Free Jet Expansion Research Articles

Observation of the FeNC molecule by laser fluorescence excitation spectroscopy

A new molecular electronic band system has been observed in the near UV by laser fluorescence excitation in a free-jet supersonic expansion of photolyzed Fe(CO)5 diluted in seed gases containing acetonitrile. The three observed bands displayed the rotational structure of a Ω′=7/2←Ω″=9/2 transition and could be assigned as an excited-state progression in a single vibrational mode. The rotational constant B″ and its change upon isotopic substitution in acetonitrile [CH313CN] was consistent with the assignment of the molecular carrier as the linear FeNC molecule. In analogy with the iron monohalides, which have electronic band systems in the same wavelength range, the transition can be described as a 4p←4s excitation. The derived structure is compared with that of other metal-containing cyanide/isocyanide molecules.

Conformational Preferences of Neurotransmitters: Norephedrine and the Adrenaline Analogue, 2-Methylamino-1-phenylethanol

The conformational preferences of the neurotransmitters (1R2S) norephedrine and 2-methylamino-1-phenylethanol (MAPE) have been examined under free-jet expansion conditions using a combination of laser-induced fluorescence (LIF), mass-selected resonant 2-photon ionization (R2PI), and infrared ion-dip spectroscopy together with ab initio calculations. Comparison of experimental infrared spectra and rotational band contours with ab initio data has allowed a full structural assignment of three conformers in MAPE and two in norephedrine. All five conformers are stabilized by intramolecular hydrogen bonding between the functional groups of the ethanolamine side chain, with the OH group acting as a proton donor. Further stabilizing interactions are provided by NH···π hydrogen bonds and by dispersive interactions between the methyl group of the side chain and the aromatic ring. A delicate balance of these factors controls both the relative stabilities of the conformers and, through cooperative effects, the strength of the primary OH···N hydrogen bond. The existence of a unique IR absorption spectrum for each individual conformer provides a powerful tool for the assignment of molecular conformation.

Spiers Memorial Lecture. Comparative studies of cluster dynamics in the gas and condensed phases.

Although an overview of the field of cluster dynamics is impossible to give in this Introductory Lecture, several important themes have emerged that can provide a backdrop for this Faraday Discussion. As a result, the first part of this paper introduces some of these themes, using results from the literature to illustrate some of the important issues. Although much of the work on clusters is being carried out in the gas phase (usually in free jet expansions and molecular beams), new approaches have recently been developed that permit the study of the same complexes in or on nanoscale clusters. In particular, recent developments in the field of superfluid helium droplet spectroscopy (SHEDS) permit high-resolution studies of atoms, molecules and clusters in this exotic quantum fluid. After introducing this new field, the author will discuss experiments from his own laboratory to illustrate some of the capabilities of the technique.

Conformational Preferences of Neurotransmitters: Ephedrine and Its Diastereoisomer, Pseudoephedrine

The conformational preferences of the diastereomeric neurotransmitters (1R2S) ephedrine and (1S2S) pseudoephedrine have been studied in the gas phase, under free jet-expansion conditions, using ultraviolet spectroscopy (both R2PI and LIF) and infrared ion-dip and hole-burning spectroscopy in combination with ab initio calculation. This has led to the identification and assignment of two conformers in ephedrine and four in pseudoephedrine. Assignments have been made by comparing their experimental infrared and LIF spectra with ab initio vibrational frequencies and ultraviolet rotational band contours. The relative stabilities of the conformers are controlled by a delicate balance between intramolecular hydrogen bonding and dispersive interactions between the methyl groups of the side chain, both with each other and with the aromatic ring. The relative conformational stabilities calculated for ephedrine do not agree with the experimental results; two of its low-lying conformers were detected, but a third, lying at an intermediate energy, was not. The possibility of its collisional relaxation into the global minimum during the supersonic expansion was not supported by the ab initio calculations, which predict a substantial barrier along the minimum energy pathway. It is possible that the combination of a relatively weak transition moment and a lack of facile pathways for relaxation from higher lying structures into the “missing” conformer may play a role.

The rotational spectrum of dichlorocarbene, C35Cl2, observed by molecular beam-Fourier transform microwave spectroscopy

The rotational spectrum of the most abundant isotopomer of dichlorocarbene, C35Cl2, in its 1A1 ground electronic state has been recorded using a molecular beam—Fourier transform microwave spectrometer. The CCl2 was generated by flash pyrolysis in a pulsed free jet expansion using CBr2Cl2 or CHCl2COCl as precursors. 43 hyperfine structure lines of 6 rotational transitions were recorded in the frequency range 8–40 GHz. The analysis of the observed spectrum yielded the rotational constants, the complete 35Cl nuclear quadrupole coupling tensor, and the nuclear spin-rotation interaction constants. In addition, a refined r0 structure of CCl2 was derived.

Mass spectrometric implications of high-pressure ion sources

The nature of supersonic free jet expansion of a gas from high pressure into vacuum is reviewed and characterized in this article. The increasingly widespread applications and implications of this jet expansion process in mass spectrometry are described. Particular attention is paid to prospective advantages and possible problems when such jets are used to transport ions from a source at high pressure into the vacuum environment of a mass analyzer.

Nuclear spin–rotation interaction in CF 2 ( [formula omitted]) observed by Fourier transform microwave spectroscopy

The rotational spectrum of difluoromethylene (CF 2) in its X ̃ 1A 1 ground electronic state has been observed using molecular beam–Fourier transform microwave spectroscopy. CF 2 was produced in a free jet expansion in Ar as inert carrier gas by ArF excimer laser photolysis of CF 2Cl 2, CF 2CHCl, or CF 2CHBr or by flash pyrolysis of CF 2Br 2 in the high-pressure region directly behind a pulsed molecular beam valve. Rotational transitions were observed in the frequency range from 8 to 40 GHz in the CF 2 (000) ground vibrational state and in the (100), (010), and (001) excited vibrational states. Several observed lines were split due to the hyperfine interaction between the 19 F nuclear spin and the overall rotation of the molecule. The results were used to determine the nuclear spin–rotation interaction parameters for CF 2.

The Electronic Spectroscopy of 2,2‘-Binaphthyl in Solution, Cryogenic Matrix and Supersonic Jet

The electronic spectroscopy of 2,2‘-binaphthyl near 330 nm has been investigated by absorption spectroscopy in room-temperature solution and Shpolskii matrix at 4 K and by fluorescence excitation spectroscopy in a supersonic free-jet expansion. In the solution and gas phase, where the molecules are free to form the minimum-energy conformation, the spectra are quite different from those of 1,1‘-binaphthyl and the parent naphthalene. However, when frozen in a matrix, aspects of the electronic spectrum resembled features of the spectra of these molecules. The fluorescence excitation spectrum in the free jet showed several long progressions in the torsional vibration with a characteristic, but anharmonic, frequency of about 30 cm-1. The spectrum was assigned as the 11B ← 11A transition with an origin transition, which, although it could not be observed, was estimated to be at 30 060 ± 30 cm-1. The frequency of 29 members of this progression allowed an accurate torsional potential to be calculated, which was flattened at low energy and approached harmonic at higher energy. The change in equilibrium torsional angle was estimated to be 32° upon electronic excitation, with the excited state being planar (either cis or trans). Several low-frequency vibrations were assigned in the gas phase that probably involve motions of the naphthalene structures as a whole about the central C−C bond. In the matrix, the torsional vibrations were frozen out, which allowed for the observation and tentative assignment of several higher-frequency vibrations resembling those of the bare naphthalene. The origin frequency was measured at 28 460 cm-1. The difference between the gas-phase and solid-phase frequencies was attributed to the constrained structure of the molecule in the matrix.

Vibronic spectroscopy and lifetime of S1 acrolein

The S1 vibronic spectrum of acrolein (CH2CHCHO) was obtained in a supersonic free-jet expansion using cavity ringdown spectroscopy. Comparison of room temperature and free-jet spectra was used to assign vibronic transitions. Computations using the configuration interaction-singles method were used to predict the values of the S1 fundamental vibrational frequencies. The current set of assignments increases the number of identified bands from 27 to 59, corrects 16 misassignments in the previous literature, and identifies two new fundamental bands. A lower limit for the lifetime of the S1 vibronic origin was determined from linewidth measurements to be 1.8–2.1 ps.

High-resolution Laser Spectroscopy of the ν6 Fundamental Band of Jet-Cooled C2F6

The ν6 parallel band of C2F6 has been measured at high resolution by using diode laser absorption spectroscopy in a free jet expansion. A comparison of the rotational intensity distribution with simulated band profiles indicated a rotational temperature of between 2 and 3 K in the jet. In addition to the fundamental band, weak Q-branches of other bands were also found, both to the red and to the blue side of the ν6 fundamental Q-branch. The band origin of the ν6 mode was measured as 713.671 05(10) cm-1. The B rotational constants were determined in different ways as described in the text. A preferred value of B0 = 0.061 580 6(22) cm-1 was selected for simulation purposes. This value is independent of the electron diffraction structure.

Molecular beam study of the Π6X 6Δ electronic transition in FeCl

The Π6–X 6Δ band system of Fe35Cl near 357 nm has been recorded at 0.3 and 0.08 cm−1 resolution by laser fluorescence excitation in a free-jet supersonic expansion of photolyzed Fe(CO)5 diluted in seed gases containing CCl4. The ΠΩ′6=Ω″−1–X 6ΔΩ″ subbands for Ω′=7/2, 5/2, and 3/2 and v′⩽4 of the Δv=1, 0, −1, and −2 sequences were assigned, and excitation energies and rotational constants were derived for the relevant upper and lower vibronic levels. With the exception of the (0,0) band, for which one subband could be rotationally analyzed, it was not possible to assign any of the subbands with Ω′=1/2, −1/2, or −3/2; these fine-structure components are expected to be strongly affected by homogeneous spin–orbit interactions with nearby sextet electronic states, leading to large Λ doubling in the Ω′=1/2 and −1/2 components. A subband with a large upper-state Λ doubling was rotationally analyzed and assigned as the Π1/26–X 6Δ3/2 (0,0) subband. The effect of the off-diagonal spin–orbit interactions is discussed in light of the present results and previous theoretical work.

Rapid expansion from supercritical to aqueous solution to produce submicron suspensions of water-insoluble drugs.

Stable suspensions of submicron particles of cyclosporine, a water-insoluble drug, have been produced by rapid expansion from supercritical to aqueous solution (RESAS). To minimize growth of the cyclosporine particles, which would otherwise occur in the free jet expansion, the solution was sprayed into an aqueous Tween-80 (Polysorbate-80) solution. Steric stabilization by the surfactant impedes particle growth and agglomeration. The particles were an order of magnitude smaller than those produced by RESS into air without the surfactant solution. Concentrations as high as 38 mg/mL for 400-700 nm particles were achieved in a 5.0% (w/w) Tween-80 solution.

The Electronic Spectrum of Tellurium Dioxide

The electronic spectrum of gas-phase tellurium dioxide has been recorded between 345 and 406 nm using the technique of laser-induced fluorescence spectroscopy. The TeO2 sample was prepared by direct heating of the solid and by seeding it in a continuous free-jet expansion in argon. Twenty-seven cold bands and thirty-two hot bands were assigned. The wavenumbers of the band origin and symmetric stretching and bending vibrational modes for the upper and lower states were determined in a simple least-squares fit: ν0 = 25526 cm−1, ω1′ = 679 cm−1, ω2′ = 220 cm−1, ω1″ = 823 cm−1, ω2″ = 282 cm−1.

Infrared laser absorption spectroscopy of the ν7 band of jet-cooled iron pentacarbonyl

The ν 7 parallel band of Fe(CO) 5 has been measured in the 620 cm −1 region using high-resolution diode laser absorption spectroscopy in a free jet expansion. A comparison with simulated band profiles indicated a rotational temperature of between 2 and 3 K in the jet. At these temperatures the K-structure of the Q-branch is partly resolved. The following molecular parameters were obtained: ν 0=619.95747(12) cm −1, B 7=0.026743(2) cm −1, A 7=0.030721(1) cm −1. Approximate values of the quartic centrifugal distortion constants were also obtained from fitting the spectra.

S 1 –S 2 vibronic coupling in cis-1,3,5-hexatriene. I. Electronic structure calculations

The estimates for the vertical excitation energy of the 2 1A1 state of cis-1,3,5-hexatriene (CHT) vary considerably and provide a good example of the difficulties that can arise in determining transition energies. The great uncertainty is surprising if one considers that this state has already been characterized by high resolution techniques such as resonance enhanced multiphoton ionization (REMPI) and fluorescence excitation spectroscopy in free jet expansions. A theoretical analysis of this problem is clearly needed and the present work, along with the following paper, represents an effort to investigate the nature of the 2 1A1 and 1 1B1 states of CHT. It is shown that a combination of ab initio electronic structure and quantum-mechanical wave packet calculations is required to systematically approach a question as involved as locating the energetical position of the 2 1A1 level. We characterize the energy dependence of the 1 1A1, 2 1A1, and 1 1B1 states of CHT as a function of the in-plane normal coordinates for small displacements from the ground-state equilibrium geometry employing the single-state multiconfigurational second-order perturbation theory (CASPT2) method. This information constitutes the basis for the construction of diabatic harmonic model potential-energy functions associated with the three electronic states in the Franck–Condon region that is essential for the treatment of nonadiabatic dynamics. Five totally symmetric modes with high Franck–Condon and/or tuning activity are identified. Vibronic interaction between the S1 and S2 states is primarily mediated by four vibrations of b1 symmetry, ν26, ν27, ν30, and ν31. ν30 and ν31 are found to be exceptionally powerful interstate coupling modes and the strong nonadiabatic effects induced by these modes in CHT are mainly responsible for the spectroscopic differences observed for the S1 and S2 states of CHT and trans-1,3,5-hexatriene.

Use of Ar–He mixed gas plasmas for furnace atomisation plasma ionisation mass spectrometry (FAPIMS)

The effect of Ar–He mixed gas plasmas on analyte signal intensities generated in a furnace atomization plasma ionization mass spectrometry (FAPIMS) source is presented. Analyte is introduced as a volatile headspace gas effluent (I2(g) and Hg(g)) and in discrete liquid sample volumes (Fe, Rb, Pd, In, Cs, Yb, Pt, Pb and Bi). The presence of as little as 2–10% (v/v) Ar increases the signal intensity of analytes having first ionization potentials greater than 6 eV by up to 10-fold. This may be attributed to the formation of more energetic plasmas whose electron density, ionization temperature and gas kinetic temperatures increase with Ar content. Consistent with this, elements with the highest ionization potentials showed the greatest enhancements. Analytes with ionization potentials of less than 6 eV, which are already presumed 100% ionized, are unaffected. Further increases in Ar content (50–100% v/v) can lead to plasma instability and accelerated erosion of graphite surfaces within the source. Most of the analytes studied showed slight improvements in the limits of detection for plasmas containing ≈5% v/v Ar; the increased signal intensity was accompanied by little or no increase in the background signal. Although Ar can significantly affect conditions within the source, the composition of the Ar–He mixture can also influence the transmission of analyte into the free jet expansion and to the MS detector. The appearance and intensity of Ar-based spectral interferences, such as ArC+, ArO+ and Ar2+, increase with Ar content and source temperature, degrading the determination of species such as 52Cr+, 56Fe+ and 80Se+. The amount of Ar required to generate significant signal enhancements is small (<10% v/v) and therefore does not significantly ease the pumping requirements on the interface.

The (B̃ 2A′←X̃ 2A′) laser induced fluorescence excitation spectrum of DCO in a supersonic jet expansion

The laser induced fluorescence (LIF) excitation spectrum of DCO ( 2A′← 2A′) has been investigated in a supersonic free jet expansion. Eleven vibrational bands including the origin band were observed in the energy range from 38600 to 42500 cm−1. The band intensities were found to decrease rapidly with increasing excitation in the state due to fast predissociation. High resolution spectra were recorded for the (000), (001), (010) and (100) vibrational bands to determine precise fundamental vibrational term energies and rotational constants and spin–rotation constants of the state. The data were used to derive the following structural parameters for DCO (): r(D–C)=1.121(6) A, r(C–O)=1.371(1) A and ∠(DCO)=105.1(2)°.

Electron ionization study of ammonia micro-clusters

An electron impact ion source on a double focusing sector field mass spectrometer was used to investigate ammonia micro-clusters produced by the adiabatic free jet expansion of ammonia gas. The appearance energies for [NH(3)](n)(+), n </= 9, ions have been determined. Results of measurements of appearance pressures of selected clusters are described for a range of operating conditions. An empirical formula describing the ammonia clusters production is proposed. Copyright 2000 John Wiley & Sons, Ltd.

Non-intrusive optical diagnostic experiments for high-speed flow generator flowfield characterization

Two non-intrusive optical diagnostic methods, electron beam fluorescence (EBF) and focused schlieren (FS) have been set up and utilized for high-speed flow characterization and visualization of the High-speed Flow Generator (HFG) at NASA LaRC. Using the EBF technique, we were able to visualize and calculate several parameters of the low-density, free jet expansion of N 2 gas through a 5.0 mm nozzle. We also developed a high-sensitivity FS system to see shock structure in front of a spherical barrier and roughly determine the lower limit of its capability to visualize the N 2 flowfield.

Direct simulation Monte Carlo study of quantum effects on the spherical expansion of 4He

Quantum effects on the translational nonequilibrium at low temperatures in a spherical expansion of 4He from room temperature are studied using the direct simulation Monte Carlo method to make a comparison with the experimental measurements along the axis of a helium free jet expansion. The quantum-mechanical scattering cross sections are obtained by a quantum phase-shift calculation for the Lennard-Jones and more elaborate Hartree–Fock dispersion potentials. It is shown that the parallel and perpendicular kinetic temperatures are higher and lower, respectively, for the quantum-mechanical scattering than for the classical-mechanical scattering. A comparison with the parallel temperature determined by fitting the ellipsoidal velocity distribution function to the measured spectral profiles indicates that the parallel kinetic temperature for the quantum-mechanical scattering is higher than the measured temperature, with which the parallel kinetic temperature for the classical-mechanical scattering is fortuitously in better agreement. Because both the parallel and perpendicular velocity distribution functions appreciably deviate from Maxwellians and the Maxwellian (half-width) fit temperatures are lower than the kinetic temperatures, the discrepancy between the quantum-mechanical and measured parallel temperatures may partly be resolved by the difference between the kinetic and fitting temperatures.