Drift Tube Research Articles

Detecting Hexafluoroisopropanol Using Soft Chemical Ionization Mass Spectrometry and Analytical Applications to Exhaled Breath

Here we explore the potential use of proton transfer reaction/selective reagent ion-time-of-flight-mass spectrometry (PTR/SRI-ToF-MS) to monitor hexafluoroisopropanol (HFIP) in breath. Investigations of the reagent ions H3O+, NO+, and O2+• are reported using dry (relative humidity (rH) ≈ 0%) and humid (rH ≈ 100%)) nitrogen gas containing traces of HFIP, i.e., divorced from the complex chemical environment of exhaled breath. HFIP shows no observable reaction with H3O+ and NO+, but it does react efficiently with O2+• via dissociative charge transfer resulting in CHF2+, CF3+, C2HF2O+, and C2H2F3O+. A minor competing hydride abstraction channel results in C3HF6O+ + HO2• and, following an elimination of HF, C3F5O+. There are two issues associated with the use of the three dominant product ions of HFIP, CHF2+, CF3+, and C2H2F3O+, to monitor it in breath. One is that CHF2+ and CF3+ also result from the reaction of O2+• with the more abundant sevoflurane. The second is the facile reaction of these product ions with water, which reduces analytical sensitivity to detect HFIP in humid breath. To overcome the first issue, C2H2F3O+ is the ion marker for HFIP. The second issue is surmounted by using a Nafion tube to reduce the breath sample's humidity prior to its introduction into drift tube. The success of this approach is illustrated by comparing the product ion signals either in dry or humid nitrogen gas flows and with or without the use of the Nafion tube, and practically from the analysis of a postoperative exhaled breath sample from a patient volunteer.

Shell effect driven fission modes in fragment mass and total kinetic energy distribution of Hg*192

Background: Asymmetric fission in sublead nuclei, especially mercury and platinum isotopes, has generated keen interest in studying fission in $A<200$ mass region. The role of proton and neutron shells on asymmetric fission in these nuclei is still under test and requires more investigation.Purpose: This paper presents measurements aimed at studying the fission modes of $^{192}\mathrm{Hg}$ formed in the reaction $^{32}\mathrm{S}+^{160}\mathrm{Gd}$ in the excitation energy range 54--74 MeV.Method: Mass distributions have been determined from the fission fragments' (FF) time of flight (TOF). Two multiwire proportional chambers were placed symmetrically on opposite sides of the beam to cover the folding angle for symmetric fission. The measured TOF of the fission fragments was used to obtain their velocities, and after clean separation of compound nuclear fission events, the velocities were further used to obtain the fission fragment mass and total kinetic energy (TKE) distribution.Results: The fission fragment mass distributions at all beam energies were found to be characteristically flat topped, which deviates significantly from a single Gaussian behavior. The distinctive features of mass and energy distributions happen to be explained by the presence of a symmetric fission mode and three asymmetric fission modes, manifested by different total kinetic energies in varying mass regions. These asymmetric fission modes are a consequence of the stabilization role of proton shells at $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 36, $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 46, and $Z$ = 28/50. The most probable mass of light and heavy fragments are found to be around 86 and 106 u, respectively. The symmetric yield is found to increase with the excitation energy.Conclusions: The present study indicates a mixture of symmetric and asymmetric modes in the fission of $^{192}\mathrm{Hg}$ nuclei and points out the relevance of deformed proton shells at $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 36 and 46.

A Comparison of the Performance of Modular Standalone Do-It-Yourself Ion Mobility Spectrometry Systems.

As the spectrum of ion mobility spectrometry (IMS) applications expands and more experimental configurations are developed, identifying the correct platform for an experimental campaign becomes more challenging for researchers. Additionally, metrics that compare performance (Rp, for example) often have nuanced differences in definition between platforms that render direct comparisons difficult. Here we present a comparison of three do-it-yourself (DIY) drift tubes that are relatively low cost and easy to construct, where the performance of each is evaluated based on three different metrics: resolving power, the ideality of resolving powers, and accuracy/precision of K0 values. The standard PCBIMS design developed by Reinecke and Clowers (Reinecke, T.; Clowers, B. H. HardwareX 2018, 4, e00030) provided the highest resolving power (>90) and the highest ideality of resolving power ratios (>90% at best) of the three systems. However, the flexible tube (FlexIMS) construction as described by Smith et al. (Smith, B. L. Anal. Chem. 2020, 92 (13), 9104-9112) exhibited the highest degree of precision of K0 values (relative standard deviation of <0.42%). Depending on the application, the drift tube variants presented and evaluated here offer a low-cost alternative to commercial drift-tube systems with levels of performance that approach theoretical maxima.

Track-based alignment for the BESIII CGEM detector in the cosmic-ray test

The Beijing Electron Spectrometer III (BESIII) is a multipurpose detector operating on the Beijing Electron Positron Collider II (BEPCII). After more than ten years operation, the efficiency of the inner layers of the Main Drift Chamber (MDC) decreased significantly. To solve this issue, the BESIII collaboration is planning to replace the inner part of the MDC with three layers of Cylindrical triple Gas Electron Multipliers (CGEM).The transverse plane spatial resolution of CGEM is required to be 120μm or better. To meet this goal, a careful calibration of the detector is necessary to fully exploit the potential of the CGEM detector. In all the calibrations, the detector alignment plays an important role to improve the detector precision. The track-based alignment for the CGEM detector with the Millepede algorithm is implemented to reduce the uncertainties of the hit position measurement. Using the cosmic-ray data taken in 2020 with the two layers setup, the displacement and rotation of the outer layer with respect to the inner layer is determined by a simultaneous fit applied to more than 160000 tracks. A good alignment precision has been achieved that guarantees the design request could be satisfied in the future. A further alignment is going to be performed using the combined information of tracks from cosmic-ray and collisions after the CGEM is installed into the BESIII detector.

Evaluation of IMS drift tube temperature on the peak shape of high boiling fragrance compounds towards allergen detection in complex cosmetic products and essential oils

Gas chromatography-ion mobility spectrometry (GC-IMS) has recently gained increasing attention for the analysis of volatile compounds due to its high sensitivity, selectivity, and robust design. Peak shape distortion, including peak tailing or broadening, are well known challenges in chromatographic analysis that result in peak asymmetry and decreased resolution. However, in IMS analysis peak tailing, which is independent on the column separation technique, have also been observed. As high boiling substances, such as monoterpenes, are mainly affected by enlarged peak tailing in GC-IMS, we propose that condensation or adsorption effects within the “cold” IMS cell, which is commonly operated at 45 °C–90 °C, are the root cause. To avoid condensation and to decrease peak tailing, we used a prototypic high temperature ion mobility spectrometry (HTIMS) in this work, which allows an increase of the IMS drift tube temperature up to 180 °C. This HTIMS was coupled to a GC column separation and used to analyse the peak shape of homologues series of ketones, alcohols, aldehydes, as well as high boiling fragrance compounds, such as monoterpenes and phenylpropanoids. While we were able to show that an increased IMS drift tube temperatures correlates well with improved peak shapes, the GC parameters of the HS-GC-HTIMS method, however, were found to have little effect on the peak shapes in IMS spectra. In particular monoterpenes, which display intense peak tailing at lower IMS drift tube temperatures, show significant improvement of the peak shape at higher IMS drift tube temperatures. This leads to the assumption that high boiling substances indeed undergo condensation effects within the IMS cell at low drift tube temperatures. For many separation tasks, such as the separation of the phenylpropanoids eugenol and isoeugenol, comparably low IMS temperatures of 120 °C are already sufficient to achieve a resolution above 1.5. However, the optimal drift tube temperature is dependent on the substance class. While the aspect ratio increases steadily for most monoterpenes, phenylpropanoids and aldehyde monomer peaks investigated, an optimal aspect ratio was found for ketones between 140 °C and 160 °C and alcohols between 120 °C and 140 °C. Lastly, the change of the reduced mobility K0 with the increase of drift tube temperature was analysed. Compounds with similar chemical structure, such as the alcoholic monoterpenes citronellol and geraniol or the phenylpropanoids eugenol and isoeugenol show similar shifts of the K0 value. Substances which differ in their chemical structure, such as the aldehyde monoterpenes citral and cinnamal have substantially different shifts of the K0 value. With a future large substance database, the temperature dependant slope of the K0 value of a substance could be used to identify the substance groups of unknown molecules. Furthermore, substances with the same drift time but different chemical composition could be separable through a change in drift tube temperature.

Metabolic signature of HepaRG cells exposed to ethanol and tumor necrosis factor alpha to study alcoholic steatohepatitis by LC-MS-based untargeted metabolomics.

Despite the high prevalence of alcoholic liver disease, its identification and characterization remain poor, especially in early stages such as alcoholic fatty liver disease and alcoholic steatohepatitis. This latter implies diagnostic difficulties, few therapeutic options and unclear mechanisms of action. To elucidate the metabolic alterations and pinpoint affected biochemical pathways, alcoholic steatohepatitis was simulated in vitro by exposing HepaRG cells to ethanol (IC10, 368mM) and tumor necrosis factor alpha (TNF-α, 50ng/mL) for 24h. This combined exposure was compared to solely ethanol-exposed as well as -nonexposed cells. Four different metabolomics platforms were used combining liquid chromatography, high-resolution mass spectrometry and drift tube ion mobility to elucidate both intracellular and extracellular metabolic alterations. Some of the key findings include the influence of TNF-α in the upregulation of hepatic triglycerides and the downregulation of hepatic phosphatidylethanolamines and phosphatidylcholines. S-Adenosylmethionine showed to play a central role in the progression of alcoholic steatohepatitis. In addition, fatty acyl esters of hydroxy fatty acid (FAHFA)-containing triglycerides were detected for the first time in human hepatocytes and their alterations showed a potentially important role during the progression of alcoholic steatohepatitis. Ethoxylated phosphorylcholine was identified as a potential new biomarker of ethanol exposure.

Breath-by-breath measurement of exhaled ammonia by acetone-modifier positive photoionization ion mobility spectrometry via online dilution and purging sampling

Exhaled ammonia (NH3) is an essential noninvasive biomarker for disease diagnosis. In this study, an acetone-modifier positive photoionization ion mobility spectrometry (AM-PIMS) method was developed for accurate qualitative and quantitative analysis of exhaled NH3 with high selectivity and sensitivity. Acetone was introduced into the drift tube along with the drift gas as a modifier, and the characteristic NH3 product ion peak of (C3H6O)4NH4+ (K0 = 1.45 cm2/V·s) was obtained through the ion-molecule reaction with acetone reactant ions (C3H6O)2H+ (K0 = 1.87 cm2/V·s), which significantly increased the peak-to-peak resolution and improved the accuracy of exhaled NH3 qualitative identification. Moreover, the interference of high humidity and the memory effect of NH3 molecules were significantly reduced via online dilution and purging sampling, thus realizing breath-by-breath measurement. As a result, a wide quantitative range of 5.87–140.92 μmol/L with a response time of 40 ms was achieved, and the exhaled NH3 profile could be synchronized with the concentration curve of exhaled CO2. Finally, the analytical capacity of AM-PIMS was demonstrated by measuring the exhaled NH3 of healthy subjects, demonstrating its great potential for clinical disease diagnosis.

A high deposition efficiency method for wire arc additive manufacturing

In this study, single wire arc additive manufacturing (SWAAM) and multi wire arc additive manufacturing (MWAAM) were used to fabricate the TC11 alloy. Microstructure and mechanical properties of TC11 alloy were studied. The results showed that the microstructure of MWAAM TC11 alloy composed of α and β phase, and the proportion of β phase increased slightly than that of SWAAM TC11 sample. Compared with the SWAAM TC11 sample, the microhardness of MWAAM TC11 alloy sample increased from 382 to 407 HV, and average ultimate tensile strength of MWAAM TC11 alloy sample increased from 922.01 to 934.56 MPa. The results indicate that the addition of wires can significantly improve both manufacturing efficiency and mechanical properties of TC11 alloy components.

Charged particle reconstruction in CLAS12 using Machine Learning

In this work, we present studies of track parameter reconstruction from raw information in CLAS12 detector's Drift Chambers, using Machine Learning (ML). We study the resolution of tracks reconstructed with different types of ML models/algorithms, including Multi-Layer Perceptron (MLP), Extremely Randomized Trees (ERT) and Gradient Boosting Trees (GBT) using simulated data. The resulting ML model is capable of reconstructing track parameters (particle momentum, and polar and azimuthal angles) with accuracy similar to Hit Based (HB) tracking code, but 150 times faster. Moreover, physics reactions can be identified using the particles reconstructed by the neural network in real-time (with a rate of about 34kHz) during experimental data collection. The developed model can be used in numerous applications, such as triggering specific physics reactions in real-time, detector performance monitoring, and real-time detector calibration.

Molecular and Structural Characterization of Isomeric Compounds in Atmospheric Organic Aerosol Using Ion Mobility-Mass Spectrometry.

Secondary organic aerosol (SOA) formed through multiphase atmospheric chemistry makes up a large fraction of airborne particles. The chemical composition and molecular structures of SOA constituents vary between different emission sources and aging processes in the atmosphere, which complicates their identification. In this work, we employ drift tube ion mobility spectrometry with quadrupole time-of-flight mass spectrometry (IM-MS) detection for rapid gas-phase separation and multidimensional characterization of isomers in two biogenic SOAs produced from ozonolysis of isomeric monoterpenes, d-limonene (LSOA) and α-pinene (PSOA). SOA samples were ionized using electrospray ionization (ESI) and characterized using IM-MS in both positive and negative ionization modes. The IM-derived collision cross sections in nitrogen gas (DTCCSN2 ) for individual SOA components were obtained using multifield and single-field measurements. A novel application of IM multiplexing/high-resolution demultiplexing methodology was employed to increase sensitivity, improve peak shapes, and augment mobility baseline resolution, which revealed several isomeric structures for the measured ions. For LSOA and PSOA samples, we report significant structural differences of the isomer structures. Molecular structural calculations using density functional theory combined with the theoretical modeling of CCS values provide insights into the structural differences between LSOA and PSOA constituents. The average DTCCSN2 values for monomeric SOA components observed as [M + Na]+ ions are 3-6% higher than those of their [M - H]- counterparts. Meanwhile, dimeric and trimeric isomer components in both samples showed an inverse trend with the relevant values of [M - H]- ions being 3-7% higher than their [M + Na]+ counterparts, respectively. The results indicate that the structures of Na+-coordinated oligomeric ions are more compact than those of the corresponding deprotonated species. The coordination with Na+ occurs on the oxygen atoms of the carbonyl groups leading to a compact configuration. Meanwhile, deprotonated molecules have higher DTCCSN2 values due to their elongated structures in the gas phase. Therefore, DTCCSN2 values of isomers in SOA mixtures depend strongly on the mode of ionization in ESI. Additionally, PSOA monomers and dimers exhibit larger DTCCSN2 values (1-4%) than their LSOA counterparts owing to more rigid structures. A cyclobutane ring is present with functional groups pointing in opposite directions in PSOA compounds, as compared to noncyclic flexible LSOA structures, forming more compact ions in the gas phase. Lastly, we investigated the effects of direct photolysis on the chemical transformations of selected individual PSOA components. We use IM-MS to reveal structural changes associated with aerosol aging by photolysis. This study illustrates the detailed molecular and structural descriptors for the detection and annotation of structural isomers in complex SOA mixtures.

A Beam Monitor for Ion Beam Therapy Based on HV-CMOS Pixel Detectors

Particle therapy is a well established clinical treatment of tumors. More than one hundred particle therapy centers are in operation world-wide. The advantage of using hadrons like protons or carbon ions as particles for tumor irradiation is the distinct peak in the depth-dependent energy deposition, which can be exploited to accurately deposit doses in the tumor cells. To guarantee this, high accuracy in monitoring and control of the particle beam is of the utmost importance. Before the particle beam enters the patient, it traverses a monitoring system which has to give fast feedback to the beam control system on position and dose rate of the beam while minimally interacting with the beam. The multi-wire chambers mostly used as beam position monitors have their limitations when a fast response time is required (drift time). Future developments such as MRI-guided ion beam therapy pose additional challenges for the beam monitoring system, such as tolerance of magnetic fields and acoustic noise (vibrations). Solid-state detectors promise to overcome these limitations and the higher resolution they offer can create additional benefits. This article presents the evaluation of an HV-CMOS detector for beam monitoring, provides results from feasibility studies in a therapeutic beam, and summarizes the concepts towards the final large-scale assembly and readout system.

The Analytical Method algorithm for trigger primitives generation at the LHC Drift Tubes detector

The Compact Muon Solenoid (CMS) experiment prepares its Phase-2 upgrade for the high-luminosity era of the LHC operation (HL-LHC). Due to the increase of occupancy, trigger latency and rates, the full electronics of the CMS Drift Tube (DT) chambers will need to be replaced. In the new design, the time bin for the digitization of the chamber signals will be of around 1 ns, and the totality of the signals will be forwarded asynchronously to the service cavern at full resolution. The new backend system will be in charge of building the trigger primitives of each chamber. These trigger primitives contain the information at chamber level about the muon candidates position, direction, and collision time, and are used as input in the L1 CMS trigger. The added functionalities will improve the robustness of the system against ageing. An algorithm based on analytical solutions for reconstructing the DT trigger primitives, called Analytical Method, has been implemented both as a software C++ emulator and in firmware. Its performance has been estimated using the software emulator with simulated and real data samples, and through hardware implementation tests. Measured efficiencies are 96 to 98% for all qualities and time and spatial resolutions are close to the ultimate performance of the DT chambers. A prototype chain of the HL-LHC electronics using the Analytical Method for trigger primitive generation has been installed during Long Shutdown 2 of the LHC and operated in CMS cosmic data taking campaigns in 2020 and 2021. Results from this validation step, the so-called Slice Test, are presented.

Precise simulation of drift chamber in the CEPC experiment

The Circular Electron Positron Collider (CEPC) is one of the future experiments which aims to precisely study the properties of the Higgs boson and to search for new physics beyond the Standard Model. To achieve that, it puts strict requirements on the particle reconstruction and identification ability of the CEPC detector. In order to obtain better particle identification performance, the drift chamber is chosen for the outer tracking detector which can provide information used by the cluster counting method for getting excellent particle identification performance. To evaluate the performance of the cluster counting method, it is necessary to develop a simulation tool that can be used to precisely simulate the response of the gaseous particle detector. This work proposes a new way to combine Geant4 and Garfield++ at the Geant4 step level for the drift chamber simulation. Due to the extreme time consumption of simulating the avalanche process in Garfield++, a fast signal response simulation method based on a neural network has been developed to replace detailed simulation. It shows the fast simulation can give consistent results with Garfield++ simulation and achieve the speedup by a factor of 200.

Using AI to predict calibration constants for the central drift chamber in GlueX at Jefferson Lab

The AI for Experimental Controls project team at Jefferson Lab has developed an AI system to control and calibrate a large drift chamber system in near-real time. The AI system will monitor environmental and experimental variables to recommend voltage settings that maintain consistent dE/dx gain and optimal resolution throughout the experiment. At present, calibrations are performed after data have been recorded and require a considerable amount of time and attention from experts. The calibrations currently require multiple iterations and depend on accurate tracking information. Our approach uses environmental data, such as atmospheric pressure and gas temperature, and beam conditions, such as the flux of incident particles, as inputs to a Gaussian Process Regression (GPR) model. For the data taken during the GlueX 2020 run period, the GPR is able to predict the existing gain correction factors to within 3.5%. This talk will briefly describe the development, testing, and future plans for this system at Jefferson Lab.

Key4hep Software Stack for Detector Studies

Detector optimisation and physics performance studies are an integral part of the development of future collider experiments. The Key4hep project aims to design a common set of software tools for future, or even present, High Energy Physics projects. Based on the iLCSoft and FCCSW frameworks an integrated solution for detector simulation, reconstruction and analyses is being developed. This presentation will give a short overview of the major ingredients of the Key4hep turnkey software; the common event datamodel – EDM4hep, the Gaudi based event processing framework, some dedicated packages developed on top of these, and the use of the Spack package manager, which enables installations on a variety of platforms. A first example highlights the synergies that were already achieved by the collaboration of the different collider communities (CEPC, CLIC, FCC, ILC). It shows the seamless integration of fast simulation with Delphes and the LCFIplus vertexing processor from iLCSoft. This combination is possible due to EDM4hep, the processor wrapper (k4MarlinWrapper), and the k4SimDelphes framework integration in Key4hep. As a second example, the multi-threaded execution of the iLCSoft processors in the Gaudi framework making use of the k4MarlinWrapper will be showcased. The third example will demonstrate the execution of a multi-threaded simulation of a drift chamber in the CEPC experiment. The chosen application will take information of primary ionizations generated by an incident charged particle as input and will create the waveform as the final output, which represents the signal collected by the chamber’s signal wire. The simulation algorithm is based on a neural network model which can be used to simulate the effects from the ionization electron’s drift and avalanches. Using these examples we review some of the challenges and issues that we encountered as well as how they were addressed in the end. We close with a brief discussion of the next steps for the Key4hep project.

Data-driven design of the Belle II track segment finder

The Belle II experiment relies on a level-1 trigger system to reduce noise background and preselect events of interest for particle physics. The Central Drift Chamber is the main track detector which makes its trigger system important for online track reconstruction. To improve its hit efficiency, an extension of the track segment finder for low angle tracks is proposed. By combining hardware and software development flows, an automated data-driven pipeline is created and three different-sized hardware concepts are implemented. The operation point is adjustable to balance hit efficiency against hit purity in the trigger system.

New developments in the MDT trigger processor for the ATLAS Level-0 muon trigger at High Luminosity LHC

The High Luminosity LHC (HL-LHC) will have an instantaneous luminosity 5–7 times higher than LHC. The ATLAS detector, trigger and data acquisition systems will undergo a significant upgrade to cope with the increased particle rate. The upgrade of the ATLAS first-level muon trigger for High-Luminosity LHC foresees incorporating the high-resolution tracking capability of the Monitored Drift Tubes (MDT) in the current system based on Resistive Plate Chambers and Thin Gap Chambers. The MDT Trigger Processor (MDTTP) processes muon trigger candidates along with MDT hits to improve the accuracy of the transverse momentum calculation at the first-level (level-0) of the muon trigger. One of the major challenges is the capability to process all candidates in a bunch crossing within a tight latency constraint. The MDTTP hardware is based on the Apollo ATCA platform [6]. A complete hardware demonstrator is available and an updated prototype has been recently developed. Recent updates in the firmware development, integration and testing are presented.

Simulation investigations of a novel L-band metamaterial relativistic oscillator with higher power and lower guiding magnetic field

In this paper, a novel high-efficiency L-band Metamaterial Relativistic Oscillator (MRO) with high input and output power under relatively low guiding magnetic field has been proposed. In order to increase the power level while decreasing the guiding magnetic field, through our theoretical and simulation investigations on previously designed metamaterial (MTM) structure, we found that the thickness of the ring and the length of the drift tube are the two key factors that can improve the power handling capacity of the MTM structure and field uniformity. Through transmission analysis and electron conductance calculation of the novel MRO based on the improved MTM structure, preliminary MRO design is done. Within CST particle simulation of the proposed MRO and after optimizations, we computationally got a pure 718 MW TE10 output signal at 1.49 GHz using a pulsed electron beam with energy of 700 keV and current of 2 kA under 1 T guiding magnetic field, and the corresponding conversion efficiency is 51.3%. Relevant results of particle simulation verified the previous high frequency analysis. A comparison between our proposed MRO and the previous one reveals that while keeping the high efficiency and miniaturization merits of MRO, the power handling capacity has been increased and the novel MRO works more stable under low guiding magnetic field.

Scanning drift tube measurements and kinetic studies of electron transport in CO

We present scanning drift tube measurements of electron swarm transport coefficients in CO as a function of the reduced electric field at room temperature under time-of-flight conditions. The measurements are compared to modeling results and other available experimental data on swarm transport over the broad range of from 2 Td to 1603 Td. The modeling results are obtained in Monte Carlo simulations and by solving the electron Boltzmann equation using a multi-term approach and the density gradient expansion procedure. We find generally good agreement between the measured and calculated transport coefficients. We propose a strategy to improve the cross-section set used to explain certain discrepancies at lower values. Measurements and calculations of electron transport coefficients under hydrodynamic conditions are complemented by Monte Carlo simulations of electron transport in an idealized steady-state Townsend (SST) setup. The ionization coefficient is calculated as a function of from the spatial density profiles of the electrons and compared to the corresponding values evaluated from the knowledge of the effective ionization frequency, drift velocity and longitudinal diffusion coefficient. Contrary to the traditional views, according to which the spatial relaxation of the mean energy and other transport properties for electrons in molecular gases is most commonly monotonic or quasi-monotonic, we find a ‘window’ of where the SST transport properties of the electrons exhibit oscillatory behavior as they relax towards the equilibrium state far downstream from the electron emitting boundary.

Probing Colossal Carbon Rings.

Carbon aggregates containing between 10 and 30 atoms preferentially arrange themselves as planar rings. To learn more about this exotic allotrope of carbon, electronic spectra are measured for even cyclo[n]carbon radical cations (-) using two-color photodissociation action spectroscopy. To eliminate spectral contributions from other isomers, the target cyclo[n]carbon radical cations are isomer-selected using a drift tube ion mobility spectrometer prior to spectroscopic interrogation. The electronic spectra exhibit sharp transitions spanning the visible and near-infrared spectral regions with the main absorption band shifting progressively to longer wavelength by ≈100 nm for every additional two carbon atoms. This behavior is rationalized with a Hückel theory model describing the energies of the in-plane and out-of-plane π orbitals. Photoexcitation of smaller carbon rings leads preferentially to neutral C3 and C5 loss, whereas rings larger than tend to also decompose into two smaller rings, which, when possible, have aromatic stability. Generally, the observed charged photofragments correspond to low energy fragment pairs, as predicted by density functional theory calculations (CAM-B3LYP-D3(BJ)/cc-pVDZ). Using action spectroscopy it is confirmed that and photofragments from rings have cyclic structures.