Katsunori Nakai

Communication: Two stages of ultrafast hydrogen migration in methanol driven by intense laser fields

Hydrogen migration in methanol induced by an intense laser field (0.2 PW/cm(2)) is investigated in real time by a pump-probe coincidence momentum imaging method. The observed temporal evolution of the kinetic energy spectra reveals that there are two distinctively different stages in the hydrogen migration processes in the singly charged methanol: ultrafast hydrogen migration occurring within the intense laser field ( approximately 38 fs) and slower postlaser pulse hydrogen migration ( approximately 150 fs).

Two-proton migration in 1,3-butadiene in intense laser fields

Ultrafast proton migration in 1,3-butadiene in an intense laser field (40 fs, 4.5 × 10(14) W cm(-2)) is investigated by using Coulomb explosion coincidence momentum imaging. The spatial distribution maps of a migrating proton reconstructed for the two three-body Coulomb explosion pathways, C(4)H(6)(3+)→ H(+) + CH(3)(+) + C(3)H(2)(+) and C(4)H(6)(3+)→ H(+) + C(2)H(+) + C(2)H(4)(+), reveal that two protons migrate within a 1,3-butadiene molecule, prior to the three body decomposition.

Electronic structures of azulene-fused porphyrins as seen by magnetic circular dichroism and TD-DFT calculations

A combination of magnetic circular dichroism (MCD), electronic absorption spectroscopy and time-dependent density functional theory (TD-DFT) calculations has been used to investigate the electronic structure of azulene-fused pi-expanded porphyrins based primarily on the spectral properties of absorption bands in the near infrared region. From MCD experiments, it was suggested that in the case of a mono-azulene-fused porphyrin DeltaHOMO approximately equal DeltaLUMO (where DeltaHOMO is the magnitude of the energy gap between the HOMO and HOMO-1 and DeltaLUMO is the magnitude of the energy gap between the LUMO and LUMO+1), while in the case of an oppositely-di-azulene-fused porphyrin, DeltaHOMO<DeltaLUMO. Since Faraday A terms are observed for both the Soret and Q bands in the MCD spectrum of tetra-azulene-fused porphyrin the corresponding excited states are clearly accidentally degenerate despite the C(2) molecular symmetry. Transition dipole moment analysis clearly demonstrates that the electronic absorption spectrum of tetra-azulene-fused porphyrin has out-of-plane electronic transitions slightly to the blue of the main Q and Soret bands. Comparison with distorted porphyrins and phthalocyanines strongly suggests that these out-of-plane transitions appear as intense Gaussian-shaped Faraday B terms in the MCD spectra.

Applications of magnetic circular dichroism spectroscopy to porphyrins and phthalocyanines

Magnetic circular dichroism (MCD) spectroscopy has widely been applied to porphyrins and phthalocyanines since around 1970, in order to elucidate their electronic structures. In this mini-review, some representative MCD results from the authors laboratory over the past 30 years are introduced, together with recent results from other laboratories. MCD studies on the following monomeric species are included: D(4h) type, adjacent vs. opposite type diaromatic ring-fused, non-planar, and reduced and oxidized species, as well as species showing temperature-dependent MCD signals. In addition, one example illustrates the use of MCD as a probe for the distal histidine residue in myoglobin. Recent results on dimers and oligomers are also reported. In particular, it is confirmed that the spectra of cofacial eclipsed dimers do not reflect the molecular symmetry of the constituent monomers. The spectra of rare-earth sandwich dimers and trimers are definitively assigned. Using spectra of planar oligomers of porphyrins, it is reiterated that it is often dangerous to assign the absorption bands of chromophores based only on the results of molecular orbital calculations. Some examples show that MCD can give information on the relative size of the DeltaHOMO (energy difference between the HOMO and HOMO-1) and DeltaLUMO (energy difference between the LUMO and LUMO+1); for example, if DeltaHOMO > DeltaLUMO, the MCD signal changes from minus to plus in ascending energy.

Theoretical investigation of the stability of highly charged C60 molecules produced with intense near-infrared laser pulses

We theoretically investigated the stability of highly charged C(60) (z+) cations produced from C(60) with an ultrashort intense laser pulse of lambda approximately 1800 nm. We first calculated the equilibrium structures and vibrational frequencies of C(60) (z+) as well as C(60). We then calculated key energies relevant to dissociation of C(60) (z+), such as the excess vibrational energy acquired upon sudden tunnel ionization from C(60). By comparing the magnitudes of the calculated energies, we found that C(60) (z+) cations up to z approximately 12 can be produced as a stable or quasistable (microsecond-order lifetime) intact parent cation, in agreement with the recent experimental report by V. R. Bhardwaj et al. [Phys. Rev. Lett. 93, 043001 (2004)] that almost only intact parent C(60) (z+) cations up to z=12 are detected by a mass spectrometer. The results of Rice-Ramsperger-Kassel-Marcus calculation suggest that the lifetime of C(60) (z+) drastically decreases by ten orders of magnitude as z increases from z=11 to z=13. Using the time-dependent adiabatic state approach, we also investigated the vibrational excitation of C(60) and C(60) (z+) by an intense near-infrared pulse. The results indicate that large-amplitude vibration with energy of >10 eV is induced in the delocalized h(g)(1)-like mode of C(60) (z+).

Communication: Long-lived neutral H2 in hydrogen migration within methanol dication

The ejection of triatomic hydrogen molecular ions HD2(+) and D3(+) from CD3OH(2+) is investigated by first-principle molecular dynamics simulation. Two C-D chemical bonds are found to be broken to form a neutral D2 moiety that vibrates, rotates, and moves for a relatively long period of time (20-330 fs) towards a transition state leading to the ejection of HD2(+) or D3(+). The formation of such a long-lived neutral D2 moiety within a hydrocarbon molecule interprets well the recent experimental findings of the long lifetime of doubly charged energized hydrocarbon molecules prior to the ejection of H3(+).

Characterization of multielectron dynamics in molecules: A multiconfiguration time-dependent Hartree-Fock picture

Using the framework of multiconfiguration theory, where the wavefunction Φ(t) of a many-electron system at time t is expanded as Φ(t)=Σ(I)C(I)(t)Φ(I)(t) in terms of electron configurations {Φ(I)(t)}, we divided the total electronic energy E(t) as E(t)=Σ(I)|C(I)(t)|(2)E(I)(t) . Here E(I)(t) is the instantaneous phase changes of C(I)(t) regarded as a configurational energy associated with Φ(I)(t). We then newly defined two types of time-dependent states: (i) a state at which the rates of population transfer among configurations are all zero; (ii) a state at which {E(I)(t)} associated with the quantum phases of C(I)(t) are all the same. We call the former time-dependent state a classical stationary state by analogy with the stationary (steady) states of classical reaction rate equations and the latter one a quantum stationary state. The conditions (i) and (ii) are satisfied simultaneously for the conventional stationary state in quantum mechanics. We numerically found for a LiH molecule interacting with a near-infrared (IR) field ε(t) that the condition (i) is satisfied whenever the average velocity of electrons is zero and the condition (ii) is satisfied whenever the average acceleration is zero. We also derived the chemical potentials μ(j)(t) for time-dependent natural orbitals ϕ(j)(t) of a many-electron system. The analysis of the electron dynamics of LiH indicated that the temporal change in Δμ(j)(t) ≡ μ(j)(t) + ε(t) · d(j)(t) - μ(j)(0) correlates with the motion of the dipole moment of ϕ(j)(t), d(j)(t). The values Δμ(j)(t) are much larger than the energy ζ(j)(t) directly supplied to ϕ(j)(t) by the field, suggesting that valence electrons exchange energy with inner shell electrons. For H2 in an intense near-IR field, the ionization efficiency of ϕ(j)(t) is correlated with Δμ(j)(t). Comparing Δμ(j)(t) to ζ(j)(t), we found that energy accepting orbitals of Δμ(j)(t) > ζ(j)(t) indicate high ionization efficiency. The difference between Δμ(j)(t) and ζ(j)(t) is significantly affected by electron-electron interactions in real time.

Coherent vibrations in methanol cation probed by periodic H 3 + ejection after double ionization

When hydrocarbon molecules are exposed to an intense laser field, triatomic hydrogen molecular ion, H3+, is ejected. Here we describe pump–probe measurements of the ejection of H3+ from methanol dication with high temporal resolution using intense few-cycle laser pulses and find a long-lasting periodic increase in the yield of H3+. We show that H3+ ejection is the lowest energy decomposition channel and that its yield is enhanced each time when the vibrational wave packet coming back to the inner turning point of methanol cation is projected onto the dication potential energy surface. We also show that the time-resolved measurement of the yield of H3+ is an efficient tool not only for probing ultrafast nuclear dynamics of hydrocarbon cations but also for deriving vibrational frequencies of hydrocarbon cations with high precision.When hydrocarbon molecules are exposed to an intense laser field, triatomic hydrogen molecular ions are ejected. Here, femtosecond spectroscopic study of the production of triatomic hydrogen ions from methanol dications offers insight into the dynamics of hydrocarbon cations.

Ab initio molecular dynamics of highly charged fullerene cations in intense near-infrared laser fields

We theoretically investigated the stability of highly charged fullerene cations produced with an ultrashort intense nearinfrared (IR) laser pulse (light intensity I~ 5 × 1014 W/cm2 and wavelength λ ~ 1800 nm). The effects of nonlinear interactions with near-IR pulses are taken into account by combining an ab initio molecular dynamics method with an time-dependent adiabatic state approach. The results indicate that large-amplitude vibration with energy of > 10 eV is induced by impulsive Raman excitation in the delocalized hg(1)-like mode of C60 z+. The field-induced large-amplitude vibration of the hg(1) mode persists for a rather long period. In conclusion, C60 and its cations created upon ionization are extremely robust against field-induced structural deformation. We found that the acquired vibrational energy is maximized at Tp ~ vib/2, where Tp is the pulse length and Tvib is the vibrational period of the hg(1) mode. We confirmed that the vibrational energy deposited in C60 can be controlled by a pulse train, i.e., by changing the intervals between pulses. Vibrational mode selectivity is also achieved by adjusting the pulse intervals.

Ultrafast Delocalization of Protons in Methanol and Allene in Intense Laser Fields

Ultrafast hydrogen migration in methanol and allene induced by intense laser fields was investigated by Coulomb explosion coincidence momentum imaging (CMI). For methanol, it was shown that there are two distinctively different stages in the hydrogen migration processes in singly charged methanol, i.e., ultrafast hydrogen migration occurring within the intense laser field, and slower post-laser pulse hydrogen migration, showing quantum mechanical nature of light protons. For allene, the distribution of a proton covering the wide spatial area in an allene molecule was visualized by the momentum correlation maps constructed from the observed momentum vectors of fragment ions.