Femtosecond transient absorption spectroscop

THEORETICAL AND SPECTROSCOPIC STUDIES There have been a considerable number of papers which fall into this category. Metal carbonyl photochemistry in organic solvents is covered comprehensively in an article dealing with femtosecond transient absorption and preliminary resonance Raman spectroscopy Turner, Poliakoff and co-workers have continued their spectroscopic investigations of organometallic chemistry in supercritical fluids 14.15,16 gjga of exceptional potential. The... 

Several years ago, Youvan and coworkers produced an important mutant RC in the bacterium Rh.capsulatus in which one BChl of the SP was changed to bacterio-pheophytin (BPh) by the removal of the Mg atom. This heterodimer mutant can successfully carry out primary charge separation, albeit at a reduced rate and with a substantially reduced quantum yield.The femtosecond transient absorption experiments of Holten, Kirmaier and coworkers revealed that a charge transfer intermediate in this system is formed very rapidly after the initial flash the species created has been identified as the internal CT state of the heterodimer.Furthermore, Stark measurements on the heterodimer reveal two features, which are hypothesized to be admixtures of the exciton and CT states.Both spectroscopic and kinetic experiments were also performed on a similar heterodimer mutant for the Rb.sphaeroides system producing similar qualitative results. 

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. 

Photoinduced electron injection is by no means a new development. This process has already been applied in areas such as silver halide photography. In this discussion, only sensitized TiC>2 surfaces will be considered. Many experiments have shown that the charge injection into the semiconductor surface is very fast. In order to study these processes, fast spectroscopic techniques are preferred. Whether or not charge injection takes place can be studied conveniently on the nanosecond time-scale by using transient absorption spectroscopy. However, to address the injection process directly, experiments are carried out on the femtosecond time-scale, while recombination and charge separation require the nanosecond to microsecond range. 

Electron injection dynamics in the conduction band of metal oxide materials from dye molecules or metal nanoparticles, which is important when applied to sensitized solar cells, can be monitored in the infrared by 100 fs time resolution. In this chapter, technical details of femtosecond visible-pump/IR-probe transient absorption spectroscopy and some typical spectroscopic data revealing the mechanism of electron injection process were described. A great advantage of this technique is that one can observe transient absorption of injected electrons easily because of the intense intraband transition of an electron at the bottom of or at the trap level just below the conduction band of the metal oxide that forms an electrode. In the case of dye-sensitized solar cells, the effects of metal oxide, dye, solvent and additive ions on the rate and efficiency of electron injection were discussed in detail. One recent discovery, plasmon-induced electron injection from a gold nanoparticle to a Ti02 nanoparticle, was presented to show how femtosecond visible-pump/IR-probe transient absorption spectroscopy is useful in studying this kind of new charge transfer dynamics in a nano-structured system. 

---