Vacuum Ultraviolet Photons Research Articles

Vacuum ultraviolet spectroscopy of cold atmospheric pressure plasma jets

AbstractDue to elevated pressure, cold atmospheric pressure plasmas generate excimer species, which can emit highly energetic photons, thus transferring energy inside the discharge and to treated substrates. However, they are difficult to assess, as they are absorbed by air or window material. Here, we present a method to measure vacuum ultraviolet photons using a monochromator with an aerodynamic window. The emission spectra of a radiofrequency‐excited atmospheric plasma jet were analyzed for typical gas mixtures. The data indicate that helium excimers contribute notably to the excitation of molecular and atomic species. The emission intensities do not follow densities of ground‐state species, underlining the variety of excitation channels and the change of the electron energy distribution function under changing gas composition.

Photo-processing of astro-PAHs

Polycyclic aromatic hydrocarbons (PAHs) are key species in astrophysical environments in which vacuum ultraviolet (VUV) photons are present, such as star-forming regions. The interaction with these VUV photons governs the physical and chemical evolution of PAHs. Models show that only large species can survive. However, the actual molecular properties of large PAHs are poorly characterized and the ones included in models are only an extrapolation of the properties of small and medium-sized species. We discuss here experiments performed on trapped ions including some at the SOLEIL VUV beam line DESIRS. We focus on the case of the large dicoronylene cation, , and compare its behavior under VUV processing with that of smaller species. We suggest that C2 H2 is not a relevant channel in the fragmentation of large PAHs. Ionization is found to largely dominate fragmentation. In addition, we report evidence for a hydrogen dissociation channel through excited electronic states. Although this channel is minor, it is already effective below 13.6 eV and can significantly influence the stability of astro-PAHs. We emphasize that the competition between ionization and dissociation in large PAHs should be further evaluated for their use in astrophysical models.

Highly-excited state properties of cumulenone chlorides in the vacuum-ultraviolet.

Recent observations of chloromethane in interstellar environments suggest that other organohalogens, which are known to be critically important in Earth's atmosphere, may also be of significance beyond our own terrestrial veil. This raises the question of how such molecules behave under extreme conditions such as when exposed to vacuum ultraviolet (VUV) radiation. VUV photons promote molecules to highly excited states that fragment in non-statistical patterns controlled by the initial femtosecond dynamics. A detailed understanding of VUV-driven photochemistry in complex organic molecules that consist of more than one functional group is a particularly challenging task. This quantum chemical analysis reports the electronic states and ionization potentials up to the VUV range (6-11 eV) of the chlorine-substituted cumulenone series molecules. The valence and Rydberg properties of lone-pair terminated, π-conjugated systems are explored for their potential resonance with lone pairs from elsewhere in the system. The carbon chain elongation within the family ClHCnO, where n = 1-4, influences the electronic excitations, associated wavefunctions, and ionization potentials of the molecules. The predicted geometries and ionization potentials are in good agreement with the available experimental photoelectron spectra for formyl chloride and chloroketene, n = 1-2. Furthermore, comparison between the regular cumulenone species and the corresponding chlorinated derivatives exhibit similar behaviors especially for n = 3, where the allene backbone in propadienone chloride is severely bent. Most notably for the excited states is that the Rydberg character becomes more dominant as the energy increases, with some retaining valence characters.

Sensitive Mass Spectrometer for Time-Resolved Gas-Phase Chemistry Studies at High Pressures.

We report a new experimental apparatus for direct time-resolved probing of high-pressure gas-phase chemical reactions by photoionization mass spectrometry. The apparatus uses a laser photolysis flow reactor, capable of operating at P = 0.3-100 bar and T = 300-1000 K. We initiate reactions in homogeneous gas mixtures by the photolysis of an appropriate radical precursor using laser pulses at repetition rates of 1-10 Hz. The reacting mixture is continuously sampled into a vacuum chamber, ionized by vacuum-ultraviolet (VUV) photons from laboratory-based discharge lamps or from a synchrotron beamline, and analyzed by a custom mass spectrometer. Soft near-threshold ionization by tunable synchrotron radiation enables spectroscopic quantification of many key intermediates and products of chemical reactions. A novel ionization scheme in the high-density region of the sample jet expansion increases the experimental sensitivity 100-fold, compared with the existing instruments, without compromising mass resolution. A 40 kHz pulsed reflectron time-of-flight mass spectrometer achieves simultaneous detection of all ionized species with 25 μs time resolution. We demonstrate the capability of this apparatus by probing the ethyl radical oxidation reaction using very dilute (<1012 molecules·cm-3) ethyl concentrations at pressures up to 25 bar.

Observation of the Carbon Elimination Channel in Vacuum Ultraviolet Photodissociation of OCS.

The textbook mechanism for OCS photodissociation mainly involves the CO + S or CS + O product channel via a single bond fission. However, a third dissociation channel concerning the cleavage of both C-S and C-O bonds yielding SO + C products, though thermodynamically allowed, has never been verified experimentally to date. By using a tunable vacuum ultraviolet laser light and time-sliced velocity map ion imaging technique, we have clearly observed the SO(X3Σ-) + C(3PJ=0) products as the vacuum ultraviolet laser photon energy gradually exceeds its thermodynamic threshold. The corresponding SO(X3Σ-) coproducts are highly vibrationally excited and show varying angular distributions from isotropic to anisotropic as the excitation photon energy increases. Theoretical analysis suggests that a fast nonadiabatic pathway plays a dominant role in the formation of the anisotropic SO products. That isotropic products arise as the excitation photon energies approach the thermodynamic threshold can be reasonably explained by the "roaming mechanism".

Non-thermal desorption of complex organic molecules

Context. The occurrence of complex organic molecules (COMs) in the gas phase at low temperature in the dense phases of the interstellar medium suggests that a non-thermal desorption mechanism is at work because otherwise, COMs should condense within a short timescale onto dust grains. Vacuum ultraviolet (VUV) photodesorption has been shown to be much less efficient for complex organic molecules, such as methanol, because mostly photoproducts are ejected. The induced photolysis competes with photodesorption for large COMs, which considerably lowers the efficiency to desorb intact molecules. Aims. We pursue an experimental work that has already shown that water molecules, the dominant ice mantle species, can be efficiently sputtered by cosmic rays. We investigate the sputtering efficiency of complex organic molecules that are observed either in the ice mantles of interstellar dense clouds directly by infrared spectroscopy (CH3OH), or that are observed in the gas phase by millimeter telescopes (CH3COOCH3) and that could be released from interstellar grain surfaces. Methods. We irradiated ice films containing complex organic molecules (methanol and methyl acetate) and water with swift heavy ions in the electronic sputtering regime. We monitored the infrared spectra of the film as well as the species released to the gas phase with a mass spectrometer. Results. We demonstrate that when methanol or methyl acetate is embedded in a water-ice mantle exposed to cosmic rays, a large portion is sputtered as an intact molecule, with a sputtering yield close to that of the main water-ice matrix. This must be even more true for the case of more volatile ice matrices, such as those that are embedded in carbon monoxide. Conclusions. Cosmic rays penetrating deep into dense clouds provide an efficient mechanism to desorb complex organic molecules. Compared to the VUV photons, which are induced by the interaction of cosmic rays, a large portion desorb as intact molecules with a proportion corresponding to the time-dependent bulk composition of the ice mantle, the latter evolving with time as a function of fluence due to the radiolysis of the bulk.

Evidence for bulk nodal loops and universality of Dirac-node arc surface states in ZrGeXc ( Xc=S , Se, Te)

We have performed angle-resolved photoemission spectroscopy (ARPES) on layered ternary compounds ZrGeXc (Xc = S, Se, and Te) with square Ge lattices. ARPES measurements with bulk-sensitive soft-x-ray photons revealed a quasi-two-dimensional bulk-band structure with the bulk nodal loops protected by glide mirror symmetry of the crystal lattice. Moreover, high-resolution ARPES measurements near the Fermi level with vacuum-ultraviolet photons combined with first-principles band-structure calculations elucidated a Dirac-node-arc surface state traversing a tiny spin-orbit gap associated with the nodal loops. We found that this surface state commonly exists in ZrGeXc despite the difference in the shape of nodal loops. The present results suggest that the spin-orbit coupling and the multiple nodal loops cooperatively play a key role in creating the exotic Dirac-node-arc surface states in this class of topological line-node semimetals.

Photoprocessing of large PAH cations.

In cosmic environments, polycyclic aromatic hydrocarbons (PAHs) strongly interact with vacuum ultraviolet (VUV) photons emitted by young stars. Trapped PAH cations ranging in size from 30 to 48 carbon atoms were irradiated by tunable synchrotron light (DESIRS beamline at SOLEIL). Their ionization and dissociation cross sections were determined and compared with TD-DFT computed photoabsorption cross sections. Evidence for radiative cooling is reported.

Vacuum-UV and Low-Energy Electron-Induced DNA Strand Breaks - Influence of the DNA Sequence and Substrate.

DNA is effectively damaged by radiation, which can on the one hand lead to cancer and is on the other hand directly exploited in the treatment of tumor tissue. DNA strand breaks are already induced by photons having an energy below the ionization energy of DNA. At high photon energies, most of the DNA strand breaks are induced by low-energy secondary electrons. In the present study we quantified photon and electron induced DNA strand breaks in four different 12mer oligonucleotides. They are irradiated directly with 8.44 eV vacuum ultraviolet (VUV) photons and 8.8 eV low energy electrons (LEE). By using Si instead of VUV transparent CaF2 as a substrate the VUV exposure leads to an additional release of LEEs, which have a maximum energy of 3.6 eV and can significantly enhance strand break cross sections. Atomic force microscopy is used to visualize strand breaks on DNA origami platforms and to determine absolute values for the strand break cross sections. Upon irradiation with 8.44 eV photons all the investigated sequences show very similar strand break cross sections in the range of 1.7-2.3×10-16 cm2 . The strand break cross sections for LEE irradiation at 8.8 eV are one to two orders of magnitude larger than the ones for VUV photons, and a slight sequence dependence is observed. The sequence dependence is even more pronounced for LEEs with energies <3.6 eV. The present results help to assess DNA damage by photons and electrons close to the ionization threshold.

Review—Progress in Understanding Host-Sensitized Excitation Processes in Luminescent Materials

Numerous technologies are in use today that employ solid state luminescent materials that are excited by high-energy sources, such as X-rays, cathode rays, or vacuum ultraviolet (VUV) photons. Absorption of this energy results in the formation of electron-hole (e-h) pairs in the host material. In a doped phosphor, this energy must be transferred to the activator for luminescence to occur. The overall efficiency of host-sensitized luminescence is therefore dictated by the efficiency of energy transfer, in addition to the activator quantum efficiency. Studies of host-sensitized luminescence tend to focus on the overall efficiency, while there are fewer reports of systematic investigations aimed at quantifying the energy transfer step. Even so, important experimental and theoretical progress has been made toward improving our understanding of these processes. In this review the development of the field is discussed, with a particular focus on the study of host-to-activator energy transfer.

Enabling liquid vapor analysis using synchrotron VUV single photon ionization mass spectrometry with a microfluidic interface.

Vacuum ultraviolet (VUV) single photon ionization mass spectrometry (SPI-MS) is a vacuum-based technique typically used for the analysis of gas phase and solid samples, but not for liquids due to the challenge in introducing volatile liquids in a vacuum. Here we present the first demonstration of in situ liquid analysis by integrating the System for Analysis at the Liquid Vacuum Interface (SALVI) microfluidic reactor into VUV SPI-MS. Four representative volatile organic compound (VOC) solutions were used to illustrate the feasibility of liquid analysis. Our results show the accurate mass identification of the VOC molecules and the reliable determination of appearance energy that is consistent with ionization energy for gaseous species in the literature as reported. This work validates that the vacuum-compatible SALVI microfluidic interface can be utilized at the synchrotron beamline and enable the in situ study of gas-phase molecules evaporating off the surface of a liquid, which holds importance in the study of condensed matter chemistry.

Branching Ratio Measurements of the Predissociation of 12C16O by Time-Slice Velocity-Map Ion Imaging in the Energy Region from 106 250 to 107 800 cm-1.

Photodissociation of CO is a fundamental chemical mechanism for mass-independent oxygen isotope fractionation in the early Solar System. Branching ratios of photodissociation channels for individual bands quantitatively yield the trapping efficiencies of atomic oxygen resulting into oxides. We measured the branching ratios for the spin-forbidden and spin-allowed photodissociation channels of 12C16O in the vacuum ultraviolet (VUV) photon energy region from 106 250 to 107 800 cm-1 using the VUV laser time-slice velocity-map imaging photoion technique. The excitations to four 1Π bands and three 1Σ+ bands of 12C16O were identified and investigated. The branching ratios for the product channels C(3P) + O(3P), C(1D) + O(3P), and C(3P) + O(1D) of these predissociative states strongly depend on the electronic and vibrational states of CO being excited. By plotting the branching ratio of the spin-forbidden dissociation channels versus the excitation energy from 102 500 to 110 500 cm-1 that has been measured so far, the global pattern of the 1Π-3Π interaction that plays a key role in the predissociation of CO is revealed and discussed.

Thermal Resilience of Imidazolium-Based Ionic Liquids-Studies on Short- and Long-Term Thermal Stability and Decomposition Mechanism of 1-Alkyl-3-methylimidazolium Halides by Thermal Analysis and Single-Photon Ionization Time-of-Flight Mass Spectrometry.

Ionic liquids are often considered as green alternatives of volatile organic solvents. The thermal behavior of the ionic liquids is relevant for a number of emerging large-scale applications at elevated temperature. Knowledge about the degradation products is indispensable for treatment and recycling of the used ionic liquids. The objective of this paper was an investigation of the short- and long-term stability of several 1-alkyl-3-methylimidazolium halides, determination of the degradation products, and the elucidation of their decomposition patterns and structure-stability relations. Short-term stability and mechanism of thermal degradation were investigated by a self-developed, innovative thermal analysis single-photon ionization time-of-flight mass spectrometry device with Skimmer coupling. The applied technology provides real-time monitoring of the forming species and allows tracing their change during the course of the decomposition. Therein, the almost fragment-free soft ionization with vacuum ultraviolet photons plays a crucial role. We have detected unfragmented molecules whose formation was only assumed by electron ionization. Nevertheless, the main decomposition products of the selected ionic liquids were alkyl imidazoles, alkenes, alkyl halides, and hydrogen halides. From the decomposition products, we have deduced the fragmentation patterns and discussed their interrelation with the length of the alkyl chain and the type of the halide anion. Our results did not suggest the evaporation of the investigated ionic liquids prior to their decomposition under atmospheric conditions. Long-term thermal stability and applicability were determined based on thermogravimetric analysis evaluated with a kinetic model. Thus, the time-dependent maximum operation temperature (MOT) for the respective ionic liquids has been calculated. As a rule, the short-term stability overestimates the long-term decomposition temperatures; the calculated MOT are significantly lower (at least 100 K) than the standardly obtained decomposition temperatures.

Formation of a Thin Luminescent Layer in LiF Crystals under Glow Discharge Radiation

The formation of thin layers of luminescent defects on the faces of planar lithium fluoride crystals located in the positive column and Faraday dark space of a glow gas discharge was studied by time-resolved confocal scanning luminescent microscopy and time-correlated single photon counting. The formation of aggregated color centers in the surface layers of crystals was established using the spectral and kinetic characteristics of luminescence appearing after irradiation. The role of gas discharge electrons, ions, and photons in the defect formation mechanism was considered. The defects were shown to be formed under the influence of vacuum ultraviolet (VUV) photons. The VUV radiation intensity distribution in the discharge gap was measured by the method of thermostimulated luminescence. The main source of this radiation was the anodic and cathodic voltage drop regions in a glow discharge.

Photons Probe Entropic Potential Variation during Molecular Confinement in Nanocavities.

In thin polymeric layers, external molecular analytes may well be confined within tiny surface nano/microcavities, or they may be attached to ligand adhesion binding sites via electrical dipole forces. Even though molecular trapping is followed by a variation of the entropic potential, the experimental evidence of entropic energy variation from molecular confinement is scarce because tiny thermodynamic energy density diverseness can be tracked only by sub-nm surface strain. Here, it is shown that water confinement within photon-induced nanocavities in Poly (2-hydroxyethyl methacrylate), (PHEMA) layers could be trailed by an entropic potential variation that competes with a thermodynamic potential from electric dipole attachment of molecular adsorbates in polymeric ligands. The nano/microcavities and the ligands were fabricated on a PHEMA matrix by vacuum ultraviolet laser photons at 157 nm. The entropic energy variation during confinement of water analytes on the photon processed PHEMA layer was monitored via sub-nm surface strain by applying white light reflectance spectroscopy, nanoindentation, contact angle measurements, Atomic Force Microscopy (AFM) imaging, and surface and fractal analysis. The methodology has the potency to identify entropic energy density variations less than 1 pJm−3 and to monitor dipole and entropic fields on biosurfaces.

(Invited) The Mechanism of Surface Modifications of Photoresist Material in Plasma Etching Nano Processes

Etching plasmas employed for the fabrication of ultra large scale integrated circuits (ULSIs) have been causing serious damages to photoresist materials such as surface roughness formations and structural changes. So far, there have been many reports on suppressing these damages by some process techniques such as the UV, HBr plasma cure and so on. However, effects of individual species and photons in the plasma on the damages of photoresist materials have not been clarified enough. Especially, the effect of vacuum ultraviolet (VUV) photons on damages have never been quantitatively understood. Furthermore, beside the VUV photons, the synergetic effects of individual species of ion and radicals enhance the damages. As a result, the chemical structure alteration and the surface roughness formation of polymer materials are considered to be brought about. In the fabrication of the sub-45 nm devices and integrated circuits, it is extremely important to clarify the reaction mechanism of surface modification of photoresist materials induced by the plasma etching process. In this article, firstly, we have focused on the effects of VUV photons on damages on the 193 ArF photoresist materials by investigating the absolute density of VUV photon fluxes measured in the range of 115nm and 200nm in HBr plasma etching processes. The etching and measurement of VUV photons in the plasma were performed in the plasma beam system for understanding the fundamental reaction on the photoresists. This plasma beam system consists of an upper chamber (plasma chamber) and a lower chamber (process chamber). This equipment provided the beam source with the HBr plasma. The plasma beam system was pumped down to a base pressure of 10-5 Pa prior to the experiment. During the whole experiment, inductively coupled plasma (ICP) was generated by applying a radio-frequency (13.56 MHz) power of 400 W to the low-impedance antenna. Pure HBr, H2 or Ar gas was introduced into the plasma chamber at a flow rate of 1 sccm. The pressure of the plasma chamber was maintained at approximately 5 Pa. The chemical compositions of the surface of photoresists before and after the plasma beam irradiation were analyzed by in-situ XPS. On the basis of measured results, effects by VUV photons emitted from HBr on the surface modifications of photoresist materials were clarified. The VUV photons cleaved the MAA groups below the photon dosage of 16 × 1016 photons/cm2. The crosslinking of the photoresist and release of volatile products simultaneously generated the surface roughness. The reaction became saturated with high dosages [1]. Secondary, the sequential irradiation by VUV/radicals from HBr or H2 plasma and Ar ions on a photoresist have been performed. The irradiation by VUV and radicals before the ion irradiation (pre-treatment) suppressed the roughness increase by the ion bombardment but the VUV and radical irradiation after the ion irradiation (post-treatment) increased the roughness. The surface morphologies were also analyzed in detail after pre- or post-treatment on the photoresist by using the power spectral density. Such pre-treatment processes generated a dense and hard crosslinked layers which improved the etch resistance of the photoresist and improved the roughness [2]. The enhancement of roughness by post-treatment was correlated with the removal of the MAA groups and the nonuniformity of crosslinking reactions on the roughened photoresist by the ion irradiation. On the basis of these results, the mechanism of photoresist modifications by individual effects of VUV, ions and radicals and their synergetic effects by sequential irradiation experiments were systematically discussed and then perspectives for the damage less processes to be established in the etching nano processes are introduced. [1] Y. Zhang et al., Journal of Vacuum Science & Technology A, 35, 060606 (2017). [2] Y. Zhang et al., Japanese Journal of Applied Physics, 56, 126503 (2017).

Organic Aerosols in Anoxic and Oxic Atmospheres of Earth-like Exoplanets: VUV-MIR Spectroscopy of CHON Tholins

Organic hazes can drastically impact the characterization and detection of exoplanet atmospheres. We explore the effects of the transition from an anoxic to oxic atmosphere on the molecular structure of organic aerosols, by producing aerosols in N2:CO2:CH4-rich plasmas with increasing CO2/CH4 gas ratios. We performed a spectroscopic study on the resulting aerosol analogs, CHON-rich tholins, from the vacuum-ultraviolet (VUV) to the mid-infrared (MIR), i.e., the 0.13–10 μm range. VUV spectroscopy revealed the presence of π–π* and n–π* transitions in the 200–500 nm range in all samples. These are attributed to electronic transitions in amine groups. As the CO2/CH4 ratio increases, new bands emerge that can be attributed to electronic transitions in hydroxyl and carboxyl functional groups. MIR spectroscopy showed that the molecular structure of oxidized aerosols is dominated by the CO2/CH4 ratio. Band deconvolution revealed the oxygenation of the organic matrix via the formation of oxygenated functional groups, including amide, hydroxyl, and carbonyl groups. For the most oxidized aerosols, absorption is greatest in the 0.13–0.3 μm and 6–10 μm regions. Some of the oxidized samples were further irradiated with VUV photons, resulting in the formation of π–π* bands in the 200–250 nm region and a decrease in the band intensities below 200 nm, attributed to amine and nitrile losses. We discuss how oxidized aerosols could provide a detectable (bio-)signature in exoplanet atmospheres.

Carbon and oxygen isotopic fractionation in the products of low-temperature VUV photodissociation of carbon monoxide

The carbon and oxygen isotope fractionations occurring during photodissociation of carbon monoxide (CO) by vacuum ultraviolet photons (91–107 nm) at 80 K were measured and the isotopic fractionations due to direct and indirect predissociation processes individually quantified. The isotopic fractionations depend on the photodissociation wavelength. Slope values (δ'17O/δ'18O) in oxygen three-isotope space range from 0.75 to 1.1. The isotopic composition of the products depends on the dissociation dynamics at the upper electronic state (perturbation and coupling associated with that state), which in turn modulates the isotope effect (in this case an enrichment of minor isotopes) inside the gas column due to the saturation of major isotopologue (isotope self-shielding). An explanation in terms of isotope self-shielding would require a quantum yield of one for photodissociation of all isotopologues which is not consistent with the data.

Response of photomultiplier tubes to xenon scintillation light

We present the precision calibration of 35 Hamamatsu R11410-22 photomultiplier tubes (PMTs) with xenon scintillation light centred near 175 nm. This particular PMT variant was developed specifically for the LUX-ZEPLIN (LZ) dark matter experiment. A room-temperature xenon scintillation cell coupled to a vacuum cryostat was used to study the full-face PMT response at both room and low temperature ( ∼ −100 °C), in particular to determine the quantum efficiency (QE) and double photoelectron emission (DPE) probability in LZ operating conditions. For our sample with an average QE of (32.4 ± 2.9)% at room temperature, we find a relative improvement of (17.9 ± 5.2)% upon cooling (where uncertainty values refer to the sample standard deviation). The mean DPE probability in response to single vacuum ultraviolet (VUV) photons is (22.6 ± 2.0)% at low temperature; the DPE increase relative to room temperature, measured here for the first time, was (12.2 ± 3.9)%. Evidence of a small triple photoelectron emission probability ( ∼ 0.6%) has also been observed. Useful correlations are established between these parameters and the QE as measured by the manufacturer. The single VUV photon response is also measured for one ETEL D730/9829QB, a PMT with a more standard bialkali photocathode used in the ZEPLIN-III experiment, for which we obtained a cold DPE fraction of (9.1 ± 0.1)%. Hence, we confirm that this effect is not restricted to the low-temperature bialkali photocathode technology employed by Hamamatsu. This highlights the importance of considering this phenomenon in the interpretation of data from liquid xenon scintillation and electroluminescence detectors, and from many other optical measurements in this wavelength region.

VUV photon emission from Ne clusters of varying sizes following photon and photoelectron excitations

Vacuum ultraviolet (VUV) fluorescence emission from neon clusters of different sizes is investigated after excitation with photons of energies between 34 and 49 eV, i.e. near and far below the Ne 2s-electron photoionization threshold. Undispersed VUV fluorescence (<120 nm) excitation functions in the Ne 2s-regime display a series of distinct features attributed to 2s → np resonant cluster excitations with subsequent cluster-specific decays. Features connected with resonant interatomic Coulombic decay are visible for all cluster sizes. For larger clusters they appear to be less prominent due to additional emissions. This emission has a threshold energy of 35.8 eV and increases with exciting-photon energy. It also increases with increasing cluster size and is interpreted as being caused by inelastically scattered 2p-photoelectrons (photoelectron impact induced luminescence).