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Volume 107, Issue 11, 15 September 1997

The influence of highfrequency modes on two pulse spectroscopy
View Description Hide DescriptionTheoretical analyses of molecular response to ultrafast pulse excitation often do not account for the influence of high frequency modes on the spectrum of the molecule. In a previous article [M. N. Kobrak, E. M. Hiller, and S. A. Rice, J. Chem. Phys. 105, 9403 (1996)], we have shown that the high frequency modes of the molecule can dramatically change the distribution of an observable in processes initiated by a single ultrashort pulse. In this article we extend our treatment to allow for twopulse processes, and apply the results to two models which have a qualitative resemblence to bacteriorhodopsin.

Dimensional perturbation theory for vibration–rotation spectra of linear triatomic molecules
View Description Hide DescriptionA very efficient largeorder perturbation theory is formulated for the nuclear motion of a linear triatomic molecule. All coupling between vibration and rotation is included. To demonstrate the method, all of the experimentally observed rotational energies, with values of almost up to 100, for the ground and first excited vibrational states of and for the ground vibrational states of and of OCS are calculated. The perturbation expansions reported here are rapidly convergent. The perturbation parameter is , where is the dimensionality of space. Increasing is qualitatively similar to increasing the angular momentum quantum number . Therefore, this approach is especially suited for states with high rotational excitation. The computational cost of the method scales only in proportion to , where is the size of the vibrational basis set.

Raman intensities of C=C stretching vibrational frequencies of polyenes: Nodal mode analysis
View Description Hide DescriptionIt is not clearly understood how and why the Raman intensity increases drastically with an increasing chain length of polyenes. We therefore investigated the vibrational intensities of the C=C stretching vibrational modes of four polyene systems of where X/Y=H/H, and The investigation was done using nodal mode analysis (based on the number of nodes formed by the alternations of stretches and contractions) combined with ab initio frequency calculations. The C=C stretching/contracting mode without node is found to have the strongest Raman intensities regardless of polyene systems because of the longrange cooperation effect by the concurrent stretch/contraction motion of all C=C bonds. The corresponding IR spectra have also the strongest intensities for the nonsymmetric polyene systems, whereas are inactive for the symmetric polyenes (by the exclusion rule). The intensities of the nonconcurrent C=C stretching/contracting modes (particularly for the Raman spectra) tend to decrease drastically (in proportion to with increasing node number though weak (or zero) intensities appear somewhat alternately because the molecules have approximate (or exact) centrosymmetry.

Temperature, pressure, and perturber dependencies of linemixing effects in infrared spectra. I. Q branches
View Description Hide DescriptionExperimental and theoretical results on the influence of line mixing on the shape of infrared Q branches of importance for atmospheric applications are presented. Two Q branches of symmetry, which lie near 618 and and belong to the and bands, have been studied for many conditions of temperature (200–300 K), total pressure (0.5–10 atm), and mixture (with He, Ar, and ). The theoretical approach used is based on the Energy Corrected Sudden approximation; its parameters have been deduced from both linebroadening data and measured absorption by the Q branches. Comparisons between experimental and computed spectra demonstrate the quality of the model, regardless of the conditions. Detailed analysis of the influences of the Qlines spectral spacing, temperature, total pressure, and collision partner are presented. They show that significantly larger linemixing effects are obtained when is considered with respect to This is analyzed in terms of the relative contributions of the short and midrange interaction forces and of propensity rules resulting from the coupling of angular momenta.

Nonadditive threebody dipoles of inert gas trimers and Longrange effects in far infrared absorption and triple vibrational transitions
View Description Hide DescriptionNonadditive, threebody dipoles are detected experimentally in farinfrared absorption by inert gas mixtures and in triple transitions, the absorption of a single photon by three molecules, each of which becomes vibrationally excited. In this work, we use perturbation theory to derive the nonadditive, threebody dipole of molecules A, B, and C of arbitrary symmetry, interacting at long range. Our results include linear induction, hyperpolarization, dispersion, and concerted induction–dispersion effects, with direct overlap damping. We derive exact, new equations for the dispersion and induction–dispersion dipoles, in terms of the polarizabilities and hyperpolarizabilities of A, B, and C, integrated over imaginary frequencies. With these, we obtain accurate numerical results for the dipoles of inert gas trimers and trimers containing isotropically averaged over the orientations of the molecular axis. For application to heavier systems, we develop a new constantratio approximation, relating threebody dipoles to the van der Waals interaction energy coefficients and the static polarizability α, and the static quadrupole hyperpolarizability For the test cases in this work, this approximation gives the integrals appearing in the induction–dispersion dipoles with rootmeansquare errors of 10–14 %, and the integrals in the pure dispersion dipoles with rootmeansquare errors of 1–4 %. Our numerical results for the dipoles of inert gas trimers should be useful in molecular dynamics simulations of farinfrared absorption, and in extracting information on intercollisional interference from the absorption line shapes.

Pump–probe spectroscopy of dissipative energy transfer dynamics in photosynthetic antenna complexes: A density matrix approach
View Description Hide DescriptionThe photoinduced ultrafast dynamics of singlet excitons in lightharvesting antennae is investigated using multilevel Redfield theory. Formulating the equations of motion for the reduced exciton density operator in terms of one and twoexciton eigenstates we focus attention on the influence of dynamic excitonvibrational coupling and static diagonal disorder on transient absorption spectra of peripheral antennae in photosynthetic purple bacteria. The simulations are discussed in view of recent experimental results obtained for the B850 absorption band of Rhodobacter sphaeroides. Further, we suggest a new way of estimating the size of the excitoncoherence domain in these systems which puts emphasis on the dynamic character of exciton localization. For the B850 pigment pool we find that at room temperature the pump–pulse initially prepares a coherent superposition of oneexciton eigenstates which can be delocalized over the whole aggregate. With increasing delay time the excitoncoherence domain shrinks to cover about four pigments in the asymptotic limit.

Wigner spectrogram representations of heterodynedetected fourwavemixing and fluorescence upconversion
View Description Hide DescriptionGated spontaneous emission and fourwavemixing signals are expressed using a mixed timefrequency representation of the fields (Wigner spectrograms) and of the material response functions. Wellseparated and overlapping pulses are described using twosided (noncausal) and onesided (causal) spectrograms, respectively. Pump–probe and fluorescence spectra are recast in an anologous form which facilitates the direct comparison of the underlying microscopic dynamics.

Manipulation of rovibrational wave packet composition in the shelf state using intermediate state selection and shaped femtosecond laser pulses
View Description Hide DescriptionThe composition of a rovibrational wave packet in the state of is controlled using a combination of intermediate state selection and femtosecond optical pulse shaping techniques. Intermediate state selection is accomplished using a cw laser to prepare a steady state population of a single rovibrational level in the state of which acts as the initial state for subsequent ultrafast pump/probe studies in the “shelf” region of the state. An initial set of relative amplitudes of the wave packet eigenstates is defined by the intermediate level. Femtosecond optical pulse shaping, performed using a grating pair and a mechanical amplitude mask, is then used to further alter the wave packet composition. Quantitative changes in the Fourier transform of the pump/probe transient demonstrate that combining these two techniques can be a facile approach to manipulating wave packet composition, with the ultimate goal of controlling wave packet evolution into a desired final form.

Sideband optical–optical double resonance Zeeman spectroscopy. II. Studies of NiH, PdD, and PtH
View Description Hide DescriptionSideband optical–optical double resonanceZeemanspectroscopy is applied here to studies of the transition metal hydrides NiH, PdD, and PtH. For both NiH and PtH, in addition to obtaining Zeemaninformation on the ground and several excited electronic states, extremely small ground stateprotonhyperfine splittings have been resolved. For PdD, subDoppler spectra of the ground state have been recorded even though the upper state in the optical transition is strongly predissociated. For all three radicals, the doubleresonance Zeeman studies reveal that the values in each state deviate from their nominal Hund’s coupling case (a) or (b) value with increasing rotational quantum number This Zeemaninformation, in addition to providing a sensitive confirmation of electronic assignments, also provides direct evidence of electronic state mixing (which is not present in ordinary nonZeeman spectra) and useful in modeling the interactions between electronic states.

Sideband optical–optical double resonance Zeeman spectroscopy. III. Analysis of composite lines and selective detection
View Description Hide DescriptionSideband optical–optical double resonanceZeemanspectroscopy is applied here to studies of the electronic spectrum of gasphase PtH. Even though each rotational transition is a composite line with closely spaced isotope and hyperfine structure, the subDoppler Zeeman spectrum of each feature has been recorded and analyzed using a systematic, quasitwodimensional approach. SubDoppler radio frequency magnetic resonances, in addition to their diagnostic power to assist rotational and electronic assignment, can also be used to detect selectively other transitions with the same Zeeman properties; if a fixed magnetic field is applied and the subDoppler signal recorded as the laser is scanned, spectral features which share the common Zeeman splitting in either the lower or upper state can be detected. Several examples illustrating the selectivity of this scheme are given.

Cluster isolated chemical reactions: Reactivity of Ba atoms and small Ba clusters with and molecules on large argon clusters
View Description Hide DescriptionThe cluster isolated chemical reaction technique was used to investigate the reactivity of the and systems in the environment of clusters. The method was extended to document several aspects of the reactivity. Notably, mass spectrometry gives insight into the full reactivity of the system deposited on the clusters.Laser induced fluorescence(LIF) and chemiluminescence are also used as detection tools. Unexpectedly, we found that a single barium atom neither reacts with nor with at the cluster temperature (32 K). In contrast, the LIF results suggest the formation of a weakly bound covalent complex. Finally, and larger barium aggregates react with and and larger aggregates react with The chemiluminescent products are in the first case, and BaF in the second. These observations are rationalized on the ground of the harpoon model.

Pair potential for water from symmetryadapted perturbation theory
View Description Hide DescriptionThe interactionenergies of over a thousand water dimer configurations have been calculated using the symmetryadapted perturbation theory. Effective, interaction optimized bases were used leading to 0.2 kcal/mol accuracy near the minimum of the dimer potential. The computed points were then fitted to two types of analytic potential energy surfaces, a sitesite form and an expansion in functions dependent on the vector connecting the centers of mass and on the Euler angles defining the orientation of each monomer. The second virial coefficient was calculated from these surfaces including the quantum correction and isotopic dependence, as well as the molar heat capacity at constant pressure. Comparison of these data to experiment shows that both of our surfaces are superior to any previously available.

Oneelectron versus multielectron effects in the nearthreshold C photoionization of acetylene
View Description Hide DescriptionThe C partial photoionization cross section and asymmetry parameter β of the molecule have been determined with highenergy resolution between threshold and 360 eV. The C shakeup satellite spectrum is richly structured; the cross section of several satellites increases strongly near threshold, which indicates that there is a conjugate contribution to their intensity. It is shown that the large enhancement in the total photoabsorption cross section between 305 and 320 eV, previously attributed to a shape resonance, is largely due to this photon energy dependence of the shakeup transitions.

Transitions through fluctuating barrier: Role of asymmetry and memory
View Description Hide DescriptionThe kinetics of single transition over a fluctuating barrier is considered. Fluctuations are modeled by dichotomous noise. The average first passage time (AFPT) is defined as the time elapsed from the beginning of the process (system in the state with probability 1) to the moment when the system attains for the first time the state with the average probability equal to It is found that the nonMarkovianity of the barrier fluctuations may introduce oscillations in the process of barrier crossing and in the effective reaction rate, and elongate the AFPTs. Especially, may become infinite, even when remains finite—the process of barrier crossing is reverted after some time. However, in some cases (strong asymmetry of barrier fluctuations, high AFPTs in the absence of fluctuations, together with long memory characteristic time of the nonMarkovian part of the fluctuations) the effective reaction rates can be enhanced and AFPTs shortened in comparison with those for static barrier.

Quasiclassical trajectory simulations of collisional deactivation of vibrationally excited I. Dependence on vibrational energy
View Description Hide DescriptionThe collisional deactivation of by He, Ne, Ar, Kr, and Xe has been studied using quasiclassical trajectories calculations, with initial vibrational energy in the range The rotational and translational energies used in the calculations corresponded to the Boltzmann distribution at 415 K. The first and second moments for the transference of translational, rotational, and vibrational energy are linearly dependent on The spread of the distribution depends quadratically on for vibrational and translational energy transfer and linearly for the rotational change. The results could also be satisfactorily adjusted to power laws on and vibrational quantum number, which is indicative of the difficulty in interpreting experimental results through the usually available information of the dependence of the total average energy loss.Analysis of the correlation coefficients and the relative changes of average translational, rotational and vibrational energy transferred per collision indicates a strong VR coupling for the heavier gases (Ar, Kr, and Xe).

Statespecific reaction and product energy disposal of electronically excited potassium with hydrogen molecule
View Description Hide DescriptionUsing a pump–probe technique, we have systematically studied the stateselected effect on the reaction, showing that the reactivity follows the trend of As long as the system is energetically allowed for reaction, the potential energy is not the key parameter, but the atomic orbital symmetry determines such a state selectivity. The observation of rotational population in the reaction of corresponds to a statistical thermal distribution at In contrast, the vibration is highly excited, yielding a Boltzmann vibrational temperature of and These results provide evidence that the attacking K atom approaches along a collinear geometry, and KH is produced via an ionpair intermediate as a likely pathway. The fraction of product energy partitioning yields 70%, 26%, and 4% for translation, vibration, and rotation. The individual energy disposal into vibration increases with the excitation energy of K. The fact indicates that the electron jumping distance elongates along the order of consistent with the prediction by the harpoon mechanism. Most available energy dissipation into translation is caused by a strong instability of the bond. The repulsive energy release from the bond rupture is seriously affected by the attraction between and Therefore, the direct interaction with product repulsion (DIPR) model may not be valid to describe the current system. As “mixed energy release” concept is considered instead, a disposal comparison of available energy among the reactions of and may be rationalized.

The ^{129}Xe nuclear shielding surfaces for Xe interacting with linear molecules and CO
View Description Hide DescriptionWe have calculated the intermolecular nuclear magnetic shieldingsurfaces for in the systems and Xe–CO using a gaugeinvariant ab initio method at the coupled Hartree–Fock level with gaugeincluding atomic orbitals (GIAO). Implementation of a large basis set (240 basis functions) on the Xe gives very small counterpoise corrections which indicates that the basis set superposition errors in the calculated shielding values are negligible. These are the first intermolecular shielding surfaces for Xemolecule systems. The surfaces are highly anisotropic and can be described adequately by a sum of inverse even powers of the distance with explicit angle dependence in the coefficients expressed by Legendre polynomials for and The Xe–CO shielding surface is well described by a similar functional form, except that were used. When averaged over the anisotropic potential function these shielding surfaces provide the second virial coefficient of the nuclear magnetic resonance(NMR) chemical shift observed in gas mixtures. The energies from the selfconsistent field (SCF) calculations were used to construct potential surfaces, using a damped dispersion form. These potential functions are compared with existing potentials in their predictions of the second virial coefficients of NMR shielding, the pressure virial coefficients, the density coefficient of the meansquare torque from infrared absorption, and the rotational constants and other average properties of the van der Waals complexes. Average properties of the van der Waals complexes were obtained by quantum diffusion Monte Carlo solutions of the vibrational motion using the various potentials and compared with experiment.

Properties of atoms in molecules: Atoms under pressure
View Description Hide DescriptionThe in situ pressure acting on the surface of an open system at the atomic level is defined and determined by the virial theorem for a proper open system, one whose spatial boundary and equations of motion are determined by the principle of stationary action. The quantum pressure is determined by the virial of the force resulting from the electronic momentum flux through the surface of the open system. A scaling procedure is used to demonstrate that the expectation value of the pressure–volume product of a proper open system is proportional to its surface virial. Previous work, in analogy with the classical virial theorem for a contained system, incorrectly relates the pressure to the external forces of constraint acting on a closed system. A neon vise consisting of a chain of three, four or five hydrogen molecules compressed between two neon atoms is used to introduce the quantum definition of pressure and study its effect on the mechanical properties of an atom and on the topology of the electron density. Pressures approaching 160 GPa have been calculated for the vise. The topology of the electron density and the homeomorphism it exhibits with the virial field are found to be invariant to an increase in pressure, the electron density accumulating to an ever increasing extent between all pairs of nuclei which serve as the sole attractors. The virial of the Ehrenfest force acting on the surface of a compressed molecule provides a measure of the increase in the electronic kinetic energy resulting from the applied pressure. The effects of pressure on the intra and intermolecular bonding are discussed in terms of pressureinduced changes in the electron density and in the mechanical properties of the atoms.

Quantum corrections to the classical path equations: Multitrajectory and Hermite corrections
View Description Hide DescriptionThe classical path theory can be derived from first principles by assuming a product type wave function and a Gaussian wave packet in those degrees of freedom for which the classical equations of motion is introduced. Here we give ways of correcting the theory so as to obtain the exact solution with enough correction terms.

Correlation states of propene
View Description Hide DescriptionHigh resolution synchrotron photoelectron spectra (PES) of propene have been obtained at several photon energies (40–70 eV). The full valence shell binding energy spectrum reveals a well resolved correlation (satellite) peak at 20.3 eV and the photon energy dependence study of its photoionization cross section indicates that this new correlation peak is a “dynamic” correlation peak. MRSDCI calculations of the theoretical spectrum show semiquantitative agreement between experiment and theory in terms of binding energies and intensities. A comparison of the full valence shell photoelectron spectra of ethylene, propene, 1butene and 1,2propadiene (allene) is also presented to illustrate trends in the correlation peaks of model alkenes.