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ABSTRACT: Molecular high-order harmonic generation (MHOHG) is studied from appropriate time-dependent Schrödinger equation (TDSE) solutions of a 2D equilateral two-electron H+3 molecular ion in interaction with an intense (5 × 1014 W cm−2) few-cycle 800 nm linearly polarized laser pulse. The nonperturbative time-dependent Born–Oppenheimer (static nuclei) two-electron wavefunction is used to calculate the MHOHG spectrum as a function of the laser–molecule orientation and internuclear distance R. Three-centre interferences in the MHOHG spectrum are shown to be sensitive functions of the laser–molecule orientation and R. A comparison is made of the MHOHG spectrum at the ground state equilibrium distance, Re, at an intermediate R where a three-photon transition (1A'1 → 1E') occurs, and finally at internuclear distance Rc where enhanced ionization can occur due to the nonlinear electron–laser interaction. Analytic expressions for MHOHG spectra interferences are derived based on a recollision model and are compared to the exact 2D TDSE simulations.
Journal of Physics B Atomic Molecular and Optical Physics 02/2011; 44(6):065601. · 1.88 Impact Factor
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ABSTRACT: We investigate theoretically the dissociative ionization of a H2+ molecule
using two ultrashort laser (pump-probe) pulses. The pump pulse prepares a
dissociating nuclear wave packet on an ungerade surface of H2+. Next, an UV (or
XUV) probe pulse ionizes this dissociating state at large (R = 20 - 100 bohr)
internuclear distance. We calculate the momenta distributions of protons and
photoelectrons which show a (two-slit-like) interference structure. A general,
simple interference formula is obtained which depends on the electron and
protons momenta, as well as on the pump-probe delay on the pulses durations and
polarizations. This interference can be interpreted as visualization of an
electron state delocalized over the two-centres. This state is an entangled
state of a hydrogen atom with a momentum p and a proton with an opposite
momentum. -p dissociating on the ungerade surface of H2+. This pump-probe
scheme can be used to reveal the nonlocality of the electron which intuitively
should be localized on just one of the protons separated by the distance R much
larger than the atomic Bohr orbit.
01/2010;
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ABSTRACT: Non-Born–Oppenheimer time-dependent Shrödinger equation numerical simulations of the nonlinear nonperturbative response of 1D H2, H+2 molecules (and their isotopes) in few cycle intense 800 nm laser pulses are presented to study the effect of nuclear motion on molecular high-order harmonic generation. A time–frequency analysis is used to identify electron recollision and recombination times responsible for the generation of attosecond pulse trains during the nuclear motion. A very strong signature of nuclear motion is seen in the time profiles of high-order harmonics. In the case of high laser intensity (I 1015 W cm−2) the nuclear motion shortens the part of the attosecond pulse train originating from the first electron contribution and may enhance the onset of the second electron contribution for longer pulses. Molecular motion thus can act as an important 'time-gating' for controlling the length of generated attosecond pulses. The shape of time profiles of harmonics can thus be used for monitoring the nuclear motion. In the case of lower laser intensity, I 4 × 1014 W cm−2, we also find in time profiles a clear signature of electron excitation due to recollision of the returning electron.
Journal of Physics B Atomic Molecular and Optical Physics 03/2009; 42(7):075602. · 1.88 Impact Factor
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ABSTRACT: We calculate harmonic spectra and shapes of attosecond-pulse trains using numerical solutions of Non-Born-Oppenheimer time-dependent Shrödinger equation for 1D H2 molecules in an intense laser pulse. A very strong signature of nuclear motion is seen in the time profiles of high-order harmonics. In general the nuclear motion shortens the part of the attosecond-pulse train originating from the first electron contribution, but it may enhance the second electron contribution for longer pulses. The shape of time profiles of harmonics can thus be used for monitoring the nuclear motion.
Physical Review Letters 11/2008; 101(15):153901. · 7.37 Impact Factor
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ABSTRACT: Time-dependent Schrödinger equation, TDSE, simulations have been performed in order to prepare and study via MPIPS the evolution of vibrational wave packets on the ion pair electronic state potentials B''B1Sigma(u)(+) and Hh1Sigma(g)(+) of the H2 molecule. Using ab initio potential surfaces and transition moments, we present two- and three-photon excitation schemes with ultrashort pulses (tau <or= 10 fs) to prepare coherent superpositions of the two ion pair H(+)H(-) and H(-)H(+) states from the doorway B1Sigma(u)(+) state, which result from the strong radiative coupling between these two electronic states. The simulations are used to estimate the time evolution and recursion times of vibrational wave packets at large internuclear distances, usually not accessible by single-photon spectroscopy. Conditions for the localization of the ion pair states are proposed.
The Journal of Physical Chemistry A 09/2007; 111(38):9340-6. · 2.95 Impact Factor
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ABSTRACT: Using numerical solutions of the time-dependent Schrödinger equation for the H2 molecule in intense laser fields we calculate the vibrational excitation induced by the Raman chirped adiabatic process (RCAP). We show that adding several higher electronic surfaces to a simple two-surface model improves the efficiency of the ladder-climbing process at intensities below the adiabaticity threshold. Furthermore, we show that although using photon energies close to the one-photon electronic transition frequency allows the use of lower pump and Stokes intensities, in general, this leads to more population transfer to the upper electronic surfaces accompanied by a loss of selectivity in the vibrational excitation on the ground-state surface. By contrast, considerable dissociation yields can be achieved when higher energy photons are used. We also investigate the structure of time-dependant vibrational wave packets prepared by RCAP. We find that at specific times the wave packet is very well localised at large inter-nuclear separations at which ionisation occurs with a probability 3 orders of magnitude larger than at the equilibrium separation. Copyright © 2007 John Wiley & Sons, Ltd.
Journal of Raman Spectroscopy 04/2007; 38(7):927 - 935. · 3.09 Impact Factor
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ABSTRACT: Using numerical solutions of a time-dependent Schrödinger equation for a hydrogen atom in a linearly polarized few-cycle laser field, we calculate the left-right photoelectron kinetic-energy spectra measured by two opposing detectors placed along the laser polarization vector, with laser focus in the center. The fastest electrons show huge asymmetries strongly dependent on the laser carrier-envelope (CE) phase which confirms the recent theoretical results [ D. B. Milosevic et al. Opt. Express 11 1418 (2003)], obtained from a modified strong field approximation model which includes rescattering by the Coulomb potential. This asymmetry can also be explained by a simple semiclassical model in which the electron after tunneling through a potential barrier returns to the proton and is elastically backscattered in the presence of the laser field thus acquiring energy close 10Up where Up is the electron ponderomotive energy in the laser field. We also present a semiclassical interpretation of counterintuitive left-right asymmetries of slow electrons discussed in our previous work [ Phys. Rev. A. 70 013815 (2004)]. Our analysis shows that the Coulomb attraction from the proton must be included in the standard tunneling model in order to account for the CE phase dependent angular asymmetry seen in our previous numerical calculations.
Phys. Rev. A. 05/2005; 71(5).
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ABSTRACT: We study theoretically the ionization and dissociation of muonic molecular ions (e.g., dd mu) in superintense laser fields. We predict that the bond breaks by tunneling of the lightest ion through a bond-softened barrier at intensity I > or =10(21) W/cm(2). Ionization of the muonic atomic fragment occurs at much higher intensity I > or =6 x 10(22) W/cm(2). Since the field controls the ion trajectory after dissociation, it forces recollision of a approximately 10(5)-10(6) eV ion with the muonic atom. Recollision can trigger a nuclear reaction with sub-laser-cycle precision. In general, molecules can serve as precursors for laser control of nuclear processes.
Physical Review Letters 09/2004; 93(8):083602. · 7.37 Impact Factor
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ABSTRACT: Using numerical solutions of the time-dependent Schrödinger equation for a hydrogen and a helium atom in a linearly polarized, few-cycle laser field, we calculate the photoelectron left-right asymmetry measured by two opposing detectors placed along the laser polarization vector, with the laser focus in the center. We find a simple dependence of this asymmetry on carrier-envelope (CE) phase phi for laser intensities slightly below the tunneling regime, which may allow us to measure (or to calibrate) and to stabilize the CE phase. In particular, we suggest that the condition of zero asymmetry for few-cycle pulses may be useful for both these goals.
Optics Letters 08/2004; 29(13):1557-9. · 3.40 Impact Factor
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ABSTRACT: Using numerical solutions of the time-dependent Schrödinger equation for a hydrogen and a helium atom in a linearly polarized laser field, we calculate the photoelectron signal P+, P− measured by two opposing detectors placed along the laser polarization vector, with laser focus in the center. Our calculations show a significant sensitivity of the normalized asymmetry coefficient a=(P+−P−)∕(P++P−) to the carrier-envelope (CE) phase for few-cycle laser pulses. We find a very simple dependence of this coefficient on the CE phase ϕ and on pulse duration for laser intensities slightly below the tunneling regime, i.e., for the intensities range between the perturbative-multiphoton and tunneling regime, for laser pulses shorter than two laser cycles. In particular, we find that in this intensity regime, the asymmetry is zero for a fixed, particular value of ϕ=ϕ0≃−0.3π and is maximum for ϕ=ϕM=ϕ0+π∕2. These regularities would allow one to measure the CE phase. In particular, the condition of zero asymmetry for few-cycle pulses can be useful for stabilizing the CE phase.
Phys. Rev. A. 07/2004; 70(1).
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ABSTRACT: A new technique for directly measuring the electric field of linearly polarized few-cycle laser pulses is proposed. Based on the solution of the time-dependent Schrödinger equation (TDSE) for an H atom in the combined field of infrared (IR) femtosecond (fs) and ultraviolet (UV) attosecond (as) laser pulses we show that, as a function of the time delay between two pulses, the difference (or equivalently, asymmetry) of photoelectron signals in opposite directions (along the polarization vector of laser pulses) reproduces very well the profile of the electric field (or vector potential) in the IR pulse. Such ionization asymmetry can be used for directly measuring the carrier-envelope phase difference (i.e., the relative phase of the carrier frequency with respect to the pulse envelope) of the IR fs laser pulse.
Physical Review Letters 01/2003; 89(28 Pt 1):283903. · 7.37 Impact Factor
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ABSTRACT: Numerical solutions of the time-dependent Schrödinger equation for a 1D model non-Born-Oppenheimer 2+
H are used to illustrate the nonlinear nonperturbative response of molecules to intense
01/1970: pages 31-54;
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ABSTRACT: Using numerical solutions of the three-dimensional time-dependent Schrödinger equation (TDSE) for the hydrogen atom in an intense, ultrashort, i.e., few-cycle, linearly polarized laser pulse, we demonstrate that ionization yields measured in the forward direction (0<θ<15°) depend strongly (by a factor of 2) on the carrier phase, leading to considerable directional (forward or backward) photoelectron asymmetries along the laser polarization vector. This effect vanishes for pulses comprising more than 15 cycles. The phase dependence of photoelectron asymmetry is intensity-dependent: the strongest asymmetry is found in the intermediate multiphoton-tunneling regime, where asymmetry originates from the Coulomb attraction after tunneling. The character of asymmetry drastically changes when an electron ionizes in the barrier-suppression regime. A measurement method of the absolute phase and width, based on this directional effect, is proposed for linearly polarized ultrashort laser pulses pulses.
Phys. Rev. A. 65(6).
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ABSTRACT: Exact non-Born–Oppenheimer simulations of a 1-D model of H2+ in an intense short laser pulse are used to investigate the non-linear multiphoton electron emission spectra, also called above-threshold ionization (ATI) spectra. Due to the fast proton motion upon dissociative-ionization, the electron ATI spectra are found to occur at the critical distance Rc≃7 a.u. by charge resonance enhanced ionization, CREI. Highly oscillatory ATI spectra occur, which are enhanced by the molecular motion, and these are the signature of incomplete localization of the electron by the laser field. It is suggested that re-scattering effects result in doubling of the ATI intensity oscillations with respect to simple coherent excitation models of the HOMO and LUMO in H2+.
Journal of Molecular Structure: THEOCHEM.
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ABSTRACT: The time-dependent Schrödinger equation describing the interaction of an HCN molecule with intense, ultrashort, chirped infrared laser pulses is solved numerically. The molecule is represented as two coupled Morse oscillators and the chirped laser pulse frequency ω(t) is adapted to the CH-bond anharmonicity in such a way that the pulse is nearly resonant with the vibration of this bond during the whole excitation process. It is shown that by controlling the chirping rate and the area of the pulse, one can selectively and efficiently excite and dissociate one particular bond and control the excitation of the other bond in a triatomic molecule. Such pulses should become important tools in photochemistry since one can thus prepare non-statistical quantum vibrational states and control the reactivity of a molecule by varying the laser phase.
Chemical Physics Letters.
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ABSTRACT: We study ionization of muonic atoms and dissociation of muonic molecular ions (dd?) in superintense laser fields. We predict that the bond breaks by tunneling at an intensity I > 1021 W/cm2. Ionization of the muonic atomic fragment occurs at a much higher intensity, I > 6 × 1022 W/cm2. Since the field controls the dissociating deuteron, it can force the deuteron return and thus the ion-?-atom recollision triggers a controlled (timed) nuclear fusion with subcycle precision. The recollision time and energy are shown to concur with classical models.
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ABSTRACT: Several theoretical models are used to explain the origin of the recently observed (unexpected) spectral progression in the Coulomb explosion spectra of the hydrogen molecule photoionized by an intense ultrashort laser pulse. In the first ionization step the molecule loses its first electron and then the H2+ molecular ion dissociates. Next, at the intermediate stage of the dissociation process, a localized electron state is created from which the second ionization occurs at each laser half-cycle. It is shown that interference between a net-two-photon and a one-photon transition introduces a dynamic structure into the nuclear wave packet corresponding to this localized electron state which leads to the regular spectral progressions seen in the experiment. We confirm these spectral progressions using numerical simulations based on a time-dependent Schr�dinger equation describing the exact three-body dynamics of H2+ in one dimension.
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ABSTRACT: We study theoretically the ionization and dissociation of muonic molecular ions (e.g., dd?) in superintense laser fields. We predict that the bond breaks by tunneling of the lightest ion through a bond-softened barrier at intensity I?1021 W/cm2. Ionization of the muonic atomic fragment occurs at much higher intensity I?6�1022 W/cm2. Since the field controls the ion trajectory after dissociation, it forces recollision of a ?105�106 eV ion with the muonic atom. Recollision can trigger a nuclear reaction with sub-laser-cycle precision. In general, molecules can serve as precursors for laser control of nuclear processes.
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