Index of content:
Volume 107, Issue 5, 01 August 1997

Microwave spectroscopic detection of HCCP in the electronic state: Phosphocarbene, phosphoallene, or phosphorene?
View Description Hide DescriptionMicrowave spectrum of the HCCP radical was detected for the first time in the ground electronic state using a sourcemodulated microwave spectrometer. In total, 24 rotational transitions of HCCP in the 90–360 GHz region, 9 rotational transitions of DCCP in the 260–360 GHz range, and 24 rotational transitions of between 130–360 GHz were measured.Hyperfine structure pertaining to the phosphorus and hydrogen nuclei was observed for HCCP, and in the case of only for phosphorus. The corresponding hyperfine coupling constants were ascertained in addition to the rotational, centrifugal distortion, and fine structure constants by a leastsquares analysis of the measured frequencies. From the hyperfine coupling constants determined, the spin density of unpaired electrons was estimated to be 76% for the phosphorus atom and 42% for the carbon adjacent to the hydrogen. The structure of HCCP was established from the rotational constants of HCCP and its isotopically substituted species: and These structural features are consistent with a linear phosphoallenic form that has been somewhat modified by a phosphorene.

High resolution absorption spectrum of jetcooled between 65 000 and Assignment of bent and linear and gerade states
View Description Hide DescriptionThe absorptionspectrum of jetcooled was photographed between 65 000 and at a resolution limit of 0.0008 nm. In the first half of the energy interval considered, a bending vibrational progression is assigned corresponding to the transition between the linear ground state and a bent excited state correlating with the state of the linear molecule. The same progression is observed in the resonance enhanced ionization (REMPI) spectrum of Baker and Couris [J. Chem. Phys. 103, 4847 (1995); 104, 6130 (1996); 105, 62 (1996)]. Another observed bending progression in the REMPI spectrum for the same region is here assigned to the other, less bent state issuing from the linear state. In both progressions, transitions are also observed. In the upper half of the energy range considered, the absorptionspectrum consists essentially of and bands associated with excitation of and states. The corresponding origin bands, as well as those of all the other twophoton allowed transitions related to the same configurations, are assigned to bands observed in the REMPI spectra. The rotational band profile associated with twophoton onecolor excitation of the supercomplex of is calculated using a program based on Hund’s case (e) representation. The band positions and relative intensities in the simulated contour are in excellent agreement with those assigned to origin transitions in the twocolor parallel polarized REMPI spectrum. All other bands of the experimental twophotonspectrum can be assigned as the bands associated with the observed electronic origins. The quantum defect values used in the final band contour calculation are consistent with those obtained in an ab initio calculation. A calculation of the same type is performed for the excitation energy from and orbitals to and from to the valence orbital. These transitions were suggested by several authors as possible assignments in this spectral region but are indeed at much higher energy. The and transition bands near, respectively, the lower and higher limits of the interval studied here, are also assigned.

Theory of double quantum twodimensional electron spin resonance with application to distance measurements
View Description Hide DescriptionA formulation is presented for calculating double quantum two dimensional electron spin resonance (DQ2D ESR) spectra in the rigid limit that correspond to recent experimental DQ2D ESR spectra obtained from a nitroxide biradical. The theory includes the dipolar interaction between the nitroxide moieties as well as the fully asymmetric and hyperfine tensors and the angular geometry of the biradical. The effects of arbitrary pulses (strong but not truly nonselective pulses) are included by adapting the recently introduced split Hamiltonian theory for numerical simulations. It is shown how arbitrary pulses in magnetic resonance create “forbidden” coherence pathways, and their role in DQ2D ESR is delineated. The high sensitivity of these DQ2D ESR signals to the strength of the dipolar interaction is demonstrated and rationalized in terms of the orientational selectivity of the “forbidden” pathways. It is further shown that this selectivity also provides constraints on the structural geometry (i.e., the orientations of the nitroxide moieties) of the biradicals. The theory is applied to the recent double quantum modulation (DQM) experiment on an endlabeled polyproline peptide biradical. A distance of 18.5 Å between the ends is found for this biradical. A new two pulse double quantum experiment is proposed (by analogy to recent NMR experiments), and its feasibility for the ESR case is theoretically explored.

Lowenergy excitations in noncrystalline arsenic trioxide
View Description Hide DescriptionThe liquid to glass transition for arsenic trioxide has been studied by performing lowfrequency Raman scattering in the temperature range 300–770 K. For this glass forming system the light scattering intensity ratio of the relaxational to the vibrational (Boson peak) contribution is much higher than that expected for low fragility systems near the glass transition temperature. A possible explanation for this unexpected feature is given by employing a structural model that is also able to interpret the hypersound propagation and absorption peculiarities found previously in this system. The depolarization ratio, in the quasielastic frequency region for different oxide glasses, is associated with local microstructural transformations causing fast relaxations in these systems. The Boson peak region is discussed in the context of different models and current theoretical approaches for the glass transition.

Isotope and temperature effects on the C and Se nuclear shielding in carbon diselenide
View Description Hide DescriptionA comprehensive theoretical and experimental study of the C and Se nuclear magnetic shieldings and their rovibrational corrections in carbon diselenide (CSe) has been undertaken. The C and Se shielding tensors as well as all their first and second derivatives with respect to the internal displacement coordinates of the molecule have been calculated by several first principles gaugeincluding atomic orbital (GIAO) methods. HartreeFock (HF), multiconfiguration HartreeFock (MCHF), and densityfunctional (DFT)theories have been compared, the latter both in the local density approximation(LDA) and by using two gradient corrected exchangecorrelation functionals. The shielding derivatives calculated with MCHF and DFT are very much smaller in magnitude than the derivatives obtained by using HF, being in reasonable mutual agreement. By using the theoretical shielding derivatives and the cubic anharmonic force constants calculated within LDA, together with an experimental harmonic force field, all the first and second order terms in the rovibrational contributions to the shielding constants and anisotropies have been worked out. The contributions to the shielding constants have been calculated for the various isotopomers of CSe at several temperatures, and the resulting theoretical temperature dependencies of the shielding constants, the isotope shifts and the temperature dependencies of the isotope shifts have been compared with the experimental results. There is excellent agreement between the theoretical and experimental results for Se. The agreement is not quite as good for the (anomalously small) shielding constant of C and its rovibrational corrections. Contrary to what has been frequently assumed, none of the first and second order terms in the rovibrational contributions to the shielding constants can safely be neglected. In particular, the first order isotope effect due to change in the bond not directly attached to the observed nucleus is very important. Furthermore, the second order terms — including the bending and even cross terms — are essential in order to give a correct description of the isotope shifts.

Nonlinear effects in dipole solvation. I. Thermodynamics
View Description Hide DescriptionThe method of Padé truncation of perturbation expansions for thermodynamic potentials of molecular liquids is extended to the calculation of the solvation chemical potential of an infinitely dilute dipolar solute in a dipolar liquid. The Padé form is constructed to include nonlinear solvation effects of dipolar saturation at large and the linear response quadratic solute dipole dependence at small solute dipoles. The theory can accommodate polarizable solvents. The limiting case of electronically rigid solvent molecules is tested on the nonlinear reference hypernetted chain (RHNC) approximation for dipolar liquids. At high solvent polarities the Padé solvation chemical potential exceeds that of the RHNC. For both treatments, the nonlinear solvation contribution is found to pass through a maximum as a function of solvent polarity indicating that orientational saturation created by the solute breaks down with increasing solventsolvent dipolar coupling. The Padé form of the chemical potential provides an analytical solution applicable to spectroscopic and electron transfer calculations involving solvation of fictitious complexvalued dipoles.

Nonlinear effects in dipole solvation. II. Optical spectra and electron transfer activation
View Description Hide DescriptionWe present a theoretical analysis of the effect of nonlinear dipole solvation on steadystate optical spectra and intramolecular electron transfer(ET)reactions. The solvation nonlinearity is attributed to saturation of a dipolar liquid produced by the solute dipole. The treatment explores the perturbation expansion over the solutesolvent dipolar interaction truncated in the form of a Padé approximant. The optical line shape and the free energies along the ETreaction coordinate are related to the chemical potential of solvation of a fictitious solute with a complexvalued dipole moment. Due to solvent dipolar saturation the spectrum of dipolar fluctuations is confined by a band of the width Solvation nonlinearity was found to manifest itself for optical transitions with high dipole moments in the initial state, most often encountered for emission lines. In this case, the spectral line approaches the saturation boundary bringing about “line squeezing” and decrease of the line shift compared to the linear response prediction. In the nonlinear region, the line shift dependence on the solute dipole variation switches from the quadratic linear response form to a linear trend . The bandwidth may pass through a maximum as a function of in the saturation region. Nonlinear solvation results thus in a narrowing of spectral lines. For a transition with solute dipole enhancement, the bandwidth in emission is therefore lower that in absorption As a result, the plot of , against the Stokes shift demonstrates the upward deviation of and downward deviation of from the linear response equality We also explored the nonlinearity effect on charge separation/charge recombination activation thermodynamics. The solvent reorganization energy was found to be higher for charge separation than for charge recombination . Both are smaller than the linear response result. For the reorganization energies, the discrepancy between and is relatively small, whereas their temperature derivatives deviate significantly from each other. The theory predictions are tested on spectroscopic computer simulations and experiment. Generally good quantitative agreement is achieved.

Control of vector properties in vibrationally mediated photodissociation near asymmetric resonances
View Description Hide DescriptionQuantum mechanical analysis is presented on the control of the vector properties of the photoproducts by vibrationally mediated photodissociation of OH. The angular distributions and alignment of fragments are calculated near isolated and overlapping asymmetric resonances. The vector properties depend very sensitively on the vibrational levels of the initial state. The variations of the anisotropy parameters as a function of the excitation energy near the asymmetric resonances change markedly depending on The widths of the variations tend to increase with increasing which is very similar to the corresponding behaviors of the product branching ratios studied earlier [J. Chem. Phys. 104, 1912 (1996)], indicating that could be a useful tool for choosing the proper linewidths in the experiments for the control of the product branching ratios, angular distributions, and distributions near asymmetric resonances. It is also found that the vector properties may exhibit splitting of the overlapping resonances for high in contrast to scalar properties.

Modespecific energy analysis for rotatingvibrating triatomic molecules in classical trajectory simulation
View Description Hide DescriptionA method for the modespecific energy analysis in a classical trajectory calculation is developed. The pure rotational energy is evaluated by invoking the Eckart condition. To evaluate the vibrational energy in each normal mode, the vibrational velocity is divided into two parts, the angular motion part and the angular motion free part, and the latter is analyzed with the Cartesian and internal coordinate systems. The potential energy of each normal mode is also evaluated in the two coordinate systems. A simple algorithm to include some anharmonicity correction is presented. Sample calculations with nonreacting triatomic molecules, and HCN, show that the internal coordinate system is more adequate than the Cartesian, especially for the linear molecule HCN. An excellent result is obtained for the product of a reaction, suggesting that the present method is adequate for the modespecific energy analysis of classical trajectory results.

Photodissociation of HCl at 193.3 nm: Spin–orbit branching ratio
View Description Hide DescriptionHCl was photodissociated by ultraviolet (uv) radiation at 193.3 nm. Timeofflightspectra of the hydrogen atom fragment provided the spin–orbit state distribution of the chlorine fragment, in excellent agreement with recent theoretical studies. The H atom angular distribution studied by changing the uv photolysis laser polarization confirmed a dominant electronic transition in the photoexcitation process ( and ).

From the sparse to the statistical limit of intramolecular vibrational redistribution in vibrational predissociation: as an example
View Description Hide DescriptionThe dynamics of intramolecular vibrational relaxation (IVR) for are examined for a wide range of vibrational and rotational excitation. In order to describe the IVR more efficiently, and characterize it more quantitatively, we propose a refinement of the traditional BixonJortner description in which the active states are prediagonalized to simplify the coupling scheme that must be considered. This allows for an explicit determination of the average density of states and average coupling strength for each initial excitation. We find that the IVR dynamics proceed from the sparse regime for =11, for which the first open dissociation channel corresponds to the loss of two quanta, to the intermediatedense regime for =25 which dissociates by the loss of 4 quanta. We find that over this range the increase in the density of states is less important than the increase in the coupling strength. For = 18 we examine the effect of rotation in considerable detail. Initial states that couple via a manifold of 6000 channels can be considered since the calculation is performed on a parallel computer. The effect of increasing , the total angular momentum excitation, is found to be less than that of increasing , the degree of rotation about the van der Waals bond. This means that the main effect is not simply an increase of the available density of states due to Coriolis coupling. Understanding the details of IVR in a relatively simple system like should help us understand the dynamics of more complicated molecules. In particular, the case of is discussed.

Quantum dissociation dynamics of and on a cluster
View Description Hide DescriptionThe dissociationdynamics of and on a rigid cluster has been investigated using a quantum mechanical model. The model is based on the spectral grid/fast Fourier transform technique and includes three variables which are treated quantum mechanically; the translational motion of the molecule normal to the cluster, the vibrational coordinate, and the polar orientation angle. The remaining three variables are fixed during the simulations. The dependence of the dissociation probability on the incident beamenergy, initial molecular state and impact site has been examined. The probabilities for rovibrational excitation of the scattered flux have also been computed as function of incident beamenergy and impact site. In addition, the dissociation probability has been averaged over the remaining three variables that define the impact site configuration.

Memory kernels and effective Hamiltonians from time dependent methods. I. Predissociation with a curve crossing
View Description Hide DescriptionNonHermitian complex effective Hamiltonians resulting from boundcontinuous partitioning techniques are built from time dependent methods. We treat predissociation processes with a curve crossing. The energy dependent shift and halfwidth matrices are obtained simultaneously by a generalization of the wave packet Golden Rule treatment, as the real and imaginary parts of the Fourier transform of a memory kernel matrix. The latter contains auto and crosscorrelation functions. They are overlap integrals among the projections on the continuum of bound states multiplied by the interchannel coupling function responsible for the predissociation. These wave packets are propagated by the propagator of the sole continuous subspace. An approximate analytical expression of this correlation matrix is established for the harmonic/linear model. The numerical method is applied to the electronic predissociation of the MgCl state, to a Morse/exponential model and to a predissociation with two coupled repulsive decay channels. The comparison between the correlation time scales and the Golden Rule lifetimes is decisive so as to justify whether the memory kernel can be considered as an impulsive kernel. This Markovian approximation implies that the two time scales are well separated. In the energy domain, this corresponds to the introduction of a mean phenomenological effective Hamiltonian that neglects the energy variation of the discretecontinuous coupling elements. We observe that the separation of the time scales is effective for weakly open systems, but not for overlapping metastable states for which the perturbativetheory widths largely exceed the mean energy spacing. This confirms from a temporal viewpoint that a nonperturbative treatment should not neglect the energy dependence of the effective Hamiltonian, as currently assumed in the study of largely open systems.

A simple physical picture for quantum control of wave packet localization
View Description Hide DescriptionBased on weak field quantum control theory, a semiclassical approximation relates the characteristic parameters of the tailored light field to corresponding classical dynamical quantities and thus reveals the underlying physical basis of wave packet focusing. A coordinatedependent twolevelsystem approximation is employed to further analyze the molecular dynamics induced by short laser pulses, thus leading to a simple interpretation of the observed correlation between the pulse chirp and vibrational focusing and defocusing. Though our study is presented in the context of quantum control, the conclusions are general, providing an intuitive picture of the quantum coherence of light–matter interaction and a guideline for the design of tailored laser fields.

A threedimensional quantum mechanical study of the system: Competition between chemical exchange and inelastic processes
View Description Hide DescriptionIn this publication is presented a threedimensional quantum mechanical study, within the coupled states approximation, of the process Both reactive (exchange) and inelastic processes were considered. The main findings are: (a) The charge transfer process takes place at large distances and so the reagents are essentially on the lower potential energy surface when they approach the close interaction region; (b) The main contributions to the reaction (exchange) are from large impact parameters; (c) The initial rotational states have at most a minor effect on the results (whether being charge transfer or chemical exchange); (d) The deep potential well in the interaction region of the lower surface has only a secondary effect on the results.

Spin relaxation by diffusion on biaxial rods
View Description Hide DescriptionA theoretical framework is presented for the analysis of orientation and frequency dependent spinrelaxation rates from nuclei diffusing on biaxial rodlike aggregates in rectangular and other amphiphilic liquid crystals. With the aid of eigenfunction expansion and integral transform techniques, the problem of calculating the spectral density functions for surface diffusion on a biaxial rod can be reduced to quadrature, without the need to specify the rod geometry. The general results are applied to ribbonlike and elliptic rods, yielding analytical results. The calculations demonstrate that spin relaxation is a considerably more sensitive probe of interface curvature than the static line shape. The relaxation anisotropy, in particular, can discriminate between closely similar rod geometries with different curvature distributions.

Vibrational coherence in nonadiabatic dynamics
View Description Hide DescriptionIn this paper we explore temporal vibrational coherenceeffects in nonadiabaticradiationless transitions between two electronic states in a large molecule or in the condensed phase, accounting explicitly for the role of the (intramolecular and/or medium) vibrational quasicontinuum of the final states. Our treatment of the time evolution of the wave packet of states and of coherenceeffects in the nonradiative population probabilities of the reactants and the products rests on the diagonalization of the Hamiltonian of the entire multimode system, with supplementary information being inferred from the effective Hamiltonian formalism. New features of the vibrational Franck–Condon quasicontinuum, which originate from weak, but finite, correlations between offdiagonal coupling terms, were established. The state dependence of the offdiagonal couplings between the doorway states manifold and the quasicontinuum was quantified by the correlation parameters where 〈 〉 denotes the average over the relevant energy range. Calculations were conducted for a Franck–Condon fourmode system consisting of doorway states and quasicontinuum states. The correlation parameters for all pairs of doorway states are considerably lower than unity obeying propensity rules with the highest values of corresponding to a single vibrational quantum difference, while for multimode changes between and very low values of are established. Quantum beats in the population probabilities of products and reactants in nonadiabatic dynamics are characterized by an upper limit for their modulation amplitudes (for ), where Γ is the decay width of the doorway states and is their energetic spacing. These low ξ values originate from a small contribution to the offdiagonal matrix elements of the nonradiative decay matrix in conjunction with low correlation parameters. The amplitudes of the quantum beats in nonradiative temporal dynamics provide dynamic information on the larger correlation parameters Our theoretical and numerical analysis was applied for temporal coherenceeffects in nonadiabatic electron transfer dynamics in a Franck–Condon quasicontinuum of Mulliken charge transfer complexes [K. Wynne, G. Reid, and R. M. Hochstrasser, J. Chem. Phys. 105, 2287 (1996)]. This accounts for the “preparation” (signature of coherent excitation), for the low amplitudes of coherent temporal modulation of reactants and products ( determined by the parameters) and for the dominating contributions to temporal coherence (subjected to propensity rules).

A femtosecond midinfrared pump–probe study of hydrogenbonding in ethanol
View Description Hide DescriptionWe present a femtosecond midinfrared pump–probe study of hydrogen bonding. It is shown that upon excitation of the OHstretching vibration of hydrogenbonded ethanol dissolved in the hydrogen bonds are predissociated on a femtosecond time scale. The measuredpredissociation time constant depends strongly on the excitation frequency, and ranged from ∼250 fs at to ∼900 fs at The time constant of the subsequent reassociation of the hydrogen bonds was found to be 15 ps, in accordance with previous picosecond studies. Furthermore, polarizationresolved measurements show that orientational relaxation takes place on a time scale much shorter than the pulse length of ∼200 fs. This rapid orientational relaxation can be explained from the fast delocalization of the O–H stretching excitation over the hydrogenbonded ethanol oligomers. The orientational anisotropy reaches a value of 0.15 instantaneously, and remains constant for all delays.

Gas phase ion chemistry and ab initio theoretical study of phosphine. I
View Description Hide DescriptionGas phase ion processes of phosphine have been studied by theoretical calculations and experimental techniques. Ab initio quantum chemical calculations have been performed on the ion/molecule reactions starting from in as they have been observed by ion trapping. gives product ions with loss of or H in different pathways and also reacts in chargeexchange processes to form The energies of transition structures, reaction intermediates, and final products, as well as their geometrical structures have been determined by theoretical methods. The initial step is formation of a triplet adduct of symmetry A hydrogen atom can either be directly lost from the tetracoordinated phosphorus, or first undergo a shift to the other P atom followed by P–H bonddissociation.Dissociation of from can also occur from both the initial and species yielding The heats of formation of the ionic species have also been computed and compared with experimental data reported in the literature.

A systematic and feasible method for computing nuclear contributions to electrical properties of polyatomic molecules
View Description Hide DescriptionAn analytic method to evaluate nuclear contributions to electrical properties of polyatomic molecules is presented. Such contributions control changes induced by an electric field on equilibrium geometry (nuclear relaxation contribution) and vibrational motion (vibrational contribution) of a molecular system. Expressions to compute the nuclear contributions have been derived from a power series expansion of the potential energy. These contributions to the electrical properties are given in terms of energy derivatives with respect to normal coordinates, electric field intensity or both. Only one calculation of such derivatives at the fieldfree equilibrium geometry is required. To show the useful efficiency of the analytical evaluation of electrical properties (the socalled AEEP method), results for calculations on water and pyridine at the SCF/TZ2P and the MP2/TZ2P levels of theory are reported. The results obtained are compared with previous theoretical calculations and with experimental values.