Volume 111, Issue 24, 22 December 1999
 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


FTICR study on hydrogenation of niobium cluster cations in seeded supersonic jet and multiplecollisioninduced dissociation of hydrides
View Description Hide DescriptionHydrogenation of niobiumcluster cations in a seeded supersonic jet of and multiplecollisioninduced dissociation (MCID) of the resulting hydrides have been studied using a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. The nascent hydrides trapped in the FTICR cell have broad m distributions with no apparent prevalence of odd or even m. A pulse of argon applied to the trapped clusters causes a dramatic squeezing of the initial m distribution (through the collisioninduced removal of weakly bound molecules), favoring several particular hydrides for each cluster size n, e.g., and for The maximum m values of these stable hydrides are close to the stoichiometric composition of for the clusters with and approach that of NbH at larger n. The hydrides observed in our experiments are different from the products of the reactions performed in the FTICR cell at room temperature, which show only even and strongly ndependent m values. The MCID of the clusters occurs through the sequential desorption of molecules yielding and as final dissociation products for odd and even m, respectively. Based on the experiments on the MCID of an explanation is suggested for different reactivities of the clusters toward in the ICR and fastflowreactor experiments.

Quantum Monte Carlo simulation of intermolecular excited vibrational states in the cage water hexamer
View Description Hide DescriptionRigidbody diffusionMonte Carlo simulations of the ground state and ten lowlying intermolecular excited vibrational states for the cage form of are reported. The excited states are found by a nodal optimization procedure in which the fundamental excitedstatenodes are constructed from the harmonic normal coordinates. The anharmonic effects in the excited states are found to be large. One of the states with relatively large transition intensity involves primarily flipping motions of the free OH bonds on the doubly bound monomers, and is assigned to the vibration–rotation–tunnelling band observed experimentally by Liu et al. [Nature 301, 501–503 (1996)].

Laserinduced fluorescence and Optical/Stark spectroscopy of PtC
View Description Hide DescriptionOptical/Stark measurements have been performed on the (0,0) bands of both the system and the system of platinum monocarbide. The PtC molecules were produced in a pulsed supersonic molecular beamsource following the reaction of laser ablated platinum vapor with a mixture of a few percent of methane in argon. The newly determined permanent electric dipole moments obtained are and These results are discussed in terms of a proposed molecular orbital correlation diagram for platinum containing diatomics. The laserinduced fluorescence spectrum of the transition of PtC has been rerecorded at high resolution (full width of halfmaximum ∼40 MHz) and analyzed to yield rotational constants for the four most abundant isotopomers of PtC, extending the previous analysis [Appelblad, Nilsson, and Scullman, Phys. Scr. 7, 65 (1973)]. The anomalously large value (∼15 MHz) for the newly derived nuclearspin rotation parameter, for the state is discussed.

Spin polarization of zero kinetic energy electrons from HBr
View Description Hide DescriptionThe range of observables in zero kinetic energy (ZEKE) electron spectroscopy of molecules, previously restricted to the total electron intensity, was extended by measuring the integral spin polarization of ZEKE electrons at the thresholds after singlephoton excitation with narrowband circularly polarized light. A comparison with calculated values from multichannel quantum defect theory underlines the importance of autoionization for the decay dynamics.

An ab initio molecular dynamics study of the reaction
View Description Hide DescriptionAn ab initiomolecular dynamics study of the reaction has been performed at the Becke, Lee, Yang, and Parr (BLYP) level of theory by the bluemoon method. The potential energy and the free energy profile along the reaction coordinate have been determined and compared with the available experimental and calculated data. An analysis of the structural parameters along the reaction pathway is presented. Results of impact studies are also reported. It is shown that, depending on impact velocity, recrossing of the barrier can occur. Strong polarization effects are reported.

Magnetic field effects on the dynamics of a Rydberg electron: The residence time near the core
View Description Hide DescriptionUsing symplectic integrator schemes, we calculate the classical trajectory of a Rydberg electron in external electric and magnetic fields. We also solve the equation of motion obtained by taking the mean values over one revolution of the electron in the undisturbed motion. The resulting secular motion is periodic. When only an electric fieldF is applied, as long as the modulation period in the orbital angular momentuml is longer than the revolution period, the motion agrees with the secular one and the duration for which l is much larger than its low initial value is stretched. The residence time (RT), namely, the probability of finding the electron at the distance r, is hence smaller than that at In crossed electric and magnetic fields, the secular motion predicts that an additional time stretching due to a magnetic field occurs up to the critical value of magnetic field strength, (n is the principal action). In the actual simulations, the RT near the core is smaller than that at even beyond regardless of the magnitude of the nonCoulombic interaction Slow modulations in l are generated by transitions to secular motions that maintain high l, in addition to the fast modulation originating from the secular motion. When the magnetic field is so strong as to induce chaotic motion (∼4000 G for the energy of −5 the RT is one order of magnitude as large as those in weak field cases around 40 G. In the intermediate region (> a few hundred Gauss), without a nonCoulombic interaction, the RT monotonically increases as B increases. In the presence of transitions from low l states to high l states occur: the RT decreases. The motions in high l states can be explained by the wellknown model in which an electron bound to the core by a harmonic force moves in a magnetic field.

One and twobody densities for the beryllium isoelectronic series
View Description Hide DescriptionOne and twobody densities in position space have been calculated for the atomic beryllium isoelectronic series starting from explicitly correlated multideterminant wave functions. The effects of electronic correlations have been systematically studied by comparing the correlated results with the corresponding Hartree–Fock ones. Some expectation values such as and where and stand for the electron–nucleus, interelectronic, and two electron center of mass coordinates, respectively, have been obtained. All the calculations have been carried out by using the Monte Carlo algorithm.

Isotopic branching in collisions: A timedependent quantum mechanical study in three dimensions
View Description Hide DescriptionA timedependent quantum mechanical approach has been used to investigate the reaction in three dimensions for total angular momentum The vibrotation stateselected reaction probability is shown to increase with over the collision energy range (0.95–2.25 eV) investigated for both the exchange channels, in accord with the experimental results. The isotopic branching ratio generally remains less than unity for different states at different in agreement with experiment. But at for Γ obtained from our calculations for of is in excellent agreement with the earlier quasiclassical trajectory calculations, but a factor of 2 less than that obtained from experiment. This difference could arise from the inclusion of nonzero states in the experimental study, as is found to be dependent for both the channels. While decreases initially with increase in from 0 to 2 and then increases when is increased further to 3, reveals an unusual dependence; it is larger for even states of than for odd As a result, Γ is strongly dependent on in contrast to the marginal dependence shown by the earlier quasiclassical trajectory calculations.

High resolution spectroscopy of MgOH in its mode: Further evidence for quasilinearity
View Description Hide DescriptionPure rotational spectraof the MgOH and MgOD radicals have been recorded in the bending vibration of their ground electronic statesusing millimeterwave direct absorptionspectroscopy. Multiple rotational transitions arising from the and substates have been measured in the frequency range 240–520 GHz for these species. Both thespinrotation and ltype doubling interactions have been resolved in thespectra. The completedata sets for MgOH and MgODhave been analyzed using alinear model for the Hamiltonian which takes into account higher order ltype interactions. The global analyses were adequate, but anomalous behavior wasapparent in both molecules. In particular, the vs relation was found to be highly nonlinear, large variations in the ltype doublingconstant q were observed with vibrational level, and and structures determined differed substantially. Such findings suggest that MgOH is highly quasilinear,comparable to HNCO. The competition between ionic and covalent bonding is therefore becomingapparent in the lighter alkaline earth hydroxide species.

Electronic transfer dynamics for bridged binuclear mixedvalence compounds: Density functional theory study on electronic structure in the ground state for the Creutz–Taube ion under asymmetric distortions
View Description Hide DescriptionIn order to explore the dynamic problem of the electronic structure in the ground state for the Creutz–Taube ion, the calculations on the electronic potential surfaces along the totally symmetric vibrational difference coordinate of the Ru–N stretch and the net charge distribution on the Ru atoms are carried out by using the density functional theory in the two schemes with or without the Ru–N(pyz) vibration. It is shown that the vibration associated with the totally symmetric vibrational difference coordinate can be regarded as a harmonic oscillation with 123.16 eV Å^{−2} of the force constant, and 430 cm^{−1} of the fundamental frequency is obtained on the basis of the fivebody vibrator model. From the electronic structure calculated in the ground state it is concluded that the asymmetric net distribution on the Ru atoms in the Creutz–Taube ion occurs in the vibration associated with the totally symmetric vibrational difference coordinate, and the charge transfer between the Ru atoms and the ligands is responsible for the asymmetric charge distribution. The total symmetric vibrations of Ru–N for the related monomer are further examined. It is found that the reduction of Ru(III) to Ru(II) corresponds to 0.05 Å of the difference in Ru–N distance between the two Rusubunits under the asymmetric distortion and to 0.012 eV of the potential energy over the zero point energy, which is comparable with kT. The Born–Oppenheimer approximation and the neglected coupling effect between the electronic and nuclear motion in the present paper are briefly discussed.

Experimental distinction between phase shifts and time delays: Implications for femtosecond spectroscopy and coherent control of chemical reactions
View Description Hide DescriptionTwo different definitions of phase shifts and time delays are contrasted and shown to match different experimental methods of generating delayed pulses. Phase shifts and time delays are usually defined in terms of a carrier wave in magnetic resonance, but definitions based on the envelope of a single pulse are useful in optics. It is demonstrated experimentally that a frequency domain measurement using spectral interferometry can simultaneously measure phase shifts with an accuracy of 0.1 rad (2σ) and time delays with a precision of 40 attoseconds (2σ) for 25 femtosecond optical pulses. Envelope time delays are generated by pathlength differences in an interferometer. Constant spectral phase shifts are demonstrated by diffracting pulses from a variable phase volume diffraction grating. Experimental requirements for phaseresolved spectroscopy are outlined. The theory of phaselocked pulse pair techniques is reexamined, and it is concluded that linear experiments with phaselocked pulse pairs are completely equivalent to Fourier transform absorption spectroscopy and do not measure the refractive index or real part of the susceptibility. It is shown that Fourier sine and cosine transformations of truncated time domain signals which do not match the symmetry of the complete signal can produce a false dispersive susceptibility because they are equivalent to Kramers–Kronig inversion of finite bandwidth absorption data. A procedure for shifting phaselocked transients by a quarter cycle of delay to generate a transient with a spectral phase shift is given. Equations used to calculate femtosecond nonlinear optical signals have assumed carrier wave delays. Modifications to these equations are required when envelope delays are generated by interferometer pathlength differences and modified equations are given. The modified equations yield significantly different results for phaseresolved or interferometric experiments. In particular, the modified equations are needed to calculate indirectly (interferometrically) detected frequencies and the real and imaginary parts of twodimensional Fourier transform spectra. The role of the refractive index and real part of the frequency domain susceptibility in nonlinear experiments with phaselocked pulse pairs is explored. It is concluded that experiments such as the heterodyne detected stimulated photon echo are insensitive to nonlinear refractive index changes under some circumstances. Finally, modifications of some equations used in the theory of coherent control are needed to match theory with experimental practice.

Fast multiquantum vibrational relaxation of highly vibrationally excited
View Description Hide DescriptionA significant fraction of the initial population of highly vibrationally excited oxygen molecules prepared by stimulated emission pumping, relaxes to much lower vibrational levels The time scale is much shorter than the known collisional lifetimes of the intervening vibrational levels and thus a sequential singlequantum relaxation mechanism can be explicitly ruled out. Statetostate measurements after preparation of and 30 provide the finalvibrational state population distribution resulting from relaxation of these two states. For at least 38%(7.9%) of the initially prepared population, undergoes multiquantum vibrational relaxation. The observed multiquantum relaxation explains, at least in part, the previously reported “dark channel” for relaxation of vibrational levels higher than but does not exclude the possibility of reactive scattering (forming ozone) for the remaining fraction of highly vibrationally excited molecules. We discuss possible explanations of this startling result including energy transfer,energy transfer, and complex formation.
