Cyclotron Resonance Techniques Research Articles

Single-ion cyclotron resonance measurement of M(CO+)/M(N2+).

We have trapped single molecular ions in a precision Penning trap and, using cyclotron resonance techniques, have measured the mass ratio M(${\mathrm{CO}}^{+}$)/M(${\mathrm{N}}_{2}^{+}$) to be 0.999 598 8876(4). Accuracy is limited at 4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ predominantly by temporal instability in the magnetic field. All other systematic sources of error are \ensuremath{\Delta}M/M\ensuremath{\lesssim}${10}^{\mathrm{\ensuremath{-}}10}$. .AE

ChemInform Abstract: Photodissociation of Gas‐Phase Halogenated Benzene Radical Cations

Photodissociation was observed and studied for a series of gas‐phase cations of fluoro‐ and chlorobenzenes, using trapped‐ion cyclotron resonance techniques. All of these dissociations were interpreted as involving absorption of two or more photons. Photodissociation (PDS) spectra of trichlorobenzene and trichloro‐trifluorobenzene ions were compared with literature fluorescence emission spectra. In both cases, the sharp fluorescence peak at the 0‐0 transition energy was not reflected in the PDS spectra; this was interpreted as showing high fluorescence quantum yield, and correspondingly low internal conversion quantum yield, for ions in the B state vibrational ground state. Collisionless (infrared radiative) relaxation rates for several of the ions were determined by chopped‐laser two‐photon dissociation, showing that the presence of several chlorines, and particularly several fluorines, enhances the rate of infrared emission. The highest radiative cooling rate constant observed was 31 s−1 for trifluorob...

Photodissociation of gas-phase halogenated benzene radical cations

Photodissociation was observed and studied for a series of gas-phase cations of fluoro- and chlorobenzenes, using trapped-ion cyclotron resonance techniques. All of these dissociations were interpreted as involving absorption of two or more photons. Photodissociation (PDS) spectra of trichlorobenzene and trichloro-trifluorobenzene ions were compared with literature fluorescence emission spectra. In both cases, the sharp fluorescence peak at the 0-0 transition energy was not reflected in the PDS spectra; this was interpreted as showing high fluorescence quantum yield, and correspondingly low internal conversion quantum yield, for ions in the B̃ state vibrational ground state. Collisionless (infrared radiative) relaxation rates for several of the ions were determined by chopped-laser two-photon dissociation, showing that the presence of several chlorines, and particularly several fluorines, enhances the rate of infrared emission. The highest radiative cooling rate constant observed was 31 s−1 for trifluorobenzene ion. For those ions whose visible–UV fluorescence quantum yields have been reported as near unity, the observation of photodissociation was accounted for by a combined mechanism involving fluorescence-pumping heating of the ions, followed by two-photon photodissociation.

Plasma-Assisted Chemical Vapor Deposition of Ceramic Films and Coatings

ABSTRACTPlasma-assisted chemical vapor deposition uses energetic electrons to decompose reactant gas molecules into more simple and more highly reactant species to achieve deposition of amorphous and crystalline films and coatings at reduced temperatures. The basis of this technique, as well as the deposition conditions and properties of several ceramic films including Al2O3, TiO2, ZnO, AN, BN, TiN, SiC, Si, GaAs, SiC, C (diamond) and GaN, are briefly described. Modifications of the classical immersed method, including remote plasmas, the use of microwave frequencies and electron cyclotron resonance techniques, are also described with material examples.

Cyclotron resonance measurements of electron effective mass in strained AlGaAs/InGaAs/GaAs pseudomorphic structures

Electron effective mass in the InxGa1−xAs conduction channel of strained AlGaAs/InGaAs/GaAs pseudomorphic structures is measured using far-infrared cyclotron resonance techniques at 4.2 K. The measured cyclotron mass is heavier than the conduction-band-edge mass in bulk InxGa1−xAs. This result is explained by the large two-dimensional electron density realized in the structure, and the lattice strain that exists in the InxGa1−xAs layer.

Formation of thioacylium ions in the gas phase

AbstractA study of the ion chemistry of benzenethioic acid using ion cyclotron resonance techniques shows that a long‐lived ion of composition C7H5S+ is formed from the reaction of the neutral acid with primary fragment ions, m/z 77 (phenyl) and m/z 105 (benzoyl). The product is assigned the thiobenzoyl structure on the basis df its mode of formation from benzoyl cations and tbe neutral acid. Other reactant ions (acetylium and thioacetylium) derived from mixtures of benzenethioic acid with ethanethioic acid or acetate esters similarly lead to thiobenzoyl ions as the major product The significance of these results as support for the thioacetylium structure of C2H3S+ ions from ethanethioic acid is discussed.

Photoluminescence and cyclotron resonance studies on highly purified ZnSe single crystals

We have prepared high purity ZnSe single crystals with donor concentrations less than 1015/cm3 and characterized the doped and undoped ZnSe single crystals using photoluminescence and cyclotron resonance techniques. Photoluminescence and photoluminescence excitation spectra on the purest crystal reveal (1) that a Zn-dip treatment increases the free exciton life time, (2) that the free excitons recombine on the lower excitonic polariton branch, (3) that the intensities of I3 become larger than those of I2, and (4) that each I3 line has excited states, which are rather more complicated than those of the I2 lines. We have observed for the first time cyclotron resonance absorption of photoexcited electrons and holes. This means that the grown crystals are not only of high purity but also of high quality. Isotropic effective masses (m∗e = 0.145m0 and m∗h = 1.04m0) and cyclotron mobilities at 4.2 K (μe = 470,000 cm2/V·s and μh = 210,000 cm2/V·s) have been obtained. Doping experiments with In and Cu reveal that the difficulties of preparing low resistivity p-type crystals, such as self-compensation and the amphoteric behavior of dopants, can be solved by heat treatments after doping.

The mechanism and product energy disposal in the reaction of C +( 2P u) with O 2( X3Σ g−)

Two separate ion cyclotron resonance techniques are used to study the details of the reaction of ground-state C + ( 2 P u ) with O 2. A tandem ICR instrument was used to measure the product branching ratio at both thermal energies and at translational energies between 0 and 2 eV. At thermal energies, O +/CO composed 60% of the products and CO +/O 40%. Comparisons are made with literature values and sources of disagreement discussed. An ion trap ICR instrument was used to measure the kinetic energy released in the O + and CO + products. Analysis of the data indicate that all products are formed in their electronic ground states but both the CO + and CO products are strongly vibrationally excited. The mechanism of the reaction is discussed in terms of correlation diagrams. Arguments are made that suggest a (C-O-O +) 1 1 We wish to dedicate this work to Eldon Ferguson on the occasion of his retirement from NOAA in Boulder, Colorado. One of us (MTB) especially wishes to thank him for his scientific counsel and personal friendship over the past 20 years. intermediate of Cs symmetry. Adiabatic correlations do not explain the product branching ratio and substantial crossings between potential energy surfaces must be invoked.

Gas‐phase protonation and methyl cation transfer from methyl halide ions

AbstractThe ion‐molecule reactions between [CH3X]+˙ [CH3XH] +, [CH3XCH3]+ ions (X = F, Cl, Br, I) and a number of nucleophiles have been studied by ion cyclotron resonance techniques. Protonation of the nucleophiles is observed to occur from both the molecular ions [CH3]X+˙ and protonated species [CH3XH]+ whereas dimethylhalonium ions [CH3XCH3]+ react principally by methyl cation transfer. A notable exception occurs in methyl iodide where the molecular ions [CH3I]+˙ act both as proton and methyl cation donors, whereas dimethyliodonium ions are found unreactive. The results are discussed with reference to the use of alkyl halides as reagent gases in chemical ionization experiments.

Effect of ligation and cluster size on the reactivity of metal clusters: reactions of rhenium carbonyl clusters (Ren(CO)m+) with cyclohexane

Gas-phase reactions between cyclohexane and Re/sub n/(CO)/sub m//sup +/ (n = 3, m = 7-13 and n = 4, m = 8-16) were characterized by ion cyclotron resonance techniques. The reactant cluster ions were formed by reaction between Re/sub 2/(CO)/sub 10/ and its electron impact produced fragments. Reactions of the clusters with cyclohexane produced species of the type Re/sub n/C/sub 6/H/sub 2(6-x)/(CO)/sub (m-y)//sup +/ corresponding to loss of x molecules of H/sub 2/ and y molecules of CO from a complex of the reactants. Reactions were observed for only the most coordinatively unsaturated clusters (m < 10 for n = 3 and m < 12 for n = 4). H/sub 2/ yield also tended to increase with coordinative unsaturation. The results are discussed in terms of a mechanism involving oxidative addition of a C-H bond to a cluster metal atom

Spectroscopy of Impurities and Complex Defects in Silicon in Zlectric and Microwave Fields

ABSTRACTWe report a detailed study of the photoluminescence (PL) intensity of bound excitons (BE:s) in silicon, related to shallow impurities and deep complex defects, as a function of DC and high frequency AC (9GHz) electric fields. Two experimental approaches are presented. The first involves a simultaneous recording of PL and photocurrent under pulsed DC excitation. The second utilizes the optically detected cyclotron resonance (ODCR) technique, which allows detection of cyclotron resonance (CR) via the resonancetransition- induced changes of BE PL intensity. The mechanism responsible for the PL changes is shown to be the impact ionization of BE:s by hot free carriers. Effects of sample inhomogeneities in these experiments are also discussed.

252Cf plasma desorption fourier transform ion cyclotron resonance mass spectrometry

The ions produced by 252Cf plasma desorption are detected by Fourier transform ion cyclotron resonance techniques. Results for CsI and N,N,N-trimethyl-p-aminophenolate iodide show the feasibility of the technique with high resolution (up to 11 000 at m/z 23) and good signal-to-noise ratio (up to 726).

Ion-molecule reactions of hydrocarbon ions in acetylene and hydrocyanic acid

Rate coefficients and product distributions have been determined for reaction of the ions C(x)H(y)+ (x ranging from 1 to 4, and y ranging from 0 to 4) with C2H2 and HCN. The measurements were obtained using the ion cyclotron resonance technique at 298 K. In several reactions an association product was observed at pressures as low as 0.000001 torr, and in these cases stabilization of the intermediate was assumed to be by photon emission. Most of the reaction studied yield ions having a larger carbon skeleton than the reactant ion. These reactions provide routes for building large organic and organonitrogen molecules in combustion zones of unsaturated hydrocarbon flames and in astrochemical environments.

Gas-phase NMR studies of alcohols. Intrinsic acidities

Abstract Gas-phase (≈100 Torr) 1H NMR spectra of eighteen simple aliphatic and unsaturated alcohols, four fluorinated alcohols, and two thiols were obtained at 148.6°C where hydrogen bonding has little effect on chemical shifts. For the methanol, ethanol, n-propanol, i-propanol, t-butanol, i- butanol, neopentanol, 2,2,2-trifluoroethanol and benzyl alcohol, the observed hydroxylic proton chemical shifts correlate with previously obtained relative gas-phase acidities from thermochemical analysis which employed equilibrium constants of proton transfer reactions measured via mass spectroscopic and ion cyclotron resonance techniques. The correlational dependence is 10.3(0.5) kcal/mol ppm with a correlation coefficient of 0.99. These results demonstrate that the trend of increasing acidity with increasing size of the alkyl substituent is also reflected in the neutral forms of the alcohols, indicating that the polarizability of the ionic forms is not the only determining factor in relative gas-phase acidities of alcohols. Although factors affecting the hydroxylic proton chemical shifts of the larger substituted and unsaturated alcohols are more complex, their observed 1H NMR spectra also reflect this trend. For methanol and ethanol observed gas-phase 1H chemical shifts are also compared with recent theoritical calculations. 3JHH coupling constants across CO bonds are ≈ 5.5 Hz, significantly smaller than typical 3JHH coupling across sp3 hybrid CC bonds.

Energy disposal in charge transfer reactions producing NH+.3: Dependence of the NH+.3+H2O reaction on NH+.3 internal energy

The division of available energy between translational and internal modes has been determined for seven charge transfer reactions which produce NH+.3. The charge transfer reagent ions studied were Ar+., Kr+., Xe+., N+.2, O+.2, CO+., and CO+.2. The average kinetic energy released in the reaction was measured directly using an ion cyclotron resonance (ICR) technique. The internal energy deposited in the NH+.3 product was determined by further reacting the NH+.3 product with H2O in a tandem ICR spectrometer. The product distribution of this subsequent reaction is shown to depend strongly on the NH+.3 internal energy; and this dependence is quantitatively determined. The energy deposited in the neutral charge transfer product is calculated using energy balance. Comparisons are made between charge transfer reagent ions of similar recombination energies to show the effects of reagent ion internal modes on the division of available reaction energy.

An ICR study of ion—molecule reactions of PH n+ ions

The reaction of PH n + ions ( n = 0–3) were examined with a number of neutrals using ion cyclotron resonance techniques. The reactions examined have significance for the distribution of phosphorus in interstellar molecules. The results indicate that interstellar molecules containing the PO bond are likely to be more abundant than those containing the PH bond.

On the nature of the tetrahedral species in the gas phase hydrolysis of esters

Ion cyclotron resonance techniques show that in the gas phase the oxygen exchange reaction in the system OH–+ HC(18O)(OMe) is at least 10–2 times slower than the hydrolysis reaction; this result, plus thermochemical estimates, suggest that the tetrahedral adduct HC(OMe)(OH)O– is more likely a local transition state rather than a stable intermediate.

Bimolecular production of proton bound dimers in the gas phase. A low pressure ion cyclotron resonance technique for examination of solvent exchange equilibria and determination of single molecule solvation energetics

A scheme is presented whereby sequences of fast bimolecular gas phase ion molecule reactions in mixtures containing (CHF2)2O may be used to generate proton bound dimer species at low pressures in an ion cyclotron resonance spectrometer. Using competitive solvent switching reactions it is demonstrated that solvent exchange equilibria may be readily established and from the thermochemical data derived from such equilibria accurate relative single molecule solvation energetics obtained.

An ion cyclotron resonance spectrometer as a gas chromatographic detector: Measurement of momentum transfer collision frequencies

The momentum transfer collision frequencies (ξ/ n) of 39K + with the three isomers of difluorobenzene are reported. The measurements have been done using ion cyclotron resonance techniques. Conventional linewidth measurements gave (ξ/ n) para = 1.71 ± 0.08 × 10 −9, (ξ/ n) meta = 2.18 ± 0.11 × 10 −9, and (ξ/ n) ortho = 2.79 ± 0.14 × 10 −9 cm 3 molecule −1 s −1. A method for measuring relative momentum transfer collision frequencies at resonance is described. The method involves coupling a gas chromatograph to the ion cyclotron resonance (ICR) spectrometer. Using this method it was found that (ξ/ n) ortho /(ξ/ n) para = 1.63±0.08, (ξ/ n) ortho /(ξ/ n) meta = 1.24±0.02, and (ξ/ n) meta /(ξ/ n) para = 1.32±0.07. The significance of the good agreement between the linewidth measurements and the measurements at resonance is discussed. The measurements are compared with those of Su and Bowers for the momentum transfer collision frequencies of 35Cl − with the difluorobenzene isomers. It is concluded that the momentum transfer rate constants are not affected significantly by a change in the sign of the ionic charge. The measurements are also compared with theory and found to agree with momentum transfer collision frequencies determined from a potential of the form Ar −8 - Br −6 - α e 2/2 r 4 + E ψ where the third term is the ion-induced dipole term and the fourth term is a statistically averaged ion-dipole term. The effect of ion/molecule reactions on the GC-ICR method and possible analytical applications of the method are discussed.

Positive-ion chemistry of cyclopentadienylcobalt dicarbonyl

Ion cyclotron resonance (ICR) techniques are used to study the positive-ion chemistry of CpCo(CO) 2, Cp = η 5-C 5H 5. An order of relative ligand binding energies to CpCo + is established: benzene > MeCN ⋍ NH 3 > tetrahydrofuran ⋍ Me 2CO > propylene ⋍ Me-CHO ⋍ Me 2O ⋍ NO ⩾ MeOH > CO. Relative ligand binding energies to CpCo(CO) + are similar. The proton affinity of CpCo(CO) 2 is indicated to be in the range 208 ∓ 4 kcal mol −1, giving a homolytic bond energy of D(CpCo(CO) 2 +H) = 80 ∓ 6 kcal mol −1. This suggests that protonation occurs on the ring. Methyl bromide reacts with CpCo + to substitute Me- for H- on the cyclopentadienyl ring. This reaction occurs four times and is followed by oxidative bromine transfer to give (η 5-Me 4C 5H)CoBr + as the final product in this sequence. The results observed with CpCo(CO) 2 are generally very similar to those previously reported for CpNi(NO).