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  • 1
    Electronic Resource
    Electronic Resource
    Rapidly fluctuating anisotropy parameter in the near-threshold photodissociation of NO2 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 3699-3709 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A novel experiment for near-threshold photodissociation studies is presented. State-selective excitation of the molecular photofragments to high-n Rydberg states is used in a variation of the ion imaging technique, allowing for undistorted detection of slow fragments produced close to the channel dissociation threshold. As a first demonstration of this method, the angular anisotropy parameter β for production of NO (J=17/2) and O 3P2 in the photodissociation of NO2 has been obtained as a function of excess energy. A classical model for β as a function of excess energy is presented, accounting for the decrease of anisotropy in the angular photofragment distribution upon approaching the channel threshold. The experimental values of β fluctuate substantially around the values predicted by the model, indicating strong underlying fluctuations in the state-to-state rate constant. This experiment offers in principle a unique route to measuring state-to-state reaction rate constants in situations where existing time- or frequency-resolved methods are inappropriate. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480522
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  • 2
    Electronic Resource
    Electronic Resource
    The Stark effect in the v+=1 autoionizing Rydberg states of NO (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 4922-4937 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Stark effect in autoionizing Rydberg states of NO, n=10–20,v+=1, is studied by a combination of experimental and theoretical methods. Double resonant excitation via selected intermediate rotation levels of the A 2Σ+, v′=1 state is carried out in the presence of fields 0–1000 V/cm. The spectra are simulated using both matrix diagonalization and full multichannel quantum defeat theory (MQDT) approaches, providing a test of quantum defect parameters and transition dipoles for Rydberg series from l=0 to l=4. The use of the same input parameters in these two types of calculation allows comparison of the validity and utility of these methods.© 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1450552
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  • 3
    Electronic Resource
    Electronic Resource
    Rotational autoionization dynamics in high Rydberg states of nitrogen (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 4509-4518 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The decay dynamics of the high Rydberg states of N2 converging on the first few rotational levels (N+=0,1,2,3) of the ground vibronic X 2Σ+g (v+=0) state of the N+2 cation have been investigated by delayed pulsed field ionization (PFI) following two-photon enhanced (2+1′) three-photon excitation via the a″ 1Σ+g (v′=0) state of N2. The experiments were carried out in the presence of a weak homogeneous dc electric field and at typical ion densities of 200–2000 ions/mm3. All Rydberg states in the range of principal quantum number n=140–200 exhibit extreme stability against autoionization and predissociation and some have lifetimes which exceed 30 μs. The decay of the highest Rydberg states beyond n=200 is induced by external perturbations (field ionization and collisional ionization) and no Rydberg states beyond n=350 can be observed by delayed PFI. The Rydberg states which converge on the N+=0 and 1 rotational levels of the ion, and which therefore are not subject to rotational autoionization, decay into neutral products (by a process presumed to be predissociation) in less than 7 μs in the range n〈100. The importance of predissociation is greatly reduced beyond n=100 and becomes negligible on our experimental timescale (30 μs) above n=140. The decay of the Rydberg states converging on the N+=2 and 3 rotational levels of the ion is more complex. Below n=100, only 30%–40% of the Rydberg population decays by fast rotational autoionization whereas 60%–70% decays by predissociation.The importance of predissociation decreases rapidly above n=100 and becomes negligible beyond n=140. The decay by rotational autoionization can be observed at all n values but becomes noticeably slower beyond n=100. In the range n=140–200 it exhibits a marked biexponential decaying behavior with 30% of the population decaying within a few microseconds and 70% displaying long term stability (τ(approximately-greater-than)30 μs). The branching between predissociation and autoionization is explained by the effect of the dc electric field which mixes strongly the optically accessible p Rydberg series with the high l manifold beyond n=100. The long lifetimes observed experimentally indicate that ml mixing becomes important as soon as l mixing sets in. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470639
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  • 4
    Electronic Resource
    Electronic Resource
    New experimental method for studying rotationally state-selected ion-molecule reactions (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 10609-10617 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new apparatus is described in which a beam of molecular ions in a selected vibration–rotation state is prepared by field ionization of high Rydberg states, in an adaptation of the zero-kinetic-energy photoelectron technique. The state-selected ions undergo low energy reactive collisions within a molecular beam and the ionic products are detected in a quadrupole mass filter. The Rydberg states are populated by two-color stepwise multiphoton excitation, and by tuning to the pseudocontinuum of high-Rydberg states associated with different vibration–rotation states of the ion core, different states of the ion are selected and the effect on reactivity determined. Some preliminary results for the H2+H+2→H+3+H reaction are reported. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467875
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  • 5
    Electronic Resource
    Electronic Resource
    On the efficient generation of Arn–SH free-radical clusters via ultraviolet photolysis of closed-shell Arn–H2S complexes (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 5149-5158 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Detailed investigations are described for the generation of open-shell radical complexes formed by unimolecular photolysis of closed-shell van der Waals precursors. As a specific test case, ultraviolet photolysis of slit-jet cooled Arn–H2S (n≤2) complexes at both 248 and 193 nm are shown to yield Ar–SH and Ar2–SH radical cluster species with surprisingly high efficiencies. Analysis of the laser induced fluorescence (LIF) spectra indicates that the radical complexes are produced with extensive van der Waals stretch/bend and overall rotational excitation, which is consistent with a simple ballistic model of the dissociation dynamics. The LIF spectra obtained as a function of expansion distance downstream provide clear evidence for remarkably efficient cooling of the newly formed radical cluster species by low-energy collisions with jet-cooled inert gas atoms at (approximate)10 K. Spectrally resolved Ar–SH and Ar2–SH LIF signals have been investigated as a function of Ar composition, which yields information on relative branching ratios for fragmentary (e.g., Ar2–H2S→Ar–SH+H+Ar) and nonfragmentary (e.g., Ar2–H2S→Ar2–SH+H) photolysis events. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478410
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  • 6
    Electronic Resource
    Electronic Resource
    "Gentle recoil'' synthesis of free-radical clusters via unimolecular photolysis of closed shell complexes (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 11360-11363 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe a novel mechanism for efficient generation of open shell free-radical complexes via UV photolysis of the corresponding closed shell precursors. Ar–SH and Ar2–SH are detected following 248 nm excitation of Ar–H2S and Ar2–H2S. Of particular dynamical interest are the nonfragmentary channels in which no Ar atoms are ejected. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472985
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  • 7
    Electronic Resource
    Electronic Resource
    Zero-kinetic-energy photoelectron spectrum of carbon dioxide (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 8430-8439 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The zero-kinetic-energy (ZEKE) photoelectron spectrum of carbon dioxide has been measured between 111 000 and 112 000 cm−1 at a resolution of 1.5 cm−1 using a coherent source of XUV radiation based on four-wave mixing in krypton. The spectrum consists of six bands corresponding to transitions from the ground X 1Σ+g(v1,v2,v3=000) state of the neutral to the two spin–orbit components of the (000) vibrational level and the four Renner–Teller states associated with the (010) vibrational level of the ground electronic state (X 2Πg) of the ion. The analysis of the partially resolved rotational structure of the various bands leads to a detailed picture of the photoionization process. The propensity rules for angular momentum transfer during photoionization are strongly dependent on the symmetry (2Πg,3/2, 2Πg,1/2, 2Δu,5/2, 2Δu,3/2, 2Σ+u, and 2Σ−u) of the different ionic states probed and on the Hund's coupling case they follow [case (a) for the Π and Δ states and case (b) for the Σ states]. A comparison of the experimental ZEKE line intensities with theoretical predictions and conventional photoelectron spectra reveals a series of anomalies which are discussed in terms of final state interactions. The ionization potential of CO2 is estimated to be 111 111.0±3 cm−1, somewhat lower than the value of 111 121±2 cm−1 determined from extrapolation of the Rydberg series by Cossart-Magos et al. [Mol. Phys. 61, 1077 (1987)].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466212
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  • 8
    Electronic Resource
    Electronic Resource
    Preparation of ions in selected rotational states by delayed pulsed field ionization (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 4213-4214 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A novel method of preparing ions in selected rovibronic states is presented. It is based on pulsed field ionization of long-lived high-n Rydberg states lying just below the different rovibronic energy levels of the ion. The potential of the method is illustrated with the example of hydrogen: A clean population of H2+ ions is prepared in the X 2∑g+ (v+=2, N+=0, 1, 2, 3) states. The method ought to be applicable to the preparation of a wide range of small molecular ions in selected rotational states and opens new possibilities in the study of state-to-state ion–molecule reactions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466119
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