Delayed Electron Emission Research Articles

What surfaces in the operation of noble liquids dark matter detectors

Though noble element dual-phase detectors have a long application history in dark matter searches, some uncertainties and differences in backgrounds persist. We compare effects caused by unextracted electrons on the liquid-gas interface in Xe and Ar dual-phase detectors with a large family of phenomena at the liquid helium surface. We pose that electron and ion accumulation on the liquid surface in detectors can lead to the formation of ordered surface states, charged liquid surface instabilities in an electric field, electrospraying, interactions with surface waves, and other effects. Not only delayed electron emission signals can be generated, but the extraction efficiency for electrons produced below the liquid surface can be altered by the presence of surface charges. Several factors lead to surface electron accumulation, and problems can become more severe with the increased detector size. We discuss possible experiments to reveal surface electron effects and design changes to alleviate electron accumulation. We conclude that studies of these effects are desirable before making final design decisions for the new multi-ton liquid Xe dark matter detector projects like DARWIN, XLZD, and large Ar dual-phase detectors.

Delayed electron emission in photomultiplier tubes

The time distributions of pulses generated by noise or external light sources in the ХР2020, ХР2232В, ХР1021, FEU-85, FEU-87, FEU-93, FEU-130, and R7600U-200 photomultiplier tubes (PMTs) were investigated using an autocorrelation delayed-coincidence time spectrometer. The observed pulse time correlations were of two types: non-exponential, related to the detection of the effects of feedback ions; and exponential, caused by the interaction of secondary-emission electrons with the PMT dynodes. Detection of multi-electron events arising from the action of external radiators leads to intensification of residual-gas ionization processes in the PMT. However, no analogy is observed in the shapes of the feedback ion spectra for different types of PMT despite the identical composition of photocathodes or dynodes. This might be related to differences in PMT manufacturing processes and to possible different restructuring of molecular electron shells in the photocathode material constituents. The detection of single-electron events (noise pulses) in some of the PMTs studied was not always characterized by nanosecond-scale emissions that might be expected to result from metastable states related to properties of their photocathodes and dynodes, and on the material processing technology used in PMT manufacture. It is difficult to describe exactly the mechanisms leading to delayed electron emission. The source of such events remains open to attempts at explanation and further research.

Thermionic Emission of Negative Ions of Molecules and Small Clusters as a Probe of Low-Energy Attachment.

We have been studying the thermionic emission of negatively charged molecules and small clusters for more than a decade. The kinetic energy released distribution (KERD) of mass-selected negative ions has been measured with a velocity map imaging spectrometer. A comparison of the experimental KERD to detailed balance models provided information on the reverse process, namely, the electron attachment to the parent. The electron attachment to neutral systems (reverse process of the electron emission from anions) is usually described in a simplified way as a single electron capture in the framework of the classical Langevin model. Our measurements show that this approach is insufficient and that, in addition to the capture step, an intramolecular vibrational redistribution (IVR) step should be included. As far as multiply charged anions are concerned, the electron attachment to anions (reverse process of the electron emission from dianions) is strongly affected by the repulsive Coulomb barrier (RCB). Previous studies assumed a pure over-the-barrier process, which is in disagreement with our study. Indeed, electron emission is measured below the RCB, revealing significant thermal tunneling. In the present review, we summarize these works on singly and doubly charged anions in an attempt to present a unified view of the involved processes. It is worth noting that the detailed measurements of KERDs in the very low kinetic energy region (typically around 0.1 eV) have been made possible thanks to electron imaging methods, without which all of this work could never have been done, with time-resolution capabilities allowing the disentangling of direct and delayed electron emission.

Laser-induced field emission from a tungsten nanotip by circularly polarized femtosecond laser pulses

We have investigated emission patterns and energy spectra of electrons from a tungsten nanotip induced by circularly polarized femtosecond laser pulses. Variations of emission patterns were observed for different helicities of circular polarization while the energy spectra remained almost identical. The physics behind this difference in emission patterns is the change in propagation directions of surface electromagnetic waves on the tip apex. The energy spectra showed the same spectroscopic signatures as the linearly polarized laser in a strong-field regime, which are a low-energy peak and a plateau feature. The low-energy peak is due to a delayed electron emission with respect to a prompt emission. The experimental data and plasmonic simulations support our previous conclusion, where the observed delayed emission processes originate from an inelastic rescattering process. This work demonstrates that the use of circular polarization is an easy means to add extra knobs to control the spatial and temporal emission from a nanotip at the nanometer and femtosecond scale. It could find applications as a helicity-driven subcycle optical switch.

Resonance-Mediated Below-Threshold Delayed Photoemission and Non-Franck-Condon Photodissociation of Cold Oxyallyl Anions.

The photoexcitation of cold oxyallyl anions was studied below the adiabatic detachment threshold at a photon energy of 1.60 eV. Photodetachment was observed through two product channels, delayed electron emission from a long-lived anionic state and dissociative photodetachment via absorption of a second photon. The former produced stable neutral C3 H4 O, while the latter resulted in the concerted elimination of CO+C2 H4 products. The neutral oxyallyl singlet state has a barrier-free route to cyclopropanone as well as zwitterionic character with a large charge separation and dipole moment. The role of long-lived dipole-bound resonances built on the singlet state below the detachment threshold is discussed. These results provide one of the first observations of delayed photoemission in a small cold molecular radical anion, a consequence of the complex electronic structure of the neutral diradical, and provide an example of resonance-mediated control of the photodissociation processes.

Resonance‐Mediated Below‐Threshold Delayed Photoemission and Non‐Franck–Condon Photodissociation of Cold Oxyallyl Anions

AbstractThe photoexcitation of cold oxyallyl anions was studied below the adiabatic detachment threshold at a photon energy of 1.60 eV. Photodetachment was observed through two product channels, delayed electron emission from a long‐lived anionic state and dissociative photodetachment via absorption of a second photon. The former produced stable neutral C3H4O, while the latter resulted in the concerted elimination of CO+C2H4 products. The neutral oxyallyl singlet state has a barrier‐free route to cyclopropanone as well as zwitterionic character with a large charge separation and dipole moment. The role of long‐lived dipole‐bound resonances built on the singlet state below the detachment threshold is discussed. These results provide one of the first observations of delayed photoemission in a small cold molecular radical anion, a consequence of the complex electronic structure of the neutral diradical, and provide an example of resonance‐mediated control of the photodissociation processes.

An experimental setup to study delayed electron emission upon photoexcitation of trapped polyatomic anions.

A Velocity Map Imaging (VMI) spectrometer has been designed and integrated with an electrostatic ion beam trap to study delayed electron emission from trapped polyatomic anions upon photodetachment. The VMI spectrometer is small in size and can record a wide range of photoelectron energies, with variable magnification. Delayed electron emission can be recorded in our experimental setup for any time duration after the photoexcitation of the polyatomic anions. Experiments were carried out with trapped O- and C5- ions to demonstrate the capability of the spectrometer. Delayed electron emissions from C5- as well as prompt photoelectrons from O- were detected by the VMI spectrometer upon photoexcitation. The design and performance of the spectrometer are presented in detail.

Effect of emission on subnanosecond breakdown in a gas diode at low pressure

The paper presents experimental and numerical research results on the operation of gas diode at low pressure. A high dispersion in the runaway electron beam current (from 20 to 100 A) with respect to the average one (∼50 A) is observed for a tubular cathode with a working edge radius of 30 μm, nitrogen pressure of 30 Torr, and an interelectrode gap of 6 mm. Numerical simulation data show that the low beam current (∼20 A) is due to the early electron emission from the cathode (at the stage of low-voltage prepulse), in which the runaway electron beam is formed from the boundary of plasma layer developing early in the breakdown. The high beam current (∼100 A) is due to the delayed electron emission from the cathode, which increases the diode voltage and the runaway electron beam current. In the latter case, the runaway electron beam is formed directly at the cathode.

Effect of emission on subnanosecond breakdown in a gas diode at low pressure

The paper presents experimental and numerical research results on the operation of a gas diode at low pressure. A large scattering in runaway electron beam current (from 20 to 100 A) with regard to the average (∼50 А) is observed for a tubular cathode with a working edge radius of 30 μm, nitrogen pressure of 40 mbar, and interelectrode gap of 6 mm. Numerical simulation data show that the low beam current (∼20 A) is due to early electron emission from the cathode (at the stage of low-voltage prepulse) in which the runaway electron beam is formed from the boundary of a plasma layer developed early in the breakdown. The high beam current (∼100 A) is due to delayed electron emission from the cathode, which increases the diode voltage and the runaway electron beam current. In the latter case, the runaway electron beam is formed directly at the cathode.

Delayed electron emission in strong-field driven tunnelling from a metallic nanotip in the multi-electron regime.

Illuminating a nano-sized metallic tip with ultrashort laser pulses leads to the emission of electrons due to multiphoton excitations. As optical fields become stronger, tunnelling emission directly from the Fermi level becomes prevalent. This can generate coherent electron waves in vacuum leading to a variety of attosecond phenomena. Working at high emission currents where multi-electron effects are significant, we were able to characterize the transition from one regime to the other. Specifically, we found that the onset of laser-driven tunnelling emission is heralded by the appearance of a peculiar delayed emission channel. In this channel, the electrons emitted via laser-driven tunnelling emission are driven back into the metal, and some of the electrons reappear in the vacuum with some delay time after undergoing inelastic scattering and cascading processes inside the metal. Our understanding of these processes gives insights on attosecond tunnelling emission from solids and should prove useful in designing new types of pulsed electron sources.

Coincidence spectroscopy of high-lying Rydberg states produced in strong laser fields

We report on the measurement of electron emission after the interaction of strong laser pulses with atoms and molecules. These electrons originate from high-lying Rydberg states with quantum numbers up to $n \lesssim 120$ formed by frustrated field ionization. Simulations show that both tunneling ionization by a weak dc field and photoionization by the black-body radiation contribute to delayed electron emission on the nano- to microsecond scale. We measured ionization rates from these Rydberg states by coincidence spectroscopy. Further, the dependence of the Rydberg-state production on the ellipticity of the driving laser field proves that such high-lying Rydberg states are populated through electron recapture. The present experiment provides detailed quantitative information on Rydberg production by frustrated field ionization.

An instrument combining an electrospray ionization source and a velocity-map imaging spectrometer for studying delayed electron emission of polyanions.

An instrument combining an electrospray ionization source and a velocity-map imaging (VMI) spectrometer has been developed in order to study the delayed electron emission of molecular anions and especially of polyanions. It operates at a high repetition rate (kHz) in order to increase the acquisition speed. The VMI spectrometer has been upgraded for nanosecond time resolution by gating the voltages applied on the position-sensitive detector. Kinetic energy release distribution of thermionic emission (without any contribution from direct detachment) can be recorded for well-defined delays after the nanosecond laser excitation. The capability of the instrument is demonstrated by recording photodetachment spectra of the benchmark C60(-) anion and C84(2-) dianion.

Observation of delayed electron emission in a two-phase liquid xenon detector

Results of the experimental study of electron emission from liquid xenon via electroluminescence of the gas phase are presented. We report on observation of a peculiar kind of delayed electroluminescent signal following initial electroluminescence caused by ionizing particles. We also present the results of a study of spontaneous single electron emission following cosmic muon signals. It was found that the rate of spontaneous single electron signals strongly depends on the time passed since the initial electroluminescence happened. The analysis of experimental data showed that both spontaneous single electron signals and delayed electroluminescent signals are associated with ionization electrons which are trapped by the potential barrier at the interface.

Photoelectron spectroscopy of hexachloroplatinate-nucleobase complexes: Nucleobase excited state decay observed via delayed electron emission.

We report low-temperature photoelectron spectra of isolated gas-phase complexes of the hexachloroplatinate dianion bound to the nucleobases uracil, thymine, cytosine, and adenine. The spectra display well-resolved, distinct peaks that are consistent with complexes where the hexachloroplatinate dianion is largely intact. Adiabatic electron detachment energies for the hexachloroplatinate-nucleobase complexes are measured as 2.26-2.36 eV. The magnitudes of the repulsive Coulomb barriers (RCBs) of the complexes are all ∼1.7 eV, values that are lower than the RCB of the uncomplexed PtCl6 (2-) dianion as a result of charge solvation by the nucleobases. In addition to the resolved spectral features, broad featureless bands indicative of delayed electron detachment are observed in the 193 nm photoelectron spectra of the four clusters. The 266 nm spectra of the PtCl6 (2-) ⋅ thymine and PtCl6 (2-) ⋅ adenine complexes also display very prominent delayed electron emission bands. These results mirror recent results on the related Pt(CN)4 (2-) ⋅ nucleobase complexes [A. Sen et al., J. Phys. Chem. B 119, 11626 (2015)]. The observation of delayed electron emission bands in the PtCl6 (2-) ⋅ nucleobase spectra obtained in this work, as for the previously studied Pt(CN)4 (2-) ⋅ nucleobase complexes, is attributed to one-photon excitation of nucleobase-centred excited states that can effectively couple to the electron detachment continuum, producing strong electron detachment. Moreover, the selective, strong excitation of the delayed emission bands in the 266 nm spectra is linked to fundamental differences in the individual nucleobase photophysics at this excitation energy. This strongly supports our previous suggestion that the dianion within these clusters can be viewed as a "dynamic tag" which has the propensity to emit electrons when the attached nucleobase decays over a time scale long enough to allow autodetachment.

Delayed electron emission from fullerene dianions: Evidence for a thermal tunneling detachment

Isolated multiply charged anions exhibit a repulsive Coulomb barrier (RCB) towards electron loss. In this paper, the role of tunneling through the RCB in the delayed electron emission is addressed. Electron kinetic energy distributions of photoexcited fullerene dianions ${\mathrm{C}}_{84}{}^{2\ensuremath{-}}$ and ${\mathrm{C}}_{90}{}^{2\ensuremath{-}}$ have been measured with a velocity-map imaging spectrometer for delays of about 150 ns after the nanosecond laser excitation. On such spectra, delayed electron emission is unambiguously separated from direct detachment. These measurements display significant delayed emission below the RCB. A detailed-balance model including electron tunneling through the RCB has been developed. The very good agreement between the measurements and the model provides quantitative evidence for a thermal tunneling electron emission.

A VMI setup to study prompt and delayed electron emission from trapped cluster anions

SynopsisWe have designed a velocity map imaging (VMI) setup and integrated it with an Electrostatic Ion Beam Trap (EIBT) to study photon induced prompt and delayed electron emission from trapped cluster anions. The operational details are discussed here. The performance of the VMI is also reported for photoionization of trapped O− ions.

Laser-induced delayed electron emission of Co−4 anions

SynopsisThe cryogenic trap for fast ion beams (CTF) located at the Max-Planck-Institut für Kernphysik has been used to study laser-induced delayed electron emission of various molecular and metal-cluster anions. We will discuss recent experiments investigating the radiative cooling respectively heating of Co−4 anions produced in hot and cold ion sources. The experiments will be continued at the cryogenic storage ring (CSR), which is presently being commissioned, to disentangle laser-induced electron emission from atom or cluster dissociation.

Deep‐UV exciton emission from Mg1 − xCaxO—A first‐principle investigation

AbstractIn order to investigate the ultraviolet and electron emission properties of rock‐salt Mg1 − xCaxO, first‐principle calculations are carried out with x ranging from 0 to 0.5. The electron–hole interaction is taken into account by solving the Bethe–Salpeter equation. In pure MgO, the calculated exciton binding‐energy value of 83 ± 3 meV is quite close to the published experimental values. The electronic properties of doped MgO are investigated based on the super‐cell model. Both the optical bandgap and the exciton binding energy of the super cell of Mg1 − xCaxO are calculated for five discrete compositions, with an increasing amount x of CaO. The results show that there are strong excitonic effects in all of these materials. A rapid reduction in bandgap value is observed for Mg1 − xCaxO, when increasing the x value. The exciton binding energy of Mg1 − xCaxO shows a minimum value of about 40 meV around x = 0.2. In all of these cases, the exciton is stable at room temperature. The lowest excitation levels in Mg1 − xCaxO are determined by dark excitons. The occurrence of these dark excitons might explain why Mg1 − xCaxO is a strong source of delayed electron emission, after being bombarded by ions in a plasma.

Appearance size of poly-anionic aluminum clusters, Aln z-, z = 2–5*

The production of poly-anionic metal clusters by simultaneous storage of electrons and cluster anions in a Penning trap has been extended to the fifth charge state. The minimum cluster size, required to attach a fifth excess electron, has been experimentally determined for aluminum clusters. A refined data evaluation method is proposed, redefining the appearance size with respect to the delayed electron emission. It has been applied to the penta-anions as well as to previous data of poly-anionic aluminum clusters. In addition, new measurements of aluminum di-anions have revealed a lower minimum appearance size than reported earlier. Comparison of the experimental results with predictions by the conducting-sphere model for the di-, tri-, tetra- and penta-anions show deviations that are probably due to thermal excitation of the cluster anions. The cluster-size dependence of the poly-anion abundance spectra is qualitatively reproduced by thermionic emission.

Kinetic energy spectra in thermionic emission from small tungsten cluster anions: Evidence for nonclassical electron capture

The delayed electron emission from small mass-selected anionic tungsten clusters W(n)(-) has been studied for sizes in the range 9 < or = n < or = 21. Kinetic energy spectra have been measured for delays of about 100 ns after laser excitation by a velocity-map imaging spectrometer. They are analyzed in the framework of microreversible statistical theories. The low-energy behavior shows some significant deviations with respect to the classical Langevin capture model, which we interpret as possibly due to the influence of quantum dynamical effects such as tunneling through the centrifugal barrier, rather than shape effects. The cluster temperature has been extracted from both the experimental kinetic energy spectrum and the absolute decay rate. Discrepancies between the two approaches suggest that the sticking probability can be as low as a few percent for the smallest clusters.