Supersonic Source Research Articles

Experimental binding energies for the metal complexes [Mg(NH3)n]2+, [Ca(NH3)n]2+, and [Sr(NH3)n]2+ for n = 4-20 determined from kinetic energy release measurements.

A supersonic source of clusters has been used to prepare neutral complexes of ammonia in association with a metal atom. From these complexes the following metal-containing dications have been generated: [Mg(NH3)n](2+), [Ca(NH3)n](2+), and [Sr(NH3)n](2+), and for n in the range 4-20, kinetic energy release measurements following the evaporation of a single molecule have been undertaken using a high resolution mass spectrometer. Using finite heat bath theory, these data have been transformed into binding energies for individual ammonia molecules attached to each of the three cluster systems. In the larger complexes (n > 6) the results exhibit a consistent trend, whereby the experimental binding energy data for all three metal ions are very similar, suggesting that the magnitude of the charge rather than charge density influences the strength of the interaction. From a comparison with data recorded previously for (NH3)nH(+) it is found that the 2+ charge on a metal ion has an effect on the binding energy of molecules in complexes containing up to 20 solvent molecules. Although subject to comparatively large experimental errors, the results recorded for Ca(2+) and Sr(2+) when n ≤ 6 show evidence for the formation of an inner solvation shell containing up to 6 molecules. However, Mg(2+) exhibits relatively low binding energies when n = 5 and 6, which suggests that a second shell starts to form before there are 6 ammonia molecules bound to the metal ion. This conclusion is supported by DFT calculations and it is proposed that these complexes could take the form [Mg(NH3)4(NH3)](2+) when n = 5 and either [Mg(NH3)4(NH3)2](2+) or [Mg(NH3)5(NH3)](2+) when n = 6. In each case, additional molecules are hydrogen bonded to one or more molecules in the inner solvation shell.

Infrared photodissociation spectroscopy of oxygen-rich Fe(O2)n(+) (n = 3-5) cation complexes.

Infrared spectra of mass-selected oxygen-rich iron dioxygen complexes Fe(O2)n(+) with n = 3-5 are measured via infrared photodissociation spectroscopy in the gas phase. These cation complexes are produced via a laser vaporization supersonic ion source. The structures are established by comparison of the experimental spectra with the simulated spectra derived from density functional calculations. All of the Fe(O2)n(+) complexes studied have a single IR-active band in the 1050-1100 cm(-1) region, arising from the O-O stretching vibration of the superoxo ligand(s). These complexes are determined to have structures with a chemically bound Fe(O2)2(+) core ion that is weakly coordinated by neutral O2 molecules. The Fe(O2)2(+) core ion has a planar D2h symmetry with two equivalent side-on superoxo ligands bound to an Fe(3+) cation center.

Supersonic Jet Noise: Main Sources and Reduction Methodologies

The large velocity ratio and the presence of Shocks in the exhaust plume from low bypass engines or supersonic jetliners cause jet noise to be dominant component of overall aircraft noise, and therefore is an important issue in design of the next generation of civil supersonic transport. Jet noise reduction technology also has application in the design of high- performance tactical aircraft. Jet noise is of particular concern on aircraft carriers where it is necessary for deck crew to be in relatively close proximity to the aircraft at takeoff and landing. In this paper, a brief discussion about supersonic jet noise sources and a review of the main passive technologies employed for the reduction of supersonic jet noise are presented.

Matrix isolation and DFT study of the conformations of diethylcarbonate

Conformations of diethylcarbonate (DEC) were studied using matrix isolation infrared spectroscopy. DEC was trapped in an Ar matrix at 12K, using both an effusive source maintained at 298 and 423K and a supersonic jet source. The experimental studies were supported by computations performed at the B3LYP/6-31++G** level of theory. The ground state conformer for DEC was found to have a “cc(tt)” geometry, where the ‘c’ refers to a cis orientation of the carbons attached to oxygen, while the ‘t’ in parenthesis refers to the trans orientation of the terminal carbon. In addition to the ground state conformer, our computations also indicated local minima corresponding to conformers with cc(tg±), cc(g±g∓) and cc(g±g±) geometries, listed in increasing order of energy. Natural bond orbital analysis was also performed to understand the role of delocalization interactions and steric effect on conformational stability.

NO Adsorption on Copper Phthalocyanine Functionalized Graphite

NO dosed on a CuPc monolayer deposited on Au(111) and HOPG is observed by scanning tunneling microscopy. After dosing NO with a supersonic molecular beam source onto CuPc/Au(111), about 7% of CuPc molecules form chemisorbates with NO. Conversely, after dosing onto CuPc/HOPG, only about 0.1% CuPc molecules form chemisorbates with NO, even though the reaction sites appear nearly identical. DFT calculations were employed to elucidate the mechanism which causes the >10× difference in saturation coverage between NO/CuPc/Au(111) and NO/CuPc/HOPG. DFT calculations show NO chemisorption with CuPc/Au(111) induces only negligible perturbation in the density of states (DOS) in Au(111) due to large density of states on Au. Conversely, for NO/CuPc/HOPG, there is a large decrease of DOS in graphene around 1 eV due to NO chemisorption on CuPc/graphene consistent with negative charge transfer from graphene to NO. This DOS perturbation of graphene results in decreased binding energy of NO chemisorption in secondary NO sit...

A collinear tandem time-of-flight mass spectrometer for infrared photodissociation spectroscopy of mass-selected ions

An apparatus based on collinear tandem time-of-flight mass spectrometer has been designed for the measurement of infrared photodissociation spectroscopy of mass-selected ions in the gas phase. The ions from a pulsed laser vaporization supersonic ion source are skimmed and mass separated by a Wiley-McLaren time-of-flight mass spectrometer. The ion of interest is mass selected, decelerated and dissociated by a tunable IR laser. The fragment and parent ions are reaccelerated and mass analyzed by the second time-of-flight mass spectrometer. A simple new assembly integrated with mass gate, deceleration and reacceleration ion optics was designed, which allows us to measure the infrared spectra of mass selected ions with high sensitivity and easy timing synchronization.

Pulsed supersonic source of vdW complexes for high-temperature applications: Spectroscopy and beam characteristics

Application of a modified version of high-temperature high-pressure all-metal pulsed source of supersonic molecular beam is demonstrated in a production of van der Waals (vdW) complexes. The vdW complexes are produced possessing controllable rotational temperature (Trot) in the range from 3 K to 19 K. An effective control over Trot is illustrated employing excitation spectrum recorded using the B31(53P1) ← X10+(51S0) transition in CdAr. First-time resolved rotational structure in the profile of the υ′ = 2←υ′′ = 0 vibrational component is reported. The control over Trot is crucial in a dissociation of the (111Cd)2 isotopologue in the supersonic beam. For the process, excitation at well defined J′← J′′ rotational transition within the (υ′,υ′′) = (40,0) vibrational band of the A10u+(51P1) ← X10g+(51S0) transition is employed. It is followed by the dissociation using A10u+(υ′ = 40,J′′) → X10g+ bound → free transition. An analysis and simulation of the (40,0) vibrational band rotational structure are presented. Parameters describing conditions in the supersonic beam, degree of rotational cooling, Doppler broadening and spectral bandwidth of the laser beam are used.

Reprint of: Negative carbon cluster ion beams: New evidence for the special nature of C60

Cold carbon cluster negative ions are formed by supersonic expansion of a plasma created at the nozzle of a supersonic cluster beam source by an excimer laser pulse. The observed distribution of mass peaks for the Cn- ions for n>40 demonstrates that the evidence previously given for the special stability of neutral C60 and the existence of spheroidal carbon shells cannot be an artifact of the ionization conditions.

Infrared Spectroscopy of Solvation in Small Zn+(H2O)n Complexes

Singly charged zinc-water cations are produced in a pulsed supersonic expansion source using laser vaporization. Zn(+)(H2O)n (n = 1-4) complexes are mass selected and studied with infrared laser photodissociation spectroscopy, employing the method of argon tagging. Density functional theory (DFT) computations are used to obtain the structures and vibrational frequencies of these complexes and their isomers. Spectra in the O-H stretching region show sharp bands corresponding to the symmetric and asymmetric stretches, whose frequencies are lower than those in the isolated water molecule. Zn(+)(H2O)nAr complexes with n = 1-3 have O-H stretches only in the higher frequency region, indicating direct coordination to the metal. The Zn(+)(H2O)2-4Ar complexes have multiple bands here, indicating the presence of multiple low energy isomers differing in the attachment position of argon. The Zn(+)(H2O)4Ar cluster uniquely exhibits a broad band in the hydrogen bonded stretch region, indicating the presence of a second sphere water molecule. The coordination of the Zn(+)(H2O)n complexes is therefore completed with three water molecules.

High-resolution electron spectroscopy and molecular structures of Cu–(2,2′-bipyridine) and Cu-(4,4′-bipyridine)

Copper complexes of 2,2′-bipyridine (22BIPY) and 4,4′-bipyridine (44BIPY) were prepared in a laser-vaporization supersonic molecular beam source and identified by laser photoionization time-of-flight mass spectrometry. Electronic spectra and molecular structures were studied with pulsed-field ionization zero electron kinetic energy (ZEKE) electron spectroscopy, density functional theory (DFT) and second-order Møller–Plesset perturbation (MP2) calculations, and spectral simulations. Adiabatic ionization energies and metal–ligand and ligand-based vibrational frequencies of Cu–22BIPY and Cu–44BIPY were measured from the ZEKE spectra. Ground electronic states and molecular structures of the two complexes were determined by comparing the spectroscopic measurements with the theoretical calculations. The ground state of Cu–22BIPY ( 2 B1, C2v) has a planar bidentate structure with Cu binding to two nitrogen atoms and two pyridine molecules in the cis configuration. The ground state of Cu–44BIPY ( 2 A, C2) has a monodentate structure with Cu binding to one nitrogen and two pyridines in a twisted configuration. The ionization energy of Cu–22BIPY is considerably lower and its bond energy is much higher than that of Cu–44BIPY. The different ionization and dissociation energies are attributed to the distinct metal binding modes of the two complexes. It has been found that the DFT calculations yield the correct structures for the Cu–22BIPY complex, whereas the MP2 calculations produce the best structures for the Cu–44BIPY complex.

Reprint of “Binding energies determined from kinetic energy release measurements following the evaporation of single molecules from the molecular clusters H+(H2O)n, H+(NH3)n and H+ (CH3OH)n”

High resolution mass spectrometry in conjunction with a supersonic source of clusters has been used to record values for the average kinetic energy release that accompanies fragmentation of the molecular cluster ions: H+(H2O)n, H+(NH3)n and H+(CH3OH)n for n in the range 3–30. Using finite heat bath theory, these data have been transformed into binding energies for molecules attached to each of the three cluster systems. For n>6 the results show quantitative agreement with the strengths of hydrogen bonds expected to be present in the clusters and it is concluded that each cluster fragments via the breaking of a single hydrogen bond.

High-Resolution Rovibrational Spectroscopy of Jet-Cooled Phenyl Radical: The ν19 Out-of-Phase Symmetric CH Stretch

Phenyl radical has been studied via sub-Doppler infrared spectroscopy in a slit supersonic discharge expansion source, with assignments for the highest frequency b2 out-of-phase C-H symmetric stretch vibration (ν19) unambiguously confirmed by ≤6 MHz (0.0002 cm(-1)) agreement with microwave ground state combination differences of McMahon et al. [Astrophys. J. 2003, 590, L61-64]. Least squares analysis of over 100 resolved rovibrational peaks in the sub-Doppler spectrum to a Watson Hamiltonian yields precision excited-state rotational constants and a vibrational band origin (ν0 = 3071.8915(4) cm(-1)) consistent with a surprisingly small red-shift (0.9 cm(-1)) with respect to Ar matrix isolation studies of Ellison and co-workers [J. Am. Chem. Soc. 2001, 123, 1977]. Nuclear spin weights and inertial defects confirm the vibrationally averaged planarity and (2)A1 rovibronic symmetry of phenyl radical, with analysis of the rotational constants consistent with a modest C2v distortion of the carbon backbone frame due to partial sp rehybridization of the σ C radical-center. Most importantly, despite the number of atoms (N = 11) and vibrational modes (3N - 6 = 27), phenyl radical exhibits a remarkably clean jet cooled high-resolution IR spectrum that shows no evidence of intramolecular vibrational relaxation (IVR) phenomena such as local or nonlocal perturbations due to strongly coupled nearby dark states. This provides strong support for the feasibility of high-resolution infrared spectroscopy in other aromatic hydrocarbon radical systems.

Note: A new design for a low-temperature high-intensity helium beam source

A high-intensity supersonic beam source is a key component of any atom scattering instrument, affecting the sensitivity and energy resolution of the experiment. We present a new design for a source which can operate at temperatures as low as 11.8 K, corresponding to a beam energy of 2.5 meV. The new source improves the resolution of the Cambridge helium spin-echo spectrometer by a factor of 5.5, thus extending the accessible timescales into the nanosecond range. We describe the design of the new source and discuss experiments characterizing its performance. Spin-echo measurements of benzene/Cu(100) illustrate its merit in the study of a typical slow-moving molecular adsorbate species.

Carbonyl Bonding on Oxophilic Metal Centers: Infrared Photodissociation Spectroscopy of Mononuclear and Dinuclear Titanium Carbonyl Cation Complexes

Mononuclear and dinuclear titanium carbonyl cation complexes including Ti(CO)(6)(+), Ti(CO)(7)(+), TiO(CO)(5)(+), Ti(2)(CO)(9)(+) and Ti(2)O(CO)(9)(+) are produced via a laser vaporization supersonic cluster source. The ions are mass selected in a tandem time-of-flight mass spectrometer and studied with infrared photodissociation spectroscopy in the CO stretching frequency region. The structures are established by comparison of the experimental spectra with simulated spectra derived from density functional calculations. Only one IR band is observed for the 15-electron Ti(CO)(6)(+) cation, which is characterized to have an octahedral O(h) structure. The Ti(CO)(7)(+) cation is determined to be a weakly bound complex involving a Ti(CO)(6)(+) core ion instead of the seventh coordinated ion. The TiO(CO)(5)(+) cation has a completed coordination sphere with a C(4v) structure. The Ti(2)(CO)(9)(+) cation is determined to have a doublet C(s) structure with two four-electron donor side-on bridging CO groups and one semibridging CO group. The Ti(2)O(CO)(9)(+) cation has a doublet C(s) structure involving a planar cyclic Ti(2)O(η(2)-μ-CO) core with a four electron donor side-on bridging CO. Bonding analysis indicates that the Ti(2)(CO)(9)(+) and Ti(2)O(CO)(9)(+) cations each have a Ti-Ti single bond. The results suggest that metal-metal multiple bonding is not favorable, and the oxophilic titanium centers failed to satisfy the 18-electron configuration in these metal carbonyl complexes.

Coordination and Solvation of the Au+ Cation: Infrared Photodissociation Spectroscopy of Mass-Selected Au(H2O)n+ (n = 1–8) Complexes

Gold cation-water complexes with attached argon atoms are produced via a laser vaporization supersonic cluster source. The [Au(H(2)O)(n)Ar(x)](+) (n = 1-8; x = 1 or 2) complexes are each mass selected and studied by infrared photodissociation spectroscopy in the OH stretching frequency region to explore the coordination and solvation structures of the Au(+) cation. Density functional calculations have been performed, and the calculated vibrational spectra are compared to the experimental spectra to identify the gas-phase structures of the Au(H(2)O)(n)(+) complexes. Confirming previous theoretical predications, the first coordination shell of the Au(+) cation contains two water molecules forming a linear O-Au(+)-O arrangement; subsequent water molecules bind to the two H(2)O ligands of the Au(H(2)O)(2)(+) core ion via hydrogen bond forming of the second hydration shell, which is complete at n = 6. For the complexes with n ≤ 7, the experimental spectrum can in general be assigned to the predicted global minimum structure. However, the spectrum suggests that two or more conformers coexist for the n = 8 complex, indicating that the identification of a single global minimum becomes less important upon increasing the number of solvating water molecules.

Binding energies determined from kinetic energy release measurements following the evaporation of single molecules from the molecular clusters H+(H2O)n, H+(NH3)n and H+(CH3OH)n

High resolution mass spectrometry in conjunction with a supersonic source of clusters has been used to record values for the average kinetic energy release that accompanies fragmentation of the molecular cluster ions: H+(H2O)n, H+(NH3)n and H+(CH3OH)n for n in the range 3–30. Using finite heat bath theory, these data have been transformed into binding energies for molecules attached to each of the three cluster systems. For n>6 the results show quantitative agreement with the strengths of hydrogen bonds expected to be present in the clusters and it is concluded that each cluster fragments via the breaking of a single hydrogen bond.

Gas-phase rotational spectroscopy of AlCCH [formula omitted]: A model system for organo-aluminum compounds

The pure rotational spectrum of AlCCH in its ground electronic state (X∼1Σ+) has been measured using Fourier transform microwave (FTMW) and mm/sub-mm direct absorption spectroscopy. AlCCH was created in a DC discharge from HCCH and aluminum vapor, either produced by a Broida-type oven, or generated from Al(CH3)3 in a supersonic jet source. Rotational transitions were measured for five isotopologues of AlCCH, with 13C and deuterium substitutions. From these data, rotational and Al and D quadrupole parameters were determined, as well as an accurate structure. AlCCH appears to exhibit an acetylenic arrangement with significant covalent character in the Al–C single bond.

A desktop supersonic free-jet beam source for a scanning helium microscope (SHeM)

A simple design for an inexpensive, compact, desktop-sized helium free-jet beam source is described. The apparatus, which is Campargue-like in design and utilizes mostly off-the-shelf parts, is capable of producing a centreline intensity of 1.2 × 1019 atoms per second per steradian. The beam performance was investigated using a conventional ion gauge and a stagnation detector, with the latter being used to produce beam flux profiles. The profile of the macroskimmed beam has been experimentally demonstrated to be strongly Gaussian.

Blackbody-induced radiative dissociation of cationic SF6clusters

The stability of cationic SF${}_{5}$${}^{+}$(SF${}_{6}{)}_{n\ensuremath{-}1}$ clusters was investigated by measuring their blackbody-induced radiative dissociation (BIRD) rates. The clusters were produced in a supersonic expansion ion source and stored in an electrostatic ion-beam trap at room temperature, where their abundances and lifetimes were measured. Using the ``master equation'' approach, relative binding energies of an SF${}_{6}$ unit in the clusters could be extracted from the storage-time dependence of the survival probabilities. The results allow for a deeper insight into the effect of a localized charge on the structure and stability of SF${}_{6}$-based clusters.

Merged-beams for slow molecular collision experiments

Molecular collisions can be studied at very low relative kinetic energies, in the milliKelvin range, by merging codirectional beams with much higher translational energies, extending even to the kiloKelvin range, provided that the beam speeds can be closely matched. This technique provides far more intensity and wider chemical scope than methods that require slowing both collision partners. Previously, at far higher energies, merged beams have been widely used with ions and/or neutrals formed by charge transfer. Here, we assess for neutral, thermal molecular beams the range and resolution of collision energy that now appears attainable, determined chiefly by velocity spreads within the merged beams. Our treatment deals both with velocity distributions familiar for molecular beams formed by effusion or supersonic expansion, and an unorthodox variant produced by a rotating supersonic source capable of scanning the lab beam velocity over a wide range.