Subpicosecond absorption spectroscopy

Isomerization process in the native and denatured photoactive yellow protein probed by subpicosecond absorption spectroscopy... 

Two bacteriochlorophyll monomers, and Bg, are located next to the primary donor D, but they are buried deeper in the membrane. Their positions are fixed by helices B, C, D, and de of the L- and M-subunits, respectively. How these so-called accessory bacteriochlorphylls are involved in the ET has been the subject of a long debate (Holzapfel et al., 1990 Kirmaier and Holten, 1991). Some evidence for their function as true electron carriers has been provided by subpicosecond absorption spectroscopy (Arlt et al., 1993 Zinth et al., 1996). The Bg molecule facilitates the triplet energy transfer between D and the carotenoid (Frank and Violette, 1989). B and Bg follow the local Cj symmetry. Their tetrapyrrole rings are superimposed by a rotation (M on L) of -175.8 (Deisenhofer and Michel, 1989a,b) which is not as perfect as for the D /Dgpair. As in the case of Dg,the phytyl side chain of Bg interacts with the M-subunit... 

Another important result from the photophysical studies on conjugated polymer/fullerene composites is the complete absence of any photoinduced interaction between fullerenes and either polyacetylene (PA) [54] or the polydiacetylenes (PDAs) [62], respectively. The results of absorption and emission spectroscopy, subpicosecond and millisecond photoinduced absorption spetroscopy, and picosecond transient and near steady state photoconductivity show that in contrast to the high quantum efficiency photoinduced electron transfer definitively established in the PPV and the P3AT composites with Cgo, photoinduced electron transfer from PA and PDAs onto Ceo is inhibited [62]. 

Xiang, J., F. S. Rondonuwu, Y. Kakitani, R. Fujii, Y. Watanabe, Y. Koyama, H. Nagae, Y. Yamano, and M. Ito. 2005. Mechanisms of electron injection from retinoic acid and carotenoic acids to Ti02 nanoparticles and charge recombination via the T, state as determined by subpicosecond to microsecond time-resolved absorption spectroscopy Dependence on the conjugation length. J. Phys. Chem. B 109 17066-17077. 

The Photoactive Yellow Protein (PYP) is the blue-light photoreceptor that presumably mediates negative phototaxis of the purple bacterium Halorhodospira halophila [1]. Its chromophore is the deprotonated trans-p-coumaric acid covalently linked, via a thioester bond, to the unique cystein residue of the protein. Like for rhodopsins, the trans to cis isomerization of the chromophore was shown to be the first overall step of the PYP photocycle, but the reaction path that leads to the formation of the cis isomer is not clear yet (for review see [2]). From time-resolved spectroscopy measurements on native PYP in solution, it came out that the excited-state deactivation involves a series of fast events on the subpicosecond and picosecond timescales correlated to the chromophore reconfiguration [3-7]. On the other hand, chromophore H-bonding to the nearest amino acids was shown to play a key role in the trans excited state decay kinetics [3,8]. In an attempt to evaluate further the role of the mesoscopic environment in the photophysics of PYP, we made a comparative study of the native and denatured PYP. The excited-state relaxation path and kinetics were monitored by subpicosecond time-resolved absorption and gain spectroscopy. 

In order to better understand the early photophysics of PYP, we have carried out a comparative study of three model chromophores, the deprotonated frans-p-coumaric acid (pCA2 ) and its amide (pCM ) and phenyl thioester (pCT) analogues, in aqueous solution (see structures in Fig. 1). The excited-state relaxation dynamics was followed by subpicosecond transient absorption and gain spectroscopy. 

Transient absorption and gain spectroscopy was performed by the pump-probe technique using a subpicosecond set-up described in detail elsewhere [10-12]. For the present experiments, the pump was set at 355 nm (pCA2- and pCM ) or 430 nm (pCT ) and the probe was a white-light continuum produced by focusing 570 nm pulses in a 1-cm water cell. The... 

Our work aims at identifying, by subpicosecond broadband transient absorption and gain spectroscopy, the primary photochemical steps of the phototransduction process in Blepharisma japonicum, more specifically in the light-adapted form of the organism (blue cell) for which the photoactive pigment is oxyblepharismin [5] (see Scheme 1) and the associated macromolecule is a large non-soluble protein (200 kDa) [6]. 

Precise measurements of the excited state lifetimes of the DNA constituents were not available till very recently, mainly due to the limited time resolution of conventional spectroscopic techniques. Studying the DNA nucleosides by transient absorption spectroscopy, Kohler and co-workers observed a very short-lived induced absorption in the visible which they assigned to the first excited state [5,6]. The lifetimes observed were all well below 1 picosecond. The first femtosecond fluorescence studies of DNA constituents were performed using the fluorescence upconversion technique. Peon and Zewail [7] reported that the excited state lifetimes of DNA/RNA nucleosides and nucleotides all fall in the subpicosecond time, thus corroborating the results obtained by transient absorption. 

The time and wavelength resolved fluorescence dynamics of bianthryl has been investigated by several groups [30, 82, 132, 133, 115, 116]. In addition, this molecule has been studied by picosecond absorption spectroscopy [115], electric field induced fluorescence anisotropy measurements [117] and optically induced dielectric absorption (microwave) measurements [118, 119]. The results are generally in accord with the theoretical model presented in Sections III.A and III.B. One of the challenges of studying the photodynamics of BA is that the LE and CT interconversion is so rapid (i.e., on the time scale of solvation) that it is necessary to employ ultraviolet subpicosecond and even femtosecond fluorescence spectroscopy which has only recently become available [30, 82, 132, 133]. 

The dynamics of the interfacial electron-transfer between Dye 2 and TiOz were examined precisely by laser-induced ultrafast transient absorption spectroscopy. Durrant et al.38) employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of Dye 2 adsorbed onto the surface of nanocrystalline Ti02 films. Detailed analysis indicates that the injection is at least biphasic, with ca. 50% occurring in <150 fsec (instrument response limited) and 50% in 1.2 0.2 psec. 

Fig. 15, were initially obtained from the DMN fluorescence lifetime data,92,102 but they have recently been measured using subpicosecond time-resolved transient absorption spectroscopy (pump-probe).105... 

In this paper we investigate the time dependent ground state hole spectrum of cresyl violet in polar solvents by means of subpicosecond transient absorption spectroscopy. The time correlation function expressed by eq (7) showed large difference in time profiles compared with the reported one expressed by eq (6). Possible mechanisms will be discussed. 

S. L. Logunov T. S. Ahmadi M. A. El-Sayed J. T. Khouey R. L. Whetten, Electron dynamics of passivated gold nanocrystals probed by subpicosecond transient absorption spectroscopy. J. Phys. Chem. B 1997, 101, 3713-3719. 

The C=0 group of coumarin (98, R = CH3) is a potential hydrogen bond acceptor. By using subpicosecond time-resolved infrared absorption spectroscopy, following photoexcitation of the cumarin chromophore of 98, in its complex with aniline, the hydrogen bond dissociation rate is in the order of femtoseconds and the aniline reorients itself by reformation of the hydrogen bond with a new geometry166. 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

Techniques and equipment used to obtain data in this type of experiment are discussed and described in quite a number of papers published during the year. The generation and properties of ultrashort pulses has been clearly presented in an article published in American Scientist . An apparatus for carrying out pump-probe broad band spectroscopy by transient absorption in the subpicosecond region has been described in detail. This equipment has been used to observe the photodissociadon of bis-(p-aminophenyl) disulphide and also to show a biexponential frequency shift arising from solvent relaxation of the photo-generated p-aminophenyl thiyl radicals. Another picosecond time resolved absorption spectrometer system using a streak camera has been reported by Japanese workers . Okamoto and Yoshihara ... 

In order to treat these observations and hypotheses in a theoretical framework as successfully as the case of electron localization in helium, we must first probe the dynamical properties of the IR absorptions in the subpicosecond regime. What perhaps is surprising and stimulating for future studies is the wealth of microscopic details that can be obtained on intermolecular interactions and electron transfer in liquids through picosecond spectroscopy, information of fundamental interest to chemical dynamics in the condensed phase. In this vein, we will conclude this chapter by an example of photoselective chemistiy in electron transfer processes that occur following laser excitation of e in the cluster. 

---