Volume 105, Issue 5, 01 August 1996
Index of content:

Solvation and melting in large benzene⋅(Ar)_{ n } clusters: Electronic spectral shifts and linewidths
View Description Hide DescriptionAlthough there has been considerable interest in solvation processes in small atomic and molecular clusters, uncertainties in the interpretation of spectral probes have made the experimental elucidation of the solvation, and in particular how it relates to bulk solvation, problematical. We demonstrate here that, through the application of a microscopic formalism which has the novel feature of accounting for the collective dielectric response of a cluster, the reported spectra of large benzene⋅(Ar)_{ n } clusters can be readily understood. Specifically, we show that the apparent lack of convergence of the benzene’s absorptionspectrum to the corresponding bulk result derives from the dominance of nonwetting cluster structures for large n. Even observed peak multiplicities and individual linewidths may be understood within this formalism if the cluster structures upon which the calculations are based are generated in a nonequilibrium (rather than thermally equilibrated) simulation. Given this detailed understanding of the relationship between spectroscopy and structure, we also can clarify the experimental consequences of the so‐called ‘‘melting’’ transition in benzene⋅(Ar)_{ n } clusters: The spectral signature of the melting is a change in the behavior of the linewidth of the absorption envelope which results from a subset, but not all, of the Ar atoms becoming fluid. This description of the melting behavior suggests an important refinement of the conventional picture of solid–fluid phase coexistence in clusters.

Identification of two ^{3}Σ_{ u } ^{−}←X ^{3}Σ_{ g } ^{−} transitions of ^{16}O_{2} near 88930 and 90780 cm^{−1}
View Description Hide DescriptionNew ^{16}O_{2}photoabsorption cross‐section measurements are presented for two bands near 88930 cm^{−1} and 90780 cm^{−1}. We have assigned these bands as the (0,0) and (1,0) bands of the E ^{′} ^{3}Σ_{ u } ^{−}←X ^{3}Σ_{ g } ^{−} system, where the E ^{′} state is a mixed state resulting from the coupling of 4pπ_{ u } and 5pπ_{ u } ^{3}Σ_{ u } ^{−}Rydberg states with the lowest ^{3}Σ_{ u } ^{−} valence state. In contrast to the E ^{3}Σ_{ u } ^{−}←X ^{3}Σ_{ g } ^{−} bands seen at lower energies, these bands show resolved rotational structure. Spectroscopic parameters for the upper levels have been derived using an effective Hamiltonian and cross‐section band models. The observed levels are perturbed by spin‐orbit interactions with nearby levels (v=1 and 2) of the 4pπ_{ u } D ^{′} ^{3}Σ_{ u } ^{+}Rydberg state. In addition, transitions to the v=2 level of the perturber state have been observed near 90920 cm^{−1}.

Rotational excitations of a symmetric top in cubic orientational potentials. II. CHD_{3} matrix‐isolated in argon
View Description Hide DescriptionInelastic neutron scattering has been performed on CHD_{3} molecules matrix isolated in solid argon. Four inelastic lines have been observed within the energy transfer range 0≤ℏω≤2.5 meV. The corresponding energy level scheme and the line intensities are explained in terms of a model based on a completely free rotation of the CHD_{3} molecule. An expression for the double differential neutron scattering cross section of the free CHD_{3} rotor is derived. In spite of a line shift of 44% with respect to the free rotor, no crystal field splitting was observed.

Mid‐infrared spectra of the proton‐bound complexes Ne_{ n }–HCO^{+} (n=1,2)
View Description Hide DescriptionThe ν_{1} band of Ne–HCO^{+} has been recorded for both ^{20}Ne and ^{22}Ne containing isotopomers by means of infrared photodissociationspectroscopy. The rotational structure of the band is consistent with a parallel Σ–Σ type transition of a linear proton‐bound complex. The following constants are extracted for ^{20}Ne–HCO^{+}: ν_{0}=3046.120±0.006 cm^{−1}, B″=0.099 54±0.000 05 cm^{−1}, D″=(5.30±0.30)×10^{−7} cm^{−1}, H″=(1.1±0.9)×10^{−11} cm^{−1}, B′=0.100 03±0.000 05 cm^{−1}, D′=(4.89±0.30)×10^{−7} cm^{−1}, H′=(1.6±0.9)×10^{−11} cm^{−1}. The ν_{1} band is redshifted by 42.5 cm^{−1} from the corresponding ν_{1} transition of free HCO^{+} indicating that the Ne atom has a pronounced influence on the proton motion. Linewidths for individual rovibrational transitions are laser bandwidth limited, demonstrating that the lifetime of the ν_{1} level is at least 250 ps. An approximate radial potential for the collinear Ne...HCO^{+} interaction is constructed by joining the mid‐range potential obtained from a Rydberg–Klein–Rees inversion of the spectroscopic data to the theoretical long‐range polarization potential. Based on this potential, the estimated dissociation energy (D _{0}) for Ne–HCO^{+} is 438 cm^{−1} in the (000) state and 454 cm^{−1} in the (100) excited state. The rotationally unresolved ν_{1} band of ^{20}Ne_{2}–HCO^{+} is slightly blueshifted with respect to that of ^{20}Ne–HCO^{+}. The observed frequency shift is compatible with a trimer structure where the second Ne atom is attached to the linear Ne–HCO^{+} dimer core.

First assignment of the rotational spectrum of a molecule containing two iodine nuclei: Spectroscopic constants and structure of CH_{2}I_{2}
View Description Hide DescriptionThe rotational spectrum of methylene iodide was assigned by combining information from broadband mm wave spectra of low‐J transitions recorded with a supersonic‐jet spectrometer and of high‐J transitions measured for a room temperature sample. In the first step, the analysis of central frequencies of hyperfine multiplets for rotational transitions with J″ up to 189 resulted in constants of the rotational Hamiltonian H _{ R }, in which the fitted quartic centrifugal distortion constants were consistent with predictions made from ab initio calculations. The knowledge of H _{ R } allowed accurate prediction and assignment of the hyperfine structure due to the two iodine nuclei, which was made on the basis of well isolated splitting patterns observed in the jet and the effects of nuclear spin statistics visible therein. The final rotational constants are A=22 034.437(2), B=620.584(2), C=605.798(2) MHz and the hyperfine coupling constants are χ_{ aa }=−1180.9(1), χ_{ bb }−χ_{ cc }=−892.42(5), χ_{ ab }=1358.9(5) MHz, and χ_{ zz }=−2030.1(5), χ_{ xx }=993.4(10), χ_{ yy }=1036.7(1) MHz. The angle θ_{ za } between the inertial a axis and the principal quadrupole axis z is 32.00(1)° and is consistent with ∠(CI⋅a)=33.0(2)° from the fitted structure. The structural parameters of the halogen in CH_{2}I_{2} are r(CI)=2.134(2)Å and ∠(ICI)=114.0(3)° and compare well with extrapolation from the remaining methylene halides.

Laser‐induced fluorescence spectroscopy of jet cooled p‐aminophenol
View Description Hide DescriptionThe laser‐induced fluorescence spectra of p‐aminophenol both in excitation and emission have been studied in a supersonic jet apparatus. The characterization of the observed spectra was done by comparison with other related substituted anilines and the IR data available in the literature. The excitation spectrum resembles that of aniline with optical activity mainly confined to 6a, 1, and the NH_{2} inversion mode. In addition, the C–X in‐plane bending mode 9b was also found to be optically active. The 6a mode dominates in most of the dispersed fluorescence spectra and shows a strong Franck–Condon activity. Unlike other similar molecules, the Δv=0 transitions were weak in the single vibronic level fluorescence spectra of 6a ^{1} and 1^{1}, which has been qualitatively explained in terms of Franck–Condon analysis. The onset of intramolecular vibrational redistribution occurs at 1135 cm^{−1}, which is much higher than many substituted anilines. The van der Waals complexes viz. p‐aminophenol–Ar_{1} and p‐aminophenol–Ar_{2} were observed. A symmetric (11) complex for the p‐aminophenol–Ar_{2} is proposed based on the redshift additivity.

Nonequilibrium phenomena in spectral diffusion physics of organic glasses
View Description Hide DescriptionWe performed two different types of spectral diffusion experiments on persistent spectral holes. In all cases, we measured the holewidth as a function of time. The two experiments differed in their initial conditions: In the type 1 experiment (the ‘‘aging experiment’’), the sample was cooled from room temperature to the final temperatures which were 100 and 800 mK, respectively. Holes were burnt at various time intervals after the final temperature was reached. In the type 2 experiment (the ‘‘cycling experiment’’), the sample was allowed to relax for a period of about 10 days. Then, a hole was burnt and subjected to a temperature cycle. In all cases, the time dependence of the holewidths was strongly nonlogarithmic. The temperature cycled hole showed a narrowing regime which prevailed for the whole observation period of roughly one week. We will show that the deviation from the logarithmic time dependences is a nonequilibrium phenomenon. All features observed could be modelled within the standard tunneling model.

Autodetachment from vibrational levels of the O^{−} _{2} A ^{2}Π_{ u } resonance across its dissociation limit by photoexcitation from O^{−} _{2} X ^{2}Π_{ g }
View Description Hide DescriptionWe report the observation of resonance structure in the photodetachment spectrum of O^{−} _{2} in the 4 eV range, which results from the excitation of autodetaching vibrational levels of the O^{−} _{2} A–X transition near the dissociation limit. The evolution of the resonances with increasing vibration is simply explained using continuity of the inner part of the vibrational wave functions across the dissociation threshold. This affords the possibility of investigating the DA process at the half‐collision, in a kind of ‘‘correspondence limit’’ where the outer turning point slowly recedes and the vibrational wave function incrementally adopts the character of the dissociation continuum. Photoexcitation near one of the resonances results in the population of significantly higher vibrational levels in the O_{2} a ^{1}Δ_{ g } state (which are cleanly resolved) than the typical ‘‘Franck–Condon’’ pattern observed for nonresonant photodetachment. Finally, hot‐band structure is also observed in the detachment spectrum, allowing us to extract a more accurate value of the O^{−} _{2} vibrational quantum (ΔG=134.4±0.8 meV) by about an order of magnitude over previous determinations.

On the origin of the dip in the KrF laser gain spectrum
View Description Hide DescriptionHigh‐resolution spectra of KrF (B–X) amplified spontaneous emission from various discharge‐pumped and electron‐beam‐pumped KrF lasers have been analyzed. An underlying structured absorptionspectrum has been discovered with a well‐resolved peak at 248.91 nm. The absorption coefficient of this peak was found to vary in exact proportion to the peak laser gain coefficient but was independent of laser gas purity. We suggest that the absorption arises internally within the KrF molecule and is due to transitions from the B state to a higher‐lying Rydberg state. This hypothesis was tested by simulating the absorptionspectrum from KrF*(B) to a weakly repulsive state dissociating to Kr*(^{3} P _{1})+F(^{2} P _{3/2}). A good agreement was obtained between simulated and experimental absorption spectra.

Tunable vacuum ultraviolet laser spectroscopy of XeAr and XeNe near 68 000 cm^{−1}: Interatomic potentials mediated by a 6s Rydberg electron
View Description Hide DescriptionThe laser induced fluorescencespectra of XeAr and XeNe have been measured near the Xe 6s[3/2]°_{1}−^{1} S _{0} transition at 68 045.663 cm^{−1}. In XeAr, nine discrete peaks were observed, attributed to excitation to vibrational levels of the excited Ω=0^{+} electronic state, merging into a continuum. From a Franck–Condon analysis, the potential for the excited Ω=0^{+} state of XeAr was determined. It was found that this potential has a shallow minimum at long range [D _{ e } ^{′}=28(1) cm^{−1}, r _{ e } ^{′}=5.44(7) Å] with a shallow repulsive wall. The shallow repulsive wall is attributed to the influence of the XeAr^{+} ion core at short range. The role of the 6sRydberg electron of Xe in determining the shape of the interatomic potentials of excited XeRg (Rg=a rare gas atom) is discussed on the basis of a simple model potential, in which the interatomic potential is described as the sum of a diatomic ion core (XeRg^{+}) potential term and a term representing the exchange repulsion between the Rydberg electron and the rare gas moiety. For XeNe, only continuum spectra were observed, indicating that the upper states are not bound.

Intrinsic non‐RRK behavior: Classical trajectory, statistical theory, and diffusional theory studies of a unimolecular reaction
View Description Hide DescriptionThe nonstatistical behavior of a unimolecular reaction at energies well in excess of the threshold is examined. This behavior is sometimes referred to as ‘‘intrinsically non‐Rice–Ramsperger–Kassel–Marcus’’ (RRKM). It is well known that microcanonical unimolecular rates computed by using classical mechanics can deviate from the predictions of statistical theories, particularly at high energies. The simplest manifestation of this behavior is that rate constants as a function of energy cannot be represented by simple expressions such as the RRK equation, k(E)=ν(1−E*/E)^{ s−1}, with a single set of parameter values over a wide energy range; more specifically, fits of the classical RRK expression to trajectory results frequently yield values for the effective number of degrees of freedom s that are significantly smaller than the ‘‘theoretical’’ values 3N−6. In the present study, rates were calculated for the unimolecular dissociation of dimethylnitramine, (CH_{3})_{2}NNO_{2}, by simple N–N bond rupture over wide energy ranges by using classical trajectories and Monte Carlo transition‐state theory. The formalism of a diffusionaltheory of chemical reactions is used to develop a model that relates classical reaction rates to intramolecular vibrational energy redistribution (IVR). This model is based on the assumption that the molecular modes can be separated into reaction coordinate and energy reservoir modes. It is shown how this model can be used to extrapolate high‐energy, nonstatistical classical trajectory rates to the low‐energy, statistical region.

Independent center, independent electron approximation for dynamics of molecules and clusters
View Description Hide DescriptionA formalism is developed for evaluating probabilities and cross sections for multiple‐electron transitions in scattering of molecules and clusters by charged collision partners. First, the molecule is divided into subclusters each made up of identical centers (atoms). Within each subcluster coherent scattering from identical centers may lead to observable phase terms and a geometrical structure factor. Then, using a mean field approximation to describe the interactions between centers we obtain A _{ I }∼∑_{ k }∏_{ ke } ^{ iδ k I } A _{ Ik }. Second, the independent electron approximation for each center may be obtained by neglecting the correlation between electrons in each center. The probability amplitude for each center is then a product of single electron transition probability amplitudes, a _{ Ik } ^{ i }, i.e. A _{ Ik }≊∏_{ ia } _{ ik } ^{ i }. Finally, the independent subcluster approximation is introduced by neglecting the interactions between different subclusters in the molecule or cluster. The total probability amplitude then reduces to a simple product of amplitudes for each subcluster, A≊∏_{ IA } _{ I }. Limitations of this simple approximation are discussed.

A study on ion–molecule reactions in the H^{+} _{3} system with the trajectory‐surface‐hopping model
View Description Hide DescriptionCross sections for ion production in the D^{+}+H_{2}, D^{+}+D_{2}, and H^{+}+D_{2} collisions have been calculated in the center‐of‐mass collision energy range of 2.5 to 8.0 eV by using trajectory‐surface‐hopping method on ab initiopotential energy surfaces. For the production of H^{+} and HD^{+} ions in the D^{+}+H_{2} collisions the present results agree with experiments. For the production of D^{+} _{2} ions in the D^{+}+D_{2} collisions the calculations show agreement with the experimental results at energies below 4.5 eV, but lead to an overestimate above 5.0 eV. For the production of D^{+} and HD^{+} ions in the H^{+}+D_{2} collisions, agreement between the calculations and experiments is good in the whole energy range, while a deviation comes out between both the results for the charge transfer H^{+}+D_{2}→D^{+} _{2}+H above 5.0 eV. As far as the energy dependence of cross sections is concerned, the present calculations well reproduce all the experiments, owing to the use of the ab initio surfaces, instead of the diatomics‐in‐molecules surfaces.

State‐selective control for vibrational excitation and dissociation of diatomic molecules with shaped ultrashort infrared laser pulses
View Description Hide DescriptionUltrafast state‐selective dynamics of diatomic molecules in the electronic ground state under the control of infrared picosecond and femtosecond shaped laser pulses is investigated for the discrete vibrational bound states and for the dissociative continuum states. Quantum dynamics in a classical laser field is simulated for a one‐dimensional nonrotating dissociative Morse oscillator, representing the local OH bond in the H_{2}O and HOD molecules. Computer simulations are based on two approaches — exact treatment by the time‐dependent Schrödinger equation and approximate treatment by integro‐differential equations for the probability amplitudes of the bound states only. Combination of these two approaches is useful to reveal mechanisms underlying selective excitation of the continuum states and above‐threshold dissociation in a single electronic state and for designing optimal laser fields to control selective preparation of the high‐lying bound states and the continuum states. Optimal laser fields can be designed to yield almost 100% selective preparation of any prescribed bound state, including those close to the dissociation threshold. State‐selective preparation of the highest bound state may be accompanied by the appearance of a quasi‐bound molecular state in the continuum with the kinetic energy of the fragments being close to zero. The respective above‐threshold dissociation spectrum contains an additional, zero‐order peak. The laser‐induced dissociation from selectively prepared high‐lying bound states is shown to be very efficient, with the dissociation probability approaching the maximal value. Flexible tools of state‐selective laser control are developed which enable one to achieve selective control of the dissociation spectra resulting in time‐selective and space‐selective control of the dissociation fragments.

Absolute partial and total cross‐section functions for the electron impact ionization of C_{60} and C_{70}
View Description Hide DescriptionElectron impact ionization of C_{60} and C_{70} was studied using a molecular/electron beam ion source in combination with a two sector field mass spectrometer operated in the ion beam deflection mode. Relative partial ionization cross sections for the production of singly and multiply charged parent ions (up to charge state z=4) and fragment ions (down to C^{+} _{44} in the case of C_{60} and down to C^{2+} _{50} in the case of C_{70}) were determined from threshold up to 1000 eV electron energy. Absolute partial and total ionization cross sections are obtained using a novel approach for the absolute calibration involving an intercomparison of the cation with the anion yield. The results obtained reveal not only an anomalous large parent ion cross section as compared to other ionization channels [e.g., σ(C^{+} _{60}/C_{60}) is more than a factor of 30 larger than σ(C^{+} _{58}/C_{60})] but also anomalies for the production of multiply charged parent and fragment ions. For instance, the maximum cross section for the formation of C^{2+} _{60} amounts to 30% of the maximum C^{+} _{60} cross section and that of C^{2+} _{70} to about 50% of C^{+} _{70}. Moreover, for all fragment ions, the formation of the doubly charged fragment ions has a larger cross section than that of the respective singly charged fragment ion. These peculiar features of the kinetics of electron impact ionization of C_{60} and C_{70} are related to the specific electronic and geometric structures of these fullerenes. The present absolute cross‐section data for the summed up partial cross sections are in good agreement with a recent semiclassical calculation from our laboratory.

On the origin of matrix elements for electronic excitation (energy) transfer
View Description Hide DescriptionThe origin of electronic energy transfer (EET) between two chromophores (D and A) is explored further for several molecular situations that may be encountered in experiment—namely, nonoverlapping active‐space orbitals of the D and A chromophores, forbidden electronic excitations for both chromophores, and an allowed and a forbidden electronic excitation for the D and A chromophores, respectively. The theory is illustrated via the results of calculations of the EET matrix elements for model systems with both four–eight active‐space electrons and all of the electrons included explicitly. In each case, it is found that any overlap contribution to these matrix elements is associated much more with charge‐transfer and penetration terms rather than it is with the Dexter exchange integral. The calculated magnitude of the latter integral is always smaller than that of the Coulomb integral.

Simulation of activation free energies in molecular systems
View Description Hide DescriptionA method is presented for determining activation free energies in complex molecular systems. The method relies on knowledge of the minimum energy path and bases the activation free energy calculation on moving along this path from a minimum to a saddle point. Use is made of a local reaction coordinate which describes the advance of the reaction in each segment of the minimum energy path. The activation free energy is formulated as a sum of two terms. The first is due to the change in the local reaction coordinate between the endpoints of each segment of the path. The second is due to the change in direction of the minimum energy path between consecutive segments. Both contributions can be obtained by molecular dynamics simulations with a constraint on the local reaction coordinate. The method is illustrated by applying it to a model potential and to the C7_{eq} to C7_{ax} transition in the alanine dipeptide. It is found that the term due to the change of direction in the reaction path can make a substantial contribution to the activation free energy.

Properties of phosphorus compounds by density functional theory: CH_{3}P species as a test case
View Description Hide DescriptionA comparison of different density functional theory(DFT) and molecular orbital (MO) methods for calculating molecular and energetic properties of low‐coordinated phosphorus compounds is reported. While DFT methods include both Becke–Lee–Yang–Parr (BLYP and B3LYP) nonlocal functionals, MO methods involve second‐order perturbation theory (MP2), quadratic configuration interaction [QCISD(T)], and coupled‐cluster theory [CCSD(T)], in conjunction with the 6‐31G(d,p), 6‐311++G(3df,2p), and 6‐311++G(3df,3pd) basis sets.Properties examined include geometrical parameters of the different CH_{3}P equilibrium structures (phosphaethene, phosphinocarbene, methylphosphinidene, and a phosphacarbyne) and relevant transition structures for isomerisations and rearrangements in both the lowest‐lying singlet and triplet states, vibrational wave numbers, relative energies, barrier heights, and singlet–triplet energy gaps. In addition, the heat of formation,ionization energy, and proton affinity of phosphaethene are also evaluated. Overall, the B3LYP method, when employed with a large basis set, yields energetic results comparable to the CCSD(T) results. Nevertheless, both DFT methods fail to predict the behavior of the addition/elimination reactions of the hydrogen atom in the triplet state.

Approaches to bifurcating reaction path
View Description Hide DescriptionThe intrinsic reaction path (IRP) often becomes unstable relative to some nontotally symmetric direction orthogonal to the path through a valley–ridge inflection point. We investigate geometric characters of the potential energy surface around the valley–ridge inflection boundary, and propose some ideas to determine a bifurcating reaction path, or to give a two‐dimensional potential energy surface which connects bifurcating point and product regions. As a demonstration, bifurcating reaction paths are calculated for the isomerizationreaction of methoxy radical (H_{3}CO→H_{2}COH) by the unrestricted Hartree–Fock (UHF) method.

The structure and binding energy of K^{+}–ether complexes: A comparison of MP2, RI‐MP2, and density functional methods
View Description Hide DescriptionThe structures and binding energies of several cation:ether complexes (K^{+}:dimethyl ether, K^{+}:dimethoxyethane, K^{+}:12‐crown‐4 and K^{+}:18‐crown‐6) were determined with second and fourth order perturbation theory using correlation consistent basis sets. Several of these are the largest correlated calculations yet attempted on crown ethers. The observed systematic convergence to the complete basis set limit provides a standard by which the accuracy of previous studies can be measured and facilitates the calibration of density functional methods. Recent Fouier transform ion cyclotron resonance experiments predicted K^{+}:18‐crown‐6 binding energies which were significantly smaller than ab initio calculations. None of the potential sources of error examined in the present study were large enough to explain this difference. Although the 6‐31+G* basis set used in an earlier theoretical study was smaller than the smallest of the correlation consistent basis sets, with suitable correction for basis set superposition error, it appears capable of yielding binding energies within several kcal/mol of the basis set limit. Perturbation theory calculations exploiting the ‘‘resolution of the identity’’ approximation were found to faithfully reproduce binding energies and conformational differences. Although the cation–ether interaction is dominated by classical electrostatics, the accuracy of density functional techniques was found to be quite sensitive to the choice of functionals. The local density SVWN procedure performed well for binding energies and conformational differences, while underestimating K^{+}O distances by up to 0.08 Å. The gradient‐corrected Becke–Lee–Yang–Parr functional underestimated the K^{+}:12c4 binding energy by 4–7 kcal/mol or 15%.