrors and thus confined over a length of about 0.35 m. The stored anions are collinearly overlapped with a 1 to 5 mW cw laser beam, which is scanned in the region around 343 nm to probe the photodetachment threshold. A laser power meter can be inserted into the laser path just in front of the vacuum-air interfacing Brewster window. As stored anions are neutralized, either by laser photodetachment or collision-induced detachment with residual gas particles at a pressure of 3 ⋅ 10−8 mbar in the present MR-ToF device, they escape as neutral atoms and are detected by a MagneTOF Mini Detector from ETP Ion Detect placed outside the ion trap. After typically half a second of anion storage, corresponding to approximately 60 thousand revolutions, the remaining 35Cl− anions are extracted from the MR-ToF device. The detector is left to acquire background data on events unrelated to anion storage for an equivalent duration, after which the measurement cycle is completed, and a new cycle restarted. More details on the experimental procedures can be found in the Methods section.

To avoid direct laser incidence onto the active area of the detector, which would generate a large background of photoelectrons, the neutral atom detector is slightly displaced off-axis. As a consequence, only about 40% of the atoms that arrive at the detector plane hit its active area. The integrated counts of neutral atoms per cycle (N) are determined by the number of counts observed while the MR-ToF device is loaded with anions, subtracted by the number of counts registered while acquiring background data after the anions are extracted. This component of the background, originating from detector dark counts and photoelectrons that are emitted when laser (stray) light hits the detector, accounts for about 3% of the total counts per cycle. The detector dark counts amount to ≈ 0.4 counts per cycle and the laser-induced background counts are ≈ 0.2 counts per cycle for each milliwatt of laser power.

As evident in Fig. 1b, the cumulative neutral atom counts per cycle grow with longer anion storage times, indicating a significant improvement in experimental sensitivity with longer laser exposure. Eventually, the growth rate diminishes as the population of anions is depleted due to neutralization. To determine the fraction of the integrated counts per cycle due to laser-induced photodetachment, measurements with laser illumination are interleaved by identical measurements with the laser off. As described in detail in the Methods section, the photodetachment signal is then defined as (S={N}_{{{{\rm{on}}}}}-{N}_{{{{\rm{off}}}}}^{{{{{\rm{int}}}}}), where Non is the number of counts of neutral atoms per cycle detected with lasers on, and ({N}_{{{{\rm{off}}}}}}{{{{\rm{int}}}}}) is the interpolated (int) counts of atoms detected between the two interleaving measurements with laser off. To properly compare measurements taken at distinct laser powers and total stored anion content, we employ the photodetachment signal strength (({{{\mathcal{S}}}})), defined as the relative, power-normalized excess counts per cycle of neutral atoms produced with laser illumination:

$${{{\mathcal{S}}}},=,\frac{S}{{N}_{{{{\rm{off}}}}}^{{{{\rm{int}}}}}\cdot P}.$$

(1)

Here, P is the average laser power and ({N}_{{{{\rm{off}}}}}^{{{{\rm{int}}}}}) is used as a proxy for the total anion content, since it is directly proportional to the number of stored anions (see Methods section).

The laser wavelength is scanned between about 340.5 and 343.5 nm, in search of the threshold above which a photodetachment signal is found. In contrast to most LPT experiments, we approach the threshold through successive approximations rather than with uniform steps, with higher statistics acquired closer to the threshold. This method focuses on the acquisition where data is more relevant, so a useful spectrum can be obtained with the least experimental time possible. The resulting LPT spectrum is shown in Fig. 2.

Fig. 2: The LPT spectrum of 35Cl.

Measured photodetachment signal strength ({{{\mathcal{S}}}}), as defined in Eq. (1) (black dots), as a function of laser photon energy. Two models are fitted to the data: the Wigner threshold function (Eq. (2), blue dashed line) and a more realistic model adapted to the MIRACLS approach (red line). A zoom of the region around the threshold is shown in the inset. The photon energy is presented in the laboratory frame, i.e., not Doppler corrected. Error bars represent 1σ uncertainty.

The electron affinity of chlorine

The LPT spectrum relates to the photodetachment cross section (σ) which is described by the Wigner threshold law63:

$$\sigma (E),=,\left{\begin{array}{lll}0,\hfill &&E\le {E}_{A}\ {A}_{w},{(E-{E}_{A})}^{l+1/2},\hfill &&E > {E}_{A}\end{array}\right.$$

(2)

where E**A is the atom’s EA, E is the photon energy in the anions’ rest frame, l the orbital angular momentum quantum number of the detached electron, and A**w is a scaling factor. The 35Cl− anion has a pure 3p6 1S0 configuration which does not exhibit any fine nor hyperfine splitting. The outgoing electron, as it is photodetached from a valence p orbital, has either l = 0 or l = 2 according to the selection rules. The latter, however, is strongly suppressed close to the threshold due to the centrifugal barrier4, permitting us to neglect it and to assume a pure s-wave emission. Finally, after the photodetachment process, the ground state of the remaining neutral atom has a configuration of 3p5 2P3/2, which splits into four hyperfine states as the 35Cl nucleus has a spin of 3/2. The maximum energy level difference between the hyperfine states is about 4.2 μeV[64](https://www.nature.com/articles/s41467-025-64581-x#ref-CR64 “Carette, T. & Godefroid, M. R. Ab initio calculations of the 33S 3p43PJ and 33S−/37,35Cl 3p5 2

$${{{\rm{P}}}}_{J}^{o}$$

P

J

o

hyperfine structures. J. Phys. B: ., Mol. Opt. Phys. 44, 105001 (2011).“), which is unresolvable given the steps in laser wavelength of the present experiment. Therefore, we approximate the photodetachment process in this experiment to occur between a pure two-level system.

The threshold position is determined by fitting Eq. (2) to the data, with the resulting curve shown as a dashed blue line in Fig. 2. The corresponding EA is extracted by applying the appropriate Doppler shift (see Methods section). The MIRACLS approach derives its strength from repeated probing of anions, which introduces additional considerations in the application of the threshold law. The two most relevant are the continuous Doppler shift and the blueshift depletion. The first refers to the non-monochromatic velocity distribution of anions as they are decelerated and re-accelerated in the mirrors of the MR-ToF device. The second effect is caused by anions moving away from the detector but still overlapping with the laser and undergoing photodetachment. Since the detector is positioned to detect photodetachment events that happen in collinear, redshifted geometry (see Fig. 1), those events that happen in anticollinear, blueshifted configuration are not detected but still cause a depletion in the overall anion population. Thus, a negative normalized photodetachment signal ({{{\mathcal{S}}}}) can arise at photon energies just below the threshold.

To account for both effects in the recorded MIRACLS data, an advanced fit function named MIRACLS-LPT is developed as shown in the Methods section. Its result, also presented in Fig. 2, is nearly identical to the Wigner law model except for details around the threshold. The extracted values of the EA are fully compatible with each other, see Methods section. Given the current level of experimental precision, the data itself does not discriminate between the two models. Therefore, we adopt the statistical uncertainty resulting from the Wigner model fit with 14 μeV and, conservatively, use the MIRACLS-LPT fit to estimate a potential systematic uncertainty arising from MIRACLS-specific factors, especially the blueshift depletion. This represents the dominant source of uncertainty in the final EA value, which - if detectable in the data at the given precision - arises from conducting the present experiment in a collinear geometry. This is due to the current experimental arrangement for neutral particle detection. If the measurement was conducted in anticollinear geometry, to be enabled by future advancements such as an efficient yet anion- and laser-transparent neutral particle detector or a laser-transparent detector combined with off-axis anion injection into the MR-ToF device, the final uncertainty could be reduced to levels comparable to those achieved with the Wigner function fit as the analogous redshift depletion effect would only occur at photon energies well above the threshold.

The present measurement results in an EA value of 3.612720(44) eV obtained from our MIRACLS-LPT fit. The 44 μeV total uncertainty combines statistical and systematic uncertainties, which are comprehensively characterized in the Methods section. Our value agrees with all previous measurements, see Fig. 3, with comparable uncertainty and hence contributes to an improved precision of the EA of 35Cl. Contrary to our experiment, however, the uncertainty of the previous results was dominated by the large spectral bandwidth of the pulsed lasers, which were required to produce the necessary intense photon flux. In our case, the prolonged laser exposure within the MR-ToF device compensates the reduced photon flux of the high-resolution cw laser employed, making laser bandwidth a negligible source of uncertainty. As such, our result demonstrates MIRACLS’ capabilities of accurate EA measurements with competitive precision to conventional LPT methods. Most notably, beyond its contribution to the overall precision in chlorine’s EA, this was accomplished with an anion sample five orders of magnitude smaller than in the previous work53, as detailed in the following sections.

Fig. 3: Electron affinity *EA* of the chlorine atom as reported from several previous experiments37,53,54(gray circles) compared to the value obtained in the present work (red star).**

Error bars represent 1σ total uncertainties, while the pink band denotes the 1σ statistical contribution to the uncertainty in our work. The systematic uncertainty is almost entirely due to a blueshift depletion when exploiting our MIRACLS scheme in collinear geometry, but would be absent in an anticollinear measurement configuration. See text for details.

Signal sensitivity

In experiments with rare radioisotopes, maximizing the use of each anion to generate experimental signals is essential. To characterize the performance of our technique under this criterion, we define the signal sensitivity

$$\Gamma=\frac{S}{{N}_{{{{\rm{anion}}}}}\cdot \Phi \cdot \sigma },$$

(3)

in which (S={N}_{{{{\rm{on}}}}}-{N}_{{{{\rm{off}}}}}^{{{{\rm{int}}}}}) is the photodetachment signal introduced above, Nanion is the number of initially stored anions per cycle, Φ is the photon flux (expressed as number of photons per second), and σ is the photodetachment cross section at the employed photon energy E above the threshold.

In Fig. 4, we show Γ for all measured points above the threshold in our experiment. Nanion is determined independently from the integrated counts/cycle of collision-induced neutralizations ({N}_{{{{\rm{off}}}}}^{{{{\rm{int}}}}}) (see Methods section) and averages to 6100(800) anions/cycle initially stored in the MR-ToF device. Φ = P/E is obtained from the photon energy E and laser power P in each measurement, typically around 2 mW. The values for σ are taken from ref. 13.

Fig. 4: Signal sensitivity of LPT experiments.

Signal sensitivity as a function of the photon energy above the threshold evaluated for this work (red circles), as well as for selected single-passage LPT measurements: Berzinsh et al.53 on Cl (gray squares) and Rothe et al.31 on I (blue open diamonds). The horizontal solid lines indicate the weighted averages for each experiment. The dashed line represents the signal sensitivity ideally required for one-atom-at-a-time LPT experiments, implying S/Nanion ≈ 1. Error bars represent 1σ uncertainty.

Our signal-sensitivity values are compared to those calculated from the data of two LPT experiments employing conventional single-pass approaches: on chlorine from ref. 53 and on iodine from ref. 31. Here, Nanion, Non, and ({N}_{{{{\rm{off}}}}}^{{{{{\rm{int}}}}}) are given as the measured anion or atom number per second. As no dedicated background measurements were conducted, for ({N}_{{{{\rm{off}}}}}}{{{{\rm{int}}}}}) the average number of detected neutral atoms for photon energies significantly below the photodetachment threshold was taken. As evident in Fig. 4, the signal sensitivities Γ for those measurements are comparable with each other. In contrast, our results represent an improvement of more than three orders of magnitude over single-passage experiments.

Our enhancement significantly advances LPT techniques, bringing them remarkably close to the capability of probing rare samples produced at the one-atom-at-a-time level. Ideally, such an LPT measurement should be able to achieve S/Nanion ≈ 1 at a reasonable target photon energy above the threshold and at a technologically attainable laser power. Figure 4 illustrates the required signal sensitivity for one-atom-at-a-time experiments (gray dashed line), assuming 1 W of laser power at the corresponding wavelength and the theoretical photodetachment cross section for chlorine. At least within the energy range of 0.01–0.1 eV above the threshold, absolute photodetachment cross-sections to the lowest bound state are also known, either theoretically for H−, Si−, Cr−65 or experimentally for F−, Cl−, Br−, I−13, O−66, and Au−34. Interestingly, when the same photon flux is assumed, the one-atom-at-a-time sensitivity limits for all these elements are within a factor of ~10 of the one shown for Cl−. Therefore, while the sensitivity target indicated by the gray dashed line in Fig. 4 is that for chlorine, it also provides a useful order-of-magnitude guidance for LPT studies of rare samples involving other species.

In particular, Fig. 4 indicates that with a respective upgrade in photon flux, our current apparatus has already achieved the necessary one-atom-at-a-time signal sensitivity — namely, the ability to successfully neutralize each available anion — at photon energies as low as ~0.1 eV above the threshold. Hence, in the case of rare samples such as accelerator-produced, short-lived (chlorine) isotopes, EA measurements with a precision of 0.1 eV are within reach, even when only a single anion is injected into the MR-ToF device at a time. At this level of precision and anion intensity, however, a limiting factor of our current MIRACLS implementation of LPT studies is the total efficiency in the detection of neutralized atoms. The total efficiency is estimated to be ≈1.5% given by the product of intrinsic detector (≈10%), geometrical (≈40%), as well as extraction efficiency for atoms neutralized in the MR-ToF device, leaving towards the detector (≈36%). The respective efficiencies were either obtained via measurements and/or ion-optical simulations. Hence, ongoing work focuses on better-suited detection schemes along with other advances to ultimately gain even higher signal sensitivity and EA precision, especially beneficial for rare samples (see Anticipated Performance Improvements).

Discussion

While the previous section focuses on the effectiveness of generating an LPT signal from a rare sample, the signal-to-noise S/N ratio is a critical characteristic of an experimental apparatus to isolate a signal from the background. In conventional LPT studies with pulsed lasers, the leading source of background arises from laser-photon-induced detector events but can be largely eliminated when selecting events based on the neutralized atoms’ delayed time of arrival after the photon pulse, as originally proposed in ref. 67. The remaining background is primarily due to collision-induced neutralization of anions. This is also the dominant source of background ({N}_{{{{\rm{off}}}}}^{{{{\rm{int}}}}}), and thus the noise, in our current implementation of LPT work at MIRACLS with Cl− anions (see below). For comparison, at a photon energy of 5 ⋅ 10−5 eV above the threshold and using the respective full data sets, the measurements on iodine reported in ref. 31 achieved an S/N ratio approximately 2.5 times higher than that of our MIRACLS-based measurement. Similarly, for the chlorine data presented in ref. 53, the S/N ratio is about 300 times higher than ours.

However, when comparing S/N ratios across different experimental campaigns, it is important to note that, generally, the signal S/N ratio increases with the number of available anions. The photodetachment studies at MIRACLS are conducted with 6100(800) anions/s, whereas the measurement reported in ref. 53 benefited from an anion rate of ≈ 9.5 ⋅ 108 anions/s, i.e., ≈ 150,000 times higher. In both cases, the actual data collection period was limited to approximately one week, hence, remaining within the same order of magnitude. Considering that the resulting (statistical) precision on the EA is within a factor of 2, comparable for both measurements, the ~5 orders of magnitude lower anion number used for our studies implies that the sensitivity for EA measurements is superior at MIRACLS.

The advantage of our technique is the consequence