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Volume 94, Issue 6, 15 March 1991

Photoionization spectroscopy of nickel clusters: The effect of ammonia adsorption on ionization potentials
View Description Hide DescriptionThe photoionization efficiency spectra of nickel clusters containing adsorbed ammonia have been recorded near threshold. Ionization potentials (IPs) have been assigned for 97 selected Ni_{ n }(NH_{3})_{ m } species, with n up to 56 atoms. We observe that nickel cluster IPs decrease upon ammonia adsorption, with the magnitude of the IP shifts varying linearly with the number of adsorbed molecules, in good agreement with a simple electrostaticmodel. Deviations from this behavior are observed for Ni_{5}(NH_{3})_{ m } and Ni_{6}(NH_{3})_{ m } clusters, in which adsorption of the first ammonia molecule produces a significantly larger IP drop than do subsequently adsorbed molecules. Such behavior may indicate the presence of unique adsorption sites on Ni_{5} and Ni_{6} preferentially occupied by the first ammonia molecule adsorbed onto these clusters, or alternatively, may indicate cluster rearrangement occurring as the result of chemisorption. Our results are discussed in light of studies of ammonia adsorption onto single crystal nickel surfaces.

The overtone dynamics of acetylene above 10 000 cm^{−1}
View Description Hide DescriptionThe 10 000–13 000 cm^{−1} C–H stretching spectra of normal acetylene have been measured with a high resolution Fourier transform spectrometer and a long path gas cell. Over 400 rovibrational lines were assigned and analyzed yielding band origins and rotational constants. Comparison of calculated and observed rotational constants and intensities confirmed the vibrational assignments made in this region. A vibrational analysis of all observed C_{2}H_{2} transitions above 10 000 cm^{−1} was made based on the normal mode constants derived earlier. Some reassignments were made, many unassigned bands were assigned, and several new Fermi resonances were analyzed. When combined with our previous results below 10 000 cm^{−1}, the standard deviation of our analysis for 64 unconstrained states up to 24 000 cm^{−1} is 9.35 cm^{−1}. Local and global rotational perturbations were noted in our spectra, and the possible identity of the interacting states is discussed. The implication of our results for theories of vibrational energy level structure, intramolecular vibrational relaxation, the C_{2}H_{2}potential surface, the structure of its phase space, and stimulated emission pumping results on acetylene are discussed.

Superradiance quenching by confined acoustic phonons in chemically prepared CdS microcrystallites
View Description Hide DescriptionWe report the luminescence spectrum and picosecond dynamics of excitons in CdS microcrystallites embedded in an acrylonitrile–styrene copolymer film by a new method. The intensity ratio of band‐edge emission due to the direct recombination of excitons to red‐shifted emission from the trapped states was much larger than any other samples previously reported. This indicates much higher quality of our microcrystallites. From the comparison of luminescence spectrum of samples prepared by different cadmium salts as raw materials, we propose a ‘‘site–substitution’’ model for the origin of the trapped states. According to the model, the trapped states yielding the red‐shifted emission are impurities substituting the sulfur sites. The band‐edge emission was found to be emitted mainly from the larger microcrystallites among the size distribution. The luminescence lifetime of the band‐edge emission was almost constant at 60 ps below a threshold temperature (45±5 K) and increases nearly proportionally with temperature above this temperature. This temperature dependence of the lifetime suggests the quenching of superradiance of excitons by acoustic phonons confined in the microcrystallites. The threshold temperature corresponds to the frequency of radial compression mode (30.0 cm^{−1}=43.2 K) of the spherical microcrystallites.

Third harmonic generation of extreme ultraviolet radiation in a nitrogen continuous free jet zone of silence
View Description Hide DescriptionThe interpretation of third harmonic generation, as obtained by crossing and focusing a pulsed laser beam on the axis of a continuous supersonic jet of nitrogen, is discussed and treated quantitatively. The geometry of the laser modes, as well as the inhomogeneity of the free jet zone of silence surrounded by the shock wave structure and the background gas, are taken into account. In this way, the variation of the phase matching factor is calculated in terms of the axial distance from the nozzle to the laser focus. This makes it possible to obtain the free jet characteristics from careful measurements of two‐photon resonant, or nonresonant, frequency conversion rates. Thus the variations along the jet axis of the number density and the rotational temperature (decreasing down to 6 K), are obtained in good agreement with theoretical predictions.

Rovibrational spectra of open‐shell van der Waals complexes: Ar–OH(X ^{2}Π)
View Description Hide DescriptionThe infrared spectrum of Ar(^{1} S _{0})–OH(X ^{2}Π) has been predicted using an a b i n i t i o potential‐energy surface. We assume that the electronic spin and orbital angular momenta remain coupled to the rotational motion of the diatom in a van der Waals complex. The rovibrational energies and wave functions are calculated using a basis‐set method involving expansion of the rovibrational wave function in terms of a product basis of optimized radial and angular wave functions. Unusual features are observed due to the nonzero electronic angular momentum of the diatom in the rovibrational levels. The effect of deuterium substitution on the predicted spectra has also been studied.

Conjugation length dependence of Raman scattering in a series of linear polyenes: Implications for polyacetylene
View Description Hide DescriptionWe have measured the solid state Raman scatteringspectra of a homologous series of linear polyenes, with the number of alternated double bonds varying from 3 to 12. While we find a linear dependence of the Raman shifts of resonantly coupled modes with inverse conjugation length, we have also followed the suggestion of previous work in examining the inverse square product of the several Raman frequencies as a function of the logarithm of the measuredenergy gap of the molecule. This provides a linear relationship, as found for t r a n s‐polyacetylene, a result which is qualitatively consistent with the amplitude mode model of Horovitz and co‐workers. We also find, consistent with previous work on polyacetylene, a monotonic decrease in the ratios of oscillator strengths of the two strongest bands with conjugation length, as recently predicted by a series of molecular dynamics calculations. Suggested interpretations of a number of qualitative observations, including splitting of modes for shorter conjugation length, are offered, and the implications for the structure of t r a n s‐polyacetylene are discussed. The present work confirms that the previously measureddispersion in the Raman spectra of t r a n s‐polyacetylene is due to a distribution of conjugation lengths and brings into question some of the quantitative aspects of the amplitude mode model.

Nonadiabatic electronic interactions in the ion‐pair states of NeICl
View Description Hide DescriptionNonadiabatic interactions in the NeICl van der Waals complex have been explored in the lowest energy triad of ICl ion‐pair states (∼39 000 cm^{−1}). Dispersed fluorescencemeasurements reveal emission characteristic of multiple ion‐pair electronic states, with the relative contributions from the E(0^{+}), β(1), and D’(2) states changing with the initial ICl vibrational excitation (v _{ICl}). Emission directly from NeICl (v _{ICl}=0) complexes indicates that the initially prepared NeICl levels have mixed electronic character and that the ICl electronic parentage changes with the initial van der Waals vibrational level selected. NeICl complexes prepared with 1–4 quanta of ICl stretch undergo rapid vibrational predissociation with a strong propensity for Δv _{ICl}=−1 relaxation. The electronic state(s) populated in the ICl fragments differ from the mixed electronic character of the initially prepared level, demonstrating that vibrational predissociation is accompanied by nonadiabatic electronic state changing processes. The observed final state selectivity may be attributed to the relative strength of the nonadiabatic couplings between the initial NeICl bound state and the final ICl states or a momentum gap rationale based on the overlap between the NeICl bound statewave function and the highly oscillatory continuum wave function of the separating fragments.

Photodissociation of acetone at 193 nm: Rotational‐ and vibrational‐state distributions of methyl fragments by diode laser absorption/gain spectroscopy
View Description Hide DescriptionDiode laser transient absorption/gain spectroscopy is used to monitor time‐dependent populations of CD_{3} fragments formed in the photodissociation of acetone‐d _{6} at 193 nm. Selected rotational lines have been measured in the ν_{2} ‘‘umbrella’’ fundamental and first two hot bands, and in the ν_{3} asymmetric stretching fundamental band. Substantial growth is observed in the vibrationless state on the time scale of vibrational relaxation. We estimate that only about 15% of the nascent CD_{3} population is formed in the vibrational states we detect: ν_{2}=1 and 2, ν_{3}=1, and the vibrationless state. Most of the nascent methyl population is evidently spread among many undetected vibrational states. These results complement previous measurements of acetone photofragments by infrared emission, multiphoton ionization, and laser‐induced fluorescence. Our inferred global vibrational distribution is consistent with a two‐step fragmentation.

Lifetime measurement of a collision complex using ion cyclotron double resonance. H_{2}C_{6}N^{+} _{2}
View Description Hide DescriptionIn the ion–molecule reaction between HC_{3}N^{+} and HC_{3}N, the lifetime of the collision complex (H_{2}C_{6}N^{+} _{2})* was long enough that ion cyclotron double resonance techniques could be used to probe the distribution of the lifetimes of the collision complex. The mean lifetime of the collision complex at room temperature was measured as 180 μs with a distribution ranging from 60 to 260 μs as measured at the half‐heights in the distribution. Lifetimes of this magnitude with respect to unimolecular dissociation allow for some stabilization of the collision complex by the slower process of infrared photon emission.

Theoretical stabilization and scattering studies of resonances in the addition reaction H+CO = HCO
View Description Hide DescriptionWe report the first coordinated stabilization and coupled channel scattering calculations of resonances for a realistic, triatomic molecule, HCO, using a global, a b i n i t i o potential‐energy surface. The two set of calculations are in excellent agreement with each other for the nine resonance energies reported.

Photodissociation of ammonia at 193.3 nm: Rovibrational state distribution of the NH_{2}(Ã ^{2} A _{1}) fragment
View Description Hide DescriptionThe rovibrational state distribution of the nascent NH_{2}(Ã ^{2} A _{1}) fragments generated by 193.3 nm photodissociation of a room temperature sample of NH_{3} is determined through an analysis of a major portion (6000–13 000 cm^{−1}) of the NH_{2}(Ã ^{2} A _{1}→X̃ ^{2} B _{1}) near infrared emission spectrum obtained by time‐resolved Fourier transform infrared emission spectroscopy. The NH_{2}(Ã) fragments are observed to be formed predominantly in their zero‐point vibrational level, with substantial rotational excitation about their a‐inertial axis up to the limit of the available energy, ∼3150 cm^{−1}, but with little excitation about the other axes. The pattern of this energy disposal is discussed within the framework of existing knowledge regarding the form of the NH_{3} Ã state potential energy surface on which the dissociation occurs. The essential features are entirely consistent with a direct carry over, into the fragment, of the out‐of‐plane bending vibrational motion introduced in the parent molecule by the photoexcitation process.

Differential cross sections for rotationally state‐resolved inelastic scattering of HF by argon
View Description Hide DescriptionWe present differential cross section (DCS) measurements for scattering of HF by Ar. These crossed‐beam experiments employ rotational state sensitivity, allowing determination of the DCS as a function of the scattered HF rotational state. The initial HF rotational distribution is generated by nozzle expansion, without further state selection. Its composition is mostly J=0 and J=1, with small admixtures for J>1. The DCS for each final state J’ is measured using a stabilized cw HF chemical laser, in conjunction with a rotatable liquid He‐cooled bolometer.Measurable signals are obtained for scattering into 0≤J’≤5, where J’=6 is the thermodynamic limit for our collision energy of 120 meV. The measured DCS’s show a strong forward peak, largely from elasticscattering. In addition, the DCS’s evolve from a broad shoulder in the θ≊25°–40° region for J’=0—through a flattening of the wide‐angle scattering for J’=2 and J’=3—to an increase in the scattering beyond ∼40° for J’=4. The DCS for scattering into J’=5 also shows increased intensity at wide scattering angles, but its onset is delayed until ∼70°. These features are shown to be independent of the laboratory → center‐of‐mass kinematic transformation. The wide‐angle scattering into J’=4 and J’=5 corresponds to transferring up to 40% and 60%, respectively, of the available kinetic energy into HF rotation. Since the center‐of‐mass scattering angles are up to ∼110°, we interpret the observed features for J’=4–5 in terms of rotational rainbow scattering from the hard core of the HF+Ar potential energy surface. The origin of the shoulder for J’=0 scattering is less clear, but it may arise from the strongly anisotropic nature of the HF+Ar van der Waals attraction.

Statistical and nonstatistical effects in bond fission reactions of SiH_{2} and Si_{2}H_{6}
View Description Hide DescriptionAn efficient implementation of microcanonical, classical variational transition‐state theory based on the use of the efficient microcanonical sampling (EMS) procedure is applied to simple bond fissions in SiH_{2} and Si_{2}H_{6} using recently constructed global potential‐energy surfaces. Comparison is made with results of trajectory calculations performed on the same potential‐energy surfaces. The predictions of the statistical theory agree well with and provide an upper bound to the trajectory derived rate constants for SiH_{2}→SiH+H. In the case of Si_{2}H_{6}, agreement between the statistical theory and trajectory results for Si–Si and Si–H bond fission is poor with differences as large as a factor of 72. Moreover, at the lower energies studied, the statistical calculations predict considerably s l o w e r rates of bond fission than those calculated from trajectories. These results indicate that the statistical assumptions inherent in the transition‐state theory method are not valid for disilane in spite of the fact that many of the mode‐to‐mode rate constants for intramolecular energy transfer in this molecule are large relative to the Si–Si and Si–H bond fission rates. There are indications that such behavior may be widespread among large, polyatomic molecules.

A wave packet Golden Rule treatment of vibrational predissociation
View Description Hide DescriptionThe time‐dependent wave packet technique is applied to the Golden Rule treatment of vibrational predissociation. The wave packet at time zero is taken as the product of the quasibound wave function and the coupling inducing predissociation. The rate for vibrational predissociation can then be obtained by Fourier transform into the energy domain of the time‐dependent wave packet autocorrelation function. The method has been applied to a model triatomic van der Waals molecule. It is shown that when the bound‐state components of the wave packet are projected out, the time‐dependent version of the Golden Rule approximation provides an alternative efficient technique to treat intramolecular decay.

Measurements of kinetic energy release following the unimolecular and collision‐induced dissociation of argon cluster ions, Ar^{+} _{ n }, for n in the range 2–60
View Description Hide DescriptionA double‐focusing mass spectrometer in conjunction with a cluster beam source has been used to measure the average kinetic energy released following the unimolecular and collision‐induced fragmentation (CID) of argon cluster ions. Measurements on unimolecular decay have been made for clusters in the range Ar^{+} _{5}–Ar^{+} _{60}, and for the CID studies the range was Ar^{+} _{2}–Ar^{+} _{30}. Within the observation time window, the kinetic energy release results for the loss of a single argon atom via unimolecular decay are consistent with internal energy being partitioned statistically. Three separate CID routes are identified: (i) loss of one Ar atom; (ii) rapid (<10^{−7} s) loss of two Ar atoms within the confines of a collision cell; (iii) sequential loss of two Ar atoms on a time scale >10^{−7} s. It is proposed that the CID of small cluster ions proceeds via electronic excitation; but that as the clusters increase in size (n>4) vibrational excitation predominates. A simple spectator model of collisional excitation accounts for the experimental CID results in cluster ions beyond Ar^{+} _{15}.

Collisional deactivation studies of the Xe(6p) states in He and Ne
View Description Hide DescriptionThe decay kinetics of the Xe(6p[1/2]_{0} or 2p _{5}), Xe(6p[3/2]_{2} or 2p _{6}) and Xe(6p[5/2]_{2} or 2p _{9}) states have been studied in He and Ne buffer gas using the two‐photon laser‐excitation technique. The pressure dependence of the fluorescence decay rates was used to measure the total quenching rate constants at 300 K. The primary product distribution from two‐body collisions with He or Ne were estimated from product fluorescence intensities, and state‐to‐state quenching rate constants were assigned. Limited information about the decay kinetics of the Xe(2p _{7}),Xe(2p _{8}), Xe(2p _{10}) states was inferred from observation of the decay of these product states at various He and Ne pressures. With the exception of Xe(2p _{5}) in He and Ne and Xe(2p _{7}) in He, the decay constants of the Xe(6p) states are less than 1.0×10^{−11} cm^{3} s^{−1}, and these rate constants are smaller then the decay constants for heavier rare gases. The collisional depolarization of Xe(2p _{6}) by Ne was studied by monitoring the decay rates of the vertical and horizontal polarization components of the fluorescence intensity.

Angularly resolved rotational energy transfer in highly vibrationally excited states: Na_{2}(v=31)–Ne
View Description Hide DescriptionThe scattering of high vibrationally excited sodium molecules Na_{2}(v=31) with Ne atoms at 180 meV collision energy is investigated in a crossed molecular beam arrangement using laser optical methods. Angularly resolved rotationally inelastic and vibrationally elastic cross sections j _{ i }→j _{ f } are measured for j _{ i } =5, 7, and 9≤j _{ f } ≤25. Pronounced rotational rainbow maxima are observed, the angular position of which indicates a significantly larger anisotropy of the interaction potential compared to that of Na_{2}(v=0)–Ne. Except for the increase of the anisotropy the vibrational excitation has little effect on the dynamics of rotational energy transfer. Good agreement with a new semiempirical Na_{2}–Ne potential surface V(r,R,γ) is found.

Separation of collisional and vibrational variables in chemical reactions: Decoupling surface in phase space
View Description Hide DescriptionA method for decoupling the collisional and vibrational variables in the collision process is developed. The present procedure of decoupling is based on a canonical transformation in phase space. The basic principle employed is the ‘‘maximal decoupling condition’’ [∂H/∂u]_{ u=v=0}=[∂H/∂v]_{ u=v=0}=0, where H is the Hamiltonian and u and v represent new vibrational coordinates and momenta. ‘‘Decoupling surface’’ is defined as the surface determined by u=v=0. The partial differential equation to be satisfied by this decoupling surface is derived. This partial differential equation can be solved easily by utilizing classical trajectories. Local vibrational frequencies along the decoupling surface are defined and their stability analysis is shown to provide the criterion for the separability. ‘‘Adiabatic approximation’’ which assumes the conservation of locally defined actions of the new vibrational variables leads to an ‘‘effective Hamiltonian’’ that describes the collision process under a given set of the initial vibrational actions. The present theory is applied to the collinear H+H_{2}→H_{2}+H chemical reaction.Analysis of the local frequencies shows that the instability occurs in the following two regions depending on the collision energy: (1) when the collision energy is just above the activation energy, the instability occurs around the saddle point of the potential energy surface and (2) at the higher collision energies, this unstable region disappears, but another one appears in the region where the reactive trajectories touch the ridge of the potential just after passing through the saddle point. In these unstable regions, bifurcation of the trajectories occurs. Analysis of the effective Hamiltonian can indicate the existence of a resonance structure corresponding to a temporal trapping of trajectory. This resonance structure is analogous to that in the coupled‐oscillator problems and we can expect that the so‐called irregular motion, or a stochastic layer, exists around the separatrix connected to this resonance structure.

Fully L ^{2} methods for multichannel scattering problems. Partial widths
View Description Hide DescriptionWe analyze the use of L ^{2} discretization methods to calculate, in the framework of the Feshbach theory, partial and total widths of resonances lying in multichannel continua. When interchannel coupling is strong, partial widths cannot be obtained without solving the scattering equations and for this purpose a fully L ^{2} method is proposed, starting from the static exchange approximation as a zeroth order result. For weak interchannel coupling, we study the validity of Born‐type approximations and we justify why neglecting this coupling may be appropriate to evaluate total widths even for cases where it is totally inadequate for the corresponding partial ones. The methods proposed are illustrated for moderately strong (He) and weak (O^{6+}) coupling in atomic resonances.

Kinetic energy release in thermal ion–molecule reactions: The Nb^{2} ^{+}–(benzene) single charge–transfer reaction
View Description Hide DescriptionWe have adapted the techniques originally developed to measure ion kinetic energies in ion cyclotron resonance (ICR) spectrometry to study the single charge–transfer reaction of Nb^{2+} with benzene under thermal conditions in a Fourier transform ion cyclotron resonancemass spectrometer (FTICRMS). The partitioning of reaction exothermicity among the internal and translational modes available is consistent with a long‐distance electron‐transfer mechanism, in which the reactants approach on an ion‐induced dipole attractive potential and cross to a repulsive potential at a critical separation of ∼7.5 Å when electron transfer occurs. The reaction exothermicity, 5.08 eV, is partitioned to translation of Nb^{+}, 0.81±0.25 eV, translation of C_{6} H_{6} ^{+}, 1.22±0.25 eV, and internal excitation of C_{6} H_{6} ^{+} to produce the la_{2u } electronic state, which is ∼3 eV above the ground state of the ion. We have also studied the kinetics of the reaction of Nb^{2+} with benzene and determined the rate constant,k = 1.4×10^{−} ^{9} cm^{3} molecule^{−} ^{1} s^{−} ^{1}, and the efficiency, 0.60, of the process. These also support the proposed charge–transfer mechanism. In addition to the charge–transfer pathway, which accounts for 95% of the reaction products, Nb^{2+} is observed to dehydrogenate benzene to form Nb^{2+} (benzyne). This process implies D(Nb^{2+} –benzyne)≥79 kcal/mol.