Browsing by Author "Shimakura, Noriyuki"

Now showing 1 - 5 of 5

Ab initio quantum dynamical analysis of ultrafast nonradiative transitions via conical intersections in pyrazine
(2015-01-01)
Kanno, Manabu
;
Ito, Yuta
;
Shimakura, Noriyuki
;
Koseki, Shiro
;
Kono, Hirohiko
We theoretically investigated the mechanism of ultrafast nonradiative transition through conical intersections in photoexcited pyrazine by ab initio quantum dynamical calculations. This work was motivated by the recent theoretical and experimental studies that presented conflicting results: the former is the on-the-fly semiclassical surface hopping calculation combined with the time-dependent density functional theory, which showed that nonadiabatic transitions from the optically bright S-2 (B-1(2u), pi pi*) state to the optically dark S-3 ((1)A(u), n pi*) and S-4 (B-1(2g), n pi*) states take place predominantly at the initial stage of electronic relaxation [U. Werner et al., Chem. Phys., 2008, 349, 319]; the latter is the pump-probe photoelectron spectroscopic measurement, which reported the S2 lifetime (22 +/- 3 fs) of nonradiative decay to the almost dark S-1 (B-1(3u), n pi*) state [Y.-I. Suzuki et al., J. Chem. Phys., 2010, 132, 174302]. We constructed adiabatic and diabatic potential energy surfaces of these pi pi* and n pi* states using the multireference configuration interaction method and calculated their diabatic couplings within two-dimensional subspaces spanned by selected ground-state normal coordinates. Contrary to the surface hopping study, our nuclear wave packet simulations demonstrated that nonadiabatic transitions to the S-3 and S-4 states are so small that the conventional two-state (S-1 and S-2) picture is valid. Ultrafast internal conversion of pyrazine, which is deemed to proceed with a 22 fs lifetime, in fact consists of three consecutive steps: (i) the wave packet excited to the S-2 state travels toward the S-2-S-1 conical intersection in 10 fs, (ii) the nonadiabatic transition to the S-1 state progresses at a rapid rate corresponding to a transient lifetime of 7 fs, and (iii) intramolecular vibrational energy redistribution occurs in the S-1 state in about 80 fs after optical excitation. To verify this prediction, time-resolved experiments with a resolution of several fs or shorter are desirable.
Ab Initio Quantum Dynamical Study on Ultrafast Nonradiative Transition Pathways of Pyrazine
(2015-01-01)
Kanno, Manabu
;
Ito, Yuta
;
Shimakura, Noriyuki
;
Koseki, Shiro
;
Kono, Hirohiko
We theoretically verified the participation of optically dark n pi* states other than S-1 in ultrafast internal conversion of pyrazine. Contrary to a recent semiclassical study, our quantum dynamical calculations demonstrated that their contributions are negligible.
Neutral-Fragmentation Paths of Methane Induced by Intense Ultrashort IR Laser Pulses: Ab Initio Molecular Orbital Approach
(2013-01-17)
Koseki, Shiro
;
Shimakura, Noriyuki
;
Teranishi, Yoshiaki
;
Lin, Sheng Hsien
;
Fujimura, Yuichi
Instantaneous (laser-field-dependent) potential energy curves leading to neutral fragmentations of methane were calculated at several laser intensities from 1.4 x 10(13) to 1.2 x 10(14) W/cm(2) (from 1.0 x 10(10) to 3.0 X 10(10) V/m) using ab initio molecular orbital (MO) methods to validate the observation of neutral fragmentations induced by intense femtosecond IR pulses (Kong et al. J. Chem. Phys. 2006, 125, 133320). Two fragmentation paths, CH2 + 2H and CH2 + H-2, in T-1(2) superexcited states that are located in the energy range of 12-16 eV were considered as the reaction paths because these states are responsible for Jahn-Teller distortion opening up reaction paths during ultrashort pulses. As field intensity increased, the low-lying excited (1)A(1) states originated from the Jahn-Teller T-1(2) states were substantially stabilized along the neutral-fragment path CH4 -> CH2 + 2H and were located below the ionization threshold. On the other hand, the low-lying excited B-1(1) states, which also originate from the Jahn-Teller T-1(2) states, were embedded on the ionized state along the dissociation path to CH2 + H-2. This indicates that ionic fragments, rather than neutral ones, are produced along the CH2 + H-2 path. The computational results support neutral fragmentations through superexcited states proposed by Kong et al.
Ultrafast nonradiative transition pathways in photo-excited pyrazine: Ab initio analysis of time-resolved vacuum ultraviolet photoelectron spectrum
(2018-11-14)
Mignolet, Benoit
;
Kanno, Manabu
;
Shimakura, Noriyuki
;
Koseki, Shiro
;
Remacle, Francoise
The internal conversion of photo-excited pyrazine, which occurs rapidly on a time scale of about 20 fs, has long been considered to proceed via a conical intersection between the optically bright S-2 (B-1(2u), pi pi*) and dark S-1 (B-1(3u), n pi*) states. Since 2008, several theoretical studies have raised the possibility that other dark states S-3 ((1)A(u), n pi*) and S-4 (B-1(2g), n pi*) may participate dominantly in the early stage of the nonradiative decay of S-2. To clarify this issue, being motivated by the recent pump-probe experiment by Horio et al. [J. Chem. Phys. 145 (2016) 044306], we calculated vacuum ultraviolet photoelectron spectra for ionization from each of the four excited states. Comparison was made with the measured time-resolved photoelectron spectrum exhibiting a temporally varying multi-band structure. We confirmed no contribution of S-3 or S-4 and thus the validity of the conventional two-state (S-2 -> S-1) picture for ultrafast nonradiative transition in pyrazine.
Ultrafast radiationless transition pathways through conical intersections in photo-excited 9H-adenine
(2010)
Hassan, Walid Mohamed Ibrahim
;
Chung, Wilfredo Credo
;
Shimakura, Noriyuki
;
Koseki, Shiro
;
Kono, Hirohiko
We performed CASSCF and MRCI calculations for determination of the effective pathways of ultrafast radiationless transitions from the optically allowed pi pi* (1)L(a) state to the ground state S(0) of 9H-adenine. The n pi*, pi sigma*, and two pp* states were taken into account as states involved in the radiationless process. Optimized geometry and conical intersections were searched in the full dimensional space for the vibrational degrees of freedom by using the suite of quantum chemistry codes MOLPRO. The MRCI transition energies to excited states are in good agreement with the experimental values. The mechanisms of three competing pathways, two indirect pathways via the ps* and np* states, 1 La-ps*-S0 and 1 La-np*-S0, and a direct pathway (1)L(a)-S(0), were examined on the basis of the structures and energies of conical intersections involved in ultrafast radiationless transitions from 1 La to S0. Any conical intersection between the ps* and np* states was not found. This suggests that the two indirect pathways are independent of each other. The pp* 1 La-ps* conical intersection lies higher than the pp* (1)L(a) state at the Franck-Condon geometry by 0.19 eV according to the present MRCI calculation, which is consistent with the experimental observation that a new channel is open at the excess energy of B0.2 eV above the band origin of the pp* 1 La state. It is concluded that relaxation from the pp* (1)L(a)-ps* conical intersection to S0 occurs mainly through the ps*-S(0) conical intersection. The pp* 1 La-np* conical intersection lies higher by 0.1 eV ( MRCI value) than the pp* 1 La state at the Franck-Condon geometry. The fast decay component in time-resolved spectra of 9H-adenine is attributed to rapid radiationless transitions to the np* state via this conical intersection followed by the transition to S0 via the np*-S(0) ( or pp*(1)L(a)-S0) conical intersection. The pp* (1)L(a)-S0 conical intersection of large out-of-plane distortion has the lowest energy among the conical intersections found in this study. We identi. ed the transition state between the pp* (1)L(a) at the Franck-Condon geometry and the pp* (1)L(a)-S0 conical intersection. The MRCI energy of the transition state on the (1)L(a) potential surface is higher by 0.21 eV than the vertical excitation energy. The possibility of strong coupling between the two close-lying states (1)L(a) and np* indicates that, besides this direct pathway, radiationless transitions to S(0) via the pp* (1)L(a)-S(0) conical intersection can also occur after rapid relaxations between (1)L(a) and np*. The analysis of the h-vector for each conical intersection has shown that the active coupling for the ps* pathway is dominated by the out-of-plane normal mode n 10, while the active coupling for the np* pathway is distributed among many normal modes. Control of the branching ratio of the two indirect pathways can be achieved by selective excitation of single vibronic levels involving active coupling modes such as the mode n v(10).