Pulsed Molecular Beam Research Articles

A method to determine the phase-space distribution of a pulsed molecular beam

Abstract We demonstrate a method to determine the longitudinal phase-space distribution of a cryogenic buffer gas cooled beam of barium-fluoride molecules based on a two-step laser excitation scheme. Temporal resolution is achieved by a transversely aligned laser beam that drives molecules from the ground state $X^2\Sigma^+$ to the $A^2\Pi_{1/2}$ state around 860\;nm, while the velocity resolution is obtained by a laser beam that is aligned counter-propagating with respect to the molecular beam and that drives the Doppler shifted $A^2\Pi_{1/2}$ to $D^2\Sigma^+$ transition around 797\;nm. Molecules in the $D$-state are detected background-free by recording the fluorescence from the $D-X$ transition at 413 nm. A temporal resolution of 11\;$\upmu$s and a velocity resolution of 6\;m/s is obtained. In order to calibrate the absolute velocity, we have determined the Doppler free transition frequencies for the $X-A$ and $X-D$ transitions with an absolute accuracy below 0.3\;MHz. The high resolution of the phase-space distributions allows us to observe a variation of the average velocity and velocity spread over the duration of the molecular beam pulse. Our method hence gives valuable insight into the dynamics in the source.

Overtone Excitation of Nitrogen Molecules via Stimulated Raman Pumping.

Nitrogen bond activation is a pivotal process in chemistry, with bond excitation being fundamental to understanding the underlying mechanisms, making the preparation of molecules in specific quantum states crucial. Here we report the first overtone excitation of the N2 molecule from X1Σg+(v = 0, j = 0, 1, and 2) to X1Σg+(v = 2, j = 0, 1, 2, and 3) using the stimulated Raman pumping (SRP) method in a pulsed molecular beam. N2 was detected using 2+1 resonance-enhanced multiphoton ionization through the a″1Σg+ state. An excitation efficiency of 4% was achieved within the excitation region in which the SRP laser intensity was saturated, indicating the low cross-sectional nature of the process. The SRP detuning spectra for different branches were measured, and the excited N2 [X1Σg+(v = 2)] was further used to access various vibrational states of a″1Σg+, enabling the determination of its vibrational constants. This research opens up new opportunities for studying the specific high vibrational excitation of nitrogen in reactions and scattering experiments and contributes additional precise spectral data for the N2 molecule.

Low duty cycle pulsed UV technique for spectroscopy of aluminum monochloride.

We present what we believe to be a novel technique to minimize UV-induced damage in experiments that employ second-harmonic generation cavities. The principle of our approach is to reduce the duty cycle of the UV light as much as possible to prolong the lifetime of the used optics. The low duty cycle is achieved by ramping the cavity into resonance for a short time during the experimental cycle when the light is used and tuning it to an off-resonant state otherwise. The necessary fast ramp and length-stabilization control of the cavity is implemented with the FPGA-based STEMlab platform. We demonstrate the utility of this method by measuring the isotope shift of the electronic transition (X1Σ ← A1Π) in AlCl at 261.5 nm in a pulsed molecular beam experiment.

Selective Oxidation of Methanol to Methyl Formate on Gold: The Role of Low-Coordinated Sites Revealed by Isothermal Pulsed Molecular Beam Experiments and AIMD Simulations.

To elucidate the role of low-coordinated sites in the partial methanol oxidation to methyl formate (MeFo), the isothermal reactivity of flat Au(111) and stepped Au(332) in pulsed molecular beam experiments was compared for a broad range of reaction conditions. Low-coordinated step sites were found to enhance MeFo selectivity, especially at low coverage conditions, as found at higher temperatures. The analysis of the transient kinetics provides evidence for the essential role of Au x O y phases for MeFo formation and the complex interplay of different oxygen species for the observed selectivity. Ab initio molecular dynamic simulations yielded microscopic insights in the formation of Au x O y phases on flat and stepped gold surfaces emphasizing the role of low-coordinated sites in their formation. Moreover, associated surface restructuring provides atomic-scale insights which align with the experimentally observed transient kinetics in MeFo formation.

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The average density of the undecelerated H atom beam is approximately an order of magnitude higher (2.9 ± 1.9) ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 105\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^5$$\\end{document} cm-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-3}$$\\end{document}, with the average density of the molecular ammonia beam over two orders of magnitude higher again (5.1 ± 2.9) ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 107\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^7$$\\end{document} cm-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-3}$$\\end{document}. The average number densities measured for the two different species of interest in this work span more than three orders of magnitude. These findings highlight the need for accurate and precise experimental measurements of number densities—for each species of interest, under the appropriate experimental conditions—before doing absolute rate coefficient calculations.Graphical abstract

Spectroscopic and computational characterization of lanthanide-mediated bond activation of ethylamine

Ln (Ln = La and Ce) atom reactions with ethylamine are conducted in a pulsed laser vaporization supersonic molecular beam source. Dehydrogenation and association metal-containing species are observed with time-of-flight mass spectrometry, and the dehydrogenated Ln ethylimido species in the formula Ln(NC2H5) are characterized by single-photon mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical calculations. The theoretical calculations include density functional theory for both Ln species and a scalar relativity correction, electron correlation, and spin-orbit coupling through multiconfiguration quasi-degenerate second-order perturbation theory for the Ce species. The MATI spectrum of lanthanum ethylimido La(NCH2CH3) has a single vibronic band system from the ionization of the doublet ground state with the La 6s1 configuration, whereas that of the cerium ethylimido Ce(NCH2CH3) displays two vibronic band system from the ionization of the two lowest-energy spin-orbit coupling states with the Ce 4f16s1 configuration. Both Ln ethylimido complexes are formed by the thermodynamically and kinetically favorable concerted dehydrogenation of the amino group. Two additional isomers of Ln(NC2H5) include a four-membered metallacycle Ln(NHCH2CH2) from the dehydrogenation of the amino and methyl groups and a three-membered cycle Ln(NHCHCH3) from the dehydrogenation of the amino and methylene groups. The cyclic isomers are not observed experimentally as they are not populated under the experimental conditions.

Nanomaterials-Based Multiple Quantum Wells for High Photovoltaic Conversion Solar Cells

Using metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and pulse Laser deposition (PLD) techniques on GaN, Silicon, Silicon Carbide and sapphire substrates, high efficiency InGaN/GaN solar cells are reported with a particular emphasis on the work and achievements made with multi-junction tandem and Nanomaterials (Quantum well (QW), Multiple Quantum Wells (MQW), and Quantum Dots (QD)). An effective method for increasing photon absorption in ultrathin cells made for the best possible photovoltaic response is the InGaN/GaN QW system. To maximize light absorption, the quantum well and barrier thicknesses and number of wells in the MQW active region must be adjusted.

Same size, same support, same spectator? Selective acetylene hydrogenation on supported Pd nanoparticles.

The selective hydrogenation of acetylene catalyzed by Pd nanoparticles is industrially used to increase the purity of ethylene. Despite the implementation of Pd based catalysts on an industrial scale, little is known about metal-support interactions on a fundamental level due to the complexity of these systems. In this study, the influence of metal-support interactions between Pd nanoparticles and two electronically modified a-SiO2 thin films on acetylene hydrogenation is investigated under ultra-high vacuum (UHV) conditions. The hydrogenation is performed under isothermal reaction conditions using a pulsed molecular beam reactive scattering (pMBRS) technique. Besides the activity and selectivity of clean Pd particles also the impact of dehydrogenated species intentionally introduced a priori is elucidated, whereas the active phase of the catalyst is additionally characterized by CO infrared reflection-absorption spectroscopy (IRRAS) and post-mortem temperature-programmed reaction (TPR). Metal-support interactions are found to influence the catalytic properties of Pd particles by charge-transfer, where positive charging leads to increased activity for acetylene hydrogenation. However, the increased activity is accompanied by formation of undesired byproducts. The active sites for acetylene and ethylene hydrogenation are shown to be different as previously proposed by the A and E model. The availability of the two different active sites on the Pd nanoparticles is determined by dehydrogenated species, whose nature and stability can be tuned by metal-support interactions. Based on these findings an electronic model is proposed how selectivity for acetylene hydrogenation can be steered solely by metal-support interactions leading to blocking of unselective sites in situ.

Design and characterization of an optical-fiber-coupled laser-induced desorption source for gas-phase dynamics experiments.

Preparation of neutral non-volatile molecules intact in the gas phase for mass spectrometry or chemical dynamics experiments remains a challenge for many classes of molecules. Here, we report the design and characterization of a fiber-coupled laser-based thermal desorption source capable of preparing intact neutral molecules at high molecular densities in the gas phase for use in velocity-map imaging experiments. Within this source, the sample is deposited onto a thin tantalum foil. Irradiation of the foil from the reverse side by a focused laser beam leads to highly localized heating of the sample, resulting in desorption of a plume of molecules into the gas phase. The fiber-coupled design simplifies the alignment of the desorption laser beam, and the ability to rotate the foil relative to the fixed laser beam allows the sample to be continually refreshed under vacuum. We use 118nm photoionization of three test molecules-uracil, adenine, and phenylalanine-to characterize the source and to demonstrate various aspects of its performance. These include the dependence of the velocity-map imaging performance on the size of the interaction region and the dependence of the laser-induced desorption source emission on desorption laser power and heating time. Signal levels recorded in these measurements are comparable to those we typically obtain in similar experiments using a pulsed supersonic molecular beam, and we, therefore, believe that the source has considerable potential for use in a wide range of chemical dynamics and other experiments.

Shielded ionisation discharge (SID) probe for spatiotemporal profiling of pulsed molecular beam.

Intense pulsed supersonic molecular beams are used in many applications such as tokamak fueling, edge plasma diagnostics, ion beam profile monitors, laser cluster experiments, chemical kinetics, etc. Measurement of absolute density is required to optimize beam sources used in these experiments. Absolute density measurement of a continuous molecular beam is challenging due to its small size and rarefied flow, which makes it even more difficult for a pulsed molecular beam due to its transient nature. In this work, we demonstrate a novel probe to measure the spatiotemporal evolution of the absolute number density of a pulsed supersonic molecular beam. The probe is named the Shielded Ionization Discharge probe. It measures density using localized discharge within the molecular beam created by the thermionic emission of electrons from a hot filament. We describe the design, calibration, and characterization of the developed probe. The performance of the probe is demonstrated by measuring the spatial and temporal profiles of a pulsed supersonic molecular beam of 1.5ms duration.

(Energy Technology Division Supramaniam Srinivasan Young Investigator Award) Fundamental Insights into the Oxygen Evolution Reaction from Epitaxial Oxide Thin Films

The intermittent nature of renewable energy sources requires a clean, scalable means of converting and storing energy. One Earth-abundant storage option is water electrolysis: storing energy in the bonds of O2 and H2, and later extracting electricity by the electrochemical reaction of gasses in a fuel cell. The efficiency of this process is primarily limited by the sluggish kinetics of the oxygen evolution reaction (OER) at the anode, resulting in extensive use of precious metal electrocatalysts in current devices. We have worked to build mechanistic understanding of the OER on benchmark catalysts rutile IrO2 and RuO2, and have investigated competition with the chlorine evolution reaction (CER) in saline feeds, where breaking scaling relations between oxidative processes can broaden the range of acceptable water sources for electrolysis. We have also investigated earth-abundant oxide materials as alternatives, especially nickel-based perovskites, focusing on fundamental understanding of the electrode/electrolyte interface and its relation to catalyst electronic structure.I will present studies of model oxide electrodes grown by pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) that display a known crystallographic orientation, strain, surface area, and path for charge transport. Such measurements can establish the intrinsic activity of oxide catalysts in a way that cannot be realized with polydisperse nanoparticle systems, and we use these findings to rationally design composition and structure to maximize activity.

Inelastic scattering of OH from a liquid PFPE surface: Resolution of correlated speed and angular distributions.

Inelastic collisions of OH with an inert liquid perfluoropolyether (PFPE) surface have been studied experimentally. A pulsed molecular beam of OH with a kinetic energy distribution peaking at 35 kJ mol-1 was directed at a continually refreshed PFPE surface. OH molecules were detected state-selectively with spatial and temporal resolution by pulsed, planar laser-induced fluorescence. The scattered speed distributions were confirmed to be strongly superthermal, regardless of the incidence angle (0° or 45°). Angular scattering distributions were measured for the first time; their reliability was confirmed through extensive Monte Carlo simulations of experimental averaging effects, described in Paper II [A. G. Knight et al., J. Chem. Phys. 158, 244705 (2023)]. The distributions depend markedly on the incidence angle and are correlated with scattered OH speed, consistent with predominantly impulsive scattering. For 45° incidence, the angular distributions are distinctly asymmetric to the specular side but peak at sub-specular angles. This, along with the breadth of the distributions, is incompatible with scattering from a surface that is flat on a molecular scale. New molecular dynamics simulations corroborate the roughness of the PFPE surface. A subtle but unexpected systematic dependence of the angular distribution on the OH rotational state was found, which may be dynamical in origin. The OH angular distributions are similar to those for kinematically similar Ne scattering from PFPE and hence not strongly perturbed by OH being a linear rotor. The results here are broadly compatible with prior predictions from independent quasiclassical trajectory simulations of OH scattering from a model-fluorinated self-assembled monolayer surface.

Heteroepitaxial growth of anatase (0 0 1) films on SrTiO3 (0 0 1) by PLD and MBE

The epitaxial growth of anatase (001) films deposited by pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) on SrTiO3 (001) (STO) single crystals has been studied using X-ray diffraction and surface sensitivity UHV techniques. The evolution of the strain represented by the microstrain and the change of the in-plane and out-of-plane lattice parameters with film growth temperature, the effect of the annealing temperature and the influence of the oxygen content of the film have been investigated.The out-of-plane lattice strain shows a compressive (−0.2 %) or expansive (+0.3 %) behavior, in the range 600–900 °C, for temperatures below or above 700 °C, respectively. The in-plane lattice parameters, as well as the cell volume of the film, remain under compression over the entire temperature range explored.PLD films grow into square islands that align with the surface lattice directions of the STO substrate. The maximum size of these islands is reached at growth temperatures close to 875–925 °C. Film annealing at temperatures of 800 °C or higher melts the islands into flat terraces. Larger terraces are reached at high annealing temperatures of 925 °C for extended periods of 12 h. This procedure allows flat surface terrace sizes of up to 650 nm to be achieved.The crystalline quality achieved in anatase films prepared by PLD or MBE growth methods is similar. The two-step anatase growth process used during the synthesis of the films with both methods: film growth and post-annealing treatment in oxygen or air at ambient pressure, using temperature and time as key parameters, allows to control the surface terrace size and stoichiometry of the films, as well as the anatase/rutile intermixing rates at sufficiently high temperatures. This growth process could allow the substitution of their equivalent single crystals. The range of applicability of these films would include their use as structural and electronic model systems, or in harsh experimental conditions due to their low production cost.

Sputter-grown c-axis-oriented PdCoO2 thin films

Metallic delafossites, ABO2 (A = Pd or Pt), are layered oxides that are as conductive as elemental metals. The high conductivity and surface polarity make metallic delafossites fascinating electrode materials for heterostructure devices. Here, we report the successful growth of c-axis-oriented PdCoO2 thin films on Al2O3 (001) substrates by magnetron sputtering that is widely used in industries. The observation of the PdCoO2 thin films through scanning transmission electron microscopy revealed layered crystal structures. A sharp interface exhibiting a layer stacking sequence of Pd/CoO2/Al2O3 was observed clearly, similar to the interfaces obtained with other growth methods such as pulsed laser deposition and molecular beam epitaxy. This layer stacking is particularly interesting because it can induce a high work function at the interface. The in-plane resistivity of the as-grown PdCoO2 thin film was 73 μΩ cm at room temperature, which decreased to 11 μΩ cm after post-annealing. The residual resistivity ratio of the annealed thin films was approximately 2.9. The impurity phases of PdOx were observed using x-ray diffraction and scanning transmission electron microscopy. The sputtering deposition of c-axis-oriented thin films could lead to the practical application of the polar surface of PdCoO2 in semiconductor devices.

Development and characterization of a narrow-pulsed molecular beam system

A narrow-pulsed and velocity-controlled molecular beam system is constructed by using a high-speed chopper and a temperature adjustable pulsed valve. The duration of the hydrogen molecular beam pulse is reduced to approximately 6.3 µs and characterized using resonance-enhanced multiphoton ionization (REMPI) at a distance of ~193 mm downstream from the nozzle. To precisely determine the velocity of the hydrogen molecules, a pump-probe technique combining stimulated Raman pumping and REMPI is employed at a fixed distance (193 mm) with nanosecond lasers. By adjusting the temperature of the pulsed valve, the velocity of the hydrogen beam varies continuously from 1290 m/s to 3550 m/s. The system exhibits potential for multiple applications in the field of chemical reaction dynamics, including its potential to integrate with ion traps and surface scattering experiments.

Optical and electrical properties of BaSnO3 and In2O3 mixed transparent conductive films deposited by filtered cathodic vacuum arc technique at room temperature

For the crystalline temperature of BaSnO3 (BTO) was above 650 °C, the transparent conductive BTO-based films were always deposited above this temperature on epitaxy substrates by pulsed laser deposition or molecular beam epitaxy till now which limited there application in low temperature device process. In the article, the microstructure, optical and electrical of BTO and In2O3 mixed transparent conductive BaInSnO x (BITO) film deposited by filtered cathodic vacuum arc technique (FCVA) on glass substrate at room temperature were firstly reported. The BITO film with thickness of 300 nm had mainly In2O3 polycrystalline phase, and minor polycrystalline BTO phase with (001), (011), (111), (002), (222) crystal faces which were first deposited at room temperature on amorphous glass. The transmittance was 70%–80% in the visible light region with linear refractive index of 1.94 and extinction coefficient of 0.004 at 550-nm wavelength. The basic optical properties included the real and imaginary parts, high frequency dielectric constants, the absorption coefficient, the Urbach energy, the indirect and direct band gaps, the oscillator and dispersion energies, the static refractive index and dielectric constant, the average oscillator wavelength, oscillator length strength, the linear and the third-order nonlinear optical susceptibilities, and the nonlinear refractive index were all calculated. The film was the n-type conductor with sheet resistance of 704.7 Ω/□, resistivity of 0.02 Ω ⋅cm, mobility of 18.9 cm2/V⋅s, and carrier electron concentration of 1.6 × 1019 cm−3 at room temperature. The results suggested that the BITO film deposited by FCVA had potential application in transparent conductive films-based low temperature device process.

Dual-Beam Circular Dichroism Spectroscopy of Jet-Cooled Chiral Molecules

Circular dichroism (CD) spectroscopy of jet-cooled molecules provides conformation-specific CD spectra. However, its widespread utilization has been limited by the weak CD effects and the low density of gas-phase molecules. Here, we developed a dual-beam method to improve the sensitivity and accuracy of gas-phase CD measurements. Circularly and linearly polarized pulses were generated from a single laser pulse and used to irradiate a single molecular-beam pulse to produce two ion peaks. The ion peaks induced by linearly polarized pulses were subtracted from those induced by circularly polarized pulses to correct the CD values for the pulse-to-pulse fluctuations in laser power and gas density. The resonant two-photon ionization CD spectrum of (1R,2R)-(-)-pseudoephedrine revealed that the standard deviations of CD values measured using the dual-beam method were three times lower than those measured using a single-beam method. The dual-beam method provides an effective, accurate, and easy-to-use tool to obtain gas-phase CD spectra.

Single-crystal thin film growth of the Mott insulator EuVO3 under biaxial substrate strain

We report the single-crystal thin film growth of EuVO3 of a Mott insulator with V3+ (d2,S=1). We used the pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) methods and obtained the well-oriented epitaxial thin films with a pseudo-tetragonal crystal structure of EuVO3 which is distinct from the orthorhombic structure in bulk. The high quality of EuVO3 films is verified by the valence state of europium and vanadium ions with X-ray absorption near-edge structure (XANES) measurements showing clear signs from Eu3+ and V3+. The electrical resistivity exhibits the insulating temperature dependence; however, the magnitude of the resistivity is reduced from that of bulk EuVO3, possibly due to biaxial strain from substrates.

Spectroscopic characterization of singlet-triplet doorway states of aluminum monofluoride.

Aluminum monofluoride (AlF) possesses highly favorable properties for laser cooling, both via the A1Π and a3Π states. Determining efficient pathways between the singlet and the triplet manifold of electronic states will be advantageous for future experiments at ultralow temperatures. The lowest rotational levels of the A1Π, v = 6 and b3Σ+, v = 5 states of AlF are nearly iso-energetic and interact via spin-orbit coupling. These levels thus have a strongly mixed spin-character and provide a singlet-triplet doorway. We here present a hyperfine resolved spectroscopic study of the A1Π, v = 6//b3Σ+, v = 5 perturbed system in a jet-cooled, pulsed molecular beam. From a fit to the observed energies of the hyperfine levels, the fine and hyperfine structure parameters of the coupled states and their relative energies as well as the spin-orbit interaction parameter are determined. The standard deviation of the fit is about 15MHz. We experimentally determine the radiative lifetimes of selected hyperfine levels by time-delayed ionization, Lamb dip spectroscopy, and accurate measurements of the transition lineshapes. The measured lifetimes range between 2 and 200ns, determined by the degree of singlet-triplet mixing for each level.

Kinetics and the crystallographic structure of bismuth during liquefaction and solidification on an insulating substrate

Here we study the kinetics of liquefaction and solidification of thin bismuth films grown on an insulating substrate by pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) and investigated by in situ electron and X-ray diffraction. By PLD, we can grow films similar to those obtained using MBE, studied by ex-situ AFM, KPFM, XRR, and XRD. The liquefaction-solidification transition is monitored in real-time by RHEED and synchrotron XRD, observing thereby the dewetting phenomenon and the formation of spherical droplets which size depends on the initial film thickness. Studying this phase transition in more detail, we find abrupt liquefaction and solidification, resulting in formation of the nanodots oriented with the (110) crystallographic plane parallel to the substrate. Furthermore, by analysis of the recorded specular diffraction rods, we propose growth by initial deposition, followed by a transformation into nandots, followed by further deposition. Overall, we demonstrate that controlling the growth scenario for Bi nanostructures, here shown by PLD and MBE, one is able to steer its resulting shape, size and corresponding (materials) properties.