# High repetition rate sub-ns electron pulses from pulsed-resonance Rydberg field-ionization (PRRFI)

Raphaël Hahn, Daniel Comparat

► To cite this version:

Raphaël Hahn, Daniel Comparat. High repetition rate sub-ns electron pulses from pulsed-resonance Rydberg field-ionization (PRRFI). The European Physical Journal D: Atomic, molecular, optical and plasma physics, 2024, 78 (6), pp.66. 10.1140/epjd/s10053-024-00851-5. hal-04873660

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# High repetition rate sub-ns electron pulses from resonant pulsed Rydberg field-ionization

R. Hahn<fnref n="1" />,<fnref n="2" /> and D. Comparat<fnref n="1" />*

*Corresponding author(s). E-mail(s): daniel.comparat@universite-paris-saclay.fr;

## Abstract

We demonstrate a novel technique, Resonant Pulsed Rydberg Field-ionization (RPRFI), to generate low-energy electron bunches at high repetition rates. By combining continuous-wave laser excitation with a pulsed electric field, this method selectively ionizes Rydberg-Stark states in cesium atoms, producing sub-ns long electron bunches (down to ~100ps) at a repetition rate of ~10MHz. The method is demonstrated to offer significant advantages in terms of flexibility in the ionization repetition rate and pulse delay adjustments. The RPRFI method holds promises for applications in high-resolution electron microscopy and spectroscopy, potentially for overcoming the limitations of traditional electron sources in terms of brightness and energy spread.

**Keywords:** electron source, Rydberg states, pulsed field-ionization

## 1 Introduction

Electron sources are essential to many applications in electron microscopy, electron spectroscopy, material sciences, nanotechnology, and biology [1, 2, 3, 4, 5, 6]. Traditional electron sources such as thermionic and field-emission guns face limitations such as low brightness and high energy spread [7]. Consequently, improvements of the existing electron sources opens up new possibilities in their application [8].

Some applications like high-resolution electron energy loss spectroscopy require electron energy spread on the order of a few meV to resolve bulk/surface phonons or molecular vibrational modes. Attaining such low energy spread requires monochromatization of an electron beam from a conventional electron source like tungsten filaments, LaB₆ crystals, or field emission guns [9], which produce electron beams with energy spread of ~250 meV at best. For this reason, recent advances in

electron microscopy have integrated monochromators in high-energy electron (transmission) microscopy columns [10]. Although this allows high-resolution spectroscopy and imaging in the same instrument, it entails high instrumental complexity and cost [11, 12]. Monochromators have additional drawbacks: they significantly attenuate the beam current and, for low energy applications, can increase angular divergence to such an extent that Low-Energy Electron Microscopy (LEEM) imaging conditions for surface characterization can no longer be fulfilled. As such, it would be advantageous to have a high-brightness electron source with low-energy width, as suggested in Ref. [13]. Such a source would allow, for example, the construction of a High-Resolution Electron Energy Loss Microscope [14, 15] that would be able to perform LEEM while gathering chemically relevant spectroscopic information on the sample by Time-of-Flight (ToF) energy analysis. The ToF method has the advantage of allowing the simultaneous detection of

<footnote n="1">¹Université Paris-Saclay, CNRS, Laboratoire Aimé Cotton, 91405, Orsay, France.</footnote>

<footnote n="2">²Department of Chemistry and Applied Biosciences, ETH Zurich, CH-8093 Zurich, Switzerland.</footnote>
all electrons that will produce the image of the sample, without needing a slit to select a portion of the image (as would be the case for a hemispherical energy analyzer). This means microscopy and spectroscopic information can be gathered at the same time thanks to time- and position-sensitive detectors. However, using ToF for energy analysis requires the electron source to be pulsed, with a temporal spread $\Delta\tau$ low enough to attain the required energy resolution.

Electron sources based on the ionization of an atomic beam/cloud appear particularly promising for generating intrinsically monochromatic electron beams [16, 15]. Their low emittance means that they can be bright enough to perform electron microscopy [17, 14]. Most previous electron sources based on the ionization of an atomic beam were continuous [16, 18, 19] but, using pulsed lasers, the generation of pulsed electron beams from these sources has also been reported [20, 21, 22, 23, 24, 25]. However, pulsed lasers limit the selectivity of the excitation and do not allow for the selection of a particularly suitable state for electron emission.

In this proof-of-principle experiment, we demonstrate a novel method for generating low-energy electrons at high repetition rates, based on the ionization of individual Rydberg-Stark states of cesium atoms. This study builds upon our previous investigations into the field-ionization of Rydberg states for the production of ion or electron beams [26, 27, 28, 24, 29]. By combining a continuous-wave laser and a predominantly DC electric field with a minor oscillating component, we selectively excite and ionize Rydberg-Stark states of Cs atoms. This process results in the emission of ~100ps to ~1ns long electron bunches at a repetition rate than can be modified arbitrarily, because it is given by a low-potential (10V) pulser. The data reported here demonstrates a repetition rate of 16MHz.

In section 2 of this article we detail the working principle of the electron source, section 3 presents the description of the experimental setup, and the results are given in section 4, where we also discuss the potential of this method for creating monochromatic electron bunches with low temporal spread and low transverse-energy spread at a high-repetition rate.

## 2 Ionization scheme: Resonant Pulsed Rydberg Field-ionization (RPRFI)

Upon exposure to a strong to moderate electric field $F$, different $l$-value Rydberg states of an atom mix, resulting in a complex “spaghetti” of Stark-Rydberg states that exhibit complex behaviors in the field but remain resolvable. Cesium, having the largest quantum defect among all stable alkali metals, displays exaggerated behaviors due to strong deviations from hydrogenic characteristics. In particular, some states of cesium have substantial ionization rates even below the classical ionization threshold, owing to the scattering of the Rydberg electron with the core. The Resonant Pulsed Rydberg Field-ionization (RPRFI) technique leverages our comprehensive investigation of this region of Rydberg excitation and ionization in Cs to identify suitable states for use in an electron source [29].

For example, we identified regions of the Stark map where isolated Rydberg-Stark states, such as the one shown in figure 1, possess a significant Stark effect and a strong variation of their linewidth with the electric field/laser wavelength, enabling us to investigate the relationship between the natural lifetime of the state and the observed ionization duration. This photoabsorption Stark map was computed using local frame transformation (LFT) theory and WKB-wavefunctions as detailed in Ref. [29]. This Rydberg-Stark state, given its strong Stark effect, can be excited by a fixed laser frequency $\nu$ only when the electric field $F$ is precisely tuned to resonance. After excitation, the atom rapidly evolves into an electron and ion pair. To produce short electron bunches at a high rate, we pulse the electric field, thereby bringing the laser in and out of resonance at a fixed and controlled frequency. Each time the field brings the Rydberg-Stark level in resonance with the laser, an electron bunch is produced. The lifetime of this state is limited by its ionization rate $\Gamma_{\text{ion}}$, so the linewidth of the transition displayed on figure 1 gives us direct access to $\Gamma_{\text{ion}}(F)$. Apart from $\Gamma_{\text{ion}}(F)$, the process’s key parameters are the Stark slope of the Rydberg-Stark energy level $\frac{\nu}{F}$, and the electric field slew rate (SR) $\frac{F}{\nu}$. We will focus on these parameters in our analysis, discussing their relationship with the temporal spread of the electron bunches. Assuming typical values for $\Gamma_{\text{ion}}$ close to $1 \times 10^9 \text{ s}^{-1}$ (see figure 1), $\frac{d\nu}{dF}$ around 100MHz/(V/cm) and $\frac{dF}{dt}$ of a few (V/cm)/ns, we
<figure><img src="image_2.png" /><figcaption>**Fig. 1** Photoabsorption spectra from the $7s(F = 4)$ hyperfine level of cesium in electric fields between 2160 V/cm and 2195 V/cm calculated using the local-frame-transformation theory [29, 30, 31]. The y-offset indicates the electric field. The spectra show a transition to a Rydberg-Stark state with a strong Stark-shift (Stark-slope of $\frac{v}{F} \approx 0.2$ GHz/(V/cm)) and field-dependent ionization rate. The red vertical lines represent the values of $\lambda_{Ti:Sa}$ at which the data in figure 3 was collected. The experimentally applied oscillating field of amplitude 25 V/cm that brings the laser in and out of resonance is represented by a black double-arrow.</figcaption></figure>

can estimate the FWHM temporal width $\Delta \tau_{FWHM}$ of the electron bunch,

$$ \Delta \tau_{FWHM} \approx \frac{\Gamma_{ion}}{2\pi} \left( \frac{dF}{dt} \frac{dV}{dF} \right)^{-1}, \quad (1) $$

to be in the ns range.

# 3 Experimental methods

## 3.1 General setup

Figure 2 presents the experimental setup, which closely mirrors that used in our prior studies [27] with the difference that now the Cs beam enters the setup laterally to mitigate Cs contamination on-axis. In brief, an effusive, recirculating oven (see [32] for details), produces a beam of atomic Cs that penetrates an electrode stack where two plates (electrodes 0 and 1) separated by approximately 4 mm establish a DC electric field of around 2200 V/cm.

Rydberg-Stark excitation is performed with three different cw-lasers with wavelengths: ~852 nm for the transition $6p_{3/2}(F = 5) \leftarrow 6s(F = 4)$, ~1470 nm for $7s(F = 4) \leftarrow 6p_{3/2}(F = 5)$, and ~794 nm for Rydberg $\leftarrow 7s(F = 4)$. Laser powers were respectively ~10 µW, ~100 µW, and 10 mW, chosen to

minimize power-broadening of the lines and keep the event rate compatible with our time-resolved measurement scheme. The first two laser radiations (852 nm and 1470 nm) are produced by DL Pro and DFB Butterfly laser diodes from Toptica. The Rydberg-excitation laser is a Ti:Sa laser, and its wavelength is monitored using a HighFinesse WS8-2 high-precision wavemeter. During data acquisition, all three lasers are frequency-locked; we use a standard vapor cell-based method for the first two lasers, and the wavemeter for the Ti:Sa laser. The waists of the lasers in the interaction region are on the order of tens of micrometers, and the last two lasers (1470 nm and Ti:Sa) cross orthogonally.

An oscillating electric field (see next section) drives the laser in and out of resonance of the excitation towards a Rydberg-Stark state. Rydberg-Stark states with short ionization lifetimes are excited and produce electrons. These electrons are accelerated by the electric field and focused by two additional electrodes (denoted by $V_2$ and $V_3$ on figure 2) onto a double stack microchannel plate (MCP), followed by a phosphor screen placed approximately 55 cm downstream. Electrode 0 is kept at -870 V, producing a potential of around -435 V where the electrons are created. These electrons are accelerated to 435 eV at the end of electrode 4, and so at the front of the MCPs, which are kept at 0 V (see figure 2 a)).

## 3.2 Electron time-of-flight measurement

As our primary interest lies in the electron pulse duration, we implemented a detection method with approximately 60 ps resolution, based on a high-performance Time-to-Digital Converter (TDC). The main clock of the experiment is a commercial pulser that provides long pulses (5 µs) and triggers the opening of the TDC. These pulses trigger another pulser that produces a burst of shorter pulses with individual width of 50 ns, rising times of 10 ns, and an amplitude of $V_1 = 10$ V. These shorter pulses are directed to electrode 1 via a short, non-shielded cable, to minimize the pulse time-broadening during propagation.

A schematic of the acquisition setup is presented in figure 2. The oscillating electric potential on electrode 1, shown in figure 2 b), creates an oscillating electric field in the laser excitation region. With electrodes 0 and 1 separated by 0.4 cm, a 10 V pulse on electrode 1 induces an electric field pulse of amplitude 25 V/cm. Electrons are detected by the MCPs, and the signal they produce is amplified and digitized
<figure><img src="image_3.png" /><figcaption>**Fig. 2** a) Diagram of the acquisition setup for pulsed resonance experiments. Electrode 0 (on the left) is maintained at -870 V and a potential $V_1$ (between 0 V and 10 V) is applied to electrode 1. This results in a field of approximately 2175 V/cm between the two electrodes. Voltages $V_2$ and $V_3$ are adjusted to produce an unfocused electron beam exiting the electrode stack. The orange spot indicates the laser position. b) Experimental recording of the time variation of the voltage $V_1$ applied to electrode 1.</figcaption></figure>

by a constant-fraction discriminator. This signal is subsequently transmitted directly to a time-to-digital converter (TDC: TDC-V4 from the DTPI platform). The TDC opens independently for 50 µs, registering two types of “stop” signals during this period with a binning time of 125 ps: pulses from the main pulser and electron hits from the MCPs. Data is accumulated for a few minutes and recorded by a dedicated acquisition software. Decreasing the event rate to around 1 electron per 100 pulses enables us to reconstruct the full time-of-flight spectra from a multitude of unique and uncorrelated events. This low event rate also guarantees that the measured temporal widths are not affected by a putative saturation of the MCP.

# 4 Results

Each time the electric field brings the laser in resonance, electrons are generated, both on the rising and falling fronts of the electric pulse applied on electrode 1, leading to doublet structures in the electron signal. A typical ToF spectrum for $\lambda_{Ti:Sa} = 794.141$ nm is shown in figure 3 a), displaying series of electron bunches.

A few doublets from the data with $\lambda_{Ti:Sa} = 794.145$ nm are shown in detail on figure 3 b). The first peak of each doublet (corresponding to the rising front) is more intense than the second, with the separation between these two peaks corresponding to the

voltage pulse length, which can thus be freely tuned by tuning the repetition rate of the burst pulser.

## 4.1 Influence of $\Gamma_{ion}$ on $\tau_{ion}$

We studied experimentally the influence of the state's linewidth on the observed bunch duration. Data was collected on the state depicted in figure 1 under the same conditions except for the tuning of the Ti:Sa's wavelength, which was adjusted from 794.141 nm to 794.149 nm in increments of 0.002 nm. The Stark map of figure 1 establishes a link between the laser wavelength, the electric field, and the state linewidth. In particular we take advantage of the quasi-linearity between the studied state's ionization rate $\Gamma_{ion}$ and its resonant laser wavelength to study the influence of $\Gamma_{ion}$ on $\Delta\tau$.

Figure 3 c) shows the most intense peak of the electron ToF spectrum for each value of $\lambda_{Ti:Sa}$ (gray rectangle on panel a)), on the same time axis, fitted with Lorentzian peak functions. We fit similarly the first 32 pairs of peaks in the spectrum with Lorentzian peak functions and report the FWHM as the temporal width $\Delta\tau$ on figure 3 d), separating peaks with positive SR (electrons created during the rise of the electric field pulse) from the peaks with negative SR (electrons created during the fall of the electric field pulse). The mean values are around 1 ns, with a minimum mean value around 930 ps for $\lambda_{Ti:Sa} = 794.145$ nm
<figure><img src="image_4.png" /><figcaption>**Fig. 3** a) Complete electron ToF spectra with $\lambda_{Ti:Sa}$ = 794.141 nm. b) Zoomed-in view on a few peaks of the ToF spectrum acquired at $\lambda_{Ti:Sa}$ = 794.145 nm. c) Shape and positions of the most intense peak in scans taken at five different laser wavelengths. The data at 794.141 nm is taken as an intensity reference and all other data are normalized to it. d) Electron temporal spread (Lorentzian FWHM) of the 32 first pairs of peaks in the spectrum as a function of $\lambda_{Ti:Sa}$ (and thus $\Gamma_{ion}$, cf. figure 1) and of the sign of the SR (see text for details). Positive and negative SRs are indicated in blue and red, respectively. Error bars represent the standard deviation.</figcaption></figure>

and a negative SR. The relation between the state's ionization rate $\Gamma_{ion}$ and the electron bunch temporal length seems to suggest an optimal ionization rate $\Gamma_{ion} = 2.7 \times 10^9 \text{ s}^{-1}$, but, surprisingly (if compared to what is expected from Eq. (1)), the influence of $\Gamma_{ion}$ on the bunch time appears to be quite limited. The intensity of the peaks present two distinct features: a marked decrease of the signal for negative slew rates, and a $\mu$s-time scale oscillatory behavior. The origin of both features is currently not well understood, but are probably due to subtle measurement artifacts. We thus focus in the rest of the analysis on the temporal width of the electron bunches, which is the key figure-of-merit for this study.

According to Eq. (1), the SR should have a large influence of the observed electron bunch duration.

Changing the SR in a controlled way is experimentally quite challenging, but we noticed that our experimental SR was actually not constant during the measurement, allowing us to study the influence of the SR on $\Delta\tau$.

## 4.2 Influence of the slew rate on $\tau_{ion}$

As visible on figure 3 c), the central ToF of the peaks decreases linearly with the laser wavelength. Because the Stark shift is also linear (see figure 1) and that other experimental conditions are kept constant, we attribute this time delay to the slightly different resonant electric fields that are reached at slightly different times, i.e. to the SR. Together with the Stark slope
<figure><img src="image_5.png" /><figcaption>Fig. 4 a) Slew rate (SR) extracted from the data presented on figure 3 for the first 32 pair of peaks (see text for details). The positive SR are about twice as large and increase during the series of pulses, while the negative SRs decrease (in amplitude) but at a smaller rate.
b) Lorentzian FWHM of the electron bunches emitted in function of the SR of all fitted peaks, for all five values of λ<sub>Ti:Sa</sub> (indicated in color), showing an absence of clear correlation.</figcaption></figure>

given by the Stark map $\frac{dv}{dF}$, the change in the cen-
tral ToF for different laser frequencies $\frac{dv}{dt}$ provides a
value for the experimental SR $\frac{dF}{dt} = (\frac{dv}{dF})^{-1} \frac{dv}{dt}$. From
the fitted position of each peak for the five different
values of $\lambda_{Ti:Sa}$ we thus determine the SR for each
peak, and plot them on figure 4 a). We see clearly that
the positive SRs are about twice as large as the neg-
atives ones. The positive SRs increase significantly in
amplitude over time, while the negative SRs decrease
slightly in amplitude. The evolution of the measured
$\Delta\tau$ in function of the SR is visible on figure 4 b) (data
with different values of $\lambda_{Ti:Sa}$ are indicated in differ-
ent colors). Again, the influence of $\frac{dF}{dt}$ on $\Delta\tau$ seems to
be very small, with no clear correlation between these
two values. The FWHM temporal spread seems to be
limited to about 1 ns for this state.

4.3 Data at $\lambda_{\text{Ti:Sa}}$ = 793.933 nm

With the intent of demonstrating the potential of this
method for creating a short (< 1 ns) electron pulse,
we targeted another state, in a different region of the
Stark map. The idea is to excite a state that has a
relatively small ionization rate just before an avoided
crossing, that increases rapidly its ionization rate. A
Stark map of such a state can be seen on figure 5 a),
where the adiabatic position of the state in the field
is highlighted in blue. The laser frequency was set
to 793.933 nm (see red vertical ribbon) and the field
was oscillating between 2055 V/cm and 2070 V/cm
so that the excitation happens during the field pulse.
The resulting electron pulses are shown on figure 5
b), which shows a large (3.4 ns) and weak pulse fol-
lowed by a very intense and narrow peak (Lorentzian
FWHM of 287(20) ps) for the rising electric field. The
falling edge of the electric field pulse shows no peak
(expected around 770 ns). The first peak is attributed
to the state visible around 2060 V/cm, and the sec-
ond, very sharp peak is attributed to the state indicated
in blue on the Stark map. The absence of bunches on
the falling edge and the one-order-of-magnitude dif-
ference in the $\Delta\tau$ between the two states are difficult
to explain from the Stark map alone and are yet to be
understood.

These features demonstrates once more that the
electron dynamics leading to ionization can be very
complex, especially in parts of the spectrum where
several states are close in energy. The ionization
dynamics can not simply be described by the state’s
ionization rate, and a more in-depth, theoretical anal-
ysis is required here, for example as done in reference
[20]. This complexity is a powerful tool to generate
asymmetric but regular electron pulses from symmet-
ric electric field pulses, as demonstrated here. Further-
more, it is remarkable to see that an electron pulse
with such a narrow temporal width can be created with
this simple and versatile method. We now investigate
whether the measured temporal width of 287(20) ps
is representative of the limit of the method, or of the
limits of our measurement scheme.

4.4 The effect of energy spread on the
measured temporal spread

The measured temporal spread $\Delta\tau$ is indeed a con-
volution of $\tau_{\text{ion}}$ and $\tau_{\Delta E}$, where $\tau_{\text{ion}}$ is the intrinsic
duration of the ionization process, and $\tau_{\Delta E}$ is the temporal spread caused by the energy spread. For energy-spread dominated bunches, $\Delta\tau$ is strongly dependent on the position of measurement: for instance sub-ps bunches have been reported when the measurement of $\Delta\tau$ is performed at the so-called self-compression point, which occurs near twice the acceleration distance [25]. However self-compression cannot yield a temporal spread smaller than the emission time $\tau_{\text{ion}}$, as it merely cancels out locally the spatial (and hence, temporal) elongation of a bunch.

To assess the influence of the electron bunch energy spread $\Delta E$ on the measured temporal spread, we conducted simulations using the SIMION software [33]. We compared the arrival temporal spread $\Delta\tau$ for electron bunches with identical $\Delta E$, but with varying $\tau_{\text{ion}}$. $\Delta E$ is chosen to best describe our experimental conditions, i.e. a Gaussian distribution of FWHM close to 14 eV. The results are shown in figure 5 c), showing clearly that the energy spread limits the measured temporal spread to be above 280 ps. Because the temporal width presented in figure 3 is around 1 ns, we can be confident that the measured $\Delta\tau$ reflects the intrinsic $\tau_{\text{ion}}$. On the contrary, for the data presented in figure 5 b), the measured $\Delta\tau$ is below 300 ps, so that $\tau_{\Delta E}$ has a large influence on $\Delta\tau$. The main consequence for this study is that $\tau_{\text{ion}}$ is very probably around or even below 150 ps.

Reducing the energy spread (by focusing the laser more tightly or using lower fields) would thus also significantly reduce the measured time spread. Calculations[17] suggests that similar states are available at much lower fields, for example around 10 V/cm, where a laser focused to 10 µm would create electron bunches with $\Delta E \approx 10$ meV. Experiments are under way to experimentally identify these states and verify that the temporal width of the electron bunches that they produce are similar to the ones measured in this work. In terms of transversal energy spread, previous measurements of ion/electron correlations have shown [17] that the transverse energy spread of these Rydberg-based electron source are on the order of 1 meV, which is comparable or even better than the best electron sources from cold atoms [22, 34].

# 5 Conclusion and future work

In conclusion, we have demonstrated the use of the RPRFI technique to create electron bunches at a high repetition rate (~16MHz) with a pulse duration on

<figure><img src="image_6.png" /><figcaption>**Fig. 5** a) Stark map with $\lambda_{Ti:Sa} = 793.933$ nm indicated in red and F pulsed from 2055 V/cm to 2070 V/cm indicated by a double-arrow. In blue we highlight one avoided crossing. b) Resulting electron pulses with Lorentzian FWHM of 3.8 ns and 287 ps corresponding to the rising edge of the electric field. c) Evolution of the measured time spread $\Delta\tau$ of the most intense peak in function of the ionization time $\tau_{\text{ion}}$, taking into account the energy spread of the electron source and our experimental conditions. The width of the line represent the scatter of the simulations results. The y = x line is shown as a dashed gray line to guide the eye.</figcaption></figure>
the order or even well below the ns range. The low-
est width we have measured is at 287(20) ps, limited
by our measurement procedure, but indicative of an
intrinsic ionization duration below 150 ps. Using a cw-
laser allows Rydberg-Stark state-selective excitation
and ionization, even in the strongly mixed region of
the Stark map, thereby allowing fine tuning of the
emission properties of the source by targeting appro-
priate states. Shaped electron bunches can be pro-
duced with this method, provided that an appropriate
part of the Stark map is used.

We studied experimentally the influence of the
states' ionization rate and of the experimental slew
rate on the temporal spread of the electron bunch and
found a surprisingly almost negligible influence. One
notable advantage of replacing a pulsed laser with
a pulsed electric field is the ability to freely adjust
the ionization repetition rate and the delay between
pulses by modifying the delays in voltage pulses.
This is much simpler than adjusting repetition rates
or delays in an optical system and can very easily
produce electrons at a higher rate if required (up to
~100MHz).

The electric field that we employed (around
2170 V/cm) currently limits the energy spread
(FWHM) to approximately 14 eV. This limitation is
due to the voltage difference among electrons gener-
ated in different parts of the cloud, which is a result of
the product of the electric field by the cloud size. Thus,
in order to further decrease the energy spread of our
source and reach $\Delta E < 10$ meV that would open vibra-
tional spectroscopic possibilities [14], we will need to
decrease the electric field by two to three orders of
magnitude, and limit the size of the ionization zone by
focusing the laser more tightly.

The lowest temporal width that we report is
already at the time-resolution of next-generation
detectors for electron microscopes, like the TimePix4
[35]. For applications that would require smaller tem-
poral width, active bunching could be considered,
particularly techniques that allow electron bunch com-
pression without increasing their energy spread [17].
By simply reversing the fields, this source could also
provide ions in a focused ion beam or ion-implantation
studies [26]. Additionally, the correlation between the
produced ions and electrons can be leveraged in dif-
ferent ways to improve the properties of the source
[36, 17]. Finally spin-polarized electrons could be eas-
ily produced by these sources, especially when using
cesium [37, 38].

Therefore, our high repetition-rate pulsed low-
energy electron source based on atomic Rydberg
ionization could find applications in various fields,
including materials science, surface science, and bio-
logical imaging.

Acknowledgments

We thank N. Barrett and L. Amiaud for fruitful discus-
sions and Y.J. Picard for sharing knowledge and access
to the data acquisition setup.

Author contributions

Funding Information

This work was supported by the Fond Unique
Interministériel (IAPP-FUI-22) COLDFIB, the labex
PALM (grant number ANR-10-LABX-0039-PALM)
the ANR-HREELM project (ANR-14-CE35-0019).

Data Availability Statement

The data that support the findings of this study are
available from the corresponding author upon reason-
able request.

Open Access

A CC-BY public copyright license has been applied
by the authors to the present document and will be
applied to all subsequent versions up to the Author
Accepted Manuscript arising from this submission, in
accordance with the grant's open access conditions.

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