Supersonic Source Research Articles

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.

Predissociation-based measurements of bond dissociation energies: US2, OUS, and USe.

The uranium-containing molecules US2, OUS, and USe have been investigated using a pulsed laser ablation supersonic beam molecular source with time-of-flight mass spectrometric detection. Spectra have been recorded using the resonant two-photon ionization method over the spectroscopic range from 277 to 238nm. These species have a myriad of excited electronic states in this spectroscopic region, leading to spectra that are highly congested and appear quasicontinuous. Sharp predissociation thresholds are observed, allowing precise bond dissociation energies to be measured. In the case of the triatomic molecules, it was necessary to use one laser for excitation and a delayed laser for ionization in order to observe a sharp predissociation threshold that allowed a precise bond dissociation energy to be measured. The resulting thermochemical values are D0(SU-S) = 4.910 ± 0.003eV, D0(OU-S) = 5.035 ± 0.004eV, and D0(USe) = 4.609 ± 0.009eV. These results provide the first measurement of D0(USe) and reduce the error limits in the previous values of D0(SU-S) and D0(OU-S) by a factor of more than 70.

Ultrahigh energy efficiency from a supersonic underwater ultrasound source

The present work is dedicated to an experimental and theoretical study of an innovative underwater ultrasound source. The source works using a technique in which a pulsed power generator using the impedance mismatch of a long high-voltage coaxial cable generates a train of voltage impulses with a very high pulse repetition frequency of the order of a few MHz. Applying this train of voltage impulses to a pair of underwater electrodes generates a streamer-initiated breakdown of water and, subsequently, a plasma column connecting the electrodes over a very large inter-electrode gap of 55 mm. The interaction of the long plasma column thus formed with the surrounding water produces a rapidly expanding vapor bubble, an “instrument” producing a strong pressure wave with an overall energy efficiency of 24%, an order of magnitude higher than most underwater pressure sources reported in the literature.

Multipole transfer matrix model-based sparse Bayesian learning approach for sound source identification

Accurate source identification is key to solving complex flow-induced noise problems, and sources often have non-uniform radiation patterns. At the same time, traditional methods have many application limitations, including the need for prior knowledge of the sound field, as well as inability to apply to coherent sources or coexisting multipoles. A multipole transfer matrix model-based sparse Bayesian learning approach for sound source identification (MT-SBL) is proposed. In this algorithm, the transfer relationship for the multipole radiation patterns of sound sources is established. Then, the signal sparse reconstruction is transformed into an iterative updating problem of the feature parameters under a sparse Bayesian learning (SBL) framework, which leads to the accurate identification of multipole sources. Numerical simulations and experiments on electric speakers in an anechoic environment validate the proposed algorithm. Compared with the existing methods, the algorithm is more advantageous in terms of accuracy, resolution, and computational efficiency. Further experiments were conducted on the identification of supersonic jet noise sources, verifying the effectiveness of the method for identifying strongly directional sound sources.

Prediction of regional infrasound produced by supersonic sources using a ray-based Mach cone source.

The geometry of the Mach cone produced by a supersonic source is analyzed and mapped into initial conditions used in acoustic ray tracing. The resulting source model is combined with spherical geometry ray tracing methods to enable propagation simulations for infrasonic signals produced by bolides, space debris, rockets, aircraft, and other fast-than-sound sources out to typical infrasonic observation distances of hundreds or thousands of kilometers. Idealized linear and parabolic trajectories typical of bolides and rockets, respectively, are used to demonstrate the calculation of regional infrasonic signals produced by such sources and characteristics of the radiated infrasonic waves are found to vary strongly with the geometry of the trajectory and atmospheric structure. Predicted regional infrasonic signals are compared with those observed from a November 2020 bolide that passed over Scandinavia using a combination of institutionally maintained infrasound stations and "citizen scientist" data from the Raspberry Shake data repository.

The field theory of collective Cherenkov radiation associated with electron beams

Classical Cherenkov radiation is a celebrated physics phenomenon of electromagnetic (EM) radiation stimulated by an electric charge moving with constant velocity in a three-dimensional dielectric medium. Cherenkov radiation has a wide spectrum and a particular distribution in space similar to the Mach cone created by a supersonic source. It is also characterized by the energy transfer from the charge's kinetic energy to the EM radiation. In the case of an electron beam passing through the middle of an EM waveguide, the radiation is manifested as collective Cherenkov radiation. In this case, the electron beam can be viewed as a one-dimensional non-neutral plasma, whereas the waveguide can be viewed as a slow wave structure. This collective radiation occurs, in particular, in traveling wave tubes (TWTs), and it features the energy transfer from the electron beam to the EM radiation in the waveguide. Based on a first principles Lagrangian field theory, we develop a convincing argument that the collective Cherenkov effect in TWTs is, in fact, a convective instability, that is, amplification. We also recover Pierce's theory as a high-frequency limit of our generalized Lagrangian theory. Finally, we derive for the first time expressions identifying low- and high-frequency cutoffs for amplification in TWTs.

Rotational detection of the silicon-carbon chains H2C3Si and HSiCCH

The rotational spectra of two new silicon-bearing carbon chains, H2C3Si and HSiCCH have been detected by means of Fourier-transform microwave spectroscopy in a supersonic jet source equipped with an electrical discharge. On the basis of measurements between 10 and 42GHz, precise spectroscopic constants have been determined for both molecules. Along with H29SiCCH and H30SiCCH, which were detected in natural abundance, several other rare isotopic species of HSiCCH were observed using samples enriched in carbon-13 and D. From this isotopic data, a precise semi-experimental equilibrium structure was derived and compared with a re-evaluated semi-experimental equilibrium structure of the parent silylene SiH2; the deuterium isotopic measurements also provide important clues as to the formation pathway of HSiCCH in our discharge. Because other small Si-bearing chains have been detected in the circumstellar envelope of the evolved carbon star IRC+10216, both chains may be of astronomical interest.

A 22-pole radiofrequency ion trap setup for laboratory astrophysical studies.

The formation of large interstellar molecules starting from small reactants constitutes the bottom-up approach in astrochemistry. An experimental setup wherein the ionic reactant is prepared and allowed to react with neutral reactants within a confined interaction space is most desirable for this approach. Here, we present our efforts in constructing a 22-pole radio-frequency ion trap experimental setup that could act as a reaction chamber for the neutral reactant and trapped ions. A compact piezo-based pulsed supersonic expansion discharge source was developed to achieve efficient production of anions, particularly metal-bearing ions. In addition, two other ion sources, namely, the electron impact ion source and the plasma supersonic discharge ion source, were developed to cater to the production of specific ions. The geometry of this experimental setup facilitates photo excitation of trapped ions. The trapping efficiency was studied by trapping SF5+ and SF6- for a few seconds. The control exhibited by various parameters on the efficiency of the trapping is discussed in detail. Potential studies that could be performed with this new experimental setup are also discussed.

Rotational spectroscopy of methyl tert-butyl ether with a new Ka band chirped-pulse Fourier transform microwave spectrometer.

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful tool for performing broadband gas-phase rotational spectroscopy, and its applications include discovery of new molecules, complex mixture analysis, and exploration of fundamental molecular physics. Here we report the development of a new Ka band (26.5-40 GHz) CP-FTMW spectrometer that is equipped with a pulsed supersonic expansion source and a heated reservoir for low-volatility samples. The spectrometer is built around a 150 W traveling wave tube amplifier and has an instantaneous bandwidth that covers the entire Ka band spectral range. To test the performance of the spectrometer, the rotational spectrum of methyl tert-butyl ether (MTBE), a former gasoline additive and environmental pollutant, has been measured for the first time in this spectral range. Over 1000 spectroscopic transitions have been measured and assigned to the vibrational ground state and a newly-identified torsionally excited state; all transitions were fit using the XIAM program to a root-mean-square deviation of 22 kHz. The spectrum displays internal rotation splitting, nominally forbidden transitions, and an intriguing axis-switching effect between the ground and torsionally excited state that is a consequence of MTBE's extreme near-prolate nature. Finally, the sensitivity of the spectrometer enabled detection of all singly-substituted 13C and 18O isotopologues in natural abundance. This set of isotopic spectra allowed for a partial r0 structure involving the heavy atoms to be derived, resolving a structural discrepancy in the literature between previous microwave and electron diffraction measurements.

Conformations of borneol and isoborneol in the gas phase: Their monomers and microsolvation clusters.

Borneol is a natural monoterpene with significant applications in various industries, including medicine and perfumery. It presents several diastereomers with different physical and chemical properties, influenced by their unique structures and interactions with molecular receptors. However, a complete description of its inherent structure and solvent interactions remains elusive. Here, we report a detailed investigation of the gas-phase experimental structures of borneol and isoborneol, along with the description of their microsolvation complexes with the common solvents water and dimethyl sulfoxide. The molecules and complexes were studied using chirped-pulse Fourier transform microwave spectroscopy coupled to a supersonic expansion source. Although three rotamers are potentially populated under the conditions of the supersonic expansion, only one of them was observed for each monomer. The examination of the monohydrated complexes revealed structures stabilized by hydrogen bonds and non-covalent C-H⋯O interactions, with water as the hydrogen bond donor. Interestingly, in the clusters with dimethyl sulfoxide, borneol and isoborneol change their roles acting as donors. We further identified a higher-energy rotamer of the borneol monomer in one of the complexes with dimethyl sulfoxide, while that rotamer was missing in the experiment for the monomer. This observation is not common and highlights a specific position in borneol especially favorable for forming stable complexes, which could have implications in the understanding of the unique physical and chemical properties of the diastereomers.

Direct observation on argon tagging nitrobenzene radical anion in gas phase: Infrared photodissociation spectroscopy and theoretical calculation.

The infrared photodissociation spectroscopy was applied to characterize nitrobenzene radical anion (NB-). NB- tagged by argon [NB(Ar)-] was prepared by a mixture of nitrobenzene/Ar through a supersonic ion source and then selected by a time-of-flight mass spectrometer. Eight strong peaks observed at 977.9, 999.6, 1059.8, 1275.7, 1309.7, 1339.7, 1367.6 and 1581.7cm-1 in the fingerprint region were assigned to NB(Ar)-, corresponding to CC bending, CC stretching, CC bending+symmetric O-N stretching vibration, antisymmetric O-N and CC stretching vibration, antisymmetric O-N stretching and CH rocking vibration, CC stretching+antisymmetric O-N stretching vibration, C-N stretching vibration, and symmetric CC stretching vibration. Most interestingly, the distinguishable CH stretching vibrations were observed at 3006.5, 3048.6 and 3084.5cm-1 absorptions. Combined with density functional theoretical (DFT) calculation, five tagging argon NB- isomers were optimized and analyzed with no imaginary frequency. The results indicated that most bond lengths in NB- become longer than those of neutral NB, except for the C1-C2/C4-C5 bonds, which are only slightly shorter than those of neutral NB, and the C-N bond, which is 0.085 A shorter in the anion. The NB- tagged by argon located on the nitro group had no change on bond parameters with Ar-tagging or not theoretically. Natural population analysis (NPA) show that the negative natural charges are mainly distributed on both oxygen atoms. And the one electron resonates between the nitro group and benzene ring. N-O bonds in NB- become much more polar than those of the neutral NB. This paper proved the usefulness to characterize NB(Ar)- and further explore the structures of NBn- (n>1) clusters by infrared photodissociation spectroscopy combining with the time-of-flight mass spectrometer.

Laser-based spectroscopy of FeD: Excitations to the g 6Φ electronic state.

Bands of the g 6Φ-X 4Δ, g 6Φ-A 4Π, g 6Φ-a 6Δ, and g 6Φ-b 6Π electronic transitions of iron monodeuteride (FeD) have been measured in laser excitation and in dispersed fluorescence. The molecules were produced both in a cold supersonic molecular jet source and in a chemical reaction between iron pentacarbonyl [Fe(CO5)] and a microwave discharge of argon and hydrogen gases. Dispersed fluorescence from the latter source was detected at high resolution with a Fourier transform spectrometer, yielding a large number of the transitions observed. The data reveal that FeD experiences strong interstate couplings that compromise fitting of the data with traditional Hamiltonians but that the problem is less severe than in corresponding spectra of FeH. This work greatly expands the available data on FeD, which were previously characterized only through the F 4Δ-X 4Δ spectrum and pure rotational data in the ground state.

∧-type doublets and lifetime broadening in the B2Π–X2Π electronic origin band of C6H

We present a high-resolution study on the B2Π-X2Π electronic origin band spectrum of the C6H radical with its center at ~18990 cm-1 The C6H radicals are produced in a pulsed supersonic slit-jet discharge source and detected in direct absorption by sensitive cavity ring-down spectroscopy. By using a home-made narrow linewidth nanosecond pulsed laser source, the experimental spectrum allows to resolve the ∧-type doublet fine structures in high-J rotational lines. A set of improved B2Π state constants, including the ∧-type doubling parameters of p′ = −1.16(9)x10−3 cm−1q′ = -1.22(7)x10−4 cm−1 are determined from analysis of the experimental spectrum. An analysis on the spectral line profile has resulted in a significant linewidth broadening of ~0.05±0.01 cm−1, corresponding to a B2Π state lifetime of ~100±20 ps. The short lifetime of the excited B2Π state is proposed due to fast internal vibronic couplings to high-lying vibronic levels of lower electronic states.

Modeling regional propagation of infrasonic Mach cone energy from a supersonic source

Supersonic sources such as bolides, aircraft, and rocket launches are known to produce a wide range of acoustic and infrasonic signals. The lower frequency infrasonic component of these signals can propagate large distances through the atmosphere due to the decreased thermo-viscous losses at low frequency and presence of temperature and wind induced infrasonic waveguides so that signals are often detected at regional distances of hundreds of kilometers. The dominant radiator of energy from such sources is the Mach cone signal that emanates from the source with very specific geometry. Ray tracing analysis using a Mach cone source model to predict infrasonic signals at regional distances will be presented and several parametric studies and comparisons with observed signals from several representative supersonic sources will be presented.

Photoelectron imaging spectroscopic signatures of CO activation by the heterotrinuclear titanium-nickel clusters

A series of heterotrinuclear Ti2Ni(CO)n– (n = 6–9) carbonyls have been generated via a laser vaporization supersonic cluster source and characterized by mass-selected photoelectron velocity-map imaging spectroscopy. Quantum chemical calculations have been carried out to identify the structures and understand the experimental spectral features. The results indicate that a building block of Ti-Ti-Ni-C four-membered ring with the C atom bonded to Ti, Ti, and Ni is dominated in the n = 6–8 complexes, whereas a structural motif of Ti-Ti-Ni triangle core is preferred in n = 9. These complexes are found to be capable of simultaneously accommodating all the main modes of metal-CO coordination (i.e., terminal, bridging, and side-on modes), where the corresponding mode points to the weak, moderate, high CO bond activation, respectively. The number of CO ligands for a specific bonding mode varies with the cluster size. These findings have important implications for molecular-level understanding of the interaction of CO with alloy surfaces/interfaces and tuning the appropriate CO activation via the selection of different metals.

Significant π bonding in coinage metal complexes OCTMCCO- from infrared photodissociation spectroscopy and theoretical calculations.

A series of coinage metal complexes in the form of TMC(CO)n - (TM = Cu, Ag, Au; n = 0-3) were generated using a laser-ablation supersonic expansion ion source in the gas phase. Mass-selected infrared photodissociation spectroscopy in conjunction with quantum chemical calculations indicated that the TMC(CO)3 - complexes contain a linear OCTMCCO- core anion. Bonding analyses suggest that the linear OCTMCCO- anions are better described as the bonding interactions between a singlet ground state TM+ metal cation and the OC/CCO2- ligands in the singlet ground state. In addition to the strong ligands to metal σ donation bonding components, the π-bonding components also contribute significantly to the metal-ligand bonds due to the synergetic effects of the CO and CCO2- ligands. The strengths of the bonding of the three metals show a V-shaped trend in which the second-row transition metal Ag exhibits the weakest interactions whereas the third-row transition metal Au shows the strongest interactions due to relativistic effects.

High-resolution infrared spectroscopy of supersonically cooled singlet carbenes: Bromomethylene (HCBr) in the CH stretch region.

First high-resolution spectra of cold (∼35K) singlet bromomethylene HCBr in the CH stretching (v1) region from 2770 to 2850cm-1 are reported using near quantum shot-noise limited laser absorption methods in a slit jet supersonic discharge expansion source. Three rovibrational bands are identified at high S/N (20:1-40:1) and rotationally assigned to (i) the CH stretch fundamental (v1) band X̃1,0,0←X̃0,0,0 and (ii) vibrational hot bands [X̃(1,1,0)←X̃(0,1,0) and X̃(1,0,1)←X̃(0,0,1)] arising from vibrationally excited HCBr populated in the discharge with single quanta in either the H-C-Br bend (v2) or C-Br stretch (v3) modes. Precision rotational constants are reported for a total of six states, with an experimentally determined CH stretch vibrational frequency (2799.38cm-1) in good agreement with previous low-resolution fluorescence studies [M. Deselnicu et al., J. Chem. Phys. 124(13), 134302 (2006)]. Detailed analysis of the fundamental v1 band highlights the presence of perturbations in the X̃1,0,0 level, which we tentatively attribute to arise from the nearby triplet state ã(0,0,1) through spin-orbit interaction or the multiple quanta X̃0,2,1 singlet state via c-type Coriolis coupling. Reduced-Doppler resolution (60MHz) in the slit-jet IR spectrometer permits for clear observation of a nuclear spin hyperfine structure, with experimental line shapes well reproduced by nuclear quadrupole/spin-rotation coupling constants from microwave studies [C. Duan et al., J. Mol. Spectrosc. 220(1), 113-121 (2003)]. Finally, the a-type to b-type transition intensity ratio for the fundamental CH stretch band is notably larger than that predicted by using a bond-dipole model, which from high level ab initio quantum calculations [CCSD(T)/PVQZ] can be attributed to vibrationally induced "charge-sloshing" of electron density along the polar C-Br bond.

Optical emission spectroscopy and Langmuir probe studies of an intermediate pressure, supersonic microplasma jet deposition source

Optical emission spectroscopy and double Langmuir probe studies were conducted on the supersonic expansion plume of a flow-through, intermediate pressure (∼10 Torr) Ar/N2 microplasma source used for material deposition. Emission from the first positive system of N2 (B3Πg → A3Σu+) was used to determine the expanding gas rotational and vibrational temperatures; both were strongly dependent on plasma drive current, and the gas (rotational) temperature could be tuned from 300 to 800 K. The effects of drive current, plasma circuit configuration, and O2 addition to the gas feed on the local electron temperature (Te) were investigated using a miniature double Langmuir probe. Electron temperatures of 1–2 eV were estimated and decreased slightly with higher O2 content in the feed and increased distance from the capillary orifice. Te dependence on drive current was more complicated, falling into two regimes, namely, a nonuniform “predischarge” regime at low currents (<12 mA) where Te varied greatly, and the normal hollow cathode regime at higher currents (>12 mA) where Te remained nearly constant as plasma current was increased. These phenomena are discussed in light of the IV characteristics of the discharge drive circuit.

Carbon-13 studies of sulphur-terminated carbon chains: chemical bonding, molecular structures, and formation pathways

The rotational spectra of the singly substituted carbon-13 isotopic species of six sulphur-terminated carbon chains have been detected between 5 and 40 GHz using a supersonic discharge jet source in combination with a cavity Fourier transform microwave spectrometer. These molecules are , a closed-shell isomer of , and five radicals: , , , and . Experiments were carried out with precursors enriched in C, either high purity , a statistical sample of C/C acetylene, or . From the C hyperfine coupling constants, the unpaired electronic density along the chain can be quantified for the radical species, while precise experimental structures () can be derived for each chain except by a least-squares fit to the rotational constants. The use of is particularly revealing in that it provides important clues as to the major formation pathway for each chain in our discharge nozzle. Somewhat surprisingly, C from this precursor appears to be substituted in one of three distinct ways: statistical, a single C site, or not at all. This propensity is molecule specific, implying that multiple formation pathways are operative.

Supersonic jet noise source distributions.

The purpose of this work is to examine the noise source distributions of shock-containing supersonic jets at various pressure ratios corresponding to fully expanded Mach numbers ranging from 1.0 to 1.4 in intervals of 0.2 for various nozzle exit diameters. Source location measurements using a phased array (beamforming), farfield jet noise measurements, and schlieren flow visualization are presented. It is found that supersonic noise source distributions are more complex than those of subsonic jets. The source distributions for supersonic jets can be divided into three different Strouhal regions. At low Strouhal numbers ( fD/U≤0.3), the noise source distributions appear very similar to those of a subsonic jet, as reported in open literature. This Strouhal region is dominated by jet-mixing noise associated with small-scale turbulence mixing. At high Strouhal numbers ( fD/U≥1.0), the noise source distributions are comprised of several repetitive sources at various discrete downstream jet locations that produce noise at all frequencies. The locations of these sources roughly correspond to the shock cells in the jet, and thus, vary with jet Mach number. Another region exists at Strouhal numbers between these two regions ( 0.3<fD/U<1.0) for which the precise location of the sources as a function of Strouhal number was determined to be ambiguous due to a limitation of the phased array used. This region roughly corresponds to the frequencies of noise where jet-mixing noise and shock noise are of similar levels. The spacing of the shock sources in this region are smaller than the beam width of the array measuring them. Their locations can no longer can be separately recorded; and instead, they are averaged together and their centroid location is plotted.