Volume 105, Issue 23, 15 December 1996
Index of content:

The argon–diacetylene complex: An example of distributed interactions and transferable potentials
View Description Hide DescriptionThe first spectroscopic study of the argon–diacetylene complex is reported here. The rotationally resolved near infrared spectrum has been analyzed in terms of a conventional asymmetric rotor Hamiltonian, yielding a set of ground and vibrationally excited staterotational constants consistent with a ‘‘T‐shaped’’ geometry, similar to that of Ar–C_{2}H_{2}. Using distributed potential parameters determined previously for the Ar–C_{2}H_{2} system we have developed an empirical potential for Ar–C_{4}H_{2}. A ground state calculation using this potential and the collocation method gives a vibrationally averaged structure in good agreement with experiment. A tentative assignment is also made for the band origins of the Ar_{2}, Ar_{3}–diacetylene complexes.

Spectroscopy of argon fluoride and krypton fluoride exciplexes in rare gas matrices
View Description Hide DescriptionThe spectroscopy of argon fluoride and krypton fluoride exciplexes in rare gas matrices is investigated using synchrotron radiation over the range of 50 to 600 nm. The diatomic Ar^{+}F^{−} and Kr^{+}F^{−} species are observed in excitation and emission spectra. Extensive vibrational progressions are observed in the excitation spectra and are used to determine the spectroscopic parameters of the ArF B state (T _{ e }=50233 cm^{−1},ω_{ e }=415.5 cm^{−1}, ω_{ e } x _{ e }=3.1 cm^{−1}) and D state (T _{ e }=51738 cm^{−1},ω_{ e }=476.7 cm^{−1},ω_{ e } x _{ e }=3.8 cm^{−1}) and of the KrF B state (T _{ e }=39024 cm^{−1},ω_{ e }=342.4 cm^{−1},ω_{ e } x _{ e }=2.0 cm^{−1}) and D state (T _{ e }=44479 cm^{−1},ω_{ e }=331.6 cm^{−1},ω_{ e } x _{ e }=1.4 cm^{−1}). Lifetimes of 4 to 8 ns are measured for the diatomic emissions. A perturbed KrF species is observed which is identified in analogy to a similarly perturbed XeF species. Excitation spectra of the triatomic exciplexes Kr^{+} _{2}F^{−} and Ar^{+} _{2}F^{−} in neon are presented and described in terms of previous ab initio calculations. Emission and excitation of argon fluoride and krypton fluoride species in other matrices are presented. Large stimulated emission cross sections on the order of 10^{−16} cm^{2} are determined for the diatomic D→X and B→X emissions, which, together with nonradiative relaxation processes within the B and D states of ArF that efficiently populate the B(v=0) regardless of initial excitation, make ArF in neon a viable solid‐state laser candidate.

Resolution of anomalies in the geometry and vibrational frequencies of monobromosilylene (HSiBr) by pulsed discharge jet spectroscopy
View Description Hide DescriptionA detailed examination of the ground and first excited singlet electronic states of HSiBr has been carried out through analysis of the 500–400 nm band system, using pulsed discharge jet and laser‐induced fluorescence techniques. HSiBr and DSiBr have been produced by an electric discharge through SiHBr_{3} and SiDBr_{3} vapor in argon. Rotational analysis of the 0^{0} _{0} bands yielded the structural parameters r _{0} ^{″}(SiH)=1.518(1) Å, r _{0} ^{″}(SiBr)=2.237(1) Å, θ_{0} ^{″}=93.4(3)°, r _{0} ^{′}(SiH)=1.497(10) Å, r _{0} ^{′}(SiBr)=2.208(2) Å, and θ_{0} ^{′}=116.4(7)°. Previous anomalies in the geometric parameters and vibrational frequencies have been resolved and the ground state bond lengths and vibrational frequencies are found to be comparable to those of SiH and SiBr. Harmonic force fields have been determined for the ground and excited states and the radiative lifetime of HSiBr has been measured to be 598±18 ns.

Internal rotation of methyl group in o‐ and m‐toluidine cations as studied by pulsed field ionization–zero kinetic energy spectroscopy
View Description Hide DescriptionInternal rotational levels of the methyl group in o‐ and m‐toluidine cations have been observed by pulsed field ionization–zero kinetic energy photoelectron spectroscopy. Level energies and transition intensities were reproduced by a one‐dimensional rotor model with a free‐rotor basis set, and the potential curves of the internal rotation in the cations have been determined. Analysis for m‐toluidine shows a drastic increase of the barrier height for internal rotational motion from the neutral to the corresponding cation. On the other hand, the barrier in o‐toluidine slightly decreases by ionization. The mechanism of the change in barrier height will be discussed in terms of geometrical and electrical change by ionization. It is suggested that the drastic change of internal rotational motion is mainly determined by the electronic structure.

Geometric isomerism in clusters: High resolution infrared spectroscopy of a noncyclic CO_{2} trimer
View Description Hide DescriptionHigh resolution infrared spectra of a previously unidentified noncyclic isomer of (CO_{2})_{3} have been obtained via direct absorption of a 4.3 μm diode laser in a slit jet supersonic expansion. Two vibrational bands (labeled ν_{I} and ν_{III}) are observed, corresponding to the two most infrared active linear combinations of the three constituent CO_{2}monomer asymmetric stretches: ν_{I} is redshifted −5.85 cm^{−1} from the monomer vibrational origin and is predominately a c‐type band of an asymmetric top, while ν_{III} is blueshifted +3.58 cm^{−1} and is predominately an a‐type band. Transitions with K _{ a }+K _{ c }=odd (even) in the ground (excited) state are explicitly absent from the spectra due to the zero nuclear spin of CO_{2}; this rigorously establishes that the noncyclic isomer has a C _{2} symmetry axis. The vibrational shifts and relative intensities of the bands are interpreted via a resonant dipole interaction model between the high‐frequency stretches of the CO_{2}monomers. Rotational constants are determined by fits of transition frequencies to an asymmetric top Hamiltonian. These results are used to determine vibrationally averaged structural parameters for the complex, which is found to be stacked asymmetric but with C _{2} symmetry about the b inertial axis. The structural parameters are then used to test several trial CO_{2}–CO_{2} interaction potentials.

Intermolecular vibrations and relaxation dynamics in complexes of OH A ^{2}Σ^{+} (v′=0,1) with N_{2}
View Description Hide DescriptionThe intermolecular vibrational energy levels supported by the OH A ^{2}Σ^{+} (v′=0,1)+N_{2} potentials have been characterized spectroscopically through excitation of OH–N_{2} complexes in the OH A ^{2}Σ^{+}–X ^{2}Π 0–0 and 1–0 spectral regions. At least 95 levels correlating with OH A ^{2}Σ^{+} (v′=0)+N_{2} are observed in fluorescence depletion experiments. OH–N_{2} complexes prepared in these levels have lifetimes with lower limits ranging from 1.4 to 8 ps due to rapid electronic quenching which precludes their detection by laser‐induced fluorescence. An onset of OH–N_{2} laser‐induced fluorescence occurs at the OH A ^{2}Σ^{+} (v′=0)+N_{2}dissociation limit, enabling determination of the ground and excited state binding energies at ∼250 and ⩾1372 cm^{−1}, respectively. In the OH A–X 1–0 region, OH–N_{2} transitions originating from a common ground state level to single or groups of intermolecular vibrational levels correlating with OH A ^{2}Σ^{+} (v′=1)+N_{2} are observed via laser‐induced fluorescence and fluorescence depletion measurements. Comparison of the OH–N_{2} spectra obtained in the OH A–X 0–0 and 1–0 regions reveals that vibrational excitation of OH A ^{2}Σ^{+} increases the OH–N_{2} binding energy by 139 cm^{−1}. OH–N_{2} complexes excited in the OH A–X 1–0 region undergo ultrafast dynamics (<200 fs) which give rise to extensive spectral line broadening. A kinetic model indicates that vibrational predissociation is the dominant decay channel for OH–N_{2} prepared in the intermolecular levels derived from OH A ^{2}Σ^{+} (v′=1)+N_{2}.

An electron spin resonance investigation of vanadium dioxide (^{51}V^{16}O_{2} and ^{51}V^{17}O_{2}) and ^{51}V^{17}O in neon matrices with preliminary assignments for VO_{3} and V^{+} _{2}: Comparison with ab initio theoretical calculations
View Description Hide DescriptionThe first spectroscopic characterization of the VO_{2} radical is reported along with new results for V^{17}O and tentative assignments for the VO_{3} and V^{+} _{2} radicals. These vanadium radicals were investigated in neon matrices at 4 K by electron spin resonance utilizing conventional high temperature vaporization and pulsed laserablation generation methods. A detailed ESR study of VO_{2} showed it to be nonlinear with a ^{2} A _{1}ground state; the gtensor analysis reveals the presence of an excited electronic state (^{2} B _{1}) approximately 1 eV above the ground state. This excited state prediction and the observed nuclear hyperfine interactions (Atensors) for ^{51}V and ^{17}O were compared with theoretical results obtained from various ab initio computational methods. Ab initio calculations with an extended basis set were performed at various levels of theory including UHF, ROHF, CAS‐SCF, and MR‐SDCI (multireference single and double configuration interaction). While UHF calculations of the hyperfine interaction were grossly in error, the better levels of theory gave qualitative agreement with experiment and provided an aid to interpretation. VO_{2} is predicted to be a bent ^{2} A _{1} state, correlating with the linear ^{2}Δ configuration having the odd electron predominantly in the V 3d orbital. VO_{3} is predicted to be planar C _{2v }, with the odd electron in a b _{2} orbital localized in the oxygen in‐plane n‐type p orbitals.

Detection of DCl by multiphoton ionization and determination of DCl and HCl internal state distributions
View Description Hide DescriptionA study of the 2+1 resonantly enhanced multiphoton ionization (REMPI) spectrum of DCl is reported. Transition energies for excitation of the F ^{1}Δ–X ^{1}Σ^{+} (0,0) and (1,0) bands, as well as the V ^{1}Σ^{+}–X ^{1}Σ^{+}(v′,0) bands, for v′=15–19, are presented. The derived molecular constants for the F–X (0,0) and the V–X bands agree well with those previously obtained from analysis of the one‐photon VUV absorptionspectrum [A. E. Douglas and F. R. Greening, Can. J. Phys. 57, 1650 (1979)]. The ion signals for excitation through various rotational lines in the E–X (0,0) and F–X (0,0) and (1,0) bands are compared with theoretical two‐photon line strengths. Extensive power‐ and J′‐dependent ion fragmentation is observed for the former band. No fragmentation is observed in the F–X bands; however, the ion signal strengths are found to vary strongly with J′. This variation of REMPI signal strengths vs J′ was shown to be due to an indirect predissociation, as in HCl. Tables of experimental line strength factors for the F–X (0,0) and (1,0) bands of HCl and DCl are reported. Finally, the relative REMPI detection sensitivities for HCl and DCl, through their respective F–X (0,0) R(1) lines, are compared.

The superposition principle and cavity ring‐down spectroscopy
View Description Hide DescriptionCavity ring‐down is becoming a widely used technique in gas phase spectroscopy. It holds promise for further important extensions, which will lead to even more frequent use. However, we have found widespread confusion in the literature about the nature of coherenceeffects, especially when the optical cavity constituting the ring‐down cell is excited with a short coherence length laser source. In this paper we use the superposition principle of optics to present a general and natural framework for describing the excitation of a ring‐down cavityregardless of the relative values of the cavity ring‐down time, the input pulse coherence time, or the dephasing time of absorption species inside the cavity. This analysis demonstrates that even in the impulsive limit the radiation inside a high finesse cavity can have frequency components only at the natural resonance frequencies of the cavity modes. As an immediate consequence, a sample absorption line can be detected only if it overlaps at least one of the cavity resonances. Since this point is of particular importance for high resolution applications of the technique, we have derived the same conclusion also in the time domain representation. Finally, we have predicted that it is possible to use this effect to do spectroscopy with a resolution much higher than that of the bandwidth of the excitation laser. In order to aid in the design of such experiments, expressions are derived for the temporal and spatial overlap of a Fourier transform limited input Gaussian beam with the TEM_{ mn } modes of the cavity. The expressions we derive for the spatial mode overlap coefficients are of general interest in applications where accurate mode matching to an optical cavity is required.

Response of a ring‐down cavity to an arbitrary excitation
View Description Hide DescriptionAn eigenmodeanalysis of the response of an empty ring‐down cavity to an arbitrary laser excitation is presented. By explicitly taking into account both the mode structure of the ring‐down cavity and the spectral content of the laser pulse, it is found that the complicated ring‐down signals commonly observed in the laboratory can be interpreted in terms of cavity mode beating. Some conclusions drawn from this analysis are verified experimentally by measurements of the time and frequency response of empty cavities. These observations provide clear evidence for the existence of longitudinal and transverse mode structures in ring‐down cavities.

Frequency‐selective heteronuclear dephasing by dipole couplings in spinning and static solids
View Description Hide DescriptionA compensated pulse sequence for the spectrally selective reintroduction of heteronuclear dipole–dipole interactions (frequency‐selective dipolar recoupling) into solid state magic angle spinning (MAS) nuclear magnetic resonance(NMR)experiments is described and shown to provide frequency‐selective dipolar dephasing in weakly coupled spin systems. The experimental dipolar spin evolution is interpreted via analytical and numerical calculations, which include a simple model for the observed losses of spin coherence in the multiple pulse experiments. In the peptide glycylglycine, the selective dipolar evolution of two spins is observed while the influence of larger internuclear couplings is suppressed. This approach is aimed at obtaining several quantitative internuclear distances independently in dipolar ‘‘recoupling’’ MAS experiments by reducing multiple spin effects in the observed dipolar evolution. Similar frequency‐selective dephasingexperiments are also introduced for static solids, where an efficient application to measuring relative tensor orientations in powdered samples is demonstrated.

Electro‐osmosis: Velocity profiles in different geometries with both temporal and spatial resolution
View Description Hide DescriptionA theoretical framework for the description of the phenomenon of electro‐osmosis is developed. The main emphasis of the work is to develop relations that describe the time and spatial resolution of the velocity of the liquid in contact with a charged surface when a train of electric field pulses are applied parallel to the surface. The work is motivated by the recent development of NMR detected electrophoresis to a powerful tool in the field of colloid chemistry. In this approach one employs pulsed electric fields, and the process of electro‐osmosis is a complication. In developing this framework, we make use of results from electro‐osmosis outside a single charged plane and in a slot, when the electric field is applied as a step‐function. Results for both planar and cylindrical geometries are presented. In both cases we present results without and with effects due to counterflow taken into account. Finally, we compare the results of our theoretical description with some recently published velocity profiles obtained from NMR imaging techniques.

Condensed phase spectroscopy from mixed‐order semiclassical molecular dynamics: Absorption, emission, and resonant Raman spectra of I_{2} isolated in solid Kr
View Description Hide DescriptionA method for spectral simulations in systems of very large dimensionality via semiclassical molecular dynamics is introduced and applied to the spectroscopy of iodine isolated in solid Kr, as a prototype of spectroscopy in condensed media in general. The method relies on constructing quantum correlation functions, C(t), using initial value propagators which correspond to the zeroth‐ and second‐order approximations in stationary phase of the exact quantum propagator. The first is used for treating modes with high thermal occupation numbers, the lattice modes, while the second is used for treating the guest mode. The limits of validity of the bare propagators are tested vs exact treatments of gas phase I_{2}, and shown to be quite broad. The mixed order simulations are then used to reproduce the structured A→X emission, the structureless B←Xabsorption, and the intensities in resonant Raman (RR) progressions of matrix isolated I_{2}, connecting spectroscopic observables to molecular motions. Decompositions of the supersystem correlations into system and bath are used to provide perspectives about condensed phase spectroscopy. The system correlation can be regarded as the sampling function for the decaying bath correlation, which in turn is a summary of the many‐body dynamics. The B←Xabsorptionspectrum is determined by the coherent ballistic motion of the excited state density: Upon stretching, I_{2} pushes the cage atoms out of overlap in position density, and C(t) never recovers. Due to the compressive nature of the cage coordinate in the A→X transition, C(t) decays more gently, after being sampled three times. RR spectra, which are reproduced with adiabatic dynamics, sample the complete history of the many‐body correlations, however, due to the breadth in space‐time of scattering into high overtones, the sampling is coarse grained. The specific dynamics that control C(t) cannot be described as dissipative.

Vibrational wave functions and spectroscopy of (H_{2}O)_{ n }, n=2,3,4,5: Vibrational self‐consistent field with correlation corrections
View Description Hide DescriptionVibrational energy levels, wave functions, and ir absorption intensities are computed for (H_{2}O)_{ n } clusters with n=2, 3, 4, and 5. The calculations were carried out by the vibrational self‐consistent field (VSCF) approximation, with corrections for correlations between the modes by perturbation theory. This correlation corrected VSCF (CC‐VSCF) is analogous to the familiar Möller–Plesset method in electronic structure theory. Test calculations indicate that this method is of very good accuracy also for very anharmonic systems. While the method is of highest relative accuracy for the stiffest modes, it works very well also for the soft ones. Some of the main results are (1) the frequencies calculated are in good but incomplete agreement with experimental data available for some of the intramolecular mode excitations. The deviations are attributed to the inaccuracy of the coupling between intramolecular and intermolecular modes for the potential function used. (2) Insight is gained into the pattern of blue‐ or redshifts from the corresponding harmonic excitation energies for the various modes. (3) Anharmonic coupling between the modes dominates in general over the intrinsic anharmonicity of individual modes in determining the spectrum. (4) The anharmonic corrections to the frequencies of some intermolecular modes (shearing, torsional) are extremely large, and exceed 100% or more in many cases. (5) An approximation of quartic potential field in the normal mode displacement is tested for the clusters. It works well for the high and intermediate frequency modes, but is in error for very soft shearing and torsional modes. (6) The relative errors of the VSCF approximation are found to decrease with the cluster size. This is extremely encouraging for calculations of large clusters, since the VSCF level is computationally simple.

Collisional removal of O_{2} (c ^{1}Σ^{−} _{ u }, ν=9) by O_{2}, N_{2}, and He
View Description Hide DescriptionThe collisional removal of O_{2} molecules in selected vibrational levels of the c ^{1}Σ^{−} _{ u } state is studied using a two‐laser double‐resonance technique. The output of the first laser excites the O_{2} to ν=9 or 10 of the c ^{1}Σ^{−} _{ u } state, and the ultraviolet output of the second laser monitors specific rovibrational levels via resonance‐enhanced ionization. The temporal evolution of the c ^{1}Σ^{−} _{ u } state vibrational level is observed by scanning the time delay between the two pulsed lasers. Collisional removal rate constants for c ^{1}Σ^{−} _{ u }, ν=9 colliding with O_{2}, N_{2}, and He are (5.2±0.6)×10^{−12}, (3.2±0.4)×10^{−12}, and (7.5±0.9)×10^{−12} cm^{3} s^{−1}, respectively. As the rate constants for O_{2} and N_{2} are similar in magnitude, N_{2} collisions dominate the removal rate in the earth’s atmosphere. For ν=10 colliding with O_{2}, we find a removal rate constant that is 2–5 times that for ν=9 and that single quantum collision cascade is an important pathway for removal.

A scrutiny of the premise of the Rice–Ramsperger–Kassel–Marcus theory in isomerization reaction of an Ar_{7}‐type molecule
View Description Hide DescriptionThe validity of the physical premise of the Rice–Ramsperger–Kassel–Marcus (RRKM) theory is investigated in terms of the classical dynamics of isomerizationreaction in Ar_{7}‐like molecules (clusters). The passage times of classical trajectories through the potential basins of isomers in the structural transitions are examined. In the high energy region corresponding to the so‐called liquidlike phase, remarkable uniformity of the average passage times has been found. That is, the average passage time is characterized only by a basin through which a trajectory is currently passing and, hence, does not depend on the next visiting basins. This behavior is out of accord with the ordinary chemical law in that the ‘‘reaction rates’’ do not seem to depend on the height of the individual potential barriers. We ascribe this seemingly strange uniformity to the strong mixing (chaos) lying behind the rate process. That is, as soon as a classical path enters a basin, it gets involved into a chaotic zone in which many paths having different channels are entangled among each other, and effectively (in the statistical sense) loses its memory about which basin it came from and where it should visit next time. This model is verified by confirming that the populations of the lifetime of transition from one basin to others are expressed in exponential functions, which should have very similar exponents to each other in each passing‐through basin. The inverse of the exponent is essentially proportional to the average passage time, and consequently brings about the uniformity. These populations set a foundation for the multichannel generalization of the RRKM theory. Two cases of the non‐RRKM behaviors have been studied. One is a nonstatistical behavior in the low energy region such as the so‐called coexistence phase. The other is the short‐time behavior. It is well established [M. Berblinger and C. Schlier, J. Chem. Phys. 101, 4750 (1994)] that in a relatively simple and small system such as H^{+} _{3}, the so‐called direct paths, which lead to dissociation before the phase‐space mixing is completed, increase the probability of short‐time passage. In contrast, we have found in our Ar_{7}‐like molecules that trajectories of short passage time are fewer than expected by the statistical theory. It is conceived that somewhat a long time in the initial stage of the isomerization is spent by a trajectory to find its ways out to the next basins.

Conventional transition state theory/Rice–Ramsperger–Kassel–Marcus theory calculations of thermal termolecular rate coefficients for H(D)+O_{2}+M
View Description Hide DescriptionSeveral ab initio studies have focused on the minimum energy path region of the hydroperoxyl potential energy surface (PES) [J. Chem. Phys. 88, 6273 (1988)] and the saddle point region for H‐atom exchange via a T‐shaped HO_{2} complex [J. Chem. Phys. 91, 2373 (1989)]. Further, the results of additional calculations [J. Chem. Phys. 94, 7068 (1991)] have been reported, which, when combined with the earlier studies, provide a global description (but not an analytic representation) of the PES for this reaction. In this work, information at the stationary points of the ab initio PES is used within the framework of conventional Transition State Theory (TST)/RRKM. Theory to compute estimates of the thermal termolecular rate coefficients for the reaction between the H(D) atom and O_{2} in the presence of two different bath gases, argon and nitrogen, as a function of pressure and temperature. These calculations span a pressure range from 1.0 Torr to the high‐pressure limit and a temperature range from 298.15 to 6000.0 K. Conventional TST/RRKM Theory was utilized within the framework of two models: an equilibrium model employing the strong collision assumption (model I), [R. G. Gilbert and S. C. Smith, Theory of Unimolecular and Recombination Reactions (Blackwell, Oxford, 1990), as implemented in the UNIMOL program suite]; and a steady‐state model that includes chemical activation (model II), using the collisional energy transfer approximation proposed by J. Troe [J. Chem. Phys. 66, 4745, 4758 (1977); 97, 288 (1992)]. In this work we first summarize the pressure‐dependent fall‐off curves (calculated with model I) and the high‐pressure limit rate coefficients (calculated with models I and II) over the entire temperature range, and then focus on the fall‐off behavior for temperatures between 298.15 and 2000.0 K. Direct comparisons are made between the experimentally determined termolecular rate coefficients (either from direct measurements or based on recommended pressure/temperature‐dependent expressions) and the estimates of these rate coefficients calculated in this work as a function of pressure at 298.15 and 500.0 K. In the fall‐off region, we find better agreement between the theoretical and experimental values at low pressures than at pressures approaching the high‐pressure limit. Significant deviations are observed between theory and experiment as the high‐pressure limit is approached. The disagreement at 298.15 K is greater for N_{2} than for Ar.

A three‐dimensional quantum mechanical study of the NH+NO reactions
View Description Hide DescriptionIn this article is described a three‐dimensional quantum mechanical study within the nonreactive infinite order sudden approximation (IOSA) of the title system. The study was performed using a recently introduced global potential energy surface [J. Chem. Phys. 102, 6696 (1995)]. Integral total cross sections for the two separate products, namely, N_{2}O+H and N_{2}+OH, were calculated as a function of kinetic energy in the range 0.05–0.50 eV. Our main findings are (a) the overall cross sections and the cross sections for N_{2}O+H are only mildly dependent on the energy; (b) the cross sections for N_{2}+OH, in conrast to those for N_{2}O+H, depend on the energy and increase as the energy increases; (c) the yield of N_{2}O+H is about 80–90 % of the total yield, in accordance with experiment; (d) the overall cross sections are about 1 to 3 times smaller than the quasiclassical‐trajectory ones and about 5 to 15 times smaller than the experimental ones.

Ground and excited states of the complex of CO with water: A diffusion Monte Carlo study
View Description Hide DescriptionWe present an analysis of the complex of water with CO which includes (a) a new potential energy surface obtained by fitting ab initio points, followed by adjustment against experimental rotational, spin–spin and quadrupole coupling constants; (b) diffusion Monte Carlo (DMC) studies of the ground vibrational state, and of three excited vibrational states, at J=0. A new approach is suggested for DMC calculation of intermolecular vibrational frequencies.

The mobilities of NO^{+}(CH_{3}CN)_{ n } cluster ions (n=0–3) drifting in helium and in helium–acetonitrile mixtures
View Description Hide DescriptionThe mobilities of NO^{+}(CH_{3}CN)_{ n } cluster ions (n=0–3) drifting in helium and in mixtures of helium and acetonitrile (CH_{3}CN) 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 cm^{2} V^{−1} s^{−1} for NO^{+}, 12.3±0.3 cm^{2} V^{−1} s^{−1} for NO^{+}(CH_{3}CN), 8.2±0.2 cm^{2} V^{−1} s^{−1} for NO^{+}(CH_{3}CN)_{2} and 7.5±0.5 cm^{2} V^{−1} s^{−1} for NO^{+}(CH_{3}CN)_{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 dissociationreactions 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^{+}(CH_{3}CN)_{2} and NO^{+}(CH_{3}CN)_{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 cm^{2} V^{−1} s^{−1} for NO^{+}(CH_{3}CN)_{2} and 0.044±0.004 cm^{2} V^{−1} s^{−1} for NO^{+}(CH_{3}CN)_{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.