Akitaka Matsuda

We investigate intramolecular hydrogen migration in acetonitrile dication in phase-controlled ω-2ω intense laser fields (800 and 400 nm, 3.3 × 1014 W/cm2) using three-dimensional coincidence ion momentum imaging. The two-body Coulomb explosion pathway, CH3CN2+→CH3++CN+, exhibits a clear phase-dependent fragment asymmetry along the laser polarization direction, showing that the tunnel ionization preferentially prepares the acetonitrile dication with the methyl group pointing toward the smaller amplitude side of the laser electric field. The Coulomb explosion pathway occurring after the migration of a single hydrogen atom, CH3CN2+→CH2++HCN+, shows a pronounced reduction in the fragment asymmetry. A clear deuteration effect observed for the asymmetry of the hydrogen-migration pathway suggests that the two-color asymmetric laser field has a significant impact on the directionality of intramolecular hydrogen migration in acetonitrile dication.

Femtosecond laser-filament reactions in gaseous hexane (C₆H₁₄) are studied by time-of-flight ion mass spectrometry. The neutral products are unambiguously distinguished from species produced by ionization for mass analysis, through velocity screening along the flight tube. In addition to hydrogen-capped polyynes, C_nH₂ (n = 4, 6, 8, 10 and 12), the mass spectrometry reveals hydrocarbon molecules in the mass range of m/z 50 -146 that had eluded identification in the previous studies, The product distributions, together with their dependence on laser pulse energy and repetition rate, provide insight into the association reaction pathways to hydrogen-capped polyynes and other products by laser filaments.

Strong-field ionization of H 2 in circularly polarized laser fields (1035 nm, 35 fs, ∼ 1 × 10 14 W / cm 2 ) has been studied by three-dimensional electron-ion coincidence momentum imaging. Both the effective ionization potential and the applied laser electric field amplitude are simultaneously determined from the parameters characterizing the tunnel-electron torus in momentum space, using the adiabatic theory of strong-field ionization extended to include higher-order nonadiabatic effects as a reference. The effective ionization potential of H 2 , obtained with an accuracy and precision of 10 − 2 a.u., reveals a clear shift from the field-free value. Dependence on both the field amplitude and the ionization pathways to H 2 + and H + + H is also resolved. These findings demonstrate that the target-specific properties can be quantitatively decoded from the structure of the photoelectron momentum torus, thus establishing a basis for quantitative ultrafast tunnel-electron spectroscopy.

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.

Three-dimensional electron–ion coincidence momentum imaging reveals that the dissociative tunneling ionization of tetrafluoromethane (CF₄) is sensitive to the helicity of the circularly polarized laser fields.

Electron-ion coincidence spectroscopy is powerful to elucidate tunneling ionization dynamics, but applicable only to molecules susceptible to dissociation. Here we introduce a novel scheme with additional pulses, which has been successfully applied to O₂ HOMO-1.