Pierre-Nicholas Roy

The structure of Nb3O and Nb3O+ determined by pulsed field ionization–zero electron kinetic energy photoelectron spectroscopy and density functional theory

The geometrical structures of the ground states of triniobium monoxide, Nb3O, and its cation, Nb3O+, have been determined by an experimental and theoretical study. Vibrationally resolved photoelectron spectra of an Nb3O cluster beam were obtained at 100 and 300 K using the pulsed field ionization‐zero electron kinetic energy technique. The spectra were simulated by calculating multidimensional Franck–Condon factors using the geometries and harmonic vibrational frequencies obtained from density functional theory for the minimum energy structures of the ion and neutral molecule. The rather remarkable agreement between the experiment and the simulated spectra establishes that Nb3O and Nb3O+ have planar C2v structures with the oxygen atom bridging two niobium atoms. These are the most complex transition metal cluster structures to date to be characterized by gas phase spectroscopic techniques.

A Feynman path centroid dynamics approach for the computation of time correlation functions involving nonlinear operators

The centroid dynamics formalism is extended to the calculation of time correlation functions of nonlinear operators. It is shown that centroid correlation functions can be related to quantum mechanical ones via higher order Kubo-type transforms. A key step is the construction of the correlation functions from a mixed classical/semiclassical centroid representation of the operators. A general methodology is developed to relate these Kubo-type transforms to the desired quantum correlation functions. The approach is tested using a one-dimensional anharmonic potential for which the 〈x2x2(t)〉 and the 〈x3x3(t)〉 correlation functions are computed. Applications of this new approach are also outlined.

Quantum Monte Carlo study of helium clusters doped with nitrous oxide: quantum solvation and rotational dynamics.

Dynamical and structural properties of small (4)He(N)-N(2)O complexes have been analyzed using ground-state and finite-temperature Monte Carlo simulations. The effective rotational constants resulting from the ground-state calculations are in excellent agreement with the results of a recent spectroscopic study [Y. Xu et al., Phys. Rev. Lett. 91, 163401 (2003)]. After an initial decrease for cluster sizes up to N=8, the rotational constant increases, signaling a transition from a molecular complex to a quantum-solvated system. Such a turnaround is not present in the rotational constants extracted from the finite-temperature Monte Carlo calculations, performed for Boltzmann statistics, thus highlighting the importance of exchange effects to explain the decoupling between a solvated dopant and the helium motion.

Path integral Monte Carlo approach for weakly bound van der waals complexes with rotations: Algorithm and benchmark calculations

A path integral Monte Carlo technique suitable for the treatment of doped helium clusters with inclusion of the rotational degrees of freedom of the dopant is introduced. The extrapolation of the results to the limit of infinite Trotter number is discussed in detail. Benchmark calculations for small weakly bound (4)He(N)--OCS clusters are presented. The Monte Carlo results are compared with those of basis set calculations for the He--OCS dimer. A technique to analyze the orientational imaginary time correlation function is suggested. It allows one to obtain information regarding the effective rotational constant for a doped helium cluster based on a model for the rotational Hamiltonian. The renormalization of the effective rotational constant for (4)He(N)--OCS clusters derived from the orientational imaginary time correlation function is in good agreement with experimental results.

Path integral ground state study of finite-size systems : Application to small (parahydrogen)N (N=2-20) clusters

We use the path integral ground state method to study the energetic and structural properties of small para-H2 clusters of sizes ranging from 2 to 20 molecules. A fourth order formula is used to approximate the short imaginary-time propagator and two interaction potentials are considered. Our results are compared to those of exact basis set calculations and other quantum Monte Carlo methods when available. We find that for all cluster sizes considered, our results show a lower ground state energy than literature values obtained by diffusion Monte Carlo and variational Monte Carlo. For the dimer and trimer, ground state energies are in good agreement with exact results obtained using the discrete variable representation. Structural properties are found to be insensitive to the choice of interaction potential. We explore the use of Pekeris coordinates to analyze the importance of linear arrangement in trimers and for trimers within clusters of larger size.

Vibrational and geometric structures of Nb3C2 and Nb3C+2 from pulsed field ionization‐zero electron kinetic energy photoelectron spectra and density functional calculations

Vibrational frequencies of three niobium normal modes of triniobium dicarbide neutral and cation have been determined from pulsed field ionization‐zero electron kinetic energy photoelectron spectra. The niobium stretching mode has a frequency of 326 cm−1 in the neutral and 339 cm−1 in the ion. The two deformation modes have frequencies of 238 and 82 cm−1 in the neutral and a degenerate frequency of 258 cm−1 in the ion. The geometry of the triniobium dicarbide has been established by comparing the experimental spectra with theoretical calculations. The cluster has a trigonal bipyramid geometry with carbon atoms capping on each face of the metal frame. The cation cluster has D3h symmetry whereas the neutral cluster has lower symmetry resulting from a Jahn–Teller distortion. A second low‐lying structure with doubly bridging carbon atoms has been identified by the calculations but has not yet been observed.

Recurrences in rotational dynamics and experimental measurement of superfluidity in doped helium clusters

We present the first experimental evidence of recurrences in the rotational dynamics of doped helium clusters. Using the dopant molecule as an experimental microscopic probe of superfluidity, we show that in small He(N)-N2O complexes, superfluidity builds up in stages correlated with the filling and completion of a solvation shell.

An evaluation of methods designed to calculate energy levels in a selected range and application to a (one‐dimensional) Morse oscillator and (three‐dimensional) HCN/HNC

In this paper we study three methods designed to calculate energy levels in a range of interest. The methods are applied to a one‐dimensional (1‐D) Morse oscillator and to HCN/HNC (in three‐dimensions). Energy levels in the chosen range are computed using the filter‐diagonalization method proposed by Neuhauser [J. Chem. Phys. 93, 2611 (1990)], a spectral transform Lanczos method, and a guided Lanczos method we suggest in this paper. In the guided Lanczos method convergence of the energy levels of interest is favored by choosing the Lanczos starting vector so that it has a substantial overlap only with eigenvectors of the eigenvalues in the chosen range. This biased starting vector is calculated from a solution of the time‐dependent Schroedinger equation. Of the three methods the guided Lanczos is the most efficient for both the Morse oscillator and HCN/HNC. None of the methods designed to favor a chosen energy range are, however, (for the two problems we considered) as efficient as a straightforward Lancz...

Path integral ground state with a fourth-order propagator: Application to condensed helium

Ground state properties of condensed helium are calculated using the path integral ground state (PIGS) method. A fourth-order approximation is used as short (imaginary) time propagator. We compare our results with those obtained with other quantum Monte Carlo (QMC) techniques and different propagators. For this particular application, we find that the fourth-order propagator performs comparably to the pair product approximation, and is far superior to the primitive approximation. Results obtained for the equation of state of condensed helium show that PIGS compares favorably to other QMC methods traditionally utilized for this type of calculation.

Path-integral Monte Carlo simulation of ν3 vibrational shifts for CO2 in (He)n clusters critically tests the He–CO2 potential energy surface

Path-integral Monte Carlo simulations of the nu(3) vibrational band origin frequency shifts of CO(2) in (He)(n) clusters for n=1-40 show that although only the asymmetric-stretch mode of CO(2) is being excited, the effect of the associated change in the average value of Q(1) cannot be ignored. When this fourth degree of freedom is taken into account, the resulting predicted vibrational frequency shifts are in excellent agreement with experiment across this whole range of cluster size. It is also shown that the quality of predictions obtained from simulations on a given potential energy surface can depend significantly on the choice of the analytic function used to represent it.