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  • 2000-2004  (12)
  • 1990-1994  (48)
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  • 1
    Electronic Resource
    Electronic Resource
    Solvent interaction with the (2p3s) Rydberg state of hexamethylenetetramine: Energetics and relaxation dynamics (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 118-125 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The (1+1) mass resolved excitation spectra are reported for the (2p3s)←(2p)2 Rydberg transition of the tricyclic, high symmetry molecule hexamethylenetetramine [HMT (C6H12N4)] and its van der Waals clusters. The solvent molecules employed include both nonpolar (Ar, CH4) and polar (NH3, CH3OCH3) species. HMT and its clusters are generated and cooled in a supersonic expansion. The observed electronic transition is assigned as T2←A1 within the Td print group. A transition blue shift of 52 cm−1 for each Ar atom and 65 cm−1 for each methane molecule in the HMT van der Waals cluster is characterized. These shifts are caused by an excited state repulsive interaction between the excited Rydberg electron and the closed shell solvent which reduces the attractive dispersion interaction between the HMT and nonpolar solvent species in the van der Waals cluster. A transition red shift of more than 600 cm−1 for NH3 and CH3OCH3 solvent/HMT clusters is observed; this large increase in interaction energy for the excited Rydberg state of HMT with respect to the ground state of HMT is associated with the delocalization of the excited electron into available (virtual) Rydberg orbitals of the solvent molecules. The interaction is characterized as an electron transfer interaction. These results and assignments are consistent with previously reported ones for DABCO/solvent clusters. Relaxation dynamics of excited HMT and its clusters are investigated via a pump/probe (ionization) technique. Relaxation of the clusters is dominated by an intersystem crossing mechanism resulting in an excited state singlet lifetime of ∼5 ns compared to a bare molecule HMT excited state lifetime of ∼1.0 μs. A triplet state of HMT lies 255 cm−1 below the first excited singlet Rydberg state as determined by two-color threshold ionization studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468494
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  • 2
    Electronic Resource
    Electronic Resource
    (σ3s) Rydberg states of cyclohexane, bicyclo[2.2.2]octane, and adamantane (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 8625-8632 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this effort the effect of Rydberg electronic excitation on the structure of cyclic and polycyclic alkanes is investigated. Two-photon resonant, one-photon ionization, mass-resolved excitation spectroscopy is employed to observe the (σ3s)←(σ)2 Rydberg transitions of cyclohexane, bicyclo[2.2.2]octane, and adamantane cooled in a supersonic jet expansion. Rydberg spectra of these three molecules display sharp, well-resolved vibronic structure. Analysis of the spectra is assisted by isotopic substitution, circular/linear polarization, vibronic feature widths (rotational selection rules), as well as comparison to the ground-state vibrational energies. A significant reduction of vibrational energies in the excited electronic state and a 381 cm−1 blue shift of the transition origin upon deuterium isotope substitution for cyclohexane are interpreted as due to the promotion of an electron from a σ-bonding orbital to a nonbonding Rydberg orbital upon optical excitation. Extensive vibronic coupling is observed for both cyclohexane and adamantane in their excited (σ3s) Rydberg electronic states. Jahn–Teller splitting is small for adamantane but quite substantial for cyclohexane. This difference is attributed to the basic stability difference for the two different ring systems (mono- and tri-cyclic). A progression in a nontotally symmetric mode is observed in the Rydberg spectrum of bicyclo[2.2.2]octane suggesting a change in the geometry of this molecule upon (σ3s)←(σ)2 excitation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466716
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  • 3
    Electronic Resource
    Electronic Resource
    Investigation of the structure, stability, and ionization dynamics of zirconium oxide clusters (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 8971-8989 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The growth dynamics, stabilities, and structures of small zirconium oxide clusters (ZrnOm) are studied by covariance mapping time-of-flight mass spectrometry and density functional theory calculations. The zirconium oxide clusters are produced by laser ablation of zirconium metal into a helium gas flow seeded with up to 7% O2. The neutral (ZrnOm) cluster distribution is examined at high and low ionization laser intensities. At high ionization laser intensities (∼107 W/cm2) the observed mass spectra consist entirely of fragmented, nonstoichiometric clusters of the type [(ZrO2)n−1ZrO]+, while in case of lower laser intensities (∼0.2×107 W/cm2), cluster fragmentation is strongly reduced and predominantly stoichiometric clusters (ZrO2)n+ appear. Under such gentle conditions, (ZrO2)5+ is found to be much more abundant than its neighboring clusters (ZrO2)n+, n=1,2,4,6,7,8. The unusually high signal intensity of the Zr5O10+ ion is found to be due to the high stability of the (ZrO2)5 neutral cluster. Density functional theory calculations show a number of different conceivable isomer structures for this cluster and reveal the most likely growth pattern that involves the sequential uptake of ZrO2 units by a (ZrO2)4 cluster to yield (ZrO2)5 and (ZrO2)6. Based on a series of different density functional theory and Hartree–Fock theory calculations, and on kinetic modeling of the experimental results, isomer structures, growth mechanisms, and stability patterns for the neutral cluster distribution can be suggested. The (ZrO2)5 structure most stable at temperatures less than 3000 K is essentially a tetragonal pyramid with five zirconium atoms at the vertices, whereas an octahedral structure is the main building block of (ZrO2)6. Modeling of the covariance matrix over a wide range of ionization laser intensities suggests that (ZrO2)n neutral clusters absorb two photons of 193 nm radiation to ionize and then, for high laser intensity, the ion absorbs more photons to fragment. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1359177
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  • 4
    Electronic Resource
    Electronic Resource
    Solvation effects on reactive intermediates: The benzyl radical and its clusters with Ar, N2, CH4, C2H6, and C3H8 (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 4339-4354 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Mass resolved excitation spectra are presented for the benzyl radical and its clusters with Ar, N2, CH4, C2H6, and C3H8. The cluster spectra exhibit small redshifts (≤50 cm−1) relative to the unclustered benzyl radical for the D1(1 2A2)←D0(1 2B2) and D2(2 2B2)←D0(1 2B2) electronic transition regions. A unique set of low energy van der Waals modes is observed for these clusters for each excited electronic state investigated. The cluster spectra also reveal significant vibronic coupling between the two excited electronic states of the benzyl radical, as evidenced by a single vibrational predissociation threshold for each cluster. Ab initio calculations are performed on the benzyl radical to examine excited electronic state structure, predict transition energies, estimate ionization energy, and determine partial atomic charges in the electronic states of interest. The resulting partial charges are used in empirical atom–atom potential energy calculations to aid in the understanding of cluster spectroscopic shifts, binding energies, and van der Waals modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464996
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  • 5
    Electronic Resource
    Electronic Resource
    The effect of nonpolar solvents on Rydberg states: van der Waals complexes of azabicyclooctanes (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 6769-6778 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of solvation by nonpolar solvents on the (n,3s) Rydberg states of 1,4-diazabicyclo[2.2.2]octane (DABCO) and azabicyclo[2.2.2]octane (ABCO) is investigated through mass resolved excitation spectroscopy of their van der Waals complexes. The solute/solvent clusters formed in a supersonic expansion include DABCO and ABCO with Ar, n-CmH2m+2 (m=1–7), and CF4 and C2F6. The resulting spectra are analyzed with the help of empirical potential energy calculations of the cluster binding energies, minimum energy structures, van der Waals modes, and potential barriers between the various cluster minimum energy structures. Good agreement is found between the calculated and experimental results for DABCO and ABCO clustered with argon and methane. The spectra of clusters with all other hydrocarbons can be ascribed to only one major geometry for each cluster stoichiometry, despite the fact that calculations yield many stable geometries for each cluster. This apparent lack of agreement between calculations and experiments can be rationalized based on cluster binding energy, zero point energy, and the potential energy barriers between the cluster minima. The observed blue shift of the cluster 000 transition energy as a function of the n-alkane chain length can be qualitatively modeled by a Lennard-Jones potential for the solute–solvent interaction for both the ground and excited states. The model reveals a strong repulsive interaction between the Rydberg state electronic distribution and the solvent molecule. This repulsion depends on the distance between the solvent molecule and the solute molecule nitrogen atom.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464769
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  • 6
    Electronic Resource
    Electronic Resource
    (n,3s) Rydberg spectra of diethyl ether, diisopropyl ether, and methyl vinyl ether: Analysis of the torsional motion (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 3703-3712 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two-photon resonance mass resolved excitation spectra are obtained for diethyl ether-h10, diethyl ether-d10, diisopropyl either, and methyl vinyl ether cooled in a supersonic jet expansion. The spectra are assigned as due to 2p3s←(2p)2 Rydberg excitations: significant progressions in a low energy vibrational mode are observed for both diethyl and diisopropyl ethers, but not for methyl vinyl ether. The transition energies for the vibronic progressions are modeled by a double well potential function of polynomial form truncated at the quartic term. The transition intensities for the progressions are calculated based on a Franck–Condon analysis of the excited state potential surface and a harmonic ground state surface. Calculations identify the progression forming mode as the antigeared torsion of the C–O–C–C dihedral angles. Excitation of diethyl ether from its ground electronic state to its 2p3s Rydberg state leads to changes in the dihedral angles (τ1, τ2) from ±180° to ±157°. Similar electronic excitation of diisopropyl ether yields changes in (τ1,τ2) from ±157° in the ground state to ±146° in the Rydberg state. The torsional displacement active in the 2p3s←(2p)2 oxygen Rydberg transition is suggested to arise from the interaction between the diffuse 3s electron and the β-methyl groups on both diethyl and diisopropyl ethers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464047
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  • 7
    Electronic Resource
    Electronic Resource
    Solvation effects on the molecular 3s Rydberg state: AZAB/CYCLO octanes clustered with argon (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 1876-1887 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two color, 1+1, mass resolved excitation spectroscopy (MRES) is used to obtain molecular Rydberg (3s←n) spectra of azabicyclo[2.2.2]octane (ABCO) and diazabicyclo[2.2.2]octane (DABCO) clustered with argon. Nozzle/laser timing delay studies are employed together with time-of-flight mass spectroscopy to identify cluster composition. Population depletion techniques are used to differentiate between clusters with the same mass, but different geometries. A Lennard-Jones 6–12 potential is used to model the intermolecular interactions and predict minimum energy cluster geometries and cluster binding energies. The experimental results are combined with the cluster geometry calculations to assign spectral features to specific cluster geometries. Three different excited state interactions are required to model the experimentally observed line shapes, spectral shifts, and cluster dissociation. The relationship between these model potentials and the cluster binding sites suggests that the form of the cluster intermolecular potential in the Rydberg excited state is dictated by the distance between the argon and chromophore atoms. A comparison of results for ABCO(Ar)1 and DABCO(Ar)1 leads to the conclusion that the nitrogen 3s Rydberg orbital in clusters of DABCO is delocalized.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464222
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  • 8
    Electronic Resource
    Electronic Resource
    Potential energy surfaces of substituted anilines: Conformational energies, deuteration effects, internal rotation, and torsional motion (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 7889-7901 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Mass resolved excitation spectra (MRES) are presented for a series of substituted anilines including 2- and 3-methylaniline, 2- and 3-ethylaniline, 2-aminobenzyl amine, and 2-aminobenzyl alcohol. The observed spectra show the following phenomena: nearly free internal rotation of the methyl substituent in the S1 state; long vibrational progressions attributed to C–C, C–N, and C–O side chain torsional motion; an inequivalence of the two amino hydrogens for both ring and side chain amino groups as determined from the spectra of deuterated species; and the existence of two conformers for 2-aminobenzyl alcohol. Semiempirical and ab initio calculations are performed on these systems to aid in the analysis of the potential energy surfaces and in the interpretation of the experimental results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463464
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  • 9
    Electronic Resource
    Electronic Resource
    Minimum energy conformation of ortho-xylene in its ground and first excited electronic states (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 8130-8136 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The geometry of ortho-xylene is studied through supersonic jet cooling and one color mass resolved excitation spectroscopy. By examining the S1←S0 transition origin region of various (d0–d2) methyl deuterated o-xylene species, the conformation of the two methyl groups in S0 and S1 can be determined. A comparison between the predicted and experimentally observed number and intensity of origin features for the various partially deuterated o-xylenes shows that two highly symmetric methyl group structures are possible, each structure having C2V point group symmetry—the antiplanar conformation (A1), in which τ1(C2–C1–Cα–Hα)=180°; τ2(C1–C2–Cα'–Hα')=180°; and the syn, planar conformation (A2) in which τ1=τ2=0°. The experimentally determined structures are consistent with ab initio calculations and microwave studies which predict the anticonformation to be most stable. Spectroscopic data, in conjunction with further ab initio calculations, are used to investigate the ground and excited state potential energy surfaces. In addition, a repulsive, intramolecular, nonbonded interaction between the two methyl groups is identified through an empirical potential energy calculation to be the most important interaction defining the lowest energy structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463434
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  • 10
    Electronic Resource
    Electronic Resource
    The effect of solvation on molecular Rydberg states: Dioxane clustered with nonpolar solvents (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 2869-2880 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: One color 2+1 mass resolved excitation spectroscopy is employed to obtain molecular Rydberg 3s←n transition spectra of 1,4-dioxane clustered in a molecular beam with nine nonpolar solvents. The solvents are Ar, Kr, CH4, CD4, CF4, SiH4, Si(CH3)4, ethane, n-propane, cyclohexane-h12, and cyclohexane-d12. Spectral results are interpreted in terms of cluster size, isotope effects, and model calculations. A Lennard-Jones–Coulomb 6-12-1 potential is used to model the intermolecular interactions and predict minimum energy cluster geometries, cluster binding energies, and intermolecular force constants which are used to calculate van der Waals vibrational frequencies. The results show that for simple solvents (i.e., Ar, CH4) the calculations offer a simple interpretation of the observed spectra in terms of multiple cluster geometries with distinct transition energies; however, as the solvent becomes more complex, the cluster spectra also become more complex, and the number of calculated minimum energy cluster geometries increases. Complex spectra are interpreted as a distribution of cluster geometries with similar transition energies. For all of the clusters, the electronic origins are blue shifted with respect to the bare 1,4-dioxane origin. This observation is consistent with a model in which the excited state intermolecular potential becomes more repulsive due to the increased radial distribution of a nonbonding electron upon excitation into the 3s Rydberg state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463029
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