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  • 1
    Electronic Resource
    Electronic Resource
    The mobilities of NO+(CH3CN)n cluster ions (n=0–3) drifting in helium and in helium–acetonitrile mixtures (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 10398-10409 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The mobilities of NO+(CH3CN)n cluster ions (n=0–3) drifting in helium and in mixtures of helium and acetonitrile (CH3CN) are measured in a flow-drift tube. The mobilities in helium decrease with cluster size [the mobility at zero field, K(0)0, is 22.4±0.5 cm2 V−1 s−1 for NO+, 12.3±0.3 cm2 V−1 s−1 for NO+(CH3CN), 8.2±0.2 cm2 V−1 s−1 for NO+(CH3CN)2 and 7.5±0.5 cm2 V−1 s−1 for NO+(CH3CN)3] and depend only weakly on the characteristic parameter E/N (electric field strength divided by the number density of the buffer gas). The size dependence is explained in terms of the geometric cross sections of the different cluster ions. The rate constants for the various cluster formation and dissociation reactions have also been determined in order to rule out the possibility that reactions occurring in the drift region influence the measurements in the mixtures. Since high pressures of acetonitrile are required to form NO+(CH3CN)2 and NO+(CH3CN)3, the mobilities of these ions are found to be dependent on the acetonitrile concentration, as a result of anomalously small mobilities of these ions in acetonitrile [K(0)0=0.041±0.004 cm2 V−1 s−1 for NO+(CH3CN)2 and 0.044±0.004 cm2 V−1 s−1 for NO+(CH3CN)3]. These values are at least an order of magnitude smaller than any previously reported ion mobility, which can be partly explained by the large ion-permanent dipole interaction between the cluster ions and acetonitrile. The remaining discrepancies may be the result of momentum transfer outside the capture cross section, dipole–dipole interactions, ligand exchange, the formation of long-lived collision complexes or the transfer of kinetic energy into internal energy of the cluster ion and the acetonitrile molecule. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472968
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  • 2
    Electronic Resource
    Electronic Resource
    Vibrational enhancement of the charge transfer rate constant of N+2(v=0–4) with Kr at thermal energies (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 5455-5466 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The charge transfer reaction of N+2(v=0–4)+Kr→N2+Kr+ is studied at thermal energy as a function of vibrational excitation in the reactant ion. The selected-ion flow tube technique coupled with laser-induced fluorescence detection is used to measure the vibrationally state specific rate constants. A dramatic vibrational enhancement is observed; measured rate constants are 1.0 (±0.6)×10−12, 2.8 (±0.3)×10−12, 2.1 (±0.2)×10−11, 5.1 (±0.2)×10−11, and 8.3 (±0.4)×10−11 cm3 molecule−1 s−1 for v=0, 1, 2, 3 and 4, respectively. Mass spectrometric kinetics experiments are also performed to confirm that vibrational relaxation, N+2(v)+Kr→N+2(v′〈v)+Kr, is a negligible process. The charge transfer for v=0 is extremely slow in spite of the large exothermicity (e.g., 0.915 eV for the production of N2(v′=0)+Kr+(2P1/2) states), yet the reaction is enhanced when the apparent energy mismatch is greater for the vibrationally excited reactant. A simple model is proposed to explain the experimental results at thermal energies ((very-much-less-than)1 eV). The model assumes that only the most energy-resonant exothermic transitions, N+2(v)+Kr→N2(v+3)+Kr+(2P1/2), occur within the duration of the ion–molecule collision complex and that the charge transfer takes place with probabilities governed by the corresponding Franck–Condon factors. However, the Franck–Condon factors are modified by a trial displacement of 0.02 A(ring) to account for the changes in vibrational wave functions of N+2 and N2 during a close approach of the (N2–Kr)+ pair; this method gives an excellent description of the experimental results. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472386
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  • 3
    Electronic Resource
    Electronic Resource
    Mobility and formation kinetics of NH4+(NH3)n cluster ions (n=0–3) in helium and helium/ammonia mixtures (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 530-538 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: NH4+(NH3)n (n=0–3) cluster ions are produced in a field-free flow tube section of a selected ion flow–drift tube (SIFDT) apparatus. Cluster ion mobilities are measured in mixtures of He and NH3 and used to obtain the individual mobilities in helium and in ammonia by applying Blanc's law to the mixtures. Mobilities of the cluster ions are also measured in pure helium by producing the ions in the ion source of a flowing afterglow, selected ion flow–drift tube apparatus (FA-SIFDT). The measurements in pure helium compare well with the mobilities in helium obtained by applying Blanc's law to the mixtures. The zero field mobilities of the cluster ions in helium are 22.1±0.4 cm2 V−1 s−1 for NH4+, 16.6±0.4 cm2 V−1 s−1 for NH4+(NH3), 12.2±0.4 cm2 V−1 s−1 for NH4+(NH3)2, and 12.1±0.4 cm2 V−1 s−1 for NH4+(NH3)3. The decrease with increasing size of the cluster can be explained in terms of the sizes of the geometric cross sections. The zero-field mobilities in NH3 are 0.94±0.35 cm2 V−1 s−1 for NH4+, 0.83±0.22 cm2 V−1 s−1 for NH4+(NH3), 0.50±0.27 cm2 V−1 s−1 for NH4+(NH3)2, and 0.25±0.20 cm2 V−1 s−1 for NH4+(NH3)3. The small values of the mobilities in these polar gas systems are understood in terms of the strong ion–dipole interactions. Calculated mobilities in NH3 are obtained by computing the collision cross section with the ion–dipole interactions taken into account; the results compare well with the measurements for NH4+ and NH4+(NH3). However, the measured mobilities of the larger cluster ions are smaller than the computed values. The discrepancies may be due to several factors including dipole–dipole interactions, ligand exchange reactions, formation of long-lived quasibound complexes, and efficient transfer of kinetic energy into internal energy of the cluster ion and the ammonia molecules. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473964
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  • 4
    Electronic Resource
    Electronic Resource
    The mobilities of ions and cluster ions drifting in polar gases (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 5937-5942 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The mobility of ions drifting in polar gases is explored both theoretically and experimentally. New experimental results are presented for (i) NO+ ions drifting in H2O (the reduced zero-field mobility K0(0) is 0.66±0.07 cm2 V−1 s−1), (ii) H3O+(H2O)3 ions drifting in H2O (K0(0)=0.43±0.06 cm2 V−1 s−1), and (iii) NO+(CH3COCH3)n ions (n=2,3) drifting in CH3COCH3 (K0(0)=0.041 ±0.010 cm2 V−1 s−1 for n=2 and K0(0)=0.050±0.015 cm2 V−1 s−1 for n=3). A number of theoretical models for ion mobilities in polar gases are described. The models are compared with the available experimental data and a reasonable agreement is obtained. For larger cluster ions the measured mobilities are considerably smaller than the calculated values. Some possible reasons for the discrepancies are discussed, including momentum transfer outside the capture cross section, dipole–dipole interactions, ligand exchange, inelastic collisions, and the validity of Blanc's law. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473609
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