Photoionization Technique Research Articles

Analysis of Trapping Effects on AlInN/GaN High Electron Mobility Transistors with Pulsed Electrical Measurements Under Visible and Infrared Illumination

The aim of this article is to show that it is possible to rapidly estimate the activation energies of electron traps in AlInN/GaN transistors operating at room temperature by combining pulsed electrical measurements with photoionization techniques using infrared illumination. This technique avoids the time‐consuming task of performing electrical measurements at different temperatures to determine the activation energies of the electron traps using Arrhenius laws. Thus, a deep level at 1.3 eV is detected and attributed to the presence of carbon in GaN buffer of the component. Moreover, we have highlighted that the trapping effect can be screened when the transistors are biased with a high drain‐source voltage during pulsed measurements.

Thermal desorption from zeolite layer for VOC detection

Volatile organic compounds (VOCs) are being considered for a wide range of applications, since they provide information on specific processes (e.g., metabolic processes in humans and plants). To date, environmental VOCs detection can be accomplished with low selectivity using both portable photoionization techniques or using gold standard laboratory techniques (e.g., gas and liquid chromatography, mass spectrometry), getting higher performances and limitations in terms of portability, availability of equipment and of consumption of time. Hereafter, a modified photoionization technique based on a thin zeolite layer for the selective adsorption of VOCs was investigated, exploiting the processes of thermal adsorption and desorption. Zeolite through the inherent nanoporous structure possesses specific electrical characteristics, strongly influenced by the adsorption of the molecules within the framework. Experimental analysis of its electrical and adsorption/desorption properties of propionic and succinic acids was performed. Results demonstrated how the use of a thin nanoporous layer allows to increase the selectivity of commercial photoionization detector during VOC detection, evidencing the potentiality of this technique for the development of low-cost and high-performance sensors in various applications.

Luminescence Characteristics of the MOCVD GaN Structures with Chemically Etched Surfaces.

Gallium nitride is a wide-direct-bandgap semiconductor suitable for the creation of modern optoelectronic devices and radiation tolerant detectors. However, formation of dislocations is inevitable in MOCVD GaN materials. Dislocations serve as accumulators of point defects within space charge regions covering cores of dislocations. Space charge regions also may act as local volumes of enhanced non-radiative recombination, deteriorating the photoluminescence efficiency. Surface etching has appeared to be an efficient means to increase the photoluminescence yield from MOCVD GaN materials. This work aimed to improve the scintillation characteristics of MOCVD GaN by a wet etching method. An additional blue photo-luminescence (B-PL) band peaking at 2.7-2.9 eV and related to dislocations was discovered. This B-PL band intensity appeared to be dependent on wet etching exposure. The intensity of the B-PL was considerably enhanced when recorded at rather low temperatures. This finding resembles PL thermal quenching of B-PL centers. The mechanisms of scintillation intensity and spectrum variations were examined by coordinating the complementary photo-ionization and PL spectroscopy techniques. Analysis of dislocation etch pits was additionally performed by scanning techniques, such as confocal and atomic force microscopy. It was proved that this blue luminescence band, which peaked at 2.7-2.9 eV, is related to point defects those decorate dislocation cores. It was shown that the intensity of this blue PL band was increased due to enhancement of light extraction efficiency, dependent on the surface area of either single etch-pit or total etched crystal surface.

Direct observation of the Yb(4f136s2)F states and accurate determination of the YbF ionization energy

YbF has been identified as a molecule that can be used to investigate charge-parity symmetry violations that are beyond the Standard Model of particle physics. Cooling to sub-milli-Kelvin is advantageous for experiments that probe manifestations of these symmetry violations. One approach involves laser cooling of YbF via the A2P1/2-X2S+ transition. However, it appears that cooling by means of this transition may be limited by the radiative loss of population from the cooling cycle. YbF has low-energy states that arise from the Yb+(4f136s2)F- configuration. Recent theoretical calculations predict (Zhang et al J. Mol. Spectrsc. 386 11625 (2022)) that radiative decay from A2{\Pi}1/2 to the 4f136s2 states occurs with a branching fraction of approximately 10-3. In the present study we have used dispersed laser induced fluorescence spectroscopy to the observe the lowest energy 4f136s2 states. These measurements were carried out using excitation of previously unobserved YbF transitions in the near UV spectral range. An accurate ionization energy (IE) for YbF is also reported. A two-color photoionization technique was used to determine the IE and observe the v+=0-3 vibrational levels of YbF+ X1S+.

Trapping Effect in AlInN/GaN HEMTs: A Study Based on Photoionization and Pulsed Electrical Measurements

The aim of this article is to detect electron traps in AlInN/GaN transistors operating at room temperature by combining pulsed electrical measurement with photoionization techniques to rapidly assess their activation energies and time constants. In addition, this technique can also reveal the presence of electron traps that cannot be observed by using pulsed measurements alone. Thus, two electron traps were identified including a deep level whose origin could be related to dislocations in the GaN buffer existing in the devices. At the same time, this study has shown that the time constants of these electron traps are inferior to 400 ns and that the electrical behavior of the components is also degraded by the presence of surface states with a time constant of 4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> . Moreover, these two traps are at the origin of the gate lag effects observed during the pulsed electrical characterization of the AlInN/GaN high electron mobility transistors (HEMTs). Likewise, a negative output conductance induced by a trapping effect has been put forward.

Solvent and Flow Rate Effects on the Observed Compositional Profiles and the Relative Intensities of Radical and Protonated Species in Atmospheric Pressure Photoionization Mass Spectrometry.

Sample preparation and instrument parameters have regularly been demonstrated to impact upon the observed results in atmospheric pressure photoionization, mass spectrometry (MS), and analytical techniques in general but may be overlooked when such methods are applied to the characterization of real-world samples. An initial investigation into different solvent systems demonstrated that the inclusion of ethyl acetate inverted the ratio of relative intensities of radical and protonated species (R/P). Design of experiments was performed and indicated that the injection flow rate is also a significant factor. The impact of the solvent system and flow rate on signal intensity, the observed compositional profile, and R/P of selected molecular groups is demonstrated further. An inversion of R/P is observed at higher flow rates in solvent systems commonly used in petroleomics studies, effecting a loss of molecular speciation. The findings presented reiterate the critical importance in considering experimental parameters when interpreting the results of analytical procedures.

Isotope-selective laser ablation ion-trap loading of Ba+137 using a BaCl2 target

The $^{133}\mathrm{Ba}^+$ ion is a promising candidate as a high-fidelity qubit, and the $^{137}\mathrm{Ba}^+$ isotope is promising as a high-fidelity qudit ($d>2$). Barium metal is very reactive, and $^{133}\mathrm{Ba}^+$ is radioactive and can only be sourced in small quantities, so the most commonly used loading method, oven heating, is less suited for barium, and is currently not possible for $^{133}\mathrm{Ba}^+$.Pulsed laser ablation solves both of these problems by utilizing compound barium sources, while also giving some distinct advantages, such as fast loading, less displaced material, and lower heat load near the ion trap. Because of the relatively low abundances of the isotopes of interest, a two-step photoionization technique is used, which gives us the ability to selectively load isotopes. Characterization of the ablation process for our $\mathrm{BaCl}_2$ targets are presented, including observation of neutral and ion ablation-fluence regimes, preparation/conditioning and lifetimes of ablation spots, and plume velocity distributions.We show that using laser ablation on $\mathrm{BaCl}_2$ salt targets with a two-step photoionization method, we can produce and trap barium ions reliably. Further, we demonstrate that with our photoionization method, we can trap $^{137}\mathrm{Ba}^+$ with an enhanced selectivity compared to its natural abundance.

Isomer-Selective Spectroscopy and Dynamics of Phenol-Arn (n ≤ 5) Clusters.

Structures and ionization-induced solvation dynamics of phenol-(argon)n clusters, PhOH-Arn (n ≤ 5), were studied by using a variety of isomer-selective photoionization and vibrational spectroscopic techniques. Several higher-energy isomers were found and assigned for the first time by systematically controlling the experimental conditions of the supersonic expansion. This behavior is also confirmed for the PhOH-Kr2 cluster. Solvation structures are elucidated by evaluating systematic shifts in the S1 ← S0 origin and ionization energies obtained by resonance-enhanced photoionization, in addition to the OH stretching frequency obtained by IR photodissociation. Isomer-selective picosecond time-resolved IR spectroscopy for the n = 2 clusters revealed that the dynamics for the ionization-induced intermolecular π → H site-switching reaction strongly depends on the initial isomeric structure. In particular, the reaction time for the (1|1) isomer is 7 ps, while that for (2|0) is <3 ps. This difference shows that the switching time is determined by the distance of the reaction coordinate between the initial π-site and the final OH-site.

Pyrolysis investigation of n-propylamine with synchrotron photoionization and molecular-beam mass spectrometry

With biofuels market expanding, amines, as an important compound class in biofuels, are attracting increasing attention. The pyrolysis of n-propylamine (NPA) was studied in a flow tube using tunable synchrotron photoionization and molecular-beam mass spectrometry techniques. 28 and 30 species, including nitrogenous compounds and hydrocarbons, were identified and quantified at 30 and 760 Torr, respectively. 1,3-Cyclopentadiene, 1,4-pentadiene, 2-methyl-1,3-butadiene and benzene, were newly detected compared with previous studies of methylamine and ethylamine. HCN, NH3, N2 and methanimine were detected as the dominant nitrogenous products. Moreover, minor aliphatic and unsaturated amines were also identified, such as methylamine, ethylamine and acetaldimine. Ethylene achieved at the highest level among hydrocarbon products. With the pressure increasing, the onset temperature of NPA pyrolysis shifts towards low temperature region and NPA decomposes faster. A comprehensive detailed kinetic model involving 405 species and 2668 reactions was developed with reasonable predictions. According to rate-of-production analysis, H-abstractions, especially at Cα position of NPA, are the major consumption pathways of NPA. In addition, unimolecular reaction breaking Cα-Cβ bond is the important consumption channel of NPA at 30 Torr, while its positive effect is seriously weakened at 760 Torr. The H-abstraction at Cα (NPA+HCH3CH2CHNH2+H2) plays a leading role for NPA consumption at 760 Torr. The reactions, CH3+CH3=C2H5+H and CH3CHCHNH2+HCH2CHCHNH2+H2, tend to present inhibiting effects on NPA consumption at 30 and 760 Torr, respectively. These results could provide a theoretical basis for further combustion kinetic studies of biofuels and amine fuels.

Experimental study of nitropropane pyrolysis with molecular-beam mass spectrometry and tunable synchrotron VUV photoionization. Part II. The flow reactor pyrolysis of 2-nitropropane

An experimental study of 2-nitropropane (2-NP) pyrolysis was performed in a flow reactor at low pressure (4.0 kPa) and in the temperature range of 523–923 K. The pyrolysis species were detected with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS) techniques. Species from m/z = 15 to 87, such as radicals of CH3, C2H5, C3H5 and C3H7; isomers of C3H4 (allene and propyne), C5H8 (1,3-pentadiene and cyclopentene), were identified from near-threshold photoionization efficiency (PIE) spectra. Four nitrogenous intermediates never reported in the homogeneous pyrolysis of 2-NP (i.e., 2-propen-1-imine, 2-propen-1-amine, acetamide and 2-nitropropene) were identified in this work. The mole fraction profiles of the pyrolysis species were evaluated at different temperatures. Based on the experimental findings, the pyrolysis pathways of 2-NP were discussed.

Experimental and kinetic modeling study of benzyl alcohol pyrolysis

The pyrolysis of benzyl alcohol (A1CH2OH) at 30 and 760 torr was studied in a flow tube reactor. The synchrotron radiation photoionization and molecular-beam mass spectrometry (MBMS) techniques were used to identify and quantify 35 intermediates and products including some small species, several monophenyl ring species, and a large number of polyaromatic hydrocarbons (PAHs). A comprehensive chemical kinetic reaction model involving 376 species and 2171 reactions was developed with reasonable prediction. According to the rate-of-production analysis, the consumption of A1CH2OH is mainly proceeded with the H-abstraction reactions with H atoms and OH radicals under both pressures. H-abstractions and unimolecular dissociation dehydrogenation reactions also play important roles in the subsequent formation of PAHs. Phenyl radicals, benzene (A1), and benzyl (A1CH2) radicals are important intermediates in the pyrolysis of A1CH2OH, which provide various ways for the formation of PAHs. Sensitivity analysis presents that the most significant sensitive reaction of A1CH2OH consumption is the unimolecular initiation reaction by the CO bond breaking from A1CH2OH under both pressures. The combination of A1CH2 radicals with H atoms to form toluene (A1CH3) has the most inhibiting influence on the A1CH2OH consumption under both pressures, while it has little sensitive effects for the oxidation process of A1CH2OH consumption. The comparison of the A1CH2OH pyrolysis and oxidation results show that the peak mole fractions of A1 increase, while the peak mole fractions of phenol (A1OH) decrease with the increase of equivalence ratio (φ) under the same pressure. It should be noted that the peak mole fractions of benzaldehyde (A1CHO) are relatively stable with various conditions. These results provide a theoretical basis for further study of the combustion chemical kinetics of A1CH2OH and its application in bio alternative fuel.

Femtosecond ionization mass spectrometry for chromatographic detection

Mass spectrometry is now in widespread use for the detection of the analytes separated by chromatography. Electron ionization is the most frequently used method in mass spectrometry. However, this ionization technique sometimes suffers from extensive fragmentation of analytes, which makes identification difficult. A photoionization technique has been developed for suppressing this fragmentation and for subsequently observing a molecular ion. A variety of lasers have been employed for the sensitive and selective ionization of organic compounds. A femtosecond laser has a high peak power and is preferential for efficient ionization as well as for suppressing fragmentation, providing valuable information concerning molecular weight and chemical structure as well. In this review, we report on applications of femtosecond ionization mass spectrometry combined with gas chromatography.

Pyrolysis study of a three-component surrogate jet fuel

The pyrolysis of three-component surrogate fuel for jet fuel has been studied experimentally in flow reactor using synchrotron photoionization and molecular beam mass spectrometry techniques with temperature range of 850–1150 K. Alkenes are the most abundant products in the decomposition process. Other important intermediates such as alkanes, alkynes, polycyclic aromatic hydrocarbons were also identified and quantified. Detailed kinetic reaction model involving 462 species and 3170 reactions have been developed by validating against the measured results as well as oxidation data reported previously with reasonable predictions. Detailed rate of production and sensitivity analysis indicated T135MCH mainly decay through demethylation and H-abstraction reactions. Moreover, n-propylbenzene consumption is more sensitive to CH 3 than H, while n-dodecane and T135MCH prefer H radical. The reactions between fuel and fuel-derived radicals show limited effect on surrogate fuel consumption. Reactivity changes of NPB and NC 12 H 26 were investigated through the comparison of fuel conversion ratio. The coupling effect was displayed via the common and important sensitive reactions which are related to the H and CH 3 radicals. Present experimental data and reaction model will contribute to a better understanding of combustion behavior of 3C surrogate fuel. Thus, the results of present work could contribute to comprehensive investigation of jet fuel combustion properties.

Photo-Assisted Corona-Charge Characterization of Wide Bandgap Interfaces with Deep Traps Invisible in Standard C-V

Wide bandgap semiconductor technology has been generating a great deal of attention due to its fundamental advantages in high power electronics. Understanding and effective control of interfacial properties belong to a group of critical issues requiring progress. In this work, we report progress in wide bandgap interface characterization, achieved using photo-ionization of deep traps under a non-equilibrium condition created by corona-charge bias in deep depletion. This characterization capability is demonstrated on oxidized n-type epitaxial SiC with deep interfacial traps invisible in standard C-V. These traps, initially present at high density, are shown to be reduced by half after a wet anneal. The photo-ionization technique is incorporated in commercially available non-contact C-V (CnCV) metrology [1,2] providing a non-invasive, cost and time saving metrology that benefits development research as well as device fabrication.

Spectroscopic and theoretical studies of UN and UN.

The low-energy electronic states of UN and UN+ have been examined using high-level electronic structure calculations and two-color photoionization techniques. The experimental measurements provided an accurate ionization energy for UN (IE = 50 802 ± 5 cm-1). Spectra for UN+ yielded ro-vibrational constants and established that the ground state has the electronic angular momentum projection Ω = 4. Ab initio calculations were carried out using the spin-orbit state interacting approach with the complete active space second-order perturbation theory method. A series of correlation consistent basis sets were used in conjunction with small-core relativistic pseudopotentials on U to extrapolate to the complete basis set limits. The results for UN correctly obtained an Ω = 3.5 ground state and demonstrated a high density of configurationally related excited states with closely similar ro-vibrational constants. Similar results were obtained for UN+, with reduced complexity owing to the smaller number of outer-shell electrons. The calculated IE for UN was in excellent agreement with the measured value. Improved values for the dissociation energies of UN and UN+, as well as their heats of formation, were obtained using the Feller-Peterson-Dixon composite thermochemistry method, including corrections up through coupled cluster singles, doubles, triples and quadruples. An analysis of the ab initio results from the perspective of the ligand field theory shows that the patterns of electronic states for both UN and UN+ can be understood in terms of the underlying energy level structure of the atomic metal ion.

Spectroscopy of defects in neutron irradiated ammono-thermal GaN by combining photoionization, photoluminescence and positron annihilation techniques

In this work, pulsed photoionization as well as photoluminescence and positron annihilation spectroscopy were combined to detect different species of defects. The GaN crystals, grown by the ammono-thermal method, doped with Mn as well as Mg impurities and irradiated with different fluences of reactor neutrons, were examined to clarify the role of the technological and radiation defects. The evolution of the prevailing photoactive centres was examined by pulsed photoionization spectroscopy. Positron annihilation spectroscopy was applied to reveal vacancy-type defects.

Investigation of high-lying even-parity levels of atomic samarium with multi-color photoionization technique: Energies and radiative lifetimes

We have investigated high-lying even-parity energy levels of atomic samarium in the energy region 32,156.3–32,886.4 cm−1 using two-color three-step photoionization technique. The experiments have been carried out in an atomic beam coupled to a linear time of flight mass spectrometer. The analysis of the photoionization spectra has resulted in the identification of twenty-five high-lying even-parity energy levels of atomic samarium. These include the confirmation of fifteen energy levels reported earlier while the remaining ten energy levels are new. The observed relative photoionization signal intensities of the corresponding transitions are also reported. The possible total angular momenta of the newly discovered energy levels have been assigned by the total angular momentum selection rules. Further, the radiative lifetimes of twenty high-lying even-parity energy levels of atomic samarium were measured for the first time by delayed three-color three-photon resonance ionization spectroscopy via a pump-probe method.

Pyrolysis study of iso-propylbenzene with photoionization and molecular beam mass spectrometry

Pyrolysis study of iso-propylbenzene (IPB) at 30 and 760 torr was carried out by using synchrotron photoionization and molecular beam mass spectrometry techniques. 0.5%IPB was diluted by helium and heated to 798–1148 K in a flow tube. Styrene was the most abundant aromatic intermediate at both pressures. Several polycyclic aromatics were only observed at 760 torr, such as 2-ethenyl-1, 4-dimethyl-benzene, fluorene, anthracene, phenanthrene, 2-methyl-anthracene and pyrene, while fulvene, 1, 3-cyclohexadiene, and 5-ethenylidene-1, 3-cyclopentadiene were only detected at 30 torr. A detail mechanism involving 306 species and 1990 reactions was developed by updating ipso-addition and unimolecular dissociation reactions of IPB as well as reactions related to styrene, indene and naphthalene. The mechanism could predict the pyrolysis of IPB and production of other intermediates well. Rate-of-production analysis shows that the formations of 2-phenethyl, 1-iso-phenylpropyl and 2-iso-phenylpropyl radicals are the main consumption reactions of IPB independent of pressure. Sensitivity analysis indicates that the unimolecular dissociation of IPB forming 2-phenethyl radical is the most significant promoting reaction at both pressures. H-abstraction reaction forming 1-iso-phenylpropyl radical tends to play a significant inhibiting effect on IPB consumption at 30 torr, while the most inhibiting reaction is self-combination of methyl radicals at 760 torr. Compared to n-propylbenzene experiments reported by Liu et al. (Combustion and Flame, 191 (2018) 53–65) and Yuan et al. (Combustion and Flame,186 (2017) 178–192.), IPB tends to produce larger quantities of benzene, styrene and indene under pyrolysis and oxidation conditions. These results will improve the understanding of the combustion and soot formation of IPB as a potential aviation surrogate fuel.

Enhancement of photoionization by applying polarization-based common level excitation scheme for the selective photoionization of odd isotopes of samarium

Due to the importance of 149Sm isotope in the nuclear industry, isotope-selective photoionization of the odd isotopes of samarium has been studied using multi-color photoionization technique. The present work has been carried out in an atomic beam coupled to a linear time-of-flight mass spectrometer using multimode dye lasers. A new photoionization scheme: 292.58 cm−1 →17,769.71 cm−1→34,929.9 cm−1→ continuum with the J-value sequence 1-1-0-continuum has been identified for the enrichment of only odd isotopes using linearly polarized lasers in the same direction. This photoionization scheme in combination with the first-step transition: 0 cm−1 →17,769.71 cm−1 (J = 0→J = 1) has been utilized to realize a common level excitation scheme which has significantly increased the photoionization yield of the odd isotopes and suppressed the ionization of the even isotopes of samarium using suitable choice of laser polarization. To enhance the photoionization yield further, a search for the third-step transition terminating into an autoionization level has been carried out. A strong autoionization resonance at the third-step laser wavelength 644.14 nm, corresponding to the transition 34,929.9 cm−1→ 50,450.3 cm−1 has been identified. The J-value of the newly discovered autoionization level at 50,450.3 (±0.5) cm−1 has been uniquely assigned as J = 1. Finally, the photoexcitation/photoionization cross sections of all the transitions involved in the common level excitation scheme including that of the third-step transition terminating into the autoionization level have been measured by saturation method.

Experimental and kinetic study on the low-temperature oxidation of pyridine as a representative of fuel-N compounds

The low-temperature oxidation (LTO) of pyridine was studied in a jet-stirred reactor over the temperature range of 700–1000 K at atmospheric pressure and equivalence ratio of 2.0. Mole fraction profiles of the reaction products were obtained based on molecular beam mass spectrometry and tunable vacuum ultraviolet synchrotron photoionization techniques. Hydrogen peroxide, methanamine, acetylenamine, ethenamine, acetaldimine, ethylamine, allyamine, and methylformamide were newly identified compared with previous studies of pyridine flame and pyrolysis. HCN was found to be the dominant N-containing species of pyridine LTO. Pyrrole, acrylonitrile, acetonitrile, and ammonia were also found at the same level of N2O and NO. Based on the new measurements and updated rate constants of several reactions including the H-abstractions of pyridine as well as the oxidation of ortho-pyridyl using density functional theory calculations, a new pyridine LTO kinetic model consisting of 588 species and 3516 reactions was developed with a reasonable agreement with the experimental results. In general, the predictions of the predominant species have been improved compared with the existing model. Rate-of-production analysis indicates that pyridine mainly consumes via C5H5N→C5H4N→C5H4NO2→HCN+CO+CH2CHC˙O, and C5H5N→C5H5NO→C2H2+HCN+CH2CO. Sensitivity analysis shows that C5H4N+O2=>C5H4NO2, and C5H5N+OHC5H4N+H2O have significant promoting effect on pyridine consumption, while the reverse of C5H4N+HO2C5H4NO+OH has strong inhibiting effect. The results will enrich the understanding of pyridine low-temperature oxidation mechanism, which can be applied to the fields of coal pre-treatment, staged combustion and mild combustion.