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  • 1
    Electronic Resource
    Electronic Resource
    State selective doubly enhanced sum-frequency mixing in CO (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 1540-1545 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Doubly enhanced sum-frequency mixing (DESMI) of two collinear laser beams is studied using a pulsed jet of carbon monoxide as the nonlinear medium. The pump laser frequency (ω1) is in two-photon resonance with a selected rotational component of the A1Π–X1Σ+ (5,0) transition. Tuning the probe laser frequency (ω2) to a sequential rotational component of the B1Σ+–A1Π (1,5) transition leads to strong enhancement of VUV generation at frequency 2ω1+ω2. The double enhancement is shown to be subject to firm overall selection rules. It is demonstrated that DESMI can be accurate and efficient for the study of molecular states in the VUV and XUV provided that low laser powers are used. The high selectivity of the four-wave mixing processes investigated allows a better understanding of the competition between four-wave mixing, reabsorption of the sum frequency, and multiphoton absorption. In particular, it is shown that resonance-enhanced multiphoton absorption is an important saturation mechanism at high laser powers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459132
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  • 2
    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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  • 3
    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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  • 4
    Electronic Resource
    Electronic Resource
    Spatial discrimination of Rydberg tagged molecular photofragments in an inhomogeneous electric field (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 7924-7934 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new approach to detecting the spatial and energetic distribution of photodissociation products is demonstrated, in which an inhomogeneous electric field is used for spatially selective field ionization of Rydberg molecules. State-selected NO fragments from NO2 photodissociation in a supersonic beam are excited to high-n Rydberg states and allowed to expand through the inhomogeneous field provided by an octopole. The field in the octopole results in the field ionization of those fragments that have moved to large radial distances (∼5 mm) from the beam axis, and therefore this device transmits only those Rydberg molecules with a low velocity component in the direction perpendicular to the beam. A detailed characterization of the properties of this "steradiancy analyzer" is carried out and factors limiting the energy resolution (currently approximately 100 cm−1 at an excess energy of 250 cm−1), along with possible improvements, are discussed in detail. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1408286
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  • 5
    Electronic Resource
    Electronic Resource
    Rydberg state decay in inhomogeneous electric fields (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 7992-8005 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An extension of the model of Merkt and Zare [J. Chem. Phys. 101, 3495 (1994)] is presented to describe the effects of static inhomogeneous electric fields, which arise experimentally from combinations of applied (or stray) homogeneous fields and the presence of charged particles, on Rydberg states of atoms and molecules. The effect of an arbitrary number of charged particles is included and the effects of nonzero quantum defects are investigated. A quantization axis rotation procedure is defined, allowing clear distinction between homogeneous and inhomogeneous field effects. Calculations are reported of the time-dependent decay of a coherent population of eigenstates for n=20, 33, and 50, involving diagonalization of the full n2×n2 matrix. Calculations are also carried out for n=100 by pre-diagonalization of the full homogeneous field perturbation followed by a restricted basis set diagonalization for the inhomogeneous part of the perturbation. The inclusion of nonzero quantum defects has a substantial impact on the ml mixing, confining significant mixing to a narrow range of radial and angular positions of the ion. An applied homogeneous field of order the Inglis-Teller field is required in combination with the field due to the ions. The dynamics are very different according to whether np or nf series carry the transition probability. For np-state population, the maximum stabilization is achieved at ion-Rydberg distances of around 5n2a0, with the ion almost perpendicular to the applied homogeneous field. For an initial nf population the ion perturbation may have a destabilizing effect at sufficiently small homogeneous field (≤0.1FIT). Significant effects of laser polarization on the stability are reported. Calculations for a realistic pseudo-random distribution of ions and Rydbergs suggest that ml mixing by ions will never reach the complete mixing limit, but that at least an order of magnitude stabilization is achievable under a restricted range of conditions. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481423
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  • 6
    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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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    Multichannel quantum defect theory simulation of the zero-kinetic-energy photoelectron spectrum of H2 (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 923-928 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The multichannel quantum defect theory (MQDT) is applied to the simulation of the v+=2 band of the zero-kinetic-energy (ZEKE) photoelectron spectrum of molecular hydrogen and also to the photoionization spectrum involving autoionizing Rydberg states in the region between the v+=1 and v+=2 thresholds. The results of the calculations are compared with previously reported experimental results [J. Chem. Phys. 96, 4149 (1992)]. The calculations are in very good agreement with experiment and provide quantitative confirmation of the previously proposed mechanisms for intensity perturbation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467746
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  • 9
    Electronic Resource
    Electronic Resource
    Multichannel quantum defect theory Stark effect calculation of autoionization lifetimes in high-n Rydberg states of Ar, N2 and H2 (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 1041-1056 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Multichannel quantum defect theory simulations of excitation spectra to autoionizing high-n Rydberg states (n=13−150) are presented for Ar (spin-orbit autoionization), H2 (vibrational autoionization) and N2 (rotational autoionization), including the l-mixing due to homogeneous electric fields (Stark effect). The calculations, the first of their kind relevant to the ZEKE (zero-kinetic energy) photoelectron spectroscopy excitation range, are compared with previously published experimental results. Although in some cases the lifetimes derived from calculated linewidths are sufficiently long for the states to be observed by delayed pulsed-field ionization, they are generally found to be too short in the highest-n regions (n〉80) to account for the very long lifetimes observed experimentally (τ〉10 μs), pointing to the importance of alternative stabilization mechanisms. The effects of rotational channel couplings in H2 and N2 are investigated; these are very weak if both channels are above the Inglis-Teller limit, but show significant effects if only one channel is strongly l mixed. In H2 it is found that a window resonance is preserved in the presence of a strong field. In Ar, ortho-H2 and N2 fine-structure of the hydogenic manifolds is predicted, and the distribution of intensity and linewidth amongst the fine-structure components is investigated. The non-zero quantum defects cause a lifting of degeneracy in the manifolds between different ml components. It is proposed that this would cause a reduction in ml-mixing by inhomogeneous fields as the homogeneous field increases. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474095
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  • 10
    Electronic Resource
    Electronic Resource
    Final-state interactions in the zero-kinetic-energy-photoelectron spectrum of H2 (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 4149-4156 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A rotationally resolved zero-kinetic-energy (ZEKE) photoelectron spectrum has been recorded for the lowest energy valence band of H2. A coherent laser-based extreme ultraviolet (XUV) source, tuned from 128 200–128 900 cm−1 excites the molecules to the ionization limit, and near-threshold electrons, in a narrow energy bandwidth (a few cm−1 ) just below the ionization limit, are detected by field ionization as a function of laser wavelength. The rotational components for the H2+X 2Σ+g–H2 X 1Σ+g (2–0) vibrational band show different relative intensities, compared to the conventional photoelectron spectrum and to the predictions of theoretical calculations. The perturbed intensities of the N+=0←J‘=0 and the 0←2 rotational lines are shown to be attributable to an interaction of the pseudocontinuum of near-threshold N+=0 np J'=1 Rydberg states, with the interloper N+=2, 26p J'=1 state. The enhanced intensity of the 2←0 rotational line is attributable to a "complex resonance'' involving the v+=3 R(0) 8p0 transition.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461870
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