Yukito Naitoh

PHOTO-INDUCED REACTIONS OF CH2I2 IN SOLUTION STUDIED BY THE ULTRAFAST TRANSIENT ABSORPTION SPECTROSCOPY

Abstract We have studied the photo-induced reactions of CH 2 I 2 upon excitation at 268 nm in CH 3 CN by femtosecond transient absorption spectroscopy. The time profile of transients observed at 400 nm consists of three components: fast rise (200 fs), fast decay (500 fs), and slow rise (7–13 ps). The first two components are independent of the solute concentration and are assigned to the photofragment CH 2 I radical. The slow rise becomes faster by increasing concentration and is assigned to the formation of a charge-transfer complex between the photofragment I and CH 2 I 2 in solute aggregate. This is followed by full electron transfer from CH 2 I 2 to I forming CH 2 I 2 + .

Formation of benzene dimer cations in neat liquid benzene studied by femtosecond transient absorption spectroscopy

Abstract Photoionization of neat liquid benzene has been studied by femtosecond pump-probe experiments in the near infrared region. A transient species is formed within 1 ps following two-photon excitation of liquid benzene at 268 nm, which shows a broad absorption band with a peak at 920 nm and extended intensities at wavelengths longer than 1000 nm. This band is attributed to a charge resonance band of a localized benzene dimer cation. The transient species show essentially no evidence of the recombination with electrons in a time shorter than 50 ps.

Ultrafast dynamics of photoexcited trans-1,3,5-hexatriene in solution by femtosecond transient absorption spectroscopy

Abstract We have investigated the ultrafast dynamics of trans-1,3,5-hexatriene in cyclohexane and acetonitrile solutions following photoexcitation to the 1 1 B u state. The transient absorption in the visible region consists of an instantaneous rise (

Photodissociation of CH2I2 and Subsequent Electron Transfer in Solution

We studied photoinduced reactions of diiodomethane (CH(2)I(2)) upon excitation at 268 nm in acetonitrile and hexane by subpicosecond-nanosecond transient absorption spectroscopy. The transient spectra involve two absorption bands centered at around 400 (intense) and 540 nm (weak). The transients probed over the range 340-740 nm show common time profiles consisting of a fast rise (<200 fs), a fast decay ( approximately 500 fs), and a slow rise. The two fast components were independent of solute concentration, whereas the slow rise became faster (7-50 ps) when the concentration in both solutions was increased. We assigned the fast components to the generation of a CH(2)I radical by direct dissociation of the photoexcited CH(2)I(2) and its disappearance by subsequent primary geminate recombination. The concentration-dependent slow rise produced the absorption bands centered at 400 and 540 nm. The former consists of different time-dependent bands at 385 and 430 nm. The band near 430 nm grew first and was assigned to a charge-transfer (CT) complex, CH(2)I(2) (delta+)I(delta-), formed by a photofragment I atom and the solute CH(2)I(2) molecule. The CT complex is followed by full electron transfer, which then develops the band of the ion pair CH(2)I(2) (+)I(-) at 385 nm on the picosecond timescale. On the nanosecond scale, I(3) (-) was generated after decay of the ion pair. The reaction scheme and kinetics were elucidated by the time-resolved absorption spectra and the reaction rate equations. We ascribed concentration-dependent dynamics to the CT-complex formation in pre-existing aggregates of CH(2)I(2) and analyzed how solutes are aggregated at a given bulk concentration by evaluating a relative local concentration. Whereas the local concentration in hexane monotonically increased as a function of the bulk concentration, that in acetonitrile gradually became saturated. The number of CH(2)I(2) molecules that can participate in CT-complex formation has an upper limit that depends on the size of aggregation or spatial restriction in the neighboring region of the initially photoexcited CH(2)I(2). Such conditions were achieved at lower concentrations in acetonitrile than in hexane.

Femtosecond TransientAbsorption Studies of SimplePolyenes in Solution

We have conducted femtosecond transient absorption experiments on the excited-state dynamics of trans- and cis-hexatriene (HT) in solution. The transient absorption in the visible region decays with the time constant of about 500 fs, indicating that the internal conversion (IC) to the ground state occurs quite efficiently. The bleach recovery signal contains several time constants. The vibrational relaxation in the ground state occurs in 10–20ps and the slow relaxation process, which may be due to the conformational change around C—C single bond, proceeds in a subnanosecond time scale.

Photodissociation of CH 2 I 2 and the Subsequent Electron Transfer in Its Cluster Formed in Solution

We have studied photodissociation and the subsequent reactions of methylene iodide (CH2I2) upon excitation at 268 nm in acetonitrile and hexane solutions by the femtosecond transient absorption spectroscopy. In addition to dissociation and geminate recombination in the femtosecond region, formation of CH2I2 + is observed in the early picosecond region. The reaction is proposed to occur by electron transfer between photofragment I and CH2I2. This indicates that the solute molecules aggregate even in a dilute solution and they are responsible for the cation formation.