Volume 98, Issue 11, 01 June 1993
Index of content:

Mass‐selective ionization‐detected stimulated Raman spectroscopy of benzene trimer and higher clusters
View Description Hide DescriptionWe present the results of mass‐selective, ionization‐detected stimulated Raman spectroscopies on jet‐cooled benzene clusters from the trimer to the octamer. The data pertain to the fundamentals of the totally symmetric ring‐breathing (ν_{1}) and C–H stretch (ν_{2}) modes of benzene and provide information about geometrical structure and vibrational dynamics. For the trimer, the results indicate a species in which all three benzenes reside in equivalent sites. For the higher clusters the results point to geometries involving two or more inequivalent sites. The ν_{1}‐excited trimer is found to decay on a nanosecond, or longer, time scale. The ν_{2}‐excited trimer decays on a time scale of greater than 5 ps. Similar timescales characterize the decays of the ν_{2}‐excited tetramer and pentamer.

Dissociation dynamics of high‐v Rydberg states of molecular hydrogen
View Description Hide DescriptionA high‐resolution, state‐selective investigation of the decay of (X ^{2}Σ_{ g } ^{+})np, v’≥9 Rydberg states of H_{2} into the H(1s)+H(3l) and H(1s)+H(4l) dissociation continua has been performed. The (X ^{2}Σ_{ g } ^{+})npRydberg states were excited by double‐resonance excitation via the E,F ^{1}Σ_{ g } ^{+}, v=6, J=0 and 1 states, and the H(3l) and H(4l) dissociation products were detected by monitoring the energy‐resolved photoelectrons produced by nonresonant photoionization of the fragments. The energy region of interest (138 800–140 000 cm^{−1}) includes the X ^{2}Σ_{ g } ^{+}, v ^{+}=9, N ^{+}=0–3 ionization thresholds, the H(1s)+H(4l) dissociation threshold, and the H^{+}+H^{−} ion‐pair threshold. Evidence of both direct and indirect dissociation involving both singly and doubly excited electronic states is observed. Comparisons to the previously observed X ^{2}Σ_{ g } ^{+}, v ^{+}=8 constant‐ionic‐state photoelectron spectrum and the ion‐pair spectrum observed in the same energy region reveal strong competition between the ionization and dissociation processes through both rovibrational and electronic interactions.

Acetylene: Synergy between theory and experiment
View Description Hide DescriptionSix anomalous vibronic feature states [∼2 cm^{−1} full‐width at half‐maximum (FWHM), each consisting of ∼20 partially resolved eigenstates] have been observed in stimulated emission pumping (SEP) spectra of C_{2}D_{2}. Of the two plausible assignments for these features, the one most consistent with spectroscopic observations would imply that the lowest energy cis‐bent triplet state of acetylene has T _{0}≤25 820 cm^{−1}, which is inconsistent with previous ab initio predictions. New higher level ab initio quantum mechanical methods have been used to predict the energy difference between X̃ ^{1}Σ_{ g } ^{+}ground state and the cis‐bent ã ^{3} B _{2} lowest triplet state of acetylene. In conjunction with a triple zeta plus double polarization plus f function (TZ2Pf) basis set, the coupled cluster including single, double, and linearized triple excitations CCSD(T) method yields T _{0}=ΔE(ã ^{3} B _{2}–X̃ ^{1}Σ_{ g } ^{+})=30 500 cm^{−1}. The true value of T _{0} for the ã ^{3} B _{2} state is estimated to be ∼500 cm^{−1} higher. At the same level of theory the zero‐point levels of the lowest triplet state of the trans‐bent (ã ^{3} B _{ u }) and vinylidene (ã ^{3} B _{2}) isomers lie at still higher energies. This result conclusively rules out any triplet assignment for the anomalous feature states. The alternative assignment, as highly excited vibrational levels of the X̃ ^{1}Σ_{ g } ^{+} state, is surprising in view of the Franck–Condon selectivity, dynamical stability, and nonselective relaxation of this special class of ‘‘bright states’’ observed in the SEP spectra.
Such an assignment would be implausible in the absence of the present ab initio calculations. Previous experimental observations [Lisy and Klemperer, J. Chem. Phys. 72, 3880 (1980) and Wendt, Hippler, and Hunziker, J. Chem. Phys. 70, 4044 (1979)] of acetylene triplet states are discussed and shown to be completely consistent with each other and with the present ab initio ordering of the cis and trans isomeric minima on the T _{1}potential energy surface:cis ã ^{3} B _{2} below trans ã ^{3} B _{ u } .

Elastic properties of water under negative pressures
View Description Hide DescriptionUsing Brillouin scattering we have investigated the elastic properties of water under negative pressures. The samples were H_{2}O liquid–vapor inclusions in α‐quartz which could be heated to their homogenization temperatures (in the range 120–370 °C); on cooling, negative pressures as high as 1000 bars were reached before a vapor bubble nucleated. The velocity of sound results obtained from our Brillouin experiments indicate that nucleation occurs long before reaching the mechanical instability region where the bulk modulus is zero.

Electron‐energy‐loss spectroscopy of condensed butadiene and cyclopentadiene: Vibrationally resolved excitation of the low‐lying triplet states
View Description Hide DescriptionLow‐energy electron‐energy‐loss spectra of 1,3‐butadiene and 1,3‐cyclopentadiene deposited on a thin film of solid argon are measured at a temperature of 15 K. This new method allows us to resolve vibrational structure within the low‐lying triplet bands. The vibrational frequencies observed in the first triplet band of butadiene are in agreement with theoretical predictions under the assumption of a metastable planar structure for the lowest triplet state (1 ^{3} B _{ u }). The lowest observable vibronic band of the second triplet transition of butadiene is located at 4.63 eV. The 0–0 transition to the first triplet state of cyclopentadiene is resolved and located at 2.555 eV. Concerning the second triplet band of cyclopentadiene, we find evidence from a comparison with optical spectra that the onset lies in the energy range from 4.55 to 4.7 eV. This is the first likely observation of T _{2} for a cis‐diene.

Rotational energy transfer in the Na_{2} b ^{3}Π_{ u } state: Propensity rules for rotation, spin–orbit component, and e/f‐parity changing collisions
View Description Hide DescriptionState‐to‐state collision‐induced transitions within the Na_{2} b ^{3}Π_{ u } state have been studied by a sub‐Doppler optical–optical double resonance (OODR) technique employing cw, single mode dye lasers for the PUMP and PROBE steps. The main experimental results are (1) strong Ω‐conservation is observed when a low‐J ^{3}Π_{0} or ^{3}Π_{1} parent level is initially prepared (we are unable to prepare low‐J ^{3}Π_{2} levels). This Ω‐conservation relaxes as J increases. (2) e/fparity conservation (i.e., propensity for even ΔJ changes in homonuclear molecules) is observed at low‐J within the ^{3}Π_{0}manifold. This propensity for e/f‐conservation relaxes rapidly as J increases. (3) The e/fparity conservation rule observed within the ^{3}Π_{0}manifold is observed not to apply to the collision‐induced transitions within the ^{3}Π_{1}manifold. At low‐J, ΔJ=±1, e→f transitions are even stronger than ΔJ=±2, e→e transitions. (4) At higher‐J, for example J=16, neither Ω‐conservation nor e/f‐conservation within the ^{3}Π_{0}manifold are observed. These results are compared with theoretical predictions for a case (a) ^{3}Π state [Alexander and Pouilly, J. Chem. Phys. 79, 1545 (1983)] and with experimental observations on another homonuclear molecule, N_{2} in the B ^{3}Π_{ g } state, which also exhibits intermediate coupling between cases (a) and (b) [Ali and Dagdigian, J. Chem. Phys. 87, 6915 (1987)].

Rotational energy transfer in the Na_{2} b ^{3}Π_{ u } state: Propensity rules for transitions between hyperfine components
View Description Hide DescriptionThe changes in the hyperfine quantum number (F’) that accompany collision‐induced ΔJ, ΔΩ, and Δ(e/f‐parity) transitions in the Na_{2} b ^{3}Π_{ u }‐state have been studied by sub‐Doppler, cw, perturbation‐facilitated optical–optical double resonance (PFOODR) spectroscopy. The Na_{2} is contained in a heat pipe oven at ∼1 Torr and the primary collision partner is Na(3s ^{2} S). The PUMP laser selectively excites a single b ^{3}Π_{0u } v’=12, J’=43e, s or 44e, a rotational level, the parent level. All F’hyperfine components of the parent J’ level are directly populated by the PUMP laser, but with different velocity projections relative to the laser propagation direction. Thus each parent hf component is labeled by its longitudinal velocity. As the PROBE laser is scanned through various 2 ^{3}Π_{Ωg } v=2, J←b ^{3}Π_{Ω’} _{ u } v’=12, J’ transitions, sub‐Doppler hyperfine structure can be resolved on each parent and daughter rotational line in the PFOODR fluorescence excitation spectrum. The collisional propensity rule ΔF=ΔJ is obeyed for (s, a permutation symmetry conserving) ΔJ’=0, ΔΩ’=0, +1, and +2 and ΔJ’=±1, ±2, ΔΩ’=0 collision‐induced transitions. No systematic exploration of the parent‐J’, Ω’ dependence of the ΔF propensity was undertaken; in particular, the present study was restricted to the high‐J limit where the Na_{2}/Na collisions are not sudden relative to the rotational (half) period and where J≫I. The ΔF=ΔJhyperfine propensity rule observed for high‐J levels of the Na_{2} b ^{3}Π_{ u } state is consistent with previous theoretical predictions of a ΔF=0 propensity for collision‐induced ΔJ=0 transitions between Λ‐doublet components of the OH X ^{2}Π state and a ΔF=ΔJ propensity for collision‐induced transitions between CaBr X ^{2}Σ^{+} rotational levels.

A random matrix approach to rotation–vibration mixing in H_{2}CO and D_{2}CO
View Description Hide DescriptionIn a previous study of highly excited states of H_{2}CO, we found that rotation–vibration mixing increases rapidly as a function of J for those states whose vibrational energy is greater than 6000 cm^{−1} [McCoy, Burleigh, and Sibert, J. Chem. Phys. 95, 7449 (1991)]. The present work elucidates the onset of this mixing in both H_{2}CO and D_{2}CO. In addition to Coriolis and centrifugal coupling, vibrational mixing between the normal modes is shown to have a strong influence on the breakdown of the separation between rotation and vibration. The role of this latter effect is accurately modeled using a random matrix approach. An ensemble of vibrational Hamiltonian matrices are defined whose matrix elements are chosen from distribution functions whose functional forms were determined by examining the matrix form of the original model Hamiltonian. In adopting such an approach, the goal is not to attempt to predict rotation–vibration mixing for a specific zero order state, but rather to model the distribution of rotation–vibration mixing for all the rotation–vibration states of a given J and within a total energy range E±ΔE. The trends observed for these distributions as J and E are increased are quantitatively reproduced using the random matrix approach. Furthermore, the random matrix approach allows one to accurately reproduce the mixing with distribution functions defined with only a few parameters.

Resonance Raman studies of the ground and lowest electronic excited state in CdS nanocrystals
View Description Hide DescriptionThe size dependence of the resonance Raman spectrum of CdSnanocrystals ranging in size from 10 to 70 Å radius has been studied. We find that while the lowest electronic excited state is coupled strongly to the lattice, this coupling decreases as the nanocrystal size is decreased. We demonstrate that the lifetime of the initially prepared excited state can influence the apparent strength of electron‐vibration coupling. Absolute resonance Raman cross section measurements can be used to determine the value of the excited state lifetime, thus removing this parameter. The coupling to the lattice, while less in nanocrystals than in the bulk, is still greater than what is predicted assuming an infinite confining potential. The width of the observed LO mode broadens with decreasing size, indicating that the resonance Raman process is intrinsically multimode in its nature. The frequency of the observed longitudinal optic (LO) mode has a very weak dependence on size, in contrast to results obtained from multiple quantum well systems. The temperature dependence of the frequency and linewidth of the observed LO mode is similar to the bulk and indicates that the LO mode decays into acoustic vibrations in 2.5 ps.

Perturbation theory approach to dynamical tunneling splitting of local mode vibrational states in ABA molecules
View Description Hide DescriptionWe introduce a point of view for treating the dynamical tunneling splitting of symmetric local mode vibrational states in ABA molecules (A=H typically) which is the one we have employed in treating the vibrational spectroscopy of CH overtones in molecules such as (CX_{3})_{3}YCCH. Namely, the vibrational coupling corresponding to the dynamical tunneling in semiclassical mechanics via many intermediate off‐resonance weak transitions between initial and final states can be treated by a standard high‐order perturbation theory. We apply that method to the present simpler problem of tunneling splittings in ABA molecules, and compare the results with those of exact diagonalization, the semiclassical method, and the periodic orbit quantization. Of all the approximate methods, the perturbation theory was found to provide the best approximation to the results of exact diagonalization for the system treated. The relationship between these three methods and application to the problem of vibrational relaxation in polyatomic molecules with tunneling mechanism of intramolecular vibrational relaxation is discussed.

Indeterminacy of molecular constants for a symmetric top molecule in an electric field
View Description Hide DescriptionIndeterminacy of molecular constants for a symmetric top molecule placed in an electric field is studied by a group‐theoretical method. For an E vibronic state of a C _{3v } molecule, the three constants τ_{ xxxz }, θ _{ x } ^{ xx }, and λ cannot be simultaneously determined. Simultaneous determination is also impossible for the set of θ _{ z } ^{ xx }, θ _{ x } ^{ xz }, and α_{⊥}, as well as for the set of η_{ J }, η_{ K }, λ, h _{19}, r, and μ_{ r }.

Visible absorption and fluorescence of AlC, B ^{4}Σ^{−}↔X ^{4}Σ^{−}, in solid argon
View Description Hide DescriptionSpectroscopic investigations of aluminumcarbide, AlC, in an argon matrix have been performed. The visible B ^{4}Σ^{−}↔X ^{4}Σ^{−} transition has been observed in absorption and laser induced fluorescence with a Fourier transform spectrometer. Carbon‐13 isotopic substitution has confirmed the carrier to be AlC. The spectroscopic constants in solid argon are found to be X ^{4}Σ^{−} (ω_{ e }=639.3 cm^{−1}, ω_{ ex } _{ e }=4.5 cm^{−1}) and B ^{4}Σ^{−} (T _{ e }=22 610.5 cm^{−1}, ω_{ e }=746.2 cm^{−1}, ω_{ ex } _{ e }=10.3 cm^{−1}).

Zero kinetic energy (ZEKE) photoelectron spectroscopy of ammonia by nonresonant two‐photon ionization from the neutral ground state
View Description Hide DescriptionRotationally resolved zero kinetic energy‐pulsed field ionization (ZEKE‐PFI) photoelectron spectra of NH_{3} were obtained by nonresonant two‐photon ionization from the X̃(^{1} A _{1} ^{’}) electronic ground state. The ZEKE spectra were recorded up to an internal ion energy of 9500 cm^{−1} by pulsed field ionization of long‐lived Rydberg states of very high principal quantum numbers. Rovibronic bands of the NH_{3} ^{+} cation with v _{2} ^{+}=1–9 (umbrella mode), v _{4} ^{+}=1, and for the first time the mode ν_{1} in the combination band v ^{+}=1^{1}2^{1} of the X̃^{+}(^{2} A _{2} ^{‘}) electronic ground state are observed in the ZEKE spectra. Rotational constants and band origins are determined in the analysis. The adiabatic ionization energy obtained by a recent ZEKE measurement is confirmed at 82 159±1 cm^{−1}.

Packing entropy of extended, hard, rigid objects on a lattice
View Description Hide DescriptionWe present a systematic method of evaluating the packing entropy for a set of mutually avoiding extended, hard, rigid objects on a lattice. The method generalizes a simple algebraic representation of the lattice cluster theory developed by Freed and co‐workers for systems composed of flexible objects. The theory provides a power series expansion in z ^{−1} for the corrections to the zeroth order mean field approximation partition function, where z is the lattice coordination number. We illustrate the general theory by calculating the packing entropy of four‐unit rigid ‘‘square’’ objects on a hypercubic lattice as a function of the volume fraction of the squares. As a particular limiting case, we also evaluate for the packing entropy of two, three, and four squares on a two‐dimensional square lattice and find agreement with the cluster expansion.

Experimental and theoretical characterization of the BAr van der Waals complex: The X ^{2}Π, A ^{2}Σ^{+}, and B ^{2}Σ^{+} electronic states
View Description Hide DescriptionThe BAr van der Waals complex and its electronic transition correlating with the B atom 3s ^{2} S–2p ^{2} P transition have been characterized in a combined experimental and theoretical investigation. The experimental portion of the study consisted of the observation by laser fluorescence excitation of rotationally resolved bands of this molecule in a supersonic jet. Specifically, four bands of the (v’,0) progression of the B ^{2}Σ^{+}–X ^{2}Π_{1/2} band system of each of the ^{11,10}BAr isotopomers were observed and analyzed, where the upper state vibrational quantum numbers v’=4–7 were determined from the isotope splittings. Vibrational and rotational constants were obtained through fits to the observed transition wave numbers. These experimental results were compared with ab initio calculations of the X ^{2}Π, A ^{2}Σ^{+}, and B ^{2}Σ^{+} electronic states of BAr. In order to obtain the interactionenergies of the excited states of this weakly bound system accurately, multireference, internally contracted, configuration‐interaction calculations were carried out, with additional provision for the effect of higher order excitations. The information derived about these states from the experiment and calculations agree reasonably well. An interesting feature of the BAr B ^{2}Σ^{+} state is the presence of a barrier in the potential energy curve.

Effect of intermolecular interactions on vibrational‐energy transfer in the liquid phase
View Description Hide DescriptionWe present an experimental and theoretical study of the relaxation after excitation of the C–H stretch vibration for dilute solutions of CHCl_{3}, CHBr_{3}, and CHI_{3} dissolved in different solvents and for the pure liquids CHCl_{3} and CHBr_{3}. Experimentally, we study the relaxation with pump–probe experiments using picosecond infrared pulses and we found that the vibrational‐energy transfer takes place via an intramolecular vibrational‐relaxation process followed by an intermolecular energy transfer. Theoretically, we develop a new model for the description of intramolecular vibrational energy transfer. In this model the important effects of the intermolecular interactions on the rate of intramolecularenergy transfer are described. From the comparison between experiment and theory the relevant parameters for intramolecular vibrational‐energy transfer can be identified.

Study of the viscoelastic properties of the metallorganic palladium mesogens Azpac and Azpac2 by photons self‐beating spectroscopy
View Description Hide DescriptionWe report on the first measurements of light scattering studies of the viscoelasticproperties of two new metallorganic nematic liquid crystals and of their parent ligand 4–4’–bis(hexyloxy)–azoxybenzene. When the magnetic properties of the liquid crystal are not known, light scattering is an efficient technique to get qualitative and quantitative information on the elastic constants and viscosities. The experimental results are discussed in the light of the molecular structure of the compounds.

Nonlinear resonance and correlated binary collisions in the vibrational predissociation dynamics of I_{2}(B,v)–Ar_{13} clusters
View Description Hide DescriptionThe vibrational predissociationdynamics of the van der Waals cluster I_{2}(B,v)–Ar_{13} are studied theoretically from a classical mechanical perspective. The focus is on the ejection of the first Ar atom from the excited cluster. This process is found to occur by two competing mechanisms (1) evaporation of a cluster atom induced by the slow vibrational relaxation of the I_{2} impurity, and (2) direct ejection of a geometrically favored Ar atom by two or more impulsive collisions with the vibrating I_{2}. In contrast with the picture of independent binary collisions, the multiple argon–iodine interactions leading to successful direct dissociation are correlated in time. The relative propensity of the direct channel is a highly structured function of v, the initial vibrational level of the I_{2}. This behavior results from the energy‐dependent frequencies of motion of the diatomic and the localized Ar cluster mode, and is a novel example of a nonlinear resonance between ‘‘system’’ and ‘‘bath’’ degrees of freedom.

Centroid‐density quantum rate theory: Variational optimization of the dividing surface
View Description Hide DescriptionA generalization of Feynman path integral quantum activated rate theory is presented that has classical variational transition state theory as its foundation. This approach is achieved by recasting the expression for the rate constant in a form that mimics the phase‐space integration over a dividing surface that is found in the classical theory. Centroid constrained partition functions are evaluated in terms of phase‐space imaginary time path integrals that have the coordinate and momenta centroids tied to the dividing surface. The present treatment extends the formalism developed by Voth, Chandler, and Miller [J. Chem. Phys. 91, 7749 (1989)] to arbitrary nonplanar and/or momentum dependent dividing surfaces. The resulting expression for the rate constant reduces to a strict variational upper bound to the rate constant in both the harmonic and classical limits. In the case of an activated system linearly coupled to a harmonic bath, the dividing surface may contain explicit solvent coordinate dependence so that one can take advantage of previously developed influence functionals associated with the harmonic bath even with nonplanar or momentum dependent dividing surfaces. The theory is tested on the model two‐dimensional system consisting of an Eckart barrier linearly coupled to a single harmonic oscillator bath. The resulting rate constants calculated from our approximate theory are in excellent agreement with previous accurate results obtained from accurate quantum mechanical calculations [McRae et al., J. Chem. Phys. 97, 7392 (1992)].

N_{4} ^{+} photodissociation: Charge exchange of N_{2} ^{+} fragments with Ar
View Description Hide DescriptionThe proportion of N_{4} ^{+} photofragmenting to yield N_{2} ^{+} in X ^{2}Σ_{ g } ^{+} v≳0 states has been measured by observing the yield of the fragments’ charge transferreaction with Ar. Such a determination is possible because N_{2} ^{+} in the v=0 level has a cross section for charge exchange with Ar that is 2 orders of magnitude smaller than in higher vibrational levels. The fraction of N_{2} ^{+}(v≳0) fragments increases from 0.30 at 620 nm to 0.37 at 266 nm. Calculations, which treat the N_{2} ^{+} and N_{2} fragments as quantum harmonic oscillators and the dissociation coordinate classically, predict an N_{2} ^{+} fragment vibrational content that is of the same order as that observed. The substantial population of vibrationally quiescent N_{2} ^{+} fragments, along with previous kinetic energy release studies of N_{4} ^{+}photodissociation, that show a limited fraction of the available energy appears as translational motion [M. F. Jarrold, A. J. Illies, and M. T. Bowers, J. Chem. Phys. 81, 214 (1984)], suggest that the fate of much of the photon’s energy is either in product rotational motion or in the neutral N_{2} vibration. Mechanisms for the dissociation are discussed.