Supersonic Molecular Jet Research Articles

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.

Verification of Microwave Effects on Molecular Clusters by Using Supersonic Molecular Jets.

The origin of the specific effect of microwaves on chemical reactions (the microwave effect) was investigated by examining the effect of microwaves on small groups of molecules such as clusters. The origin of the effect was verified by introducing 2.45 GHz microwaves into a system equipped with a supersonic molecular jet and a special microwave feedthrough to record the fluorescence excitation spectrum of molecules. The carrier gas was bubbled through water and introduced into a phenol-filled sample holder to generate phenol-water clusters. Subsequently, it was confirmed that exposure of the phenol-water clusters contained in the molecular jet ejected from the pulse valve to microwave radiation increased the fluorescence derived from the phenol monomer. This is considered to occur because the phenol-water clusters in the molecular jet absorb microwaves and collapse, thereby increasing the abundance of phenol monomers. This result suggests that microwaves affect not only bulk systems but also small groups of molecules, and that local selective heating, which is one of the causes of the microwave effect, may occur.

Gas-Phase Hydration of Perillaldehyde Investigated by Microwave Spectroscopy Assisted by Computational Chemistry.

The microsolvated complexes of two equatorial conformers of perillaldehyde were experimentally investigated in a supersonic molecular jet coupled to a cavity-based Fourier transform microwave spectrometer, in the 2.3-8 GHz frequency range. The structures of hydrates C10H14O·(H2O)n (n = 1,2,3) were first optimized at the MP2/6-311++G(d,p) and B3LYP-D3BJ/def2-TZVP levels of theory. The spectral signatures of four monohydrates and of two dihydrates could then be obtained. Additional rotational constants from the analysis of the spectra of their 18O isotopologues allowed the calculation of the substitution coordinates of the water oxygen atoms of each hydrate. They were found to be in good agreement with those of the optimized structures. SAPT2 calculations and noncovalent interaction analysis highlight the role of dispersion and quasi-hydrogen bonds in the stabilization of the structures.

C-H···O Hydrogen Bond Anchored Water Bridge in 1,2,4,5-Tetracyanobenzene-Water Clusters.

The hydrogen-bonded chain of water molecules is known to play a very important role in proton/H transfer in chemistry and biology. This kind of water chain mainly starts from a conventional hydrogen bond (HB) donor/acceptor site. Here, we report the experimental evidence of water chain formation on an unconventional C-H HB donor site in 1,2,4,5-tetracyanobenzene (TCNB). Laser-induced fluorescence (LIF) spectra and fluorescence dip IR (FDIR) spectra of 1: n ( n = 1-3) clusters of TCNB with water prepared in a supersonic molecular jet are presented. Quantum chemical calculations of several intuitive conformers of 1: n ( n = 1-3) clusters were performed, and the computed IR spectra were compared with the experimental FDIR spectra in order to get the structural information on the experimentally observed clusters. We find that the first water molecule binds to the C-H moiety of TCNB (1:1 cluster) and the subsequent water molecules form a water chain (1:2 and 1:3 clusters) that constitutes a bridge between the C-H and the proximal -CN moiety of TCNB, forming a cyclic ring structure. NBO calculations show that upon addition of water molecules significant change in the charge distribution takes place, mainly on the atoms which are involved in cyclic ring structure. This redistribution of charges causes a cooperativity effect in the higher water clusters of TCNB.

Microwave spectra and nuclear quadrupole structure of the NH3-N2 van der Waals complex and its deuterated isotopologues.

The microwave spectrum of the NH3-N2 van der Waals complex has been observed in a supersonic molecular jet expansion via broadband (2-8 GHz) chirped-pulse Fourier-transform microwave spectroscopy. Two pure rotational R(0) transitions (J = 1 - 0) with different hyperfine structure patterns were detected. One transition belongs to the (ortho)-NH3-(ortho)-N2 nuclear spin isomer in the ground K = 0 state reported earlier [G. T. Fraser et al., J. Chem. Phys. 84, 2472 (1986)], while another one is assigned to the (para)-NH3-(para)-N2 spin isomer in the K = 0 state not reported before (K is the projection of the total angular momentum J on the intermolecular axis). The complicated hyperfine structure arising from three quadrupole 14N nuclei of NH3-N2 was resolved for both transitions, and the quadrupole coupling constants associated with the NH3 and N2 subunits were precisely determined for the first time. These constants provided the dynamical information about the angular orientation of ammonia and nitrogen indicating that the average angle between the C 3 axis of NH3 and the N2 axis is about 66°. The average van der Waals bond lengths are slightly different for (ortho)-NH3-(ortho)-N2 and (para)-NH3-(para)-N2 and amount to 3.678 Å and 3.732 Å, respectively. Similar results for the deuterated isotopologues, ND3-N2, NHD2-N2, and NH2D-N2, and their nuclear spin isomers were also obtained thus confirming and extending the analysis for the parent NH3-N2 complex.

Flexibility at the Fringes: Conformations of the Steroid Hormone β-Estradiol.

We present the accurate experimental structure of the steroid hormone β-estradiol obtained with high-resolution rotational spectroscopy under the solvent free, isolated, and cold conditions of a molecular jet. This is the first time that the rotational signature of a steroid hormone is reported. β-Estradiol is a primary female sex hormone and features a rigid steroidal ring system. Three conformers could be identified in the cold environment of a supersonic molecular jet, which only differ in the orientation of the two hydroxy groups attached to the steroidal backbone. The conformers are almost isoenergetic and have very similar rotational constants but still could be clearly resolved and assigned. The high sensitivity of the technique allowed us to record and identify all singly substituted 13 C isotopologues in natural abundance for the dominant conformer. The additional spectroscopic constants led to an accurate determination of its experimental molecular structure. Along with a previous comparison of β-estradiol structures in the solid state and the liquid phase, this study completes the investigations in all three main phases by providing the interaction free gas-phase structure.

Millimeter-wave observations of the NH3–D2and ND3–H2van der Waals complexes

The rotational spectra of the NH 3 –D 2 and ND 3 –H 2 van der Waals complexes have been observed in a supersonic molecular jet for the first time. The OROTRON intracavity spectrometer was used to detect pure rotational transitions in the frequency range from 75 to 120 GHz. The search of the spectral lines was based on the initial observation of the main isotopic species of the NH3−H2. The measured transition frequencies including hyperfine splitting were analyzed in order to determine the energy levels, molecular parameters and structure of the ortho ND 3 − ortho H 2 and ortho NH 3 − para D 2 complexes.

Supersonic Jet Spectroscopy and Density Functional Theory Study of Isomeric Diazines: 1,4- and 1,8-Diazatriphenylene. Why Do They Differ So Deeply?

We report on laser-induced fluorescence excitation and dispersed fluorescence spectra of two isomeric compounds: 1,4- and 1,8-diazatriphenylene (1,4- and 1,8-DAT) isolated in supersonic molecular jets, and their 1:1 complexes with protic solvents. We found that the ground and excited state vibronic patterns of bare 1,4-DAT differ significantly from those of 1,8-DAT, and those of the complexes of both isomers. A marked activity of several out-of-plane vibrations in 1,4-DAT and the symptoms of the distortion of the S1 excited molecule were diagnosed from the vibronic spectra, whereas planar structures were predicted for 1,8-DAT in S0 and S1 states. An anharmonic double-minimum potential along an out-of-plane coordinate has been derived and used to predict higher overtones of the S1 state vibration at 113 cm-1. Large enhancement of fluorescence was observed upon formation of 1:1 complexes of 1,4-DAT with water or methanol, which is explained in terms of an increased separation of interacting (n,π*) and (π,π*) electronic states in the H-bonded complexes, and/or a suppression of the intersystem crossing process.

Experimental Observation of Dynamical Localization in Laser-Kicked Molecular Rotors.

The periodically kicked rotor is a paradigm system for studying quantum effects on classically chaotic dynamics. The wave function of the quantum rotor localizes in angular momentum space, similarly to Anderson localization of the electronic wave function in disordered solids. Here, we observe dynamical localization in a system of true quantum rotors by subjecting nitrogen molecules to periodic sequences of femtosecond pulses. Exponential distribution of the molecular angular momentum-the hallmark of dynamical localization-is measured directly by means of coherent Raman scattering. We demonstrate the suppressed rotational energy growth with the number of laser kicks and study the dependence of the localization length on the kick strength. Because of its quantum coherent nature, both timing and amplitude noise are shown to destroy the localization and revive the diffusive growth of energy.

An overview of recent HL-2A experiments

For the first time supersonic molecular beam injection (SMBI) and cluster jet injection (CJI) were applied to mitigate edge-localized modes (ELMs) in HL-2A successfully. The ELM frequency increased by a factor of 2–3 and the heat flux on the divertor target plates decreased by 50% on average after SMBI or CJI. Energetic particle induced modes were observed in different frequency ranges with high-power electron cyclotron resonance heating (ECRH). The high frequency (200–350 kHz) of the modes with a relatively small amplitude was close to the gap frequency of the toroidicity-induced Alfvén eigenmode. The coexistent multi-mode magnetic structures in the high-temperature and low-collision plasma could affect the plasma transport dramatically. Long-lived saturated ideal magnetohydrodynamic instabilities during strong neutral beam injection heating could be suppressed by high-power ECRH. The absolute rate of nonlinear energy transfer between turbulence and zonal flows was measured and the secondary mode competition between low-frequency (LF) zonal flows (ZFs) and geodesic acoustic modes (GAMs) was identified, which demonstrated that ZFs played an important role in the L–H transition. The spontaneously generated E × B shear flow was identified to be responsible for the generation of a large-scale coherent structure (LSCS), which provided unambiguous experimental evidence for the LSCS generation mechanism. New meso-scale electric potential fluctuations (MSEFs) at frequency f ∼ 10.5 kHz with two components of n = 0 and m/n = 6/2 were also identified in the edge plasmas for the first time. The MSEFs coexisted and interacted with magnetic islands of m/n = 6/2, turbulence and LF ZFs.

ChemInform Abstract: A Supersonic Molecular Jet for a Fourier Transform Interferometer: The ν3 Band of OCCCO.

Abstract A supersonic molecular jet apparatus has been constructed for use with a Fourier transform infrared spectrometer (Bruker IFS 120 HR) or with the millimeter-wave and diode laser spectrometers available in the Giessen laboratory. Design criteria were chosen to provide optimum conditions for obtaining spectral simplification for relatively large molecules with large amplitude motions. Use with unstable species is planned. Thus, throughput could not be too high, and temperature need not be extremely low. First results with the jet system attached to the interferometer are presented. Test measurements with CO2 show a rotational temperature of 22 K for a hole nozzle, and are used to contrast the spectra from a hole and a slit nozzle. A trial jet spectrum of OCCCO shows significant cooling of the low-lying ν7 bending mode and considerable simplification, revealing some new features not detected in an earlier room-temperature study.

ChemInform Abstract: Solid State Proton Spin Relaxation in Ethylbenzenes: Methyl Reorientation Barriers and Molecular Structure

We have investigated the dynamics of the ethyl groups and their constituent methyl groups in polycrystalline ethylbenzene (EB), 1,2‐diethylbenzene (1,2‐DEB), 1,3‐DEB, and 1,4‐DEB using the solid state proton spin relaxation (SSPSR) technique. The temperature and Larmor frequency dependence of the Zeeman spin‐lattice relaxation rate is reported and interpreted in terms of the molecular dynamics. We determine that only the methyl groups are reorienting on the nuclear magnetic resonance time scale. The observed barrier of about 12 kJ/mol for methyl group reorientation in the solid samples of EB, 1,2‐DEB, and 1,3‐DEB is consistent with that of the isolated molecule, implying that in the solid state, intermolecular electrostatic interactions play a minor role in determining the barrier. The lower barrier of 9.3±0.2 kJ/mol for the more symmetric 1,4‐DEB suggests that the crystal structure is such that the minimum in the anisotropic part of the intramolecular potential is raised by the intermolecular interactions leading to a 3 kJ/mol decrease in the total barrier. We are able to conclude that the methyl group is well away from the plane of the benzene ring (most likely orthogonal to it) in all four molecules, and that in 1,2‐DEB, the two ethyl groups are in the anticonfiguration. Our SSPSR results are compared with the results obtained by microwave spectroscopy and supersonic molecular jet laser spectroscopy, both of which determine molecular geometry better than SSPSR, but neither of which can determine ground electronic state barriers for these molecules.

Plasma behaviour with hydrogen supersonic molecular beam and cluster jet injection in the HL-2A tokamak

The experimental results of low pressure supersonic molecular beam injection (SMBI) fuelling on the HL-2A closed divertor indicate that during the period of pulsed SMBI the power density convected at the target plate surfaces was 0.4 times of that before or after the beam injection. An empirical scaling law used for the SMBI penetration depth for the HL-2A plasma was obtained. The cluster jet injection (CJI) is a new fuelling method which is based on and developed from the experiments of SMBI in the HL-1M tokamak. The hydrogen clusters are produced at liquid nitrogen temperature in a supersonic adiabatic expansion of moderate backing pressure gases into vacuum through a Laval nozzle and are measured by Rayleigh scattering. The measurement results have shown that the averaged cluster size of as large as hundreds of atoms was found at the backing pressures of more than 0.1 MPa. Multifold diagnostics gave coincidental evidence that when there was hydrogen CJI in the HL-2A plasma, a great deal of particles from the jet were deposited at a terminal area rather than uniformly ablated along the injecting path. SMB with clusters, which are like micro-pellets, will be of benefit for deeper fuelling, and its injection behaviour was somewhat similar to that of pellet injection. Both the particle penetration depth and the fuelling efficiency of the CJI were distinctly better than that of the normal SMBI under similar discharge operation. During hydrogen CJI or high-pressure SMBI, a combination of collision and radiative stopping forced the runaway electrons to cool down to thermal velocity due to such a massive fuelling.

Synthetic Spitzer Infrared Array Camera band maps from simulations of protostellar jets

The bandpasses for Spitzer images contain numerous molecular hydrogen emission lines. With the additional complication of the non-uniform spectral response, it is necessary to calculate model images in order to analyse observations of molecular outflows. We employ three-dimensional hydrodynamic simulations of a range of dense supersonic molecular jets, including various degrees of velocity pulsation and precession, to demonstrate how the observed structure depends on the band. Features in band 1, which is dominated by emission from vibrational levels, are even more concentrated than in K-band images. In contrast, bands 2-4 are dominated by rotational lines and bow shock wings appear extended. Especially in band 4, broken cavity walls can be detectable. An initial comparison with Spitzer data is made.

Formation of Ammonia Clusters in a Free Molecular Jet of Binary Mixtures

The formation of small ammonia clusters is investigated experimentally with a supersonic free molecular jet of NH3−X binary mixtures (X = Ar, He, N2), with a special attention paid to the effect of second species X on the cluster formation. The ion currents of protonated ammonia clusters (NH3)nH + (n = 2−6) were measured with a combined cluster source system and a double focussing sector field mass spectrometer. Experiments were made both by changing the species concentration at a constant total pressure and by changing the total pressure with a constant composition of each mixture. The NH3 mole fraction in the tested gas mixtures ranges from 10 to 100% (pure ammonia). It is observed that the formation process of ammonia clusters strongly depends on the nature of the carrier gas and its percentage in the gas mixture. The most likely explanation seems to depend on the unique properties of the free jets; the thermal anisotropy, the spatial cooling, and the mass separation effects.

Prevalence of the alkyl/phenyl-folded conformation in benzylic compounds C6H5CH2-X-R (X=O, CH2, CO, S, SO, SO2): significance of the CH/pi interaction as evidenced by high-level ab initio MO calculations.

Ab initio MO calculations were carried out to examine the conformational energies of various benzylic compounds C(6)H(5)CH(2)XR (X=O, CH(2), CO, S, SO, SO(2); R=CH(3), C(2)H(5), iC(3)H(7), tC(4)H(9)) at the MP2/6-311G(d,p)//MP2/6-31G(d) level. Rotamers with R/Ph in gauche relationship are generally more stable than the R/Ph anti rotamers. In these stable geometries, the interatomic distance in the interaction of alpha- or beta-CH in the alkyl group and the ipso-carbon atom of the phenyl ring is short. The computational results are consistent with experimental data from supersonic molecular jet spectroscopy on 3-n-propyltoluene and NMR and crystallographic data on structurally related ketones, sulfoxides, and sulfones. In view of this, the alkyl/phenyl-congested conformation of these compounds has been suggested to be a general phenomenon, rather than an exception. The attractive CH/pi interaction has been suggested to be a dominant factor in determining the conformation of simple aralkyl compounds.

Low-Temperature Epitaxial Growth of Cubic Silicon Carbide on Si(100) for Submicron-Pattern Fabrication

SiC films were selectively grown on patterned SiO2/Si(100) substrates using supersonic molecular jet epitaxy. For film growth, we employed methylsilane seeded in a carrier gas (He or H2) as a reactant gas. Due to the high translational kinetic energy of methylsilane molecules in the supersonic jet, the growth temperature was as low as 670°C. The reduction of the growth temperature is explained in terms of a decrease in the activation energy and enhancement of the reaction efficiency from methylsilane to the SiC film during film growth. More importantly, a high growth rate of SiC at low temperature yielded submicron patterns of SiC without degradation of the SiO2 mask.

Growth of Boron Nitride Thin Films on Silicon Substrates Using New Organoboron Precursors

Thin films of BN have been deposited on silicon substrates in the temperature range of 600 to 900 °C by supersonic molecular jet epitaxy and plasma-assisted MOCVD using new organoboron precursors such as borane-triethylamine complex (BTEA) and tris(sec-butyl)borane (TSBB). Hydrogen was used as carrier gas, and additional nitrogen and ammonia as reactive gases were supplied by either through nozzle or via remoted plasma. Crack-free, polycrystalline hexagonal BN films with nano-size crystals were successfully grown at temperature as low as 750 °C using BTEA. With increasing substrate temperatures to 900 °C, however, the h-BN films chenged polycrystalline with smoother surface and larger crystals. The changes of relative film quality with deposition temperature and reactive gas have been confirmed with FTIR. Nitrogen plasma enhanced the film quality and the stoichiometry, and removed the surface carbon effectively rather than ammonia and hydrogen plasmas molecular beam. Wide band gaps were calculated using optical transmittance measurements.

Growth of Boron Nitride Thin Films on Silicon Substrates Using New Organoboron Precursors

Thin films of BN have been deposited on silicon substrates in the temperature range of 600 to 900 °C by supersonic molecular jet epitaxy and plasma-assisted MOCVD using new organoboron precursors such as borane-triethylamine complex (BTEA) and tris(sec-butyl)borane (TSBB). Hydrogen was used as carrier gas, and additional nitrogen and ammonia as reactive gases were supplied by either through nozzle or via remoted plasma. Crack-free, polycrystalline hexagonal BN films with nano-size crystals were successfully grown at temperature as low as 750 °C using BTEA. With increasing substrate temperatures to 900 °C, however, the h-BN films chenged polycrystalline with smoother surface and larger crystals. The changes of relative film quality with deposition temperature and reactive gas have been confirmed with FTIR. Nitrogen plasma enhanced the film quality and the stoichiometry, and removed the surface carbon effectively rather than ammonia and hydrogen plasmas molecular beam. Wide band gaps were calculated using optical transmittance measurements.

Direct epitaxial growth of submicron-patterned SiC structures on Si(001)

We report on the direct epitaxial growth of submicron-patterned SiC structures on Si(001) substrates using supersonic molecular jet epitaxy and resistless e-beam lithography. Prior to SiC film growth, an electron beam was scanned on hydrogen-passivated Si substrates in order to produce silicon oxide lines with widths ⩾60 nm. The SiC nucleation and growth rates were significantly reduced on the oxidized regions during the subsequent supersonic jet epitaxial growth of SiC, which yielded epitaxial, submicron-patterned SiC films. The effects of the growth temperature and e-beam dose on the SiC growth and pattern linewidth are discussed.