Index of content:
Volume 103, Issue 2, 08 July 1995

Excited ν_{3} vibrational state of the Ar–HCN and Kr–HCN dimers
View Description Hide DescriptionRotational spectra have been observed for an excited vibrational state of the Ar/Kr–HCN dimers in a Balle–Flygare Fourier transformmicrowave spectrometer with a high temperature nozzle that can be heated to ∼1300 K. The B, D _{ J }, and H _{ J }rotational constants found for the excited parent Ar–HCN isotopic species are 1602.475(3) MHz, 164.2(6) kHz, and 309(5) Hz, and for Kr–HCN they are 1176.4493(3) MHz, 41.7(1) kHz, and 56(1) Hz. The ^{14}N quadrupole interaction χ_{ a }(J=1) observed in the excited state is −2.824 (−3.239) MHz for Ar(Kr)–HCN, slightly smaller than the −2.844 (−3.268) MHz in the ground state. A substitution analysis based on the C and N isotopic species reveals that the C–N distance r _{ s } (CN) increases by ∼0.013 (0.020 Å) on excitation of Ar(Kr)–HCN. A similar analysis for free DC^{15}N gives an increase in r _{ s }(CN) upon ν_{3} excitation of 0.0022 Å. Assignment of the excited state in the dimer as the ν_{3}C–N stretch of the HCN is proposed and discussed.

Electron spin resonance and theoretical studies of the PO_{2} and AsO_{2} radicals in neon matrices at 4 K: Laser vaporization and x‐irradiation radical generation techniques
View Description Hide DescriptionThe utilization of high energy generation techniques for trapping ion radicals and other reactive intermediates in neon matrices at 4 K is discussed. Electron spin resonance(ESR) results for several small radicals are presented to demonstrate the application of x irradiation and reactive laser vaporization for neon matrix isolation studies. Detailed ESR studies of the PO_{2} and AsO_{2} radicals, along with ab initiotheoretical computations of their nuclear hyperfine interactions, are presented. No previous ESR observations have been reported for these radicals, although PO_{2} has been studied under high resolution in the gas phase utilizing far‐infrared laser magnetic resonance and microwave spectroscopies.

Density dependence of the ionization potential of CH_{3}I in argon and of the quasi‐free electron energy in argon
View Description Hide DescriptionField ionization has been employed as a new technique to determine the ionization potential of an impurity doped in a nonpolar fluid. This has been shown for a model system, CH_{3}I doped into argon for argon densities ranging from the gas up to the triple point liquid. The ionization potential in the medium reflects the different possible configurations of the medium atoms around the dopant at the moment of excitation and, therefore, is given by a distribution. This distribution is identified with that of the polarization energy between the positive ion and the medium. The first and second moments of the polarization distribution were calculated at the densities at which the experiments were performed. Simulated spectra (generated using the experimental results obtained in pure CH_{3}I, the calculated moments, and a Gaussian shape for the polarization distribution) reproduce closely the experimental ionization potential. Furthermore, by combining the experimental data with the calculated distributions, we obtained the density dependence of the quasi‐free electron energy in argon. These results are consistent with previous experimental data and with recent theoretical calculations.

Infrared spectroscopy of the siliconium ion, SiH^{+} _{5}
View Description Hide DescriptionThe infrared spectrum for the H–H stretching mode of the siliconium ion SiH^{+} _{5} in the frequency range of 3650–3740 cm^{−1} is presented. The observed vibration–rotation transitions were fitted with the A‐type rotational transitions of an asymmetric top using the Watson S‐type asymmetric top rotational Hamiltonian. The results suggested that the siliconium ion SiH^{+} _{5} can be described as a complex between SiH^{+} _{3} and a freely internally rotating H_{2} groups, with a highly localized three‐center two‐electron bond.

Infrared spectroscopy of the molecular hydrogen solvated carbonium ions, CH^{+} _{5}(H_{2})_{ n } (n=1–6)
View Description Hide DescriptionThe infrared spectra for the molecular hydrogen‐solvated carbonium ions, CH^{+} _{5}(H_{2})_{ n } (n=1–6) in the frequency range of 2700–4200 cm^{−1} are presented. Spectroscopic evidence was found in support of the scrambling of CH^{+} _{5} through the large amplitude motions such as the CH_{3} internal rotation and the in‐plane wagging motion of three‐center two‐electron bond. More importantly, the scrambling motions of CH^{+} _{5} cores were slowed down considerably by attaching the solvent H_{2} molecules to the core ion. The complete freezing of the scrambling motions was found when the first three H_{2} molecules were bound to the CH^{+} _{5} core. A good agreement between the experimental and the theoretical predictions was found in the dynamics of CH^{+} _{5}.

Resonant ion‐dip infrared spectroscopy of benzene–H_{2}O and benzene–HOD
View Description Hide DescriptionResonant ion‐dip infrared spectra of C_{6}H_{6}–H_{2}O and C_{6}H_{6}–HOD have been recorded in the OH stretch fundamental region. The spectra provide further evidence for the unique, large‐amplitude motions present in these π hydrogen‐bonded complexes. In C_{6}H_{6}–H_{2}O, transitions out of the lowest ortho (Π) and para (Σ) ground state levels are observed. A transition at 3634 cm^{−1} is assigned as an unresolved pair of parallel transitions (Σ→Σ and Π→Π) involving the symmetric stretch fundamental (at 3657 cm^{−1} in free H_{2}O). In the antisymmetric stretch region, transitions at 3713, 3748, and 3774 cm^{−1} are assigned as Π→Σ, Σ→Π, and Π→Δ transitions, respectively. The spacing of the transitions is consistent with nearly free internal rotation of H_{2}O about benzene’s sixfold axis in both ground and vibrationally excited states. The intensities of combination bands depends critically on the mixing of some local mode character into the symmetric and antisymmetric stretches at asymmetric positions of H_{2}O on benzene. Surprisingly, in C_{6}H_{6}–HOD, five transitions are observed in the OH stretch region, all arising from the ground state zero point level. Even more unusual, the higher‐energy combination bands are many times stronger than the OH stretch fundamental. The local mode OH stretch has components both parallel and perpendicular to benzene’s sixfold axis, leading to strong parallel and perpendicular transitions in the spectrum. A two‐dimensional model involving free internal rotation and torsion of HOD in its plane is used to account for the qualitative appearance of the spectrum. The form of the OH(v=0) and OH(v=1) torsional potentials which reproduce the qualitative features of the spectrum are slightly asymmetric, double‐minimum potentials with large‐amplitude excursions for HOD over nearly 180°.

Kinetics of crystallizing D_{2}O water near 150 K by Fourier transform infrared spectroscopy and a comparison with the corresponding calorimetric studies on H_{2}O water
View Description Hide DescriptionCrystallization kinetics of hyperquenched D_{2}O liquid has been studied by Fourier transform infrared spectroscopy by determining the irreversible change in the spectra of stretching vibrations of the decoupled OH oscillator in ≊12 mol % HOD in D_{2}O. The kinetics follows an almost exponential behavior with time. The exponent to the crystallization time, n, increases from its value of 1.0 at 152 K to 1.2 at 147 K and its value is slightly lower than that of H_{2}O. The combined results of H_{2}O and D_{2}O indicate that temperature rather than the mass of the molecules determines n. Interpretation of n in terms of the morphology of grains formed seems inconsistent in view of the observed temperature effect. It is suggested that an alternative interpretation of n in terms of distribution of diffusion times, or time independent activation energy (barrier height to crystallization), is more appropriate. Calorimetric measurements for the crystallization of H_{2}O yield results consistent with those from infrared spectroscopic study.

Magnetic resonance and spin dynamics in radical ion pairs: Pulsed time‐resolved fluorescence detected magnetic resonance
View Description Hide DescriptionPulsed time‐resolved fluorescence detected magnetic resonance (FDMR) was applied to examine spin dynamics in radical ion pairs. Two time‐domain FDMR phenomena were examined; microwave‐induced quantum beats and the formation of the ‘‘echo’’ signal. These experiments demonstrate the fast decay of the first‐order coherence generated by a microwave field on the time scale of 0.05–1 μs. This decay originates through the phase relaxation in the triplet manifold of the pairs and is probably caused by electron dipole–dipole interaction of radical ions in spurs.

Symmetry breaking and localization in resonant photon emission
View Description Hide DescriptionThe photon emission following the excitation to an intermediate decaying state which interacts with other electronic states is discussed in some detail. Particular attention is paid to resonant x‐ray emission in symmetric polyatomic systems. The decaying core‐excited state always possesses partner states to which it couples through the nontotally symmetric nuclear motion. The resulting dynamical symmetry breaking induces localization of the core holes and may have considerable impact on the intensity and selection rules. The effect of vibronic coupling, lifetime of the decaying state and of temperature are analyzed and interpreted.

Overtone Raman spectrum and molecular polarizability surface of CO_{2}
View Description Hide DescriptionThe Q branches of the Raman bands associated to the 2ν_{1}:4ν^{0} _{2}:ν_{1}+2ν^{0} _{2} Fermi resonance of ^{12}C ^{16}O_{2} have been observed in the gas phase at 2543, 2671, and 2797 cm^{−1} and, in addition, at 2514 cm^{−1}, one hot band from their first excited vibrational state is seen. From the analysis of their cross sections, jointly with those of the main Fermi diad, ν_{1}:2ν^{0} _{2} and its hot bands, an improved description of the molecular polarizabilitysurface has been achieved. The following derivatives of the mean polarizability have been obtained: ∂ᾱ/∂q _{1}=12.43×10^{−42} CV^{−1} m^{2}; ∂^{2}ᾱ/∂q ^{2} _{2σ}=2.81×10^{−42} CV^{−1} m^{2} (σ=a,b); ∂^{2}ᾱ/∂q ^{2} _{1}=0.45×10^{−42} CV^{−1} m^{2}; ∂^{2}ᾱ/∂q ^{2} _{3}=0.67 or 0.15×10^{−42} CV^{−1} m^{2}; ∂^{3}ᾱ/∂q _{1}∂q ^{2} _{2σ}=−0.06×10^{−42} CV^{−1} m^{2}, in terms of the dimensionless normal coordinates, and ∂ᾱ/∂S _{1}=3.15×10^{−30} CV^{−1} m; ∂^{2}ᾱ/∂S ^{2} _{2σ}=0.36×10^{−20} CV^{−1}; ∂^{2}ᾱ/∂S ^{2} _{1}=2.9×10^{−20} CV^{−1}; ∂^{2}ᾱ/∂S ^{2} _{3}=2.0 or 0.5×10^{−20} CV^{−1}; ∂^{3}ᾱ/∂S _{1}∂S ^{2} _{2σ}=−1.7×10^{−10} CV^{−1} m^{−1}, in terms of the symmetry coordinates. The thermal dependence of the mean polarizability is discussed in terms of these quantities.

Hyperfine constants of bromine and iodine monofluoride
View Description Hide DescriptionThe J′−J″=1−0 rotational transitions of ^{79}BrF, ^{81}BrF, and IF have been reinvestigated in the ground and first excited vibrational states using microwave Fourier transform spectroscopy. For IF, lines were also detected in the second excited vibrational state. Bromine and iodine nuclear quadrupole coupling constants, spin–rotation constants for all nuclei, and tensor and scalar spin–spin coupling constants (S and J), have been determined along with the rotational constants. Antishielding occurs at the fluorine nucleus for both BrF and IF, as has been found earlier for ClF. The antishielding has been rationalized following a model developed for ClF. The ratio 1.197 0514 (32) has been found for the equilibrium bromine quadrupole coupling constants of ^{79/81}BrF.

Structure and vibrations of phenol⋅CH_{3}OH (CD_{3}OD) in the electronic ground and excited state, revealed by spectral hole burning and dispersed fluorescence spectroscopy
View Description Hide DescriptionThe intermolecular vibrations of phenol(CH_{3}OH)_{1} and its deuterated isotopomer d‐phenol(CD_{3}OD)_{1} were examined by comparing the vibrational frequencies of the electronic ground and excited state with the results of ab initionormal mode calculations at the Hartree–Fock level, using the 4‐31G* and 6‐31G** basis sets. Full energy minimization showed a translinear structure similar to phenol(H_{2}O)_{1} or to the water dimer. Dispersed fluorescence spectra have been recorded via excitation of the electronic cluster origin and several intermolecular vibrational transitions. The Franck–Condon intensity pattern allowed an assignment of the ground state vibrational frequencies to the excited state frequencies, which were examined by resonance enhanced multiphoton ionization and hole burning spectroscopy. The existence of another conformer that possibly absorbs in the region of interest was ruled out by hole burning spectroscopy of the phenol(CH_{3}OH)_{1} cluster. A full assignment of all intermolecular vibrations of this hydrogen bonded cluster in the S _{0} state could be given for the first time on the basis of ab initio calculations and a combination of different spectroscopical methods.

Nonequilibrium photoinduced electron transfer
View Description Hide DescriptionWe consider photoinduced electron transfer, which is intrinsically a three‐state system consisting of electronic ground, electronic excited (electron donor), and electron acceptor states. It is assumed that the bath consists of a collection of harmonic oscillators. Using an elementary time‐dependent perturbation theory, it is found that the nonequilibrium Golden rule formula proposed by Coalson et al. [J. Chem. Phys. 101, 436 (1994)] can be rigorously obtained in a certain limit of our results. Invoking a stationary phase approximation, a simple result analogous to the Marcus expression is obtained, except for the presence of time‐dependent reorganization energy. The multidimensional nature of the solvation coordinate system is discussed further. Finally a few numerical calculations are presented.

Disposal of reactant vibrational excitation in adiabatically endothermic reactions. I. H+D_{2}(v″=1, j″=2)→HD(v′, j′)+D
View Description Hide DescriptionWe report absolute partial and total cross sections for the H+D_{2}(v″=1, j″=2)→HD(v′, j′)+D reaction at E _{rel}=1.3 eV. Addition of D_{2} reactant vibrational energy increases the total reactive cross section from 1.2 to 2.5 Å^{2}. That a similar amount of increased collision energy does not increase the cross section to such an extent distinguishes reactant vibrational energy from reactant translational energy. The average rotational energy for the HD product increases from 0.25 to 0.44 eV, but the effect is caused entirely by increased rotational energy in the v′=0 vibrational ground state. Reactant vibrational energy does not enhance the rotational energy for v′=1 and only modestly enhances HD vibrational energy. The average vibrational energy 〈E _{ v }〉 is 0.10 eV for the v″=0 reaction and 0.16 eV for the v″=1 reaction. These results contrast with those of the D+H_{2}(v″=j″=1)→HD(v′,j′)+H reaction at ∼1.4 eV, in which the vibrational energy of the HD product is three times as great for the v″=1 reaction as for the v″=0 reaction. This difference in reactions may be explained by the reactant H_{2} vibrational energy, as opposed to the reactant D_{2} vibrational energy, exceeding one quantum of vibration of the product HD. There is no specific or selective channeling of reactant vibration into product rotation in the present case, but reactive trajectories that allow channeling into v′=0, high j′ quantum states are enhanced upon the addition of D_{2} vibrational energy.

Kinetic energy dependence of the reactions of Ru^{+}, Rh^{+}, Pd^{+}, and Ag^{+} with O_{2}
View Description Hide DescriptionReactions of Ru^{+}, Rh^{+}, Pd^{+}, and Ag^{+} with molecular oxygen are studied as a function of kinetic energy by using guided ion beammass spectrometry. By using a flow tube ion source, it has been possible to create Ru^{+}, Rh^{+}, Pd^{+}, and Ag^{+} ions in their electronic ground state terms and primarily in the lowest spin–orbit levels. All reactions are observed to be endothermic. The reactivity of ground stateAg^{+} is found to be particularly inefficient and is believed to occur through an impulsive pairwise mechanism. Excited states of Ag^{+} are observed to react efficiently at thermal energies. Analyses of the endothermic reaction cross sections yield 0 K bond dissociation energies of D _{0}(Ru^{+}–O)=3.81±0.05 eV, D _{0}(Rh^{+}–O)=3.02±0.06 eV, D _{0}(Pd^{+}–O)=1.46±0.11 eV, and a speculative value of D _{0}(Ag^{+}–O)=1.23±0.05 eV. The reactivity differences among all four metal systems and the electronic states of Ag^{+} are explained by using simple molecular orbital concepts.

Trajectory simulations of collisional energy transfer in highly excited benzene and hexafluorobenzene
View Description Hide DescriptionQuasiclassical trajectory calculations of the energy transfer of highly vibrationally excited benzene and hexafluorobenzene (HFB) molecules colliding with helium, argon and xenon have been performed. Deactivation is found to be more efficient for HFB in accord with experiment. This effect is due to the greater number of low frequency vibrational modes in HFB. A correlation between the energy transfer parameters and the properties of the intramolecular potential is found. For benzene and HFB, average energies transferred per collision in the given energy range increase with energy. Besides weak collisions, more efficient ‘‘supercollisions’’ are also observed for all substrate–bath gas pairs. The histograms for vibrational energy transfer can be fitted by biexponential transition probabilities. Rotational energy transfer reveals similar trends for benzene and HFB. Cooling of rotationally hot ensembles is very efficient for both molecules. During the deactivation, the initially thermal rotational distribution heats up more strongly for argon or xenon as a collider, than for helium, leading to a quasi‐steady‐state in rotational energy after only a few collisions.

Finite‐difference approach to solving Heisenberg’s operator equations of motion: Application to one‐dimensional time dependent Hamiltonians
View Description Hide DescriptionWe reviewed and expanded on the finite difference approach introduced by Moncrief [Phys. Rev. D 28, 2485 (1983)] and Bender [Phys. Rev. Lett. 55, 901 (1985)] for solving Heisenberg’s operator equations of motion. In this approach, finite‐difference recurrence relations are used to evolve the matrix representation of the operators in time. The advantages and disadvantages of this approach for the study of quantum processes in real time are discussed. The approach performed very well as illustrated by examples of harmonic and Morse oscillatorsinteracting with continuous wave and pulsed laser fields.

Electronic structures of Pd_{4} and Pt_{4}
View Description Hide DescriptionComplete active space multiconfiguration self‐consistent field (CAS‐MCSCF) followed by multireference configuration interaction computations which included up to 4.1 million configurations and correlated all 40 electrons of Pd_{4} and Pt_{4} were made. Relativistic effective core potentials (RECPS) were employed for both Pt and Pd atoms. We found 44 electronic states for Pd_{4} within the 2.2 eV region and 51 electronic states for Pt_{4} within 1.2 eV. Two nearly‐degenerate electronic states with tetrahedral geometries were found as candidates for the ground states of Pd_{4} and Pt_{4} with ^{3} T _{1} and ^{1} A _{1} symmetries at the highest level of theory. The metal–metal bond lengths for Pd_{4} and Pt_{4} were found to be 2.686 and 2.602 Å for the ^{3} T _{1} state and 2.696 and 2.595 Å for the ^{1} A _{1} state, respectively. The atomization energies of Pd_{4} and Pt_{4} were computed as 5.63 and 11.8 eV, respectively, suggesting that Pt_{4} is considerably more bound compared to Pd_{4}. Relativistic effects are attributed to the enhanced stability of Pt_{4}. The Mulliken population analysis reveals enhanced Pt(6s) and reduced Pt(5d) populations for the electronic states of Pt_{4} while the electronic states of Pd_{4} exhibit the opposite trend.

Anharmonic contributions to the inversion vibration in 2‐aminopyrimidine
View Description Hide DescriptionThe out‐of‐plane vibrations of the amino group in 2‐aminopyrimidine have large amplitudes, and cannot be properly described within the harmonic approximation. The normal modeanalysis carried out at this level of approximation at the restricted Hartree–Fock level and at the second‐order Mo/ller–Plesset perturbation theory level failed to match the experimental transition frequency of ν≊200 cm^{−1} of the inversion vibration in this compound. In an effort to better understand this vibration motion, we went beyond the harmonic approximation. The inversion vibration was treated as being uncoupled from all other nuclear degrees of freedom. An internal coordinate (ω) was chosen whose displacement mimicked the out‐of‐plane distortion of the amino group during the inversion vibration. Electronic energy was calculated at the second‐order Mo/ller–Plesset perturbation theory level at selected values of ω to form a double‐well curve describing a model potential within which the nuclei move during the vibration. This potential was incorporated into a one‐dimensional Hamiltonian, and vibrational energy expectation values were variationally determined by utilizing the harmonic wavefunctions as the basis set. Two sets of calculations were performed: one in which the mirror plane of symmetry was preserved throughout the vibrational deformation limiting the internal coordinates to 17, and another in which the symmetry was unconstrained permitting description by 3N−6=30 internal coordinates. These calculations resulted in prediction of the v=0→v=1 transition energy of ν=130.1 cm^{−1} and ν=206.7 cm^{−1}, respectively, reasonably matching the experimental value of ≊200 cm^{−1}.

A general quantum chemical approach to study the locally perturbed periodic systems: A new development of the ab initio crystal elongation method
View Description Hide DescriptionWe have recently proposed the elongation method which is a novel molecular orbital method at the Hartree–Fock level to calculate the electronic structures of large periodic or aperiodic polymers efficiently. This method has the idea of the successive connection of any fragments to obtain the electronic properties of large molecules with any units. In this approach, the stationary conditions of the electronic states against the size extension have been formulated. Studies for molecular systems have suggested that the elongation technique with the stationary conditions may be applicable to periodic systems described by the crystal orbital. A one‐dimensional polymer, a two‐dimensional surface, and a three‐dimensional crystal with a local disordering part can be treated systematically by introducing the elongation technique into a large extended supercell model. In the present study, we develop a new quantum chemical approach for the study of locally perturbed periodic systems by the ab initio crystal orbital calculation. The description for the methodology of this approach is given in detail. Results of test applications to a perturbed two‐dimensional surface are shown. A local adsorption of carbon monoxide on (001) surface composed with magnesium oxide is examined as a sample model to confirm the accuracy of ab initio crystal elongation method. The utility of our method is clarified by an application to the perturbed surface.