Picosecond Time-Resolved Measurement

Aromatic hydroxy compounds are known to show remarkable enhancement in their acidity in the excited singlet state and often serve as prototypes in this area of research. They are some of the earliest ESPT molecules studied by Forster [2-4], Their ESPT behavior has been extensively studied over the past three decades [1,7-12,17-19], The ultrafast nature of ESPT rate constants was realized as early as 1979 [30], Picosecond time-resolved measurements by Webb et al. 

For fluorescence decay curves of the J-aggregate LB films of [CI-MC] mixed with various matrix agents, measured with a picosecond time-resolved single photon counting system, three components of the the lifetimes fitting to exponential terms in the following equation ... 

Aramaki and Atkinson were also active in work on the spiro-oxazines [65]. They noted that for NOSH in many polar and nonpolar solvents the picosecond time-resolved resonance Raman spectra simply built up over 50 psec with no shape evolution. The same finding was concluded from transient absorption measurements over the same time scale. The spectra/absorbances were then constant for 1.5 nsec. These authors suggest that only two isomers can be expected to contribute to the merocyanine spectra because those trans about the y-methene bridge bond attached to the naphthalene ring are sterically crowded due to short interproton distances. There was no evidence for the X transient in their study however, the 50-psec convoluted pulse profile may be expected to mask this sortlifetime species even if it were present. 

When an electron is injected into a polar solvent such as water or alcohols, the electron is solvated and forms so-called the solvated electron. This solvated electron is considered the most basic anionic species in solutions and it has been extensively studied by variety of experimental and theoretical methods. Especially, the solvated electron in water (the hydrated electron) has been attracting much interest in wide fields because of its fundamental importance. It is well-known that the solvated electron in water exhibits a very broad absorption band peaked around 720 nm. This broad absorption is mainly attributed to the s- p transition of the electron in a solvent cavity. Recently, we measured picosecond time-resolved Raman scattering from water under the resonance condition with the s- p transition of the solvated electron, and found that strong transient Raman bands appeared in accordance with the generation of the solvated electron [1]. It was concluded that the observed transient Raman scattering was due to the water molecules that directly interact with the electron in the first solvation shell. Similar results were also obtained by a nanosecond Raman study [2]. This finding implies that we are now able to study the solvated electron by using vibrational spectroscopy. In this paper, we describe new information about the ultrafast dynamics of the solvated electron in water, which are obtained by time-resolved resonance Raman spectroscopy. 

The first subsurface bone tissue Raman spectroscopic measurements were performed using picosecond time-resolved Raman spectroscopy on excised equine cortical bone [56, 57], In these experiments it was shown that a polystyrene backing could be detected through 0.3 mm of bone. The same picosecond technology was used to perform the first transcutaneous Raman spectroscopic measurements of bone tissue [58]. In this study, the cortical bone mineral/matrix ratios of excised limbs of wild type and transgenic (oim/oim) mice were compared and the differences demonstrated. 

Stimulated fluorescence appears with a delay of about 500 fs relative to the A [ absorption, although both absorption and fluorescence stem from the same state, proved by decay measurements with picosecond time-resolved spectroscopy. This delayed appearance of stimulated fluorescence is caused by a quickly increasing and short-lived absorption A0 located in nearly the same spectral range as the fluorescence. The authors assumed that the instant absorption A0 is caused by a state located slightly below the level reached by excitation with 4eV. This state 2,... 

Figure 14. Experimental apparatus for picosecond, time-resolved CD measurements using a mode-locked, Q-switched, cavity dumped pump laser. P, polarizer PC, Pockels cell Q, quarter-wave plate RHP, rotating half-wave plate S, sample cell PMT, photomultiplier tube. From ref. [42].

Another approach of great importance for studies of excited state dynamics is sub-picosecond time resolved spectroscopy. A number of authors have reported femtosecond pump-probe measurements of excited state lifetimes in A, C, T, and G [13-16] and base pair mimics [17]. Schultz et al. have reported time resolved photoelectron spectroscopy and electron-ion coincidence of base pair mimics [18]. these studies can also be compared with similar measurements in solution [19-24], While time resolved measurements provide direct lifetime data, they do have the limitation that the inherent bandwidth reduces the spectral resolution, required for selecting specific electronic states and for selecting single isomers, such as cluster structure and tautomeric form. 

The time-resolved Raman spectra were measured with a picosecond time-resolved Raman spectrometer which employs a standard pump-probe technique. The details of the spectrometer have been publish elsewhere. The followings are concise description of the apparatus The output from a synchronously pumped mode-locked dye laser is amplified with the output from a cw Nd YAG regenerative amplifier. The second harmonic (294 nm, 2 kHz, 1-2 mW) of the amplified light (588 nm, 3.2 ps, 2 kHz, 15 mW) was used as a... 

The rotational reorientation times of the sample in several solvents at room temperature were measured by picosecond time-resolved fluorescence and absorption depolarization spectroscopy. Details of our experimental setups were described elsewhere. For the time-correlated single photon counting measurement of which the response time is a ut 40 ps, the sample solution was excited with a second harmonics of a femtosecond Ti sapphire laser (370 nm) and the fluorescence polarized parallel and perpendicular to the direction of the excitation pulse polarization as well as the magic angle one were monitored. The second harmonics of the rhodamine-640 dye laser (313 nm 10 ps FWHM) was used to raesisure the polarized transient absorption spectra. The synthesis of the sample is given elsewhere. All the solvents of spectro-grade were used without further purification. 

The next chapter by Ohshima, Kajimoto and Fuke reviews some results obtained with linked systems, in which charge transfer is facilitated by torsional motion around a single bond. Picosecond time-resolved experiments allowed the direct measurement of the decay of the LE state and the rise of the CT state fluorescence. In some cases a third excited state, also characterized as a charge-transfer state, was observed. It was assigned to a charge-transfer state in which the two connected moieties (anthracene and aniline in this case) are twisted from the planar conformation characteristic of the ground state, but not to a fully perpendicular conformation. The photophysics of twisted intramolecular charge-transfer (TICT) states, were studied extensively in the gas phase and in solution (see the previous chapter by Herbich and Brutschy). 

These studies offer great promise, and with better temporal resolution it will be possible to obtain accurate product buildup times for small prototypical systems. Such time-resolved measurements can offer remarkable insight into the physics of chemical change, as has been amply demonstrated by the Zewail group [156, 157], and we look forward to new insights on the CO2-HX and NjO-HX systems. For example, take the case of NjO-HX (described below), where both the NH(X Z) -I- NO(X Il) and OH(X n) -1-N2(X Z) product channels are known to be open under bulk as well as complexed conditions [38, 39]. Do the NH and OH product buildups occur on the same time scales How do the time scales of these chemically distinct channels vary with energy Are NH and NO produced synchronously, as they must form an HNNO precursor We believe that such questions can be answered by sub-picosecond resolution measurements. 

We also measured picosecond time-resolved TFD-IR images from —10 to 50ps. Figure 29.9 shows the picosecond time-resolved TFD-IR images obtained around a 0 ps delay time. For these TFD-IR images, the population decay of the vibrationally excited Rhodamine-6G molecule in a cell is demonstrated as the delay-time dependent fluorescence. At a — 5 ps delay time, when visible light is applied before IR light,... 

Ultrafast techniques have been used to observe energy transfer directly. For example, sub-picosecond time resolved intramolecular transfer examined in flexible bichromophoric coumarin molecules shows that exchange occurs within 1 to 20 ps depending polymethylene chain length . The distribution of interchromophoric distances in donor/acceptor coumarin supermolecules has been measured analysis of data ftom time resolved energy transfer . 

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