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  • 1990-1994  (22)
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  • 1
    Electronic Resource
    Electronic Resource
    Real-time dynamics of clusters. II. I2Xn (n=1; X=He, Ne, and H2), picosecond fragmentation (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 8037-8047 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this second paper (II) of a series, we report our picosecond time-resolved studies of the state-to-state rates of vibrational predissociation in iodine–rare gas (van der Waals) clusters. Particular focus is on the simplest system, I2He, which serves as a benchmark for theoretical modeling. Comparisons with I2Ne and I2H2 are also presented. The results from measurements made in real time are compared with those deduced from linewidth measurements, representing a rare example of a system studied by both methods under identical conditions and excited to the same quantum (v'i) states. The discrepancies are discussed in relation to the origin of the broadening and preparation of the state. The rates as a function of v'i display a nonlinear behavior which is examined in relation to the energy-gap law. The measured absolute rates and their dependence on v'i are compared with numerous calculations invoking classical, quantum, and semiclassical theories. In the following paper (III in this series), the cluster size of the same system, I2Xn, is increased (n=2–4) and the dynamics are studied.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463426
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Femtosecond real-time probing of reactions. IX. Hydrogen-atom transfer (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 9046-9061 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The real-time dynamics of hydrogen-atom-transfer processes under collisionless conditions are studied using femtosecond depletion techniques. The experiments focus on the methyl salicylate system, which exhibits ultrafast hydrogen motion between two oxygen atoms due to molecular tautomerization, loosely referred to as intramolecular "proton'' transfer. To test for tunneling and mass effects on the excited potential surface, we also studied deuterium and methyl-group substitutions. We observe that the motion of the hydrogen, under collisionless conditions, takes place within 60 fs. At longer times, on the picosecond time scale, the hydrogen-transferred form decays with a threshold of 15.5 kJ/mol; this decay behavior was observed up to a total vibrational energy of ∼7200 cm−1. The observed dynamics provide the global nature of the motion, which takes into account bonding before and after the motion, and the evolution of the wave packet from the initial nonequilibrium state to the transferred form along the O–H—O reaction coordinate. The vibrational periods (2π/ω) of the relevant modes range from 13 fs (the OH stretch) to 190 fs (the low-frequency distortion) and the motion involves (in part) these coordinates. The intramolecular vibrational-energy redistribution dynamics at longer times are important to the hydrogen-bond dissociation and to the nonradiative decay of the hydrogen-transferred form.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463331
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  • 3
    Electronic Resource
    Electronic Resource
    Real-time dynamics of clusters. III. I2Nen (n=2–4), picosecond fragmentation, and evaporation (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 8048-8059 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper (III) we report real-time studies of the picosecond dynamics of iodine in Ne clusters I*2Nen(n = 2–4) → I*2 + nNe. The results are discussed in relation to vibrational predissociation (VP), basic to the I2X systems, and to the onset of intramolecular vibrational-energy redistribution (IVR). The latter process, which is a precursor for the evaporation of the host atoms or for further fragmentation, is found to be increasingly effective as the cluster size increases; low-energy van der Waals modes act as the accepting (bath) modes. The reaction dynamics for I2Ne2 are examined and quantitatively compared to a simple model which describes the dynamics as consecutive bond breaking. On this basis, it is concluded that the onset of energy redistribution is observed in I2Ne2. Comparison of I2Ne and I2Ne2 to larger clusters (n=3,4) is accomplished by introducing an overall effective reaction rate. From measurements of the rates and their dependence on v'i, the initial quantum number of the I2 stretch, we are able to examine the dynamics of direct fragmentation and evaporation, and compare with theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463427
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  • 4
    Electronic Resource
    Electronic Resource
    Femtosecond probing of bimolecular reactions: The collision complex (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7763-7766 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461349
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  • 5
    Electronic Resource
    Electronic Resource
    Femtosecond vibrational transition-state dynamics in a chemical reaction (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 8801-8804 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the femtosecond dynamics of the vibrational motion in the transition-state evolution of an isomerization reaction. The observed nonstatistical and bound behavior reflects the localization in selective torsional and bending modes (of the 72 normal modes of the system). The multidimensionality of the potential energy surface is examined by comparing experiments with theoretical calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463350
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  • 6
    Electronic Resource
    Electronic Resource
    Real-time clocking of bimolecular reactions: Application to H+CO2 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 5239-5259 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An experimental methodology is described for the real-time clocking of elementary bimolecular reactions, i.e., timing the process of formation and decay of the collision complex. The method takes advantage of the propinquity of the potential reagents in a binary van der Waals (vdW) "precursor'' molecule. An ultrashort pump laser pulse initiates the reaction, establishing the zero-of-time (e.g., by photodissociating one of the component molecules in the vdW precursor, liberating a "hot'' atom that attacks the nearby coreagent). A second ultrashort, suitably tuned, variably delayed probe laser pulse detects either the intermediate complex or the newly born product. From an analysis of this temporal data as a function of pump and probe wavelengths, the real-time dynamics of such a "van der Waals-impacted bimolecular (VIB)'' reaction can be determined. Chosen as a demonstration example is the VIB reaction H+CO2→HOCO
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458531
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  • 7
    Electronic Resource
    Electronic Resource
    Real-time probing of reactions in clusters (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 805-807 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458384
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  • 8
    Electronic Resource
    Electronic Resource
    Femtosecond real-time probing of reactions. VIII. The bimolecular reaction Br+I2 (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 4127-4148 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper, we discuss the experimental technique for real-time measurement of the lifetimes of the collision complex of bimolecular reactions. An application to the atom–molecule Br+I2 reaction at two collision energies is made. Building on our earlier Communication [J. Chem. Phys. 95, 7763 (1991)], we report on the observed transients and lifetimes for the collision complex, the nature of the transition state, and the dynamics near threshold. Classical trajectory calculations provide a framework for deriving the global nature of the reactive potential energy surface, and for discussing the real-time, scattering, and asymptotic (product-state distribution) aspects of the dynamics. These experimental and theoretical results are compared with the extensive array of kinetic, crossed beam, and theoretical studies found in the literature for halogen radical–halogen molecule exchange reactions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463917
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  • 9
    Electronic Resource
    Electronic Resource
    Real-time dynamics of clusters. I. I2Xn (n=1) (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 198-212 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational predissociation of I2Xn (X=Ne, Ar) van der Waals clusters are studied in real-time using picosecond pump–probe (LIF) and molecular beam techniques. The state-to-state rates of vibrational predissociation are measured for specific vibrational levels v'i by monitoring the rise of nascent I2. Here, we report our study of I2X(B,v'i)k(v i,v'f) →I2(B,v'f)+X. For I2Ne, the values of τ=k−1(vi,v'f) decrease nonlinearly from 216 ps for vi=13 to 53 ps for v'i=23. For I2Ar (B,vi), which undergo electronic and vibrational predissociation, the risetime of the nascent I2 is found to be 70 ps when v'i=18 and 77 ps when vi=21. A number of theoretical models for vibrational predissociation (the energy-gap law, the momentum-gap law, quantum and classical calculations) are compared with our experimental results in an attempt to understand the key features of the dynamics and the potential energy surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462499
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  • 10
    Electronic Resource
    Electronic Resource
    Direct observation of the picosecond dynamics of I2–Ar fragmentation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 9180-9184 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Picosecond real-time observations of the dynamics of I2–Ar fragmentation are reported. The state-to-state rates, k(νi';νf'), are directly measured and related to the homogeneous broadening of the initial state, and to product state distributions in the exit channel. Comparisons with different theories of vibrational (and electronic) predissociation are made.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459207
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