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  • Journal article
    Seideman T, Ivanov MY, Corkum PB, 1996,

    The use of intense-field ionization in time-resolved measurements

    , CHEMICAL PHYSICS LETTERS, Vol: 252, Pages: 181-188, ISSN: 0009-2614

    We show that intense radiation introduces a length-scale in molecules which translates into a dramatic enhancement of the rate of multiple ionization of molecules at characteristic internuclear separations. This feature is common to a wide range of systems and is insensitive to variations in frequency and intensity. The electronic properties responsible for this effect are explored. We propose the use of the sensitivity of intense-field ionization to the molecular configuration as a probe in time-resolved studies of dissociative dynamics.
  • Journal article
    Thachuk M, Ivanov MY, Wardlaw DM, 1996,

    A semiclassical approach to intense-field above-threshold dissociation in the long wavelength limit

    , JOURNAL OF CHEMICAL PHYSICS, Vol: 105, Pages: 4094-4104, ISSN: 0021-9606

    A new semiclassical Formalism has been developed to treat Hamiltonians having explicit time dependence with particular application to the dissociation of diatomic ions in intense laser fields. Based on this formalism, a hopping algorithm is presented which specifies how classical trajectories should be moved between coupled electronic surfaces. The theory is laid out in a rigorous, general form and an analysis is also presented for the case where only two electronic surfaces are strongly coupled. In addition, valuable physical insight into the hopping process is obtained by considering the theory in a number of physically relevant limiting cases. From this insight a number of guidelines are proposed which detail the manner in which trajectory hopping should be implemented when lime-dependent potential energy surfaces are present, including the effects of phase coherence and conservation principles. (C) 1996 American Institute of Physics.
  • Journal article
    Villeneuve DM, Ivanov MY, Corkum PB, 1996,

    Enhanced ionization of diatomic molecules in strong laser fields: A classical model

    , PHYSICAL REVIEW A, Vol: 54, Pages: 736-741, ISSN: 1050-2947

    A classical model is developed to study molecular ionization in strong laser fields. The model is compared with a one-dimensional quantum-mechanical model for the case of H-2(+). The intensity threshold for ionization of H-2(+) is studied as a function of laser polarization, wavelength, pulse duration, and internuclear separation. A range of internuclear separations is found where the ionization rate is enhanced over that at the equilibrium separation, in agreement with the quantum-mechanical model. The classical model is then extended to multielectron diatomic molecules. A regime is found where simultaneous two-electron ionization occurs. An intensity range is suggested for looking for enhanced ionization effects in dissociating diatomic molecules.
  • Conference paper
    Sanpera A, Protopapas M, Watson JB, Burnett K, Knight PLet al., 1996,

    High harmonic generation efficiency

    , New York, 9th international symposium on ultrafast processes in spectroscopy (ups 95) OCT 30-NOV 03, 1995 TRIESTE, ITALY, Publisher: Plenum Press Div Plenum Publishing Corp, Pages: 295-298
  • Journal article
    Ivanov MY, Brabec T, Burnett N, 1996,

    Coulomb corrections and polarization effects in high-intensity high-harmonic emission

    , PHYSICAL REVIEW A, Vol: 54, Pages: 742-745, ISSN: 1050-2947

    We describe a quasiclassical approach to correct electron motion following tunneling ionization in intense laser fields, for the presence of the Coulomb potential. The technique is applied to model the dependence of high-harmonic generation on the ellipticity of the incident laser field. We present experimental data on the harmonics with photon energies close to the ionization potential of an atom, identify their source, and explain their unusual dependence on the ellipticity of the incident laser field. Our calculations are in good agreement with experimental data.
  • Journal article
    Ivanov MY, Matusek DR, Wright JS, 1996,

    Altered reaction dynamics: Lowered barriers and bound states induced by intense infrared laser fields

    , CHEMICAL PHYSICS LETTERS, Vol: 255, Pages: 232-237, ISSN: 0009-2614

    This Letter reports a study on the effect of the Stark shift for exchange reactions in intense infrared laser fields. It is shown for the H + H-2 exchange reaction that such fields reduce the potential barrier, thus enhancing the reaction rate at low temperatures. Another interesting effect, caused by the change in molecular polarizability as reactants approach, is the creation of (laser-induced) potential minima along the reaction path.
  • Journal article
    Dietrich P, Ivanov MY, Ilkov FA, Corkum PBet al., 1996,

    Two-electron dissociative ionization of H-2 and D-2 in infrared laser fields

    , PHYSICAL REVIEW LETTERS, Vol: 77, Pages: 4150-4153, ISSN: 0031-9007

    We present a quantitative physical model of above-threshold dissociation of homonuclear molecular ions in intense midinfrared laser fields. This allows us to describe self-consistently all three processes that occur during dissociative ionization of H-2 and D-2 (ionization of the neutral molecule, dissociation of the molecular ion, and its secondary ionization) using a semiclassical approach. The kinetic energy spectrum of the dissociating fragments is obtained and analyzed.
  • Journal article
    Corkum P, Ivanov MY, 1996,

    Ultrafast VUV lasers eye first commercial applications

    , PHOTONICS SPECTRA, Vol: 30, Pages: 104-105, ISSN: 0731-1230
  • Journal article
    Brabec T, Ivanov MY, Corkum PB, 1996,

    Coulomb focusing in intense field atomic processes

    , PHYSICAL REVIEW A, Vol: 54, Pages: R2551-R2554, ISSN: 1050-2947

    In the intense field (long-wavelength) limit, the oscillating motion of an electron wave packet leads to multiple passes by the scattering Coulomb center. The influence of the Coulomb focusing, in combination with multiple returns, focuses parts of the electron wave function, increasing the efficiency of such intense field processes as multiphoton double ionization. Our calculations show enhancement by more than an order of magnitude for multiphoton double ionization of He at 0.8 mu m.
  • Conference paper
    Dalton BJ, Guerra ES, Knight PL, 1996,

    Input-output from optical cavities: Quasi-mode theory

    , New York, 7th rochester conference on coherence and quantum optics JUN 07-10, 1995 ROCHESTER, NY, Publisher: Plenum Press Div Plenum Publishing Corp, Pages: 577-578
  • Conference paper
    Knight PL, Keitel CH, Burnett K, Watson JBet al., 1996,

    High harmonic generation as a recollision process

    , New York, 9th international symposium on ultrafast processes in spectroscopy (ups 95) OCT 30-NOV 03, 1995 TRIESTE, ITALY, Publisher: Plenum Press Div Plenum Publishing Corp, Pages: 305-309
  • Conference paper
    Vitanov NV, Knight PL, 1996,

    Coherent excitation of two-level atoms by pulse trains

    , New York, 7th rochester conference on coherence and quantum optics JUN 07-10, 1995 ROCHESTER, NY, Publisher: Plenum Press Div Plenum Publishing Corp, Pages: 521-522
  • Conference paper
    Knight PL, Garraway BM, 1996,

    Quantum superpositions in dissipative environments: Decoherence and deconstruction

    , 44th Scottish Universities Summer School in Physics - Quantum Dynamics of Simple Systems, Publisher: IOP PUBLISHING LTD, Pages: 199-238
  • Conference paper
    Keitel CH, Watson JB, Knight PL, Burnett Ket al., 1996,

    Quantum signatures in strong field physics

    , New York, 7th rochester conference on coherence and quantum optics JUN 07-10, 1995 ROCHESTER, NY, Publisher: Plenum Press Div Plenum Publishing Corp, Pages: 535-537
  • Journal article
    Fraser DJ, Hutchinson MHR, Marangos JP, Shao YL, Tisch JWG, Castillejo Met al., 1995,

    High harmonic generation in butane and butadiene

    , JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, Vol: 28, Pages: L739-L745, ISSN: 0953-4075
  • Journal article
    AVERBUKH V, MOISEYEV N, MIRBACH B, KORSCH HJet al., 1995,

    DYNAMICAL TUNNELING THROUGH A CHAOTIC REGION - A CONTINUOUSLY DRIVEN RIGID ROTOR

    , ZEITSCHRIFT FUR PHYSIK D-ATOMS MOLECULES AND CLUSTERS, Vol: 35, Pages: 247-256, ISSN: 0178-7683
  • Journal article
    VITANOV NV, KNIGHT PL, 1995,

    CONTROL OF ATOMIC TRANSITIONS BY THE SYMMETRY OF EXCITATION PULSES

    , OPTICS COMMUNICATIONS, Vol: 121, Pages: 31-35, ISSN: 0030-4018
  • Journal article
    WATSON JB, KEITEL CH, KNIGHT PL, BURNETT Ket al., 1995,

    Quantum signatures in the stabilization dynamics

    , Physical Review A: Atomic, Molecular and Optical Physics, Vol: 52, Pages: 4023-4028, ISSN: 1050-2947

    We investigate the phase-space dynamics of a hydrogenic atom in a very intense, high-frequency laser field, by comparing the time-dependent quantum Wigner functions with the corresponding classical distributions calculated using the Monte Carlo method. In both cases we model the atom by a one-dimensional soft-core potential in order to simplify the calculation. We demonstrate the importance of nonclassical interferences in the Wigner function and show how negative parts of it observed at the peak of the pulse are associated with the formation of a coherent superposition of dressed ‘‘Kramers-Henneberger’’ (KH) eigenstates. These signatures of the quantum dynamics are eliminated towards the end of the pulse when the coherence between the KH eigenstates degrades so that the classical and the quantum mechanical phase-space distributions come into agreement.
  • Journal article
    Knight PL, 1995,

    Rudolf Peierls (1907-1995) - Obituary

    , CONTEMPORARY PHYSICS, Vol: 36, Pages: 375-375, ISSN: 0010-7514
  • Journal article
    PROTOPAPAS M, KNIGHT PL, 1995,

    PHASE-CONTROL IN 2-COLOR MULTIPHOTON IONIZATION

    , JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, Vol: 28, Pages: 4459-4479, ISSN: 0953-4075
  • Journal article
    VIVIRITO RMA, KNIGHT PL, 1995,

    ADIABATIC EXCITATION AND STABILIZATION IN SHORT-RANGE POTENTIALS

    , JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, Vol: 28, Pages: 4357-4375, ISSN: 0953-4075
  • Journal article
    PROTOPAPAS M, SANPERA A, KNIGHT PL, BURNETT Ket al., 1995,

    High-intensity two-color interactions in the tunneling and stabilization regimes

    , Physical Review A: Atomic, Molecular and Optical Physics, Vol: 52, Pages: R2527-R2530, ISSN: 1050-2947

    Two-color excitation of a hydrogen atom has been studied by solving the time-dependent Schrödinger equation. We have chosen the two laser frequencies to be the fundamental and its third harmonic with an adjustable phase between the two laser fields. This has been proposed and successfully implemented in the past for "coherent control" of atomic and chemical processes. Here we have extended the study to the high-intensity domain and well into the tunneling and the stabilization regimes. We show that in the tunneling regime the two-color excitation can enhance the intensity of harmonics by more than two orders of magnitude compared to a single color at the same effective intensity. In the stabilization regime we confirm that two-color excitation can be used to generate higher harmonics.
  • Journal article
    Hinds EA, Berkelend DJ, Boshier MG, 1995,

    Precise Optical Measurement of Lamb Shifts in Atomic Hydrogen

    , Physical Review Letters, ISSN: 0031-9007
  • Journal article
    NEW GHC, WILSON DC, 1995,

    SIMULATIONS OF RAMAN CONVERSION IN LARGE-APERTURE AMPLIFIERS

    , OPTICS COMMUNICATIONS, Vol: 119, Pages: 673-681, ISSN: 0030-4018
  • Journal article
    VITANOV NV, KNIGHT PL, 1995,

    Coherent excitation of a two-state system by a train of short pulses

    , Physical Review A: Atomic, Molecular and Optical Physics, Vol: 52, Pages: 2245-2261, ISSN: 1050-2947

    A theoretical nonperturbative study of the coherent excitation of a two-state system by N consecutive equally spaced identical pulses is presented. General relations between the evolution matrix elements in the cases of one and N pulses are obtained in a closed form. For pulse envelopes allowing analytical solutions, these relations enable analytical treatment of pulse-train excitation; for pulses that can only be treated numerically, they shorten the computations by a factor of N. The relations show that the multiple-pulse excitation of a two-state system can be considered as a quantum analog of the diffraction grating. The interaction dynamics substantially depends on the way the train is produced because the phase shift that is accumulated by the probability amplitudes due to the free evolution of the system during and between the pulses differs. In the limit of weak excitation, the results recover earlier conclusions based on perturbative treatments. Simple formulas are derived for the conditions for complete population inversion (CPI) and complete population return (CPR), which differ from the single-pulse ones. These general results are applied to four particular cases that allow analytical solutions: resonant, rectangular, Rosen-Zener, and Allen-Eberly pulses. A common feature in all these cases is that the number and the amplitude of oscillations in the state populations increase with the number of pulses, while their width decreases. For rectangular pulses, CPI is possible below a given value of the ratio between the detuning and the pulse area; this value increases nearly linearly with the number of pulses.For the Rosen-Zener model, CPI is found to be possible for two and more pulses, while it is impossible for a single non-resonant pulse. It is shown that for a given number of pulses there is an upper limit on the detuning for which CPI can be observed; this limit increases logarithmically with the number of pulses. For the Allen-Eberly model, CPR is esta
  • Journal article
    GILES AJ, POSTHUMUS JH, THOMPSON MR, FRASINSKI LJ, CODLING K, LANGLEY AJ, SHAIKH W, TADAY PFet al., 1995,

    INTERFEROMETRIC SHAPING OF FEMTOSECOND LASER-PULSES

    , OPTICS COMMUNICATIONS, Vol: 118, Pages: 537-541, ISSN: 0030-4018
  • Journal article
    GUERRA ES, DUTRA SM, KNIGHT PL, ZAGURY Net al., 1995,

    Influence of spontaneous emission on trapping states and start-up times of degenerate two-photon micromasers

    , Physical Review A: Atomic, Molecular and Optical Physics, Vol: 52, Pages: 1651-1664, ISSN: 1050-2947

    We study degenerate two-photon micromasers pumped by cascade three-level atoms, where each atomic energy level has a different linewidth. Our model allows us to recover the ordinary two-photon micromaser situation when these widths vanish. In the limit of large linewidths we obtain a two-photon laser model. We show that trapping states are extremely sensitive to atomic decay, even when the decay time is small in comparison to the transit time through the cavity. Unlike the one-photon micromaser, they are sensitive to the linewidth of the upper, as well as the lower energy level. We have also studied the effects of atomic decay on the starting time of micromasers and on the noise. We found that there is a significant difference between the behavior of two-photon micromasers and lasers as we change the ratio of the widths of the upper and lower levels.
  • Journal article
    KEITEL CH, KNIGHT PL, NARDUCCI LM, SCULLY MOet al., 1995,

    RESONANCE FLUORESCENCE IN A TAILORED VACUUM

    , OPTICS COMMUNICATIONS, Vol: 118, Pages: 143-153, ISSN: 0030-4018
  • Journal article
    GAUTHEY FI, KEITEL CH, KNIGHT PL, MAQUET Aet al., 1995,

    Role of initial coherence in the generation of harmonics and sidebands from a strongly driven two-level atom

    , PHYSICAL REVIEW A, Vol: 52, Pages: 525-540, ISSN: 1050-2947

    We investigate the coherent and incoherent contributions of the scattering spectrum of strongly driven two-level atoms as a function of the initial preparation of the atomic system. The initial ‘‘phasing’’ of the coherent superposition of the excited and ground states is shown to influence strongly the generation of both harmonics and hyper-Raman lines. In particular, we point out conditions under which harmonic generation can be inhibited at the expense of the hyper-Raman lines. Our numerical findings are supported by approximate analytical evaluation in the dressed state picture.
  • Journal article
    GARRAWAY BM, KIM MS, KNIGHT PL, 1995,

    QUANTUM JUMPS, ATOMIC SHELVING AND MONTE-CARLO FLUORESCENCE-SPECTRA

    , OPTICS COMMUNICATIONS, Vol: 117, Pages: 560-569, ISSN: 0030-4018

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