1990 - 1993 : Excited states of diatomic molecules
I studied the excimer emission spectra and the interaction potential energy of the ground and excited states of
He and alkali, metal ion systems for graduation thesis. The rare gas-alkali molecular ions, HeLi+, HeNa+ and HeK+,
are investigated with the ab initio MO CI method. The simulated theoretical emission spectra from the first
excited state of these molecules were in good agreement with experimental spectra. Because a competitive pathway
is found in the excited singlet state of HeK+, HeNa+ is the rare-gas-alkali-metal diatomic ion which shows
the shortest wavelength emission among these possible candidates for VUV lasing systems.
After that I studied theoretical assignment of the vibronic bands in the photoelectron spectra of
N2, HCl, and CO below 30 eV for master thesis. The high-resolution photoelectron spectra of
N2, HCl and CO up to 30 eV are analyzed with ab initio MO CI calculations.
I was able to identify the vibrational quantum numbers of the spectra in the bands between 23 and 30 eV.
From this, I discovered that previously unknown excited state had been hidden within the N2 photoelectron spectra.
These days, these data are used to assign synchrotron radiation photoionization spectra.
1993 - 2006 : Super excited states of molecules
The super excited state is defined as the neutral molecule excited state of which energy
is higher than the ionization threshold. There are two kinds of super exited state:
one is the electronic excited state of neutral molecule and the other is vibrational excited state of Rydberg state.
Autoionization
I investigated the characteristics and dynamics of superexcited states of
diatomic molecules using original general theoretical procedure for doctor thesis.
I also studied the photoelectron spectra of atoms and molecules with an autoionizing state in intense laser fields.
Strong mixing of auto- and photoionizations. Photoelectron spectra of an atom with an autoionizing state
in intense laser fields are investigated theoretically. In intense fields, photoelectron spectra are found
to have a profile totally different from that in weak fields.
A 'phase transition' is found to take place at critical laser
intensity and the spectral profile changes drastically at this critical point.
This phenomenon originates from mixing of auto- and photoionizations in intense laser fields.
R matrix / multichannel quantum defect theory (R-matrix/MQDT)
To obtain the potential energy curves of both Rydberg and valence excited states,
I have made the R-matrix/ multichannel quantum defect theory (R-matrix/MQDT)
method by a combination of ab initio and multichannel quantum defect theories.
I have shown that this method is used to calculate resonant changes in the MQDT
scattering matrix arising from valence state interactions with the Rydberg
channels of nitrogen monoxide. The resonant couplings between the
Rydberg channels and four valence states are shown to vary sufficiently
smoothly with both bond length and energy to allow easy interpolation.
I have extended the range of application of R-matrix/MQDT,
and this method cab be used to analyze core excitation spectra.
Both valence and core excitations of the NO molecule were studied using the R matrix/ MQDT method.
If we use the standard ab initio program code and R matrix/MQDT method,
we can obtain not only the valence excited states but also core excited states of molecules and analyze the experimental spectra.
2006-2014 : Molecular Dynamics Simulations for Rigid-Body Systems
For dynamics simulation including enough water molecules in biological systems,
an approximation for water molecules will be needed.
Although some water molecules in biological systems play
important roles in the reactions, the others work as a filed.
The latter kind water molecules can be approximate as rigid molecules.
Angular Momentum Verlet (AMV) algorithm
A new simple algorithm named 'the angular momentum Verlet (AMV) algorithm '
was presented for solving the equations of motion for rigid molecules.
For obtaining a high accuracy in molecular dynamics simulations,
the scaling method with the constraint by Lagrange's method of
undetermined multipliers was introduced in this algorithm.
Fast Time-Reversible Molecular Dynamics Algorithm (FT)
In association with AMV algorithm, a new algorithm for molecular dynamic simulation for rigid molecules was made
with Prof. Kajima and Prof. Ogata's gourp in Nagoya Institute of Technology.
We developed a remarkable algorithm in the molecular dynamics to numerically enforce the complete time-reversal symmetry
2010 - : Mechanisms of Photochemistry in Biomolecules
To elucidate the mechanism of photochemistry in biomolecules theoretically,
we need the information about the electronic states of biomolecules,
the nuclear dynamics of them, and the effects from the environment around them.
I pay attention to the electron transition in the photochemistry of
photosynthesis and bioluminescences.
These processes are considered as essential processes in each biological system.
Electron Transfer in Photochemical Reaction Center of Bacterial Photosystems
Photosynthesis is a biochemical reaction that transforms solar
energy into chemical energy. The essence of light-dependent
reactions in the early stages of photo- synthesis is the transformation of
the absorbed excitation energy into the electron transfer energy.
There are two light-dependent reaction systems in plants and algae,
photosystem I (PSI) and photosystem II (PSII).
Bacterial photosystems are suitable for investigating the electron transfer
in these reaction centers because they are categorized as either PSI or PSII.
A typical example for PSI systems is the reaction center of purple bacteria.
I investigated that the electron transfer in the reaction centers of purple bacteria
using ab initio method with Prof. Koga of Nagoya University.
We found that the presence of the longest substitution on porphyrin rings
play rules of promoting the electron transition in the reaction center of purple bacteria.
Emission Color in Firefly Bioluminescence
To understand the mechanism of firefly bioluminescence, the spectroscopic property has been paid attention to
and investigated over the past few decades. In the firefly bioluminescence, not only its substrate and emitter
but also the effects from proteins may decide the emission color.
However, the spectroscopic characteristics of firefly luciferin (luciferin here after) and oxyluciferin,
which are substrate and emitter of firefly bioluminesccence, were unclear.
Firefly Luciferin
To understand the mechanism of firefly bioluminescence, the spectroscopic property has been paid attention to
Because of unstable of oxyluciferin in aqueous solutions, its experimental spectra were quite few.
On the other hand, there are many experimental data for luciferin in aqueous solutions.
Therefore, first of all we paid attention to the spectroscopic characteristics of luciferin.
We studied the photoluminescence of luciferin and clarified its ground and excited states with the quantum chemical calculations.
These results are in good agreement with its experimental spectra.
We found that the main peak position in photoluminescence spectra was changed with the pH value of its solution
because the kinds of most abundant chemical species were changed.
Recently, the photo absorption spectra of lucifein in gas phase were reported.
Then, we analyzed these photo absorption spectra of luciferin in gas phase using the first-principles GW+Bethe-Salpeter method
made by Dr. Noguchi of ISSP and compared with the results obtained using a recent experiment in vacuum.
This method well reproduces the line shape at the photon energy corresponding to the Rydberg and resonance excitations.
Firefly Oxyluciferin
Next, we studied the theoretical absorption spectra of oxyluciferin using similar methods for luciferin.
The absorption spectra and photoluminescence pathway of oxylucifein in aqueous solutions were estimated.
We also determined the full width at half maximum of absorption and emission spectra from vibrational analysis.
The pH dependence of theoretical molar concentration is in good agreement with experimental one.
We elucidated the photoluminescence process for green emissions above and below pH 8,
the photoluminescence process for blue emission, and the photoluminescence process for red emission in strong acidic conditions.
2014 - : Core Absorption Spectra
Recent developments in experimental techniques enable us to obtain sufficiently accurate X-ray absorption spectra (XAS)
and discuss the electronic and atomic properties of a wide range of materials under various experimental conditions.
The oxygen 1s XAS of acetone and acetic acid molecules in vacuum were calculated by utilizing
the first-principles GW+Bethe-Salpeter method with an all-electron mixed basis.
The theoretical excitation energies show good agreement with the available experimental data without an artificial shift.
Our method reproduces the first and second isolated peaks and broad peaks at higher photon energies, corresponding to Rydberg excitations.