M. Hayashi

Experimental and theoretical investigations of absorption and emission spectra of the light-emitting polymer MEH-PPV in solution

Abstract We report the absorption and photoluminescence spectra of the MEH-PPV polymer in a chloroform solvent. In the analysis, we assume that the polymer consists of short conjugated segments which are called oligomers with different lengths. The appearance of each chain length is controlled by a distribution function. Based on the conformational disorder caused by torsional motion, we found that the distribution function should take a Gaussian form with its center and width being adjustable parameters. While the energy level of the emissive S1 state of the oligomer and the corresponding oscillator strength are calculated from exciton theory. In addition, the associated vibrational motions are considered in determining the spectral shape in which both displacement and distortion of the potential surfaces are taken into account. With this information, the calculation result can fit the experiment reasonably well.

Ab initio study of the n-pi(*) electronic transition in acetone: Symmetry-forbidden vibronic spectra

Ab initio calculations of geometry and vibrational frequencies of the first singlet excited 1A2(1A″) state of acetone corresponding to the n-π* electronic transition have been carried out at the CASSCF/6-311G** level. The major geometry changes in this state as compared to the ground state involve CO out-of-plane wagging, CO stretch and torsion of the methyl groups, and the molecular symmetry changes from C2v to Cs. The most pronounced frequency changes in the 1A″ state are the decrease of the CO stretch frequency v3 by almost 500 cm−1 and the increase of the CH3 torsion frequency v12 from 22 to 170 cm−1. The optimized geometries and normal modes are used to compute the normal mode displacements which are applied for calculations of Franck–Condon factors. Transition matrix elements over the one-electron electric field operator at various atomic centers calculated at the state-average CASSCF/6-311+G** level are used to compute vibronic couplings between the ground 1A1, 1A2, and Rydberg 1B2(n-3s), 2u200a1A1(n-3...

Ab initio calculations of radiationless transitions between excited and ground singlet electronic states of ethylene

General expressions for internal conversion (IC) rate constant calculations have been derived by taking into account displacements, distortions, and rotation (mixing) of normal modes. The electronic part of the rate constant has been computed through the ab initio calculations of vibronic coupling. The corresponding expressions for the simplest two-mode case as well as for the general n-mode case have been derived. We demonstrate the effect of rotated (mixed) normal modes on the IC rate constants based on a model consisting of one promoting and two mixed modes. The dynamics of excited states of C2H4 has been investigated based on the internal conversion mechanism. The calculated rate of internal conversion show that the lifetimes of the excited π–3p and π–π* states of C2H4 are on the picosecond scale. We predict that if the molecule is excited to a Rydberg π–3p state, it relaxes to the ground state via the cascade mechanism, π–3p→π–3s(1B3u)→π–π*(1B1u)→1Ag.

Femtosecond pump-probe study of molecular vibronic structures and dynamics of a cyanine dye in solution

Time-resolved pump–probe spectra of 1,1′,3,3,3′,3′-hexamethyl-4,4′,5,5′-dibenzo-2,2′indotricarbocyanine (HDITC), a cyanine dye, in ethylene glycol are obtained using 11 fs and 90 fs duration pulses and analyzed in order to study its potential energy surfaces and vibrational dynamics. Ten oscillatory frequencies ranging from 30 cm−1 to 1400 cm−1 are observed in the 11 fs duration wavelength-resolved pump–probe measurements. They are assigned as fundamental vibrational frequencies of HDITC. The relative displacements of the equilibrium position between electronic excited and ground states along the resolved ten vibrational modes are determined through the wavelength dependence of the oscillatory amplitude. After considering the contributions of the ten vibrational modes, it is found that most of the Stokes shift and the early fast decays of the pump–probe signals are due to relaxation along the low frequency overdamped modes of the chromophore. The overdamped modes are characterized by the 90 fs pump–probe ...

Ab initio calculations of vibronic coupling. Applications to symmetry-forbidden vibronic spectra and internal conversion in ethylene

Abstract A formalism for the calculation of vibronic coupling and transition dipole moments for symmetry-forbidden transitions has been presented. The approach involves ab initio calculations of vibrational frequencies, normal coordinates, transition moments for allowed transitions and transition matrix elements over the one-electron electric field operator at the atomic centers. The approach is applied to the calculations of vibronic spectra for the π-3p transitions in ethylene. The vibronic coupling is used to calculate the rates of internal conversion between the π-3p and π-3s states of C 2 H 4 which are found to be in the 10 9 –10 11 s −1 range.

Branching ratios of C2 products in the photodissociation of C2H at 193 nm

Experimental and theoretical results are combined to show that vibrationally excited C2H radicals undergo photodissociation to produce C2 radicals mainly in the Bu200a1Δg state. Infrared (IR) emissions from the photolysis of acetylene with a focused and unfocused 193 nm excimer laser have been investigated using step-scan Fourier transform infrared (FTIR) emission spectroscopy at both low and high resolution. With an unfocused laser, the low-resolution infrared emission spectra from the C2H radicals show a few new vibrational bands in addition to those previously reported. When the laser is focused, the only emissions observed in the 2800–5400 cm−1 region come from the electronic transitions of the C2 radicals. Most of the emissions are the result of the Bu200a1Δg→Au200a1Πu transition of C2 although there are some contributions from the Ballik–Ramsay bands C2(bu200a3Σg−→au200a3Πu). A ratio of [Bu200a1Δg]/[bu200a3Σg−]=6.6 has been calculated from these results. High quality theoretical calculations have been carried out to determine ...

Theoretical analyses on femtosecond time-resolved spectra of initial electron transfer of photosynthetic reaction centers at low temperatures

Abstract In this paper, based on the perturbative density matrix method, we calculate femtosecond (fs) transient spectra of initial electron transfer (ET) in the bacterial reaction centers (RCs) of Rhodobacter (Rb.) capsulatus at 20 K and compare them with experimental data. It is believed that the lowering of temperature will decrease the intermolecular distance in the special pair, which will in turn affect the energy levels of the excitonic-coupled dimer (special pair) P, i.e., P − ∗ , P + ∗ , P − + and P + + where, for instance, P − ∗ and P + + represent the lowest excited and the second charge-separated states of the P, respectively. The absorption spectrum of the P − ∗ band is analyzed to obtain the small vibrational modes and their coupling constants (Huang-Rhys factor). By taking into account the anion and cation states of the bacteriochlorophyll a molecule (B) and the cation state of the bacteriopheophytin a molecule (H), the excitonic-vibronic model including the six electronic states P − ∗ BH, P − + B − H, P − + BH − , PB + H − , P + + B − H and P + + BH − is applied to simulate the fs time-resolved three-dimension spectra of Rb. capsulatus . We find only the three electronic states P − ∗ BH, P + + B − H and P + + BH − are mainly involved in the ET dynamics. We shall theoretically analyze the quantum beats appearing in the fs pump-probe stimulated emission profiles of the D ll mutant RCs of Rb. capsulatus at 10 K. The conventional ET theory assumes that vibrational relaxation is much faster than ET so that vibrational equilibrium is established before ET takes place. However, several recent fs measurements on RCs show quantum beats in their time-resolved profiles. This implies that the non-equilibrated vibrational motions of RCs should be investigated. The D ll mutant RC, lacking of the H ll , is incapable of ET on a time scale of the fs measurements. Thus, analyzing the quantum beats appearing in the fs time-resolved profiles, we shall extract the information of transient vibrational motions and their decays of the electronically lowest excited state P − ∗ . The multi-mode effect on the quantum beats is also investigated.

Ultrafast dynamics of excitations in conjugated polymers: A spectroscopic study

We construct a microscopic model to describe the excited states of poly(2-methoxy, 5-(2′-ethylhexoxy)-p-(phenylenevinylene) in thin film. Within this model, we deduce that in the high energy region, the nature of excited states in the film is very similar to the species observed in solution phase. Moreover, we propose that the decay process of these excited states involves energy transfer, vibrational relaxation, and dissipation simultaneously, in contrast to the usual argument that assumes exciton migration occurs after vibrational motion reaches thermal equilibrium. As a result, the simulation of time-resolved photoluminescence spectra is in agreement with the experiment.

THEORETICAL INVESTIGATION OF NEAR-FIELD OPTICAL PROPERTIES OF TAPERED FIBER TIPS AND SINGLE MOLECULE FLUORESCENCE

We investigate herein the propagation and tunneling of monochromatic light through a tapered fiber tip, modeled as a tapered perfectly conducting waveguide. The opening aperture of the waveguide is much smaller than the wavelength of light. The transmission rate through the tip as well as the profiles of electric fields are obtained. A detailed formulation for calculating the fluorescent spectra of a single molecule under near-field excitation is also presented. In addition, the limitations of fluorescence measurement for obtaining near-field pattern is discussed on the basis of the present formulation.

Theoretical wave packet study on pump-probe stimulated emission signals from electron transfer systems in condensed phases

The pump-probe stimulated emission signals from the electron transfer system in a condensed phase are studied theoretically. To show how the system dynamics affect the stimulated emission signal, both weak and strong electronic coupling cases are investigated. The effects of temperature on the dynamics of the system are reported.