Shinichirou Minemoto

Quantum interference during high-order harmonic generation from aligned molecules.

High-order harmonic generation (HHG) from atoms and molecules offers potential application as a coherent ultrashort radiation source in the extreme ultraviolet and soft X-ray regions. In the three-step model of HHG, an electron tunnels out from the atom and may recombine with the parent ion (emitting a high-energy photon) after undergoing laser-driven motion in the continuum. Aligned molecules can be used to study quantum phenomena in HHG associated with molecular symmetries; in particular, simultaneous observations of both ion yields and harmonic signals under the same conditions serve to disentangle the contributions from the ionization and recombination processes. Here we report evidence for quantum interference of electron de Broglie waves in the recombination process of HHG from aligned CO2 molecules. The interference takes place within a single molecule and within one optical cycle. Characteristic modulation patterns of the harmonic signals measured as a function of the pump–probe delay are explained with simple formulae determined by the valence orbital of the molecules. We propose that simultaneous observations of both ion yields and harmonic signals can serve as a new route to probe the instantaneous structure of molecular systems.

Feedback optimization of shaped femtosecond laser pulses for controlling the wavepacket dynamics and reactivity of mixed alkaline clusters

Abstract In this paper we describe the branching control of different ionization and fragmentation channels of coherently excited Na 2 K by means of feedback optimization of shaped femtosecond laser pulses. For this purpose the system was first excited with one photon into a pair of intersecting electronic states, from where it was ionized in a two-photonic process, so the resulting ions could size selectively be detected. By employing an evolutionary algorithm for optimizing phase and amplitude of the applied laser field, the relative signal intensities of the resulting mother and fragment ions could significantly be influenced. The peak intensities in the obtained pulse shapes correspond very well to the cross sections of the irradiated transitions; their temporal structure reflects perfectly the wavepacket dynamics of the resulting particles. Hence it was possible to extract with the self-learning algorithm – as postulated by Judson and Rabitz [Phys. Rev. Lett. 68 (1992) 1500] – intrinsic properties from the reactive system in real time.

Analysis and control of laser induced fragmentation processes in CpMn(CO)3

Abstract In this paper we present experimental and theoretical findings about the dynamics of ultrafast fragmentation processes which occur during resonant multiphoton ionization of CpMn(CO) 3 with femtosecond laser pulses. Employing a two-color pump and probe scheme, it was possible to retrieve lifetimes of the electronically excited parent molecule and its first fragment CpMn(CO) 2 . The observed time of 66 fs for the loss of the first CO-ligand is in good agreement with the results of one-dimensional quantum dynamical model simulations based on three relevant adiabatic ab initio potentials and the related components of the transition dipole matrix elements, in C s symmetry. Subsequently, smaller fragments appear somewhat later during approximately 100 fs. Based on these findings we performed feedback control experiments on the system in order to optimize individual fragmentation/ionization paths. With the routine a considerable increase of – for example – the CpMn(CO) + /CpMn(CO) + 3 intensity ratio was achieved. The obtained optimized laser pulses correlate well with the fast dynamics of the photoinduced preparation of CpMn(CO) + 3 versus CpMn(CO) + product ions, respectively.

Observation of molecular orientation by the combination of electrostatic and nonresonant, pulsed laser fields

The orientation of polar molecules is demonstrated by the combination of electrostatic and nonresonant, nanosecond Nd:YAG laser fields. The orientation is probed by Coulomb exploding the molecules with a femtosecond laser pulse and detecting the fragment ions with the time-of-flight mass spectrometer. A significant asymmetry is observed in the signal magnitudes of the forward and the backward fragments, which is well explained in terms of the above-mentioned combined-fields scheme proposed by Friedrich and Herschbach [J. Phys. Chem. A 103, 10280 (1999)]. The degree of orientation is enhanced by increasing the peak intensity of the laser field and the magnitude of the electrostatic field, or by lowering the initial rotational temperature. The experimental results obtained are compatible with our numerical simulations.

Polarizability anisotropies of rare gas van der Waals dimers studied by laser-induced molecular alignment

The molecular alignment technique utilizing the interaction between the intense nonresonant laser field and the induced dipole moment is applied to the homonuclear rare gas dimers Rg2 (Rg=Ar, Kr, and Xe). The degree of alignment is investigated by Coulomb exploding Rg2 and by measuring the angular distributions of the fragment ions. At the same peak intensity of the laser field, the degree of alignment ≪cos2 θ ≫ becomes larger in order of Ar2, Kr2, and Xe2, reflecting the order of magnitudes of their polarizability anisotropy Δα. By taking I2 molecules as a reference, Δα of Ar2, Kr2, and Xe2 are estimated to be 0.5, 0.7, and 1.3 A3, respectively.

Electronic structures of cobalt cluster cations: Photodissociation spectroscopy of Co+nAr (n=3–5) in the visible to near‐infrared range

Photodissociation spectra of cobalt cluster ion–argon atom complexes, Co+nAr (n=3–5), were measured by detecting the product ions, Co+n and Con−1+, with a tunable laser pulse from an optical parametric oscillator in the photon‐energy range from 0.7 to 2.8 eV. The photodissociation spectra thus obtained are equivalent to the optical absorption spectra of the underlying cobalt cluster ions, Co+n, because the argon atom is weakly bound to Co+n. The spectrum was analyzed by means of a spin‐polarized DV‐Xα calculation, and the electronic and the geometric structures of Co+3 and Co+4 were determined. The analysis shows that all the transitions in the entire energy range studied occur between occupied and unoccupied energy levels associated with 3d atomic orbitals (AOs) having the minority spin. The spin difference (difference in population per Co atom between the majority and the minority spins) was evaluated from the electronic structures thus obtained. The spin differences of 2.00 for Co+3 and 1.75 for Co+4 i...

Ferromagnetic spin coupling in the manganese trimer ion evidenced by photodissociation spectroscopy

The optical spectrum of the manganese trimer ion, Mn3+, was obtained by measurement of the photodissociation cross section in the photon-energy range between 1.43 and 4.13 eV. Analysis of the spectrum by quantum-chemical calculations derived its electronic and geometric structures. The geometric structure was found to be an isosceles triangle (C2v) with bond lengths of 3.03 A and an apex angle of 144°. The ground electronic state was found to be 17B2. The electronic structure of the valence orbitals indicates that the chemical bond is formed weakly by the 4s electrons. The 3d electrons are localized on the atomic sites, as is suggested by the nonbonding nature of the nearly degenerate occupied orbitals. All of the local spins are in the majority-spin state, and give rise to a total spin magnetic moment as large as 16μB. The ferromagnetic nature is due to the weak binding among constituent atoms and to the strong 3d-4s exchange interaction in the manganese atom. This finding is in marked contrast to the an...

Energetics of the manganese trimer and tetramer ions

The photodissociation processes of the manganese cluster ions Mnn+ (n=3 and 4) were investigated in the visible and near-infrared photon-energy ranges. The threshold energies were determined for the two-atom-loss channels by measurements of the spectra of the partial photodissociation cross sections. The binding energies of Mnn+ (n=3 and 4) were obtained by using these threshold energies and the known bond dissociation energy of Mn2+. The bond dissociation energies of Mn3+, D0(Mn2+⋯Mn), and Mn4+, D0(Mn3+⋯Mn), were determined to be 0.83±0.05 and 1.04±0.07 eV, respectively. Thus the binding energies (per atom) of Mn3+ and Mn4+ turned out to be 0.74±0.03 and 0.82±0.05 eV, respectively. These findings show that the interaction between manganese atoms is exceptionally weak, compared with other transition-metal cluster ions.

Photoelectron diffraction from laser-aligned molecules with X-ray free-electron laser pulses

We report on the measurement of deep inner-shell 2p X-ray photoelectron diffraction (XPD) patterns from laser-aligned I2 molecules using X-ray free-electron laser (XFEL) pulses. The XPD patterns of the I2 molecules, aligned parallel to the polarization vector of the XFEL, were well matched with our theoretical calculations. Further, we propose a criterion for applying our molecular-structure-determination methodology to the experimental XPD data. In turn, we have demonstrated that this approach is a significant step toward the time-resolved imaging of molecular structures.

Measuring polarizability anisotropies of rare gas diatomic molecules by laser-induced molecular alignment technique

The polarizability anisotropies of homonuclear rare gas diatomic molecules, Ar(2), Kr(2), and Xe(2), are investigated by utilizing the interaction of the induced electric dipole moment with a nonresonant, nanosecond laser pulse. The degree of alignment, which depends on the depth of the interaction potential created by the intense laser field, is measured, and is found to increase in order of Ar(2), Kr(2), and Xe(2) at the same peak intensity. Compared with a reference I(2) molecule, Ar(2), Kr(2), and Xe(2) are found to have the polarizability anisotropies of 0.45 ± 0.13, 0.72 ± 0.13, and 1.23 ± 0.21 Å(3), respectively, where the uncertainties (one standard deviation) in the polarizability anisotropies are carefully evaluated on the basis of the laser intensity dependence of the degree of alignment. The obtained values are compared with recent theoretical calculations and are found to agree well within the experimental uncertainties.