Volume 86, Issue 7, 01 April 1987
Index of content:

Infrared spectra of HF complexes with methane, silane, and germane
View Description Hide DescriptionHF complexes with methane, silane, and germane were prepared in noble gas matrices and studied using infrared spectroscopy and Hartree–Fock (SCF) calculations. The spectra indicate that two types of 1:1 complexes were formed, a normal one in which the hydrogen of HF is interacting with one hydrogen of silane or germane, and a reverse complex in which the fluorine of HF is interacting with one hydrogen atom of methane. The IR inactive symmetric C–H stretch in CH_{4} was observed in the CH_{4}‐‐FH complex as a weak band at 2914 cm^{−} ^{1}. In the silane‐‐HF and germane‐‐HF complexes, the Si–H and Ge–H stretches were perturbed approximately 50 cm^{−} ^{1} to higher energy relative to the antisymmetric stretch ν_{3} in each parent molecule, but the ν_{1} modes were masked by the strong ν_{3} parent bands. Higher order 1:2 (AH_{4}:HF) complexes were also observed and support the normal or reverse‐type geometry of the 1:1 complexes.

Absolute I* quantum yields for the ICN Ã state by diode laser gain‐vs‐absorption spectroscopy
View Description Hide DescriptionAbsolute I* quantum yields have been measured as a function of wavelength for room temperature photodissociation of the ICN Ã state continuum. The yields are obtained by the technique of time‐resolved diode laser gain‐vs‐absorption spectroscopy. Quantum yields are evaluated at seven wavelengths from 248 to 284 nm. The yield at 266 nm is 66.0±2% and it falls off to 53.4±2% and 44.0±4% at 284 and 248 nm, respectively. The latter values are significantly higher than those obtained by previous workers using infrared fluorescence. Estimates of I* quantum yields obtained from analysis of CN photofragment rotational distributions, as discussed by other workers, are in good agreement with the I* yields reported here. The results are considered in conjunction with recent theoretical and experimental work on the CN rotational distributions and with previous I* quantum yield results.

Infrared spectra of the very weak H_{2}‐‐HF and O_{2}‐‐HF complexes in solid neon
View Description Hide DescriptionVery weak, hydrogen bonded complexes of molecular hydrogen and oxygen with HF have been prepared by condensing the neon diluted reagents at 4–5 K. Infrared spectra of the H_{2}‐‐HF complex revealed a ν_{ s }(HF) mode at 3938 cm^{−} ^{1} and ν^{ c }(H_{2}) mode at 4155 cm^{−} ^{1} which are red shifted from isolated molecule values in agreement with theoretical predictions. Increasing the HF concentration produced a 1:2 complex, H_{2}–(HF)_{2}. Similar experimental results were obtained for molecular oxygen complexes using HF and DF; the oxygen fundamental in this complex, however, exhibited a 6 cm^{−} ^{1}blue shift. Spectra of these complexes in argon matrices showed that argon is not an effectively inert medium for very weak complexes where interaction with the solid argon environment is competitive with the H_{2}–HF and O_{2}–HF interactions.

Interaction of Xe^{+} and Cl^{−} ions and their formed molecules with a Xe solid matrix
View Description Hide DescriptionThe aim of the present work is the calculation of the energy of ionic xenon–chlorine systems which can be formed in solid Xe by irradiation. The energy levels of these ionic systems differ from those in the gas phase due to polarization and dispersion interactions with solid Xe atoms. It is shown that the Xe^{+} _{2}Cl^{−} molecule is responsible for experimentally observed emission. The activation energy of the Xe^{+} _{2}Cl^{−} formation is found to form a broad band.

Time resolved resonance Raman spectra of anilino radical and aniline radical cation
View Description Hide DescriptionWe report, in this paper, submicrosecond time resolvedresonanceRaman spectra of anilino radical and its radical cation as observed in pulse radiolytic studies of the oxidation of aniline in aqueous solution. By excitation in resonance with the broad and weak electronic transition of anilino radical at 400 nm (ε∼1250 M^{−} ^{1} cm^{−} ^{1}) we have observed, for the first time, the vibrational features of this radical. The Wilson ν_{8a } ring stretching mode at 1560 cm^{−} ^{1} is most strongly resonance enhanced. The ν_{7a } CN stretching band at 1505 cm^{−} ^{1}, which is shifted to higher frequency by 231 cm^{−} ^{1} with respect to aniline, is also prominent. The frequency of this latter mode indicates that the CN bond in the radical has considerable double bond character. The Raman spectrum of aniline radical cation, excited in resonance with the ∼425 nm electronic absorption (ε∼4000 M^{−} ^{1} cm^{−} ^{1}), shows features which are similar to phenoxyl radical. Most of the observed frequencies of this radical in solution are in good agreement with vibrational energies determined by recent laser photoelectron spectroscopic studies in the vapor phase. The bands most strongly enhanced in the resonanceRaman spectrum are, however, weak in the photoelectron spectrum. While the vibrational frequencies observed for anilino radical and its isoelectronic cation are quite similar, the resonance enhancement patterns are very different. In particular the ν_{1} _{4} b _{2} mode of anilino radical observed at 1324 cm^{−} ^{1} is highly resonance enhanced because of strong vibronic coupling between the 400 nm ^{2} A _{2}–^{2} B _{1} and the higher ^{2} B _{1}–^{2} B _{1} electronic transitions.

Temperature dependence of the EPR zero‐field splitting of Mn^{2} ^{+} in ZnSiF_{6}⋅6H_{2}O
View Description Hide DescriptionThe temperature dependence of the EPR spectrum of Mn^{2} ^{+} in ZnSiF_{6}⋅6H_{2}O was measured at 9.2 GHz between 15 and 297 K. Both the g and Atensors were found to be isotropic with values close to 2.000 and −90×10^{−} ^{4} cm^{−} ^{1}, respectively. A slight decrease in the magnitude of A with increasing temperature was observed. The zero‐field splitting parameters D and a–F were found to vary from −130.5×10^{−} ^{4} and 9.9×10^{−} ^{4} cm^{−} ^{1}, respectively, at 15 K to −170.2×10^{−} ^{4} and 7.8×10^{−} ^{4} cm^{−} ^{1}, respectively, at 297 K. The data for D below 100 K were fitted to the orbit–lattice interaction in the long‐wavelength approximation with a Debye temperature of 138 K. An estimate of the low‐temperature ratio of D for Ni^{2} ^{+} and Mn^{2} ^{+} in ZnSiF_{6}⋅6H_{2}O based on the long‐wavelength fits was in good agreement with the measured ratio of 10 at 4 K.

Infrared diode laser spectroscopy of the PF radical
View Description Hide DescriptionThe infrared absorptionspectrum of the PF radical in the X ^{3}Σ^{−}ground electronic state has been observed using a Zeeman modulated diode laserspectrometer. The radical was produced in a multiple‐reflection absorption cell by a dc glow discharge in a mixture of PH_{3} and CF_{4}. The F _{1} and F _{3} spin components of five vibration–rotation transitions of the v=1–0 band have been recorded in the region of 823 to 830 cm^{−} ^{1}. By constraining molecular constants of the v=0 state to the values previously determined by microwave spectroscopy, the band origin, the vibration–rotation constant α^{ B }=B _{0}−B _{1}, and the vibrational change of the spin–spin interaction constant α^{λ}=λ_{1}−λ_{0} have been determined from the observed spectrum to be 837.816 36(70), 0.004 639 4(70), and −0.005 04(91) in cm^{−} ^{1}, respectively, with one standard deviation in parentheses. The sign of α^{λ} is the same as those of NCl and PCl, but is opposite to those of O_{2}, SO, S_{2}, and SeO.

Radiative decay lifetimes of CH^{−} _{2}
View Description Hide DescriptionRecently the presence and radiative decay of vibrationally excited CH^{−} _{2}, generated in a hot cathode discharge of methane, was established by measuring the time dependent photodetachment from excited states of CH^{−} _{2} as it radiatively relaxed in a high vacuum ion trap. The time dependence of the photodetachment was found to be consistent with an electron affinity of 5250 cm^{−} ^{1} (0.65 eV) for ground stateX̃ ^{3} B _{1} methylene. The radiative decay lifetimes of the first three excited bending vibrations of CH^{−} _{2} were also tentatively assigned. Here, we report a more refined analysis of the experimental data along with theoretical a b i n i t i o determinations of the radiative decay lifetimes of the first four excited bending vibrational levels of CH^{−} _{2}. There is some discrepancy between the a b i n i t i o values (431, 207, 118, and 68 ms for the v _{2}=1, 2, 3, and 4 levels respectively) and the experimental values (525, 70, and 14 ms for v _{2}=1, 2, and 3 respectively) for v _{2}=2 and 3. Possible reasons for this discrepancy are discussed but none of the alternatives are entirely satisfactory.

Fourier transform NMR at low temperatures
View Description Hide DescriptionIn NMR at low temperatures the steady‐state absorption/dispersion spectrum in general differs from the Fourier transform of the free induction decay. The nature of this difference is discussed and an experimental example is given.

Accurate width and position of lowest ^{1} S resonance in H^{−} calculated from real‐valued stabilization graphs
View Description Hide DescriptionThe use of real‐valued stabilization graphs for calculation of resonance energies and widths is considered in connection with the lowest Feshbach resonance state of H^{−}, nominally (Φ_{2s })^{2}. One problem that arises is the difficulty of generating stabilization graphs with well defined avoided crossings between the resonance state of interest and nearby interacting continuum states. For this purpose, criteria are developed for selection of basis sets and CI lists and for determination of suitable stabilization parameters. Another problem is the extraction of resonance parameters from the stabilization graph. We study one particular analytic continuation procedure recently proposed by Isaacson and Truhlar. Criteria for separation of physical from nonphysical solutions of the complex energy stationary point, for determination of the necessary numerical precision for the input real eigenvalues, and for other details of the method have all been examined. The results for H^{−}, even with a modest Gaussian basis set and partial CI list, are in excellent agreement both with experiment and with more elaborate calculations by other methods. It is concluded that the method is capable of describing electronic resonances to high accuracy and shows promise for application to systems more complicated than H^{−}.

Considerations of the rates and lifetimes of intermediate complexes for the association of various ligands to metal ions: Ag^{+} and Cu^{+}
View Description Hide DescriptionRates were measured for the association of CO, CH_{4}, CH_{3}F, NH_{3}, ND_{3}, CH_{3}Cl, and CH_{3}Br onto Ag^{+} and Cu^{+} at 298 K. In the order given above, the three‐body association rate constants for Ag^{+} range from 2.5×10^{−} ^{3} ^{0} to 3.9×10^{−} ^{2} ^{7} cm^{6} s^{−} ^{1}. The rate constants for Cu^{+} are about four to six times larger for a given neutral reactant. The rate constants display trends in the order expected considering the relative bond energies of the clusters, although the enormous range of reactivity is not reflected simply by differences in dipole moments and polarizabilities. There is a very large isotope effect, where the rate constant for ND_{3} association was found to be about three times greater than NH_{3} in the case of both ions. (Results for Na^{+} follow the same trend.) This suggests a coupling of ligand vibrations with orbiting motion which leads to enhanced lifetimes of the cluster intermediates. The trend found for the methyl halides also supports the involvement of this effect.

Stochastic dynamic models of curve crossing phenomena in condensed phases
View Description Hide DescriptionTwo stochastic dynamic models are used to study several aspects of curve crossing phenomena in dissipative systems. A surface hopping model is used to test the qualitative predictions of earlier theories. The simulation results agree well with the qualitative picture. Results are obtained for an alternate semiclassical model based on a vector spin representation which is derived via a variational principle. The vector model shows some differences in behavior as compared to the hopping model. In certain regimes the vector model shows chaotic behavior.

Mutual neutralization and chemiionization in collisions of alkali metal and halogen atoms
View Description Hide DescriptionA semiempiricalvalence bond method is used to obtain realistic potential curves for the three lowest ^{1}∑^{+} curves of NaF, KF, LiCl, NaCl, KCl, Lil, and Nal. They are used together with a model functional form for the diabiatic coupling to compute quantal chemiionization and mutual neutralization cross sections. The accuracy of one‐electron estimates of the interaction energies in the crossing regions is explored. The Landau–Zener method is found to be very reliable except for collisions involving iodine, for which modifications are necessary.

A simple kinetic theory model of reactive collisions. III. Convex loaded bodies
View Description Hide DescriptionDerived is a classical ‘‘line‐of‐normals’’ model for the treatment of the orientation dependence of the reaction cross section for nonspherical convex bodies intended to represent loaded diatoms in reactive collisions with (spherical) atoms. For the case of nonspherical molecules with small loading, simple formulas are obtained which display explicitly the dependence of the orientational cross section on the nonsphericity and the loading parameters. Applications to realistic systems are presented.

Angular momentum relaxation in atom–diatom dilute gas mixtures
View Description Hide DescriptionThe angular momentum relaxation cross sections for a diatomic molecule in a dilute atomic gas are estimated subject to the assumption that the intermolecular torque is dominated by the hard, impulsive contribution (evaluated using Boltzmannkinetic theory for nonspherical molecules). For carbon monoxide in a variety of gases, the kinetic theory derived contribution to the angular momentum cross section is in qualitative agreement with the experimental results of Jameson, Jameson, and Buchi.

Photodissociation of carbon cluster cations
View Description Hide DescriptionMass resolved carbon cluster cations from C^{+} _{3} to C^{+} _{20} have been photofragmented using 248 and 351 nm light. For an initial cluster C^{+} _{ n } , the dominant fragment observed is C^{+} _{ n−3} . Fluence dependence measurements of photofragmentation yield bracketed dissociation thresholds, photofragmentation cross sections, and product branching ratios. Photodissociation with 248 nm light is found to be primarily linear with laser fluence for n>5, but there are indications that C^{+} _{3} and C^{+} _{5} fragment only with the absorption of two or more photons. For 351 nm light, clusters with six and more atoms all show a linear dependence. The photofragmentation cross sections for both 351 and 248 nm light show a significant change as a function of cluster size.

Theory of initial yields of ions generated by electrons in binary mixtures. II
View Description Hide DescriptionEggarter [J. Chem. Phys. 8 4, 6123 (1986)] showed a method of calculation of the yield of ionization in a mixture by electrons, and carried out a numerical solution for the Ar–H_{2} mixture. At sufficiently high electron energies, the yields N ^{Ar} and N ^{H2 } of each component species closely follow the relation N ^{Ar}/N ^{H2 } =K⋅C ^{Ar}/C ^{H2 }, where C ^{Ar} and C ^{H2 } represent the concentration fractions, and K is a constant. We now present a theoretical interpretation of that relation, and show the microscopic meaning of the constant K; it is the ratio of the ionization cross sections of the two component species, evaluated for an electron of a certain kinetic energy, which is much greater than the ionization threshold energy. The interpretation rests on the fact that the Spencer–Fano electron degradation spectrum depends on electron energy only mildly and smoothly except near the initial source energy and near the first ionization threshold energy. The same relation should hold approximately for any combination of ordinary molecules, although for the Ar–H_{2} case the relation is obeyed especially close. We also discuss the connection of our treatment with earlier treatments based on certain assumptions about the radiation‐energy partition among the mixture components.

Collision induced dissociation of metal cluster ions: Bare aluminum clusters, Al^{+} _{ n } (n=3–26)
View Description Hide DescriptionThe collision‐induced dissociation of aluminum clusters, Al^{+} _{ n } (n=3–26), by argon, at a center of mass collision energy of 5.25 eV, has been studied using a low energy ion beam apparatus. Product branching ratios and collision induced dissociation cross sections are presented and discussed. The main product is Al^{+} for the smaller clusters and Al^{+} _{ n−1} for the larger ones. The cross sections rise to a peak at Al^{+} _{6}−Al^{+} _{9} and then decrease with increasing cluster size. Cross sections for Al^{+} _{7}, Al^{+} _{13}, Al^{+} _{14}, and Al^{+} _{23} are significantly smaller than their neighbors. A crude kinetic model is used to derive approximate cluster ionization potentials from the product branching ratios. The IPs initially rise with cluster size, peak at Al_{6} and then decrease. The IP of Al_{7} is particularly low and there is a sharp drop in IP at Al_{1} _{4} where the IP falls below that of the atom. The results suggest that the dissociation energies increase for the larger clusters and there is evidence that Al^{+} _{7}, Al_{1} _{3}, Al^{+} _{13}, Al^{+} _{14}, and Al^{+} _{23} have enhanced stability. The results are compared to the predictions of the electronic shell model which can account for some of the results but predicts additional features which are not observed.

On understanding the relationship between structure in the potential surface and observables in classical dynamics: A functional sensitivity analysis approach
View Description Hide DescriptionThe relationship between structure in the potential surface and classical mechanical observables is examined by means of functional sensitivity analysis. Functional sensitivities provide maps of the potential surface, highlighting those regions that play the greatest role in determining the behavior of observables. A set of differential equations for the sensitivities of the trajectory components are derived. These are then solved using a Green’s function method. It is found that the sensitivities become singular at the trajectory turning points with the singularities going as η^{−} ^{3} ^{/} ^{2}, with η being the distance from the nearest turning point. The sensitivities are zero outside of the energetically and dynamically allowed region of phase space. A second set of equations is derived from which the sensitivities of observables can be directly calculated. An adjoint Green’s function technique is employed, providing an efficient method for numerically calculating these quantities. Sensitivity maps are presented for a simple collinear atom–diatom inelastic scattering problem and for two Henon–Heiles type Hamiltonians modeling
intramolecular processes. It is found that the positions of the trajectory caustics in the bound state problem determine regions of the highest potential surface sensitivities. In the scattering problem (which is impulsive, so that ‘‘sticky’’ collisions did not occur), the positions of the turning points of the i n d i v i d u a l t r a j e c t o r y c o m p o n e n t s determine the regions of high sensitivity. In both cases, these lines of singularities are superimposed on a rich background structure. Most interesting is the appearance of c l a s s i c a l interference effects. The interference features in the sensitivity maps occur most noticeably where two or more lines of turning points cross. The important practical motivation for calculating the sensitivities derives from the fact that the potential is a f u n c t i o n, implying that any direct attempt to understand how local potential regions affect the behavior of the observables by repeatedly and systematically altering the potential will be prohibitively expensive. The functional sensitivity method enables one to perform this analysis at a fraction of the computational labor required for the direct method.

Infrared laser kinetic spectroscopy of a photofragment CS generated by photodissociation of CS_{2} at 193 nm: Nascent vibrational–rotational–translational distribution of CS
View Description Hide DescriptionCarbon monosulfide fragments generated by CS_{2}photodecomposition at 193 nm were examined by time‐resolved observation of their vibration–rotation spectral lines with infrared diode laser kinetic spectroscopy. The CS molecules were found to be initially spread over a wide range of vibrational and rotational levels which were accessible with available energy, for both the triplet and singlet channels leading to sulfur atoms in the ^{3} P ground and ^{1} Dexcited states, respectively. The analysis of the observed line shape has allowed us to obtain information also on translational energy of CS fragments and to distinguish the contributions of the two channels. The branching ratio was thus estimated to be approximately one to one.