Coherent control of photofragment separation in the dissociative ionization of IBr

Dissociative ionization and Coulomb explosion of CH4 in two-color asymmetric intense laser fields

Directional fragment ejection from a tetrahedral molecule CH4 in linearly polarized two-color (ω and 2ω) asymmetric intense laser fields (50 fs, 1.4 × 1014 W cm-2, 800 nm and 400 nm) has been studied by three-dimensional ion coincidence momentum imaging. The H+ fragment produced from dissociative ionization, CH4 → H+ + CH3 + e-, is preferentially ejected on the larger amplitude side of the laser electric fields. Comparison with theoretical predictions by weak-field asymptotic theory shows that the observed asymmetry can be understood by the orientation selective tunneling ionization from the triply degenerated highest occupied molecular orbital (1t2) of CH4. A similar directional ejection of H+ was also observed for the low kinetic energy components of the two-body Coulomb explosion, CH4 → H+ + CH3+ + 2e-. On the other hand, the fragment ejection in the opposite direction were observed for the high energy component, as well as H2+ produced from the Coulomb explosion CH4 → H2+ + CH2+ + 2e-. Possible origins of the characteristic fragmentation are discussed.

Rotational dynamics and transitions between Λ-type doubling of NO induced by an intense two-color laser field

We numerically investigate the rotational dynamics of NO in the electronic ground X²Π state induced by an intense two-color laser field (10 TW/cm²) as a function of pulse duration (0.3-25 ps). In the short pulse duration of less than 12 ps, rotational Raman excitation is effectively induced and results in molecular orientation. On the contrary, when the pulse duration is longer than 15 ps, the rotational excitation is suppressed. In addition to the rotational excitation, we find that transitions between Λ-type doubling are induced. Significantly, the maximum coherent wave packet between Λ-type doubling in J = 0.5 is generated using the pulse duration of 19.8 ps. The wave packet changes to the eigenstates of Λ = +1 or -1 alternatively, where Λ is the projection of the electronic orbital angular momentum on the N-O axis, which is regarded as the unidirectional rotation of an unpaired 2π electron around the N-O axis in a space-fixed frame as well as in a molecule-fixed frame. The experimental method to observe the alternation of the rotational direction of the electron around the N-O axis is proposed.

Exploring the influence of experimental parameters on the interaction of asymmetric ω/2ω fields with water isotopologues

Water isotopologues are doubly ionized by phase-controlled asymmetric ω/2ω laser fields, and their two-body fragmentation channels leading to pairs of OH⁺/H⁺ [channel (I)] and H₂ ⁺/O⁺ [channel (II)] are systematically investigated. The dependence of the ionic fragments on phase distinguishes between two dissociation channels, while a quantity that is proportional to the directionality of the ejected fragments, called asymmetry parameter (β), is measured as a function of composite field's phase. The dependence of the two channels' asymmetry amplitude (β₀) on the experimental parameters that characterize the composite field (wavelength, anisotropic shape, and total intensity) is found to differ significantly. The channel leading to H₂ ⁺ and O⁺ ions' ejection shows increased asymmetry compared to the other channel and is found to be dependent on excitation of overtones and combinations of vibrational modes as well as from the field's shape and intensity. The asymmetry (β) of the channel leading to the release of a H⁺ and an OH⁺ ions is far less sensitive to the experimental parameters. Inspection of the individual OH⁺ peak's dependence on phase reveals information on the effect of the field's profile, which is unclear when asymmetry (β) is inspected.

Quantum Interference Visibility Spectroscopy in Two-Color Photoemission from Tungsten Needle Tips

When two-color femtosecond laser pulses interact with matter, electrons can be emitted through various multiphoton excitation pathways. Quantum interference between these pathways gives rise to a strong oscillation of the photoemitted electron current, experimentally characterized by its visibility. In this Letter, we demonstrate the two-color visibility spectroscopy of multiphoton photoemissions from a solid-state nanoemitter. We investigate the quantum pathway interference visibility over an almost octave-spanning wavelength range of the fundamental (ω) femtosecond laser pulses and their second harmonic (2ω). The photoemissions show a high visibility of 90% ±  5%, with a remarkably constant distribution. Furthermore, by varying the relative intensity ratio of the two colors, we find that we can vary the visibility between 0% and close to 100%. A simple but highly insightful theoretical model allows us to explain all observations, with excellent quantitative agreements. We expect this work to be universal to all kinds of photo-driven quantum interference, including quantum control in physics, chemistry, and quantum engineering.

Population transfer through multiple channels in two harmonic laser pulses

The processes of population transfer in the ground electronic state of HCl molecule through the three transition schemes are investigated by numerically solving the time-dependent Schrödinger equation. Two harmonic pulses are employed to induce population transfer and the relative phase of the two pulses can control the final population distributions. In the ladder transition scheme, the variation range of the target population with the relative phase is nearly 100% which is larger than that in the multi-photon transition scheme. It is more efficient for the mixed transition scheme to control population transfer between the initial and target states by using the relative phase. Comparing with the multi-photon and ladder schemes, the transition probability of the target population is more sensitive to the two pulse amplitudes in the mixed transition scheme.

A laser-plasma accelerator driven by two-color relativistic femtosecond laser pulses

A typical laser-plasma accelerator (LPA) is driven by a single, ultrarelativistic laser pulse from terawatt- or petawatt-class lasers. Recently, there has been some theoretical work on the use of copropagating two-color laser pulses (CTLP) for LPA research. Here, we demonstrate the first LPA driven by CTLP where we observed substantial electron energy enhancements. Those results have been further confirmed in a practical application, where the electrons are used in a bremsstrahlung-based positron generation configuration, which led to a considerable boost in the positron energy as well. Numerical simulations suggest that the trailing second harmonic relativistic laser pulse is capable of sustaining the acceleration structure for much longer distances after the preceding fundamental pulse is depleted in the plasma. Therefore, our work confirms the merits of driving LPAs by two-color pulses and paves the way toward a downsizing of LPAs, making their potential applications in science and technology extremely attractive and affordable.

Controlling intramolecular hydrogen migration by asymmetric laser fields: the water case

Hydrogen and deuterium intramolecular migration in water's isotopomer dications has been found to depend on the wavelength of the laser used for the excitation. This is imprinted in H₂⁺ and D₂⁺ fragment ions' observation in the mass spectra induced by single color fs laser irradiation with 800 nm ≤λ≤ 1570 nm. Based on these findings, experiments with ω/2ω asymmetric laser fields (1400/700 nm) have been performed. The dissociation channels of the dications exhibit different dependence on the phase between the ω and 2ω components of the field thus offering an ability for controlling the fragmentation. For the interpretation of these observations, a tunneling mechanism is invoked.

Strong laser field control of fragment spatial distributions from a photodissociation reaction

The notion that strong laser light can intervene and modify the dynamical processes of matter has been demonstrated and exploited both in gas and condensed phases. The central objective of laser control schemes has been the modification of branching ratios in chemical processes, under the philosophy that conveniently tailored light can steer the dynamics of a chemical mechanism towards desired targets. Less explored is the role that strong laser control can play on chemical stereodynamics, i.e. the angular distribution of the products of a chemical reaction in space. This work demonstrates for the case of methyl iodide that when a molecular bond breaking process takes place in the presence of an intense infrared laser field, its stereodynamics is profoundly affected, and that the intensity of this laser field can be used as an external knob to control it.

Two-Color Coherent Control of Femtosecond Above-Threshold Photoemission from a Tungsten Nanotip

We demonstrate coherent control of multiphoton and above-threshold photoemission from a single solid-state nanoemitter driven by a fundamental and a weak second harmonic laser pulse. Depending on the relative phase of the two pulses, electron emission is modulated with a contrast of the oscillating current signal of up to 94%. Electron spectra reveal that all observed photon orders are affected simultaneously and similarly. We confirm that photoemission takes place within 10 fs. Accompanying simulations indicate that the current modulation with its large contrast results from two interfering quantum pathways leading to electron emission.

Coherent phase control of population transfer through ladder system in two laser pulses with ω and nω

The ladder transitions controlled by two harmonic pulses are investigated theoretically using a time-dependent quantum wave packet method for the ground electronic state of HF molecule. By choosing [Formula: see text], [Formula: see text] and [Formula: see text] schemes, the population can be transferred to target states [Formula: see text]. The population distribution can be controlled by the average amplitude of total electric field which depends on the relative phase of two pulses. With the variation of the relative phase between [Formula: see text] and [Formula: see text] pulses, the variation of population has a period of [Formula: see text]. For [Formula: see text] and [Formula: see text] schemes, the population distributions show oscillation behavior with a period of [Formula: see text] by varying the relative phase. The two harmonic pulses can realize a nearly complete population transfer to the target state.

Coherent control of population transfer between vibrational states in an optical lattice via two-path quantum interference

We demonstrate coherent control of population transfer between vibrational states in an optical lattice by using interference between a one-phonon transition at 2ω and a two-phonon transition at ω. The ω and 2ω transitions are driven by phase- and amplitude-modulation of the lattice laser beams, respectively. By varying the relative phase of these two pathways, we control the branching ratio between transitions to the first excited state and those to the higher states. Our best result shows a branching ratio of 17±2, which is the highest among coherent control experiments using analogous schemes. Such quantum control techniques may find broad application in suppressing leakage errors in a variety of quantum information architectures.

Directional emission of multiply charged ions during dissociative ionization in asymmetric two-color laser fields

Intense, asymmetric 1ω+2ω laser fields are used to affect the directional ejection of multiply charged ion fragments from a variety of molecules, including N2, O2, CO, HBr, and CO2. By tuning the relative phase, ϕ, between the two fields, we observe large forward-backward dissociation asymmetries. The largest asymmetries are obtained at the same values of ϕ for all species, suggesting a common dynamical mechanism. Following an independent phase calibration, the sign of the asymmetry appears to be opposite that expected from the standard enhanced ionization model.

Vibrationally selective optimal control of alignment and orientation using infrared laser pulses: application to carbon monoxide

Optimal control methods are used to study molecular alignment and orientation using infrared laser pulses. High order molecule-field interactions are taken into account through the use of the electric-nuclear Born-Oppenheimer approximation [G. G. Balint-Kurti et al., J. Chem. Phys. 122, 084110 (2005)]. High degrees of alignment and orientation are achieved by optimized infrared laser pulses of duration on the order of one rotational period of the molecule. It is shown that, through the incorporation of a vibrational projection operator into the optimization procedure, it is possible not only to maximize the alignment and orientation but also to bring the whole system into a single prescribed vibrational manifold. Numerical calculations are performed for carbon monoxide using ab initio potential energies computed in the presence of external electric fields.

Observation of the phase lag in the asymmetric photoelectron angular distributions of atomic barium

We have observed and measured the phase lag in the phase-dependent variation of the asymmetric photoelectron angular distribution of atomic barium. For these measurements, we photoionize the 6s6p(1)P(1) intermediate state of barium with concurrent one-photon and two-photon (omega-2omega) interactions. The laser interactions ionize the atoms in the vicinity of the series of autoionizing states converging upon the 5d(2)D(5/2) threshold. We study the variation of the phase lag as a function of the laser frequency. The variation shows strong correlation to the location of the autoionizing resonances, with full range exceeding 2pi, confirming the critical role of these resonances.

Coherent spectroscopy in dissipative media: Time-domain studies of channel phase and signal interferometry

We extend a recently formulated coherence spectroscopy of dissipative media [J. Chem. Phys. 122, 084502 (2005)] from the stationary excitation limit to the time domain. Our results are based on analytical and numerical solutions of the quantum Liouville equation within the Bloch framework. It is shown that the short pulse introduces a new, controllable time scale that allows better insight into the relation between the coherence signal and the phase properties of the material system. We point to the relation between the time-domain coherence spectroscopy and the method of interferometric two-photon photoemission spectroscopy, and propose a variant of the latter method, where the two time-delayed excitation pathways are distinguishable, rather than identical. In particular, we show that distinguishability of the two excitation pathways introduces the new possibility of disentangling decoherence from population relaxation.

Selective ionization of oriented nonpolar molecules with asymmetric structure by phase-controlled two-color laser fields

We report on the selective ionization of oriented nonpolar molecules with asymmetric structure by using phase-controlled two-color omega + 2omega laser pulses with an intensity of 1.0 x 10(13) W/cm(2) (tunneling ionization regime) and a pulse duration of 130 fs. The orientation of 1-bromo-2-chloroethane was monitored by the directional asymmetries of the forward-backward emission in dissociative ionization. The observed direction of orientation clearly confirms that molecular orientation is induced not by dynamic orientation but by selective ionization of oriented molecules, which reflects the structure of the highest occupied molecular orbital. This method can be applied for the vast majority of molecules.

Ab initio investigation of potential energy curves of the 23 electronic states of IBr correlating to neutral2P atoms

Potential energy surfaces for all Born-Oppenheimer electronic states of IBr molecule correlating to the neutral (2)P ((2)P(3/2) and (2)P(1/2)) iodine and bromine are calculated for the first time. Electric dipole and polarizability curves (static and transition) are also determined. Calculations include scalar and spin-orbit relativistic effects within all-electron Douglas-Kroll two-component Hamiltonian. Electron correlation is treated with quasi-degenerate multi-reference second-order perturbation theory. Seven adiabatic electronic states (X (1)Sigma(+), A'(3)Pi(2), A (3)Pi(1), 1 (3)Pi(0-), B (3)Pi(0+), B'(3)Sigma, and 2 (3)Pi(0+)) exhibit significant covalent bonding, and can support vibrational states. Calculated spectroscopic parameters agree with experiment to better than 1000 cm(-1) (T(e)), 10 cm(-1) (omega(e)), and 0.05 Angstrom (r(e)). A new 1 (3)Pi(0-) state correlating to ground-state atoms is predicted at T(e) approximately 14 000 cm(-1), omega(e) approximately 80 cm(-1), and r(e) approximately 3.0 Angstrom. The second new state (2 (3)Pi(0+)) correlates to excited iodine atom, with T(e) approximately 20 000 cm(-1), omega(e) approximately 115 cm(-1), and r(e) approximately 3.3 Angstrom. Non-adiabatic coupling parameters are calculated for the four avoided crossings, which arise due to electronic spin-orbit interaction. Estimated parameters of the B (3)Pi(0+)/B'(3)Sigma crossing (R(c) approximately 3.32 Angstrom; V approximately 120 cm(-1)) agree with experimental values. The previously unsuspected 2 (3)Pi(0-)/1 (1)Sigma(-) crossing of two repulsive surfaces provides a new collisional deactivation channel for Br* atoms at relative velocities above 1000 m s(-1). Several repulsive states (including 1 (1)Pi(1) and 2 (3)Pi(1)) intersect the B/B' system near the avoided crossing point, and may affect dynamics of IBr in strong laser fields.

Interpreting closed-loop learning control of molecular fragmentation in terms of wave-packet dynamics and enhanced molecular ionization

We interpret a molecular fragmentation experiment using shaped, ultrafast laser pulses in terms of enhanced molecular ionization during dissociation. A closed-loop learning control experiment was performed to maximize the CF3+CH3+ production ratio in the dissociative ionization of CH3COCF3. Using ab inito molecular structure calculations and quasistatic molecular ionization calculations along with data from pump-probe experiments, we identify the primary control mechanism which is quite general and should be applicable to a broad class of molecules.

Transformations to diagonal bases in closed-loop quantum learning control experiments

This paper discusses transformations between bases used in closed-loop learning control experiments. The goal is to transform to a basis in which the number of control parameters is minimized and in which the parameters act independently. We demonstrate a simple procedure for testing whether a unitary linear transformation (i.e., a rotation amongst the control variables) is sufficient to reduce the search problem to a set of globally independent variables. This concept is demonstrated with closed-loop molecular fragmentation experiments utilizing shaped, ultrafast laser pulses.