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.

Nanosecond simulations of the dynamics of C60 excited by intense near-infrared laser pulses: impulsive Raman excitation, rearrangement, and fragmentation.

Impulsive Raman excitation of C(60) by single or double pulses of near-infrared wavelength λ = 1800 nm was investigated by using a time-dependent adiabatic state approach combined with the density functional theory method. We confirmed that the vibrational energy stored in a Raman active mode of C(60) is maximized when T(p) ~ T(vib)/2 in the case of a single pulse, where T(p) is the pulse length and T(vib) is the vibrational period of the mode. In the case of a double pulse, mode selective excitation can be achieved by adjusting the pulse interval τ. The energy of a Raman active mode is maximized if τ is chosen to equal an integer multiple of T(vib) and it is minimized if τ is equal to a half-integer multiple of T(vib). We also investigated the subsequent picosecond or nanosecond dynamics of Stone-Wales rearrangement (SWR) and fragmentation by using the density-functional based tight-binding semiempirical method. We present how SWRs are caused by the flow of vibrational kinetic energy on the carbon bond network of C(60). In the case where the h(g)(1) prolate-oblate mode is initially excited, the number of SWRs before fragmentation is larger than in the case of a(g)(1) mode excitation for the same excess vibrational energy. Fragmentation by C(2) ejection C(60) → C(58) + C(2) is found to occur from strained, fused pentagon/pentagon defects produced by a preceding SWR, which confirms the earliest mechanistic speculations of Smalley et al. [J. Chem. Phys. 88, 220 (1988)]. The fragmentation rate of C(2) ejection in the case of h(g)(1) mode excitation does not follow a statistical description as employed for instance in the Rice-Ramsperger-Kassel (RRK) theory, whereas the rate for a(g)(1) mode excitation does follow the prediction by RRK. We also found for the h(g)(1) mode excitation that the nonstatistical nature affects the distribution of barycentric velocities of fragments C(58) and C(2). This result suggests that it is possible to control rearrangement and subsequent bond breaking in a "nonstatistical" way by initial selective mode excitation.

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.

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).

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

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 author's 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+).

Time-dependent density functional theory investigation of electric field effects on absorption spectra of meso-meso-linked zinc porphyrin arrays: Role of charge-transfer States.

By using time-dependent density functional theory, we calculated the transition energies of a zinc porphyrin monomer and its meso-meso-linked arrays. In line with the prediction of the molecular exciton model, the calculated splitting energy of the Soret band increased as the number of linked porphyrins increased. We then examined how the transition energies of the dimer array were shifted by an applied electric field. For reproduction of an electroabsorption spectrum (EA), i.e., the field-induced change in absorption intensity, a model Hamiltonian constructed from five states is proposed. It is concluded for the dimer that the field-induced coupling between the lower-energy Soret band Se and the lower-lying ionic character (charge-transfer) states is responsible for the experimentally observed blue shift of Se as well as the second-derivative profile in the EA spectrum.