K. H. Welge

Bielefeld University, Bielefeld, North Rhine-Westphalia, Germany

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Publications (118)233.34 Total impact

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    ABSTRACT: The scattering of highly excited hydrogen Rydberg atoms, H* (n = 36), with deuterium molecules in their rovibrational ground state, D2(v = 0, j = 0), has been investigated at a relative collision energy of 0.53 eV. Time-of-flight distributions of elastically/inelastically scattered H* Rydberg atoms and reactively scattered D* Rydberg atoms have been measured at different laboratory angles. The extracted rovibrationally resolved state distributions of the HD product molecules from reactive collisions resemble closely those reported for the corresponding ion-molecule reaction, H+ + D2 --> HD + D+. This similarity is rationalised using the free electron model which predicts that the Rydberg electron acts as a spectator while the ionic reaction takes place.
    Physical Chemistry Chemical Physics 04/2005; 7(7):1577-82. · 4.20 Impact Factor
  • The Journal of Physical Chemistry. 04/2002; 88(22).
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    ABSTRACT: The H+D2(v = 0,j = 0)→HD(v′,j′)+D isotopic variant of the hydrogen atom exchange reaction has been studied in a crossed molecular beam experiment at a collision energy of 2.20 eV. Kinetic energy spectra of the nascent D atoms were obtained by using the Rydberg atom time-of-flight technique. The extensive set of spectra collected has permitted the derivation of rovibrationally state-resolved differential cross sections in the center-of-mass frame for most of the internal states of the HD product molecules, allowing a direct comparison with theoretical predictions. Accurate 3D quantum mechanical calculations have been carried out on the refined version of the latest Boothroyd–Keogh–Martin–Peterson potential energy surface, yielding an excellent agreement with the experimentally determined differential cross sections. The comparison of the results from quasi-classical trajectory calculations on the same potential surface reveals some discrepancies with the measured data, but shows a good global accordance. The theoretical calculations demonstrate that, at this energy, reactive encounters are predominantly noncollinear and that collinear collisions lead mostly to nonreactive recrossing. The experimental results are satisfactorily accounted for by theoretical calculations without consideration of Geometric Phase effects. © 1999 American Institute of Physics.
    The Journal of Chemical Physics 05/1999; 110(20):9971-9981. · 3.12 Impact Factor
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    ABSTRACT: The authors have studied atomic hydrogen in intense, short, visible laser pulses (I<or approximately=1.2*1014 W cm-2, tau approximately=0.5 ps, lambda approximately=608 nm). At high intensities (I>6*1013 W cm-2) they observe a new series of higher order above-threshold ionization peaks in the photoelectron energy spectrum. They discuss some tentative explanations. Two models are described in some detail. One relies on ionization saturation, the second one may be named 'parking of atoms as Rydberg wavepackets'.
    Journal of Physics B Atomic Molecular and Optical Physics 12/1998; 26(2):L15. · 2.03 Impact Factor
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    ABSTRACT: Multiphoton ionisation of strontium atoms to Sr+ and Sr2+ has been investigated with a Nd-YAG laser ( lambda =10642 AA), its second, third and fourth harmonics and a tunable dye laser in a wavelength region around 5321 AA. The power dependence of the Sr+ and Sr2+ production has been obtained. The creation of Sr2+ is discussed with respect to two models: the simultaneous excitation and elimination of two electrons and a stepwise ionisation via Sr+.
    Journal of Physics B Atomic and Molecular Physics 12/1998; 15(11):1663.
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    ABSTRACT: The strong radiative single photon coupling between two excited states of sodium atoms (3p and 4d) has been studied at high intensities, exceeding 109 W cm-2. These excited states are probed by Raman-like resonant five-photon ionization processes. Using two tunable lasers the authors have observed that the doubly resonant process does not occur. In the model picture of photon-dressed atomic levels, this situation can be described as an avoided crossing, with a large minimum separation of 30 cm-1.
    Journal of Physics B Atomic Molecular and Optical Physics 12/1998; 26(2):271. · 2.03 Impact Factor
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    ABSTRACT: The authors present a comparison of theoretical and experimental data for the first three above-threshold ionization channels of the photoelectron kinetic energy spectrum of atomic hydrogen when the atom is irradiated by a short pulse of linearly polarized light of wavelength 608 nm and peak intensity 6*1013 W cm-2 or 1.2*1014 W cm-2.
    Journal of Physics B Atomic Molecular and Optical Physics 12/1998; 25(12):L275. · 2.03 Impact Factor
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    ABSTRACT: The Sr2+ yield has been measured in the wavelength region around 561 nm. Maxima of the yield can be correlated with energy levels of Sr II, indicating that the double ionisation is a step-wise process via Sr+.
    Journal of Physics B Atomic and Molecular Physics 12/1998; 15(15):L529.
  • D Feldmann, G Otto, D Petring, K H Welge
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    ABSTRACT: Although the structure in the total three-photon ionisation cross section around an autoionising resonance at 13.134 eV predicted by a recent calculation has not been observed, the angular distribution of photoelectrons changes considerably at the corresponding wavelength.
    Journal of Physics B Atomic and Molecular Physics 12/1998; 19(5):L141.
  • D Feldmann, K H Welge
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    ABSTRACT: Ionisation spectra of strontium have been measured using tunable laser radiation between 555 and 577 nm. The influence of bound two-photon resonances and autoionising three-photon resonances on the energy spectra and the angular distribution of the photoelectrons is studied for selected wavelengths. Four-photon ionisation via three-photon autoionising resonances is observed.
    Journal of Physics B Atomic and Molecular Physics 12/1998; 15(11):1651.
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    ABSTRACT: Precision measurements of photoabsorption lines of atomic hydrogen in magnetic fields up to 6 T are compared with exact quantum mechanical calculations for energies reaching beyond the regime of approximate separability of the Schrodinger equation. The agreement between measurement and calculation is excellent.
    Journal of Physics B Atomic and Molecular Physics 12/1998; 19(17):L557.
  • E Flothmann, J Main, K H Welge
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    ABSTRACT: For the hydrogen atom in crossed, perpendicular, static electric and magnetic fields we show the existence of three elementary Kepler ellipses, two in the plane perpendicular to the magnetic field and one, three-dimensional, largely perpendicular to the electric field. They are determined from the symmetry properties in the low field limit, occurring for all external field combinations. Their parameter dependences and stability properties are analysed in detail. In the case that the orbits are stable the corresponding maximum orientated quantum states are identified. For states with sufficient low principal quantum numbers the total splitting of a given n-shell is described by semiclassical quantization of these three Kepler ellipses.
    Journal of Physics B Atomic Molecular and Optical Physics 12/1998; 27(13):2821. · 2.03 Impact Factor
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    ABSTRACT: The three-photon ionisation of strontium atoms via resonance in the last photon absorption with some autoionising states was observed in the excitation range 5560-5640 AA using a tunable dye laser. The spectra as a function of the laser intensity are reported. Very rapid line broadening and ion enhancement together with very asymmetric line profiles are observed.
    Journal of Physics B Atomic and Molecular Physics 12/1998; 14(14):2353.
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    ABSTRACT: The state selective photodissociation of acetylene, C2H2/C2D2, was studied in the wavelength range 121.2–132.2 nm by high resolution Rydberg atom time-of-flight measurements on the atomic fragment, H/D. In the wavelength region studied members of all four Rydberg series and the highly excited valence state were state selectively excited using tunable vacuum-ultraviolet laser radiation. The lifetime of the excited states which were studied varied from 58 fs to more than 2 ps. Formation of the ethynyl radical in its electronic ground state and its first electronically excited state is observed with practically no indication of state fragments. Two decay channels with different dissociation dynamics were also observed. In both channels the observed decay dynamics depended strongly on the excited state of the parent molecule. Further there are major differences between these two dissociation pathways with respect to the measured internal energy and angular distributions. In one channel the dissociation is dominated by dynamical effects and the C2H fragments are formed with a high degree of vibrational excitation. In contrast to this in the second channel a smooth internal energy distribution is observed indicating that the fragment quantum state distribution is spread over a considerable range of the available phase space. Moreover, this second channel can be fit with a phase space model constrained only by conservation of energy and angular momentum. This is further evidence for the randomization of internal energy during the dissociation process. © 1998 American Institute of Physics.
    The Journal of Chemical Physics 09/1998; 109(13):5231-5246. · 3.12 Impact Factor
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    ABSTRACT: The photodissociation of jet-cooled HCCH molecules following excitation to their S1 state has been investigated further, at a number of wavelengths in the range 205–220 nm, using the H atom photofragment translational spectroscopy (PTS) technique. Analysis of the rovibrational structure evident in the total kinetic energy release (TKER) spectra so obtained confirms previous reports that the resulting C2H() fragments are formed in most (if not all) of the v2 bending vibrational levels permitted by energy conservation, and that there is a clear preference for populating those states in which the axial projection of this vibrational angular momentum is maximized (i.e., states with l = v2). The distribution of H atom recoil velocity vectors resulting from photolyses at the shorter excitation wavelengths (e.g., λphot=205.54 nm) shows bimodal rotational distributions, and a marked anisotropy—especially in the case of those H atoms that are formed in association with C2H() fragments carrying little rotational excitation. Two competing dissociations mechanisms have been identified. Our discussion of these observations is guided by the recent ab initio calculations of Cui and Morokuma [Chem. Phys. Lett. 272, 319 (1997)]. Channel I conforms to their proposal that the S1 molecules reach the H+C2H() asymptote as a result of sequential nonadiabatic couplings via the T3, T2, and T1 potential energy surfaces. The product energy disposal at the longest excitation wavelengths is rationalized in terms of the forces acting as the dissociating molecule traverses a late barrier in the C–H exit channel on the T1 surface, while the propensity for populating states with l = v2 reflects the importance of parent torsional motion in promoting the S1→T3, T3→T2, and T2→T1 surface couplings. The population of low rotational states with high recoil anisotropy at shorter excitation wavelengths is ascribed to channel II, involving a direct nonadiabatic transition from S1 to T1 for a structure with one near linear CCH angle. In contrast to channel I, there is no extensive torsional motion and the anisotropy of the initial excitation is retained through to fragmentation. Excitation of the ν1′ mode of HCCH enhances the branching to channel II. © 1998 American Institute of Physics.
    The Journal of Chemical Physics 01/1998; 108(2):519-526. · 3.12 Impact Factor
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    ABSTRACT: We have carried out a systematic crossed molecular beam study of the hydrogen exchange reaction in the H+D2→HD+D isotopic form at two collision energies: 0.53 and 1.28 eV. The Rydberg atom time-of-flight method was used to measure the D-atom product angle-velocity distribution. For the first time ro-vibrational quantum state resolved differential cross sections for the title reaction were measured, which can directly be compared to theoretical predictions at this detailed level. Experimental results are compared to theoretical predictions from both quasi classical and quantum mechanical calculations on different potential energy surfaces as well as to earlier experiments. A general good agreement is found for the converged quantum mechanical calculations with indications that the Boothroyd-Keogh-Martin-Peterson potential energy surface is better suited to describe the dynamics of the reaction. For the higher collision energy the quasi classical trajectory calculations reproduce the experimental data quite well, whereas they fail to describe the situation at the lower collision energy especially with respect to angular resolved differential cross sections. © 1997 American Institute of Physics.
    The Journal of Chemical Physics 10/1997; 107(16):6175-6195. · 3.12 Impact Factor
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    ABSTRACT: Scattering measurements performed at an energy slightly higher than that of the lowest crossing between the ground and the first electronically excited state of the H3 system are reported. The essentials of the reactive dynamics are describable in terms of a classical motion of the nuclei on the lowest adiabatic electronic potential.© 1997 American Institute of Physics.
    The Journal of Chemical Physics 05/1997; 106(18):7862-7864. · 3.12 Impact Factor
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    ABSTRACT: We present the first observations of cylindrical symmetry breaking in highly excited diamagnetic hydrogen with a small crossed electric field, and we give a semiclassical interpretation of this effect. As the small perpendicular electric field is added, the recurrence strengths of closed orbits decrease smoothly to a minimum, and revive again. This phenomenon, caused by interference among the electron waves that return to the nucleus, can be computed from the azimuthal dependence of the classical closed orbits. Comment: 4 page REVTeX file including 5 postscript files (using psfig) Accepted for publication in Physical Review Letters. Difference from earlier preprint: we have discovered the cause of the earlier apparent discrepancy between experiment and theory and now achieve excellent agreement
    Physical Review Letters 01/1997; · 7.73 Impact Factor
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    ABSTRACT: The reactivity of H + D2(v = 0, j = 0) → HD(v′,j′) + D has been investigated in a high resolution crossed molecular beam experiment at a collision energy of 2.2 eV. Time-of-flight (TOF) spectra of D atoms at different laboratory (LAB) scattering angles and their total angular distribution were measured using the technique of Rydberg atom TOF spectroscopy. The resolution of this technique allows the identification of individual rovibrational states of the corresponding HD product molecule at the different LAB angles. A detailed simulation of the experimental measurements has been performed by using state resolved differential cross sections from quasiclassical trajectory (QCT) calculations on the Liu-Siegbahn-Truhlar-Horowitz (LSTH) and on the double-many-body-expansion (DMBE) potential energy surfaces (PES). A general good agreement between experiment and theory is found, although there are some discrepancies which are discussed.
    Chemical Physics Letters 01/1997; · 2.15 Impact Factor