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Excitation-wavelength dependence of the transient absorbance changes of photosystem I reaction center particles with a reduced number of antenna chlorophylls is studied in an effort to understand the energy equilibration among the chlorophylls in the electron-transfer system. Implications of the results for understanding the primary processes in more intact photosystem I are discussed.

The ultrafast photoinduced electron transfer between excited oxazine-1 dye and electron donating solvents aniline (AN) and N,N-dimethylaniline (DMA) has been investigated. The results of femtosecond fluorescence up-conversion analysis showed the ultrafast and highly nonexponential fluorescence decay of oxazine-1 in AN and DMA. The fluorescence decays were found to be strongly dependent on the wavelength of observation in DMA and moderately dependent in AN.

Higher-order nonlinear spectroscopy was used to investigate vibrational dephasing of the overtone bands of chloroform and its deuterated derivative. A single-exponential function represents the overtone vibrational dephasing signals as well as the fundamental ones. Stochastic theory does not explain these dephasing signals. Effects of resonant transfer and population relaxation are not factors in the deviation from theoretical prediction, with the polyatomic natures of the molecules suggested as behind this deviation.

The CO ligation process in a mutant (H77G) of CooA, the CO-sensing transcriptional regulator of Rhodospirillum rubrum, is studied with fematosecond time-resolved transient absorption spectroscopy in the mid-infrared region. The results reveal a conformational alteration by the mutation and indicate the function of an amino acid residue at the proximal side that maintains the correct protein conformation for activation and the heme also changes its position/orientation with mutation.

A semiempirical theoretical model is developed for estimating the rates of collision-induced V-T vibrational relaxation. The model is useful for estimating the absolute magnitude of state-to-field vibrational relaxation rate coefficients within an overall accuracy of thirty percent and with an average error of -10 percent.

A variety of rotational-level population distributions are prepared in the 6(super 1) vibrational level of (super 1)B(sub 2u) benzene, and the rotational distributions within 0 degree that result from collisions with H2, D2, or CH4 are observed. The 0 degree distributions for D2 are broader than for H2, illustrating that the reduced mass and rotational-level spacings play an important role in determining the rotational changes that occur.

The low temperatures and high resolution associated with helium droplet spectroscopy are used to obtain vibrational spectra for clusters containing a single HF molecule. The results suggest that, for n=9 and larger, the solvent cage becomes highly isotropic allowing the HF molecule to undergo nearly free rotation.

We have observed fifth-order optical response from liquid CS2 by six-wave mixing with five different short pulses and investigated the inhomogeneity in the spectral broadening of the low-frequency intermolecular vibrational modes. There are a couple of distinct differences in the time profile of the fifth-order signal from that of the third-order signal. The fifth- and third-order signals obtained experimentally are compared with the prediction by the theory of Tanimura and Mukamel. We simulate the third- and fifth-order signals in the two cases, the homogeneous limit case and the partially inhomogeneously broadened case with a Gaussian distribution function. The experimental results can be reproduced well in terms of two vibrational modes, one overdamped mode in the homogeneous limit and one underdamped mode with an inhomogeneous width of 2.5 ps-1. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied the inhomogeneity of the low-frequency modes of binary mixtures of CS2 with various cosolvents by the temporally two-dimensional spectroscopy with femtosecond pulses. The inhomogeneity in the underdamped mode is found to be decreased upon dilution for all the solvents investigated. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Resonance enhanced two photon ionization spectrum of RbCs is obtained in a very cold pulsed molecular beam. We observe three vibrational series in the range of 540–590 nm. The excited electronic state that shows a very long vibrational series is assigned to the (4) 1Σ+ state. The absolute vibrational quantum numbers are determined from the isotopic shift of the vibrational energy. Rotational constants of several vibrational levels are determined in comparison with the Fourier transform spectroscopic data reported previously. Another electronic state that shows a short and intense vibrational series is assigned to the (3) 1Π state and the vibrational quantum numbers are determined. The term values and vibrational frequencies of 85RbCs are determined as Te=16 628.6±1 cm-1 and ωe=24.51±0.2 cm-1 for the (4) 1Σ+ state and Te=17 418.9±1 cm-1 and ωe=22.53±0.2 cm-1 for the (3) 1Π state. [ABSTRACT FROM AUTHOR]

Photodissociation of 11 alkylbenzenes (toluene, ethylbenzene, butylbenzene, and their deuterated compounds) have been studied by the ArF (193 nm) laser photolysis. Formation rate constants of benzyl radical were measured under collision free conditions. The major relaxation process of the photoexcited state is internal conversion to highly excited vibrational levels of the ground electronic state, i.e., hot alkylbenzene formation, followed by decomposition yielding benzyl radical. Deuterium isotope effects demonstrate that the bond fission occurs after intramolecular vibrational redistribution is complete. [ABSTRACT FROM AUTHOR]

Time-resolved absorption spectra of hexafluorobenzene vapor have been observed in the time range of 0–2 μs after pulsed ArF laser (193 nm) excitation. The absorption spectrum observed at the time t=0 is attributed to HFB* (S0) (hot hexafluorobenzene with an internal energy of 639 kJ/mol). This transient absorption spectrum can be simulated as a part of the spectrum of the S3(1E1u)←S0 transition at 3050 K on the basis of a modified Sulzer–Wieland model. The collisional deactivation of HFB* molecules is explained in terms of an energy transfer model which assumes that the energy removed per collision depends upon the internal energy. [ABSTRACT FROM AUTHOR]

Excited state enol-keto isomerization in salicylic acid (SA) monomer and dimer has been studied in a supersonic free jet expansion. Two carboxylic group rotamers of SA with significantly different photophysical properties are found in the expansion. Rotamer I, the major form of SA in the expansion, has an intramolecular hydrogen bond and can undergo excited state tautomerization reaction. Its S1 origin is at 335.34 nm. Single vibronic level emission spectra are dominated by progressions in OH stretching (3230 cm-1), and in-plane bending of the carboxylic group (240 cm-1). The spectra appear to consist of two components, normal (UV) and tautomer (BLUE) emissions, even at the origin. The intensity of the BLUE relative to the UV emission depends on the vibronic state, rather than the excess vibrational energy between the origin and 1100 cm-1. The fluorescence decay time profiles for both the emission components of rotamer I are identical within ∼1 ns experimental time resolution. A nonradiative decay process with an activation energy of ∼1100 cm-1 is deduced from an abrupt decrease in fluorescence lifetimes above this energy. The rotamer II cannot undergo excited state tautomerization. Its electronic origin is at 311.52 nm and emits only UV fluorescence. Upon increasing the concentration of the SA sample, a new spectrum is observed. Due to a nonlinear concentration dependence of the intensity and the propensity of SA to form dimers in solution, it is assigned to the SA dimer. This spectrum shows possible evidence of double proton transfer in the S1 state. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

One- and two-photon fluorescence excitation spectra of the S1←S0 transitions of the all-trans isomers of 1,3,5,7-octatetraene, 1,3,5,7-nonatetraene, and 2,4,6,8-decatetraene have been obtained in free jet expansions. Comparison of the one- and two-photon spectra allows the unambiguous identification of electronic and vibronic origins and, for octatetraene and decatetraene, provides clear evidence for molecular inversion symmetry. One-photon spectra show ag progressions built on Herzberg–Teller, bu promoting modes, while two-photon spectra are built on progressions of ag modes starting from the 2 1Ag←1 1Ag electronic origins. In nonatetraene, the absence of inversion symmetry results in an allowed electronic origin in both the one- and two-photon spectra. Nevertheless, bands built on vibronic origins dominate the one-photon spectrum. The S1←S0 spectra of nonatetraene and decatetraene exhibit characteristic splittings of vibronic bands that can be quantitatively explained by the tunneling of the methyl groups through low energy, torsional barriers in the S1 states. Couplings between methyl torsions and low frequency skeletal modes further complicate the optical spectra. Fluorescence lifetimes indicate abrupt onsets of nonradiative decay processes (tentatively attributed to trans→cis isomerization) at ∼2100 cm-1 excess energy. Systematic differences in the energy dependencies of S1 nonradiative decays in the three polyenes can be explained by the higher densities of vibronic states in the methyl-substituted compounds. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Temperature dependence of intermolecular electron transfer (ET) between oxazine 1 (OX 1) in the excited state and electron donor solvents such as aniline (AN) and N,N-dimethylaniline (DMA) was studied by observing fluorescence dynamics of OX1. The fluorescence decay of OX1 in DMA showed a single exponential behavior with a time constant of ∼280 fs which was independent of temperature over the range of 280 to 353 K. In AN, the ET caused nonexponential fluorescence decay whose time constants range from a few hundred femtoseconds to a few picoseconds depending on temperature. The time constants of these ET were smaller than the solvation times obtained by dynamic fluorescence Stokes shift of coumarin 102. This indicates the importance of vibrational nuclear motion in ET. The extended Sumi–Marcus two-dimensional reaction coordinate model which concerns the effect of high frequency mode was applied to explain the experimental observations. Good agreements were obtained between the experiments and calculations in terms of necessary parameters such as electronic matrix element and solvent reorganization energy. The difference between the ET of OX1 in DMA and AN can be mainly explained by the free energy difference between the reactant and the product. [ABSTRACT FROM AUTHOR]

One- and two-photon fluorescence excitation and emission spectra of the S1S0 transition of trans,trans-1,3,5,7-octatetraene have been measured for the first time in free jet expansions. The one-photon excitation spectrum is the same, with the exception of significant differences in the intensities of a few lines, as the two-color, resonance-enhanced, two-photon ionization spectrum, previously assigned to the 2 1A’←1 1A’ transition of cis,trans-1,3,5,7-octatetraene. However, comparison of the one- and two-photon fluorescence excitation spectra shows clearly that the carrier of the spectrum has inversion symmetry, as expected for trans,trans-1,3,5,7-octatetraene. The one-photon spectrum is built on bu Herzberg–Teller promoting modes, which are origins of progressions in ag modes, while the two-photon spectrum is due to a single progression in ag modes starting from the 2 1Ag←1 1Ag electronic origin. The appearance of out-of-plane vibrations, possibly including torsions of the polyene framework, suggests large differences in force constants and perhaps in the geometries of the 2 1Ag and 1 1Ag potential surfaces. For 2 1Ag vibronic levels with energies ≤1000 cm-1, the fluorescence lifetimes vary between 170 and 450 ns due to the dependence of radiative and nonradiative decay rates on the vibronic state. An abrupt increase in the nonradiative decay rates at ∼2100 cm-1 excess energy is tentatively ascribed to trans→cis isomerization. This work demonstrates that the one- and two-photon cross sections of the 2 1Ag←1 1Ag transitions of all-trans linear polyenes are sufficiently large to allow the study of 2 1Ag states under isolated, unperturbed conditions. [ABSTRACT FROM AUTHOR]

First observation of fluorescence for simple, linear trienes is reported. S1←S0 fluorescence excitation spectra of hexatriene and octatriene indicate large differences between the S0 and S1 potential energy surfaces. Activation energy of <200 cm-1 for the S1 state nonradiative decay is tentatively ascribed to isomerization. [ABSTRACT FROM AUTHOR]

Fluorescence and fluorescence excitation spectra of all-trans-2,4,6,8-decatetraene have been obtained in free jets and in inert-gas clusters. In isolated decatetraene, excitation into 1 1Bu (S2) results in emission from both S2 (1 1Bu→1 1Ag) and S1 (2 1Ag→1 1Ag) on time scales that are faster than the 10 ns experimental resolution. In clusters, rapid electronic and vibrational relaxation leads to long-lived (360 ns) emission from thermally relaxed levels of S1. Direct excitation of low-lying, S1 vibronic levels in cold, isolated molecules also results in long-lived S1→S0 fluorescence, as expected for this symmetry-forbidden transition. The detection of S1 emission in free decatetraene has permitted the first detailed study of the vibronic structure and kinetics of the 2 1Ag state of an isolated, all-trans linear polyene. The S1←S0 fluorescence excitation spectrum is rich in low-frequency vibronic progressions. Analysis of this spectrum suggests that the transition not only is made allowed by vibronic coupling involving low-frequency bu skeletal modes (Herzberg–Teller coupling), as for polyenes in condensed phases, but also gains intensity from interactions between the electronic motion and the hindered rotations (torsions) of the terminal methyl groups. Preliminary analysis suggests that the barriers to internal rotation of the methyl groups must be substantially reduced in the 2 1Ag (S1) state. For isolated decatetraene, the 2 1Ag fluorescence lifetimes show a monotonic decrease with increasing vibrational energy, presumably due to increased mixing with the 1 1Bu state. [ABSTRACT FROM AUTHOR]

Absorption spectra of phenoxy radical in the gas phase are observed in the range of 210–300 nm in the ArF excimer laser (193 nm) flash photolysis of phenol and anisole. An intense absorption band is obtained around 230 nm and a broad band around 270–300 nm. In the photolysis of anisole, simultaneous formation of methyl and phenoxy radicals makes it possible to determine the molar extinction coefficient of phenoxy radical which is determined to be 7800 M-1 cm-1 at 230 nm. The phenoxy radicals are produced in the time range of shorter than 1 ns from phenol, anisole, pentafluorophenol, and pentafluoroanisole on irradiation with the ArF laser light. The photodissociation is not explained in terms of the hot molecule mechanism but of predissociation. [ABSTRACT FROM AUTHOR]