Femtosecond time-resolved spectroscopy

Science during this period was led by nanomaterials design, inspired by the discovery of Ceo fullerene in 1985 and by photochemistry inspired by the development of femtosecond time-resolved spectroscopy in 1987. This scientific tide may have had an important impact on quantum chemistry during the latter half of this period DFT, enabling fast calculations, rapidly grew in use, and many excited state theories were developed for photochemical reaction calculations. 

The main luminescence parameters traditionally measured are the frequency of maximal intensity Vmax, intensity I, the quantum yield < >, the hfetime of the exited state T, polarization, parameters of Raman spectroscopy, and excited-state energy migration. The usefulness of the fluorescence methods has been greatly enhanced with the development of new experimental techniques such as nano-, pico-, and femtosecond time-resolved spectroscopy, single-molecule detection, confocal microscopy, and two-photon correlation spectroscopy. 

FEMTOSECOND TIME-RESOLVED SPECTROSCOPY OF THE DYNAMICS AT CONICAL INTERSECTIONS... 

We have outlined the theoretical description of femtosecond time-resolved spectroscopy of the photophysical and photochemical dynamics at conical... 

To illustrate the practical application of the theoretical formalism, we have discussed several representative examples that demonstrate the potential of femtosecond time-resolved spectroscopy. Since typically only a few degrees of freedom (say, 2-4) are involved in the ultrafast photodynamics via a conical intersection, explicit simulations of spectroscopic experiments for polyatomic molecules are nowadays possible on a routine basis. As an example of a state-of-the-art simulation of a femtosecond experiment on a polyatomic system, we have presented first-principles calculations of the time-resolved photoelectron spectra of pyrazine. Although in this case the interpretation was found to be quite complicated, the investigations nevertheless demonstrated the considerable potential of a joint experimental/theoretical study of the dynamics at conical intersections. Employing... 

We next study how the results of wavepacket dynamics can be tracked experimentally as a real-time history of chemical or physical events. Femtosecond time-resolved spectroscopy enables us to observe nuclear dynamics and to chart the path of chemical reactions in real time, and has been exploited in numerous applications ranging from fundamental studies of real-time motion in the photodissociation of diatomic molecules to stud-... 

V. Blanche , M.A. Bouchene, and B. Girard, Femtosecond Time-Resolved Spectroscopy and Coherent Control in CS2 in Fast Elementary Processes in Chemical and Biological Systems, Vol. 364 A IP Conference Proceedings, A. Tramer (ed.) (AIP Press, Woodbury, New York, 1996), p 619. 

Recently, the femtosecond time-resolved spectroscopy has been developed and many interesting publications can now be found in the literature. On the other hand, reports on time-resolved vibrational spectroscopy on semiconductor nanostructures, especially on quantum wires and quantum dots, are rather rare until now. This is mainly caused by the poor signal-to-noise ratio in these systems as well as by the fast decay rates of the optical phonons, which afford very fast and sensitive detection systems. Because of these difficulties, the direct detection of the temporal evolution of Raman signals by Raman spectroscopy or CARS (coherent anti-Stokes Raman scattering) [266,268,271-273] is often not used, but indirect methods, in which the vibrational dynamics can be observed as a decaying modulation of the differential transmission in pump/probe experiments or of the transient four-wave mixing (TFWM) signal are used. 

Finally, Klein et al. report on the femtosecond time-resolved spectroscopy of a-6T single crystals and explain the induced absorption and stimulated emission by the decay of the singlet excitons to the lower Davydov band [113]. 

Meyer S and Engel V 2000 Femtosecond time-resolved CARS and DFWM spectroscopy on gas-phase I, a ... 

Joe T and Albrecht A C 1993 Femtosecond time-resolved coherent anti-Stokes Raman spectroscopy of liquid benzene a Kubo relaxation function analysis J. Chem. Phys. 99 3244-51... 

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

Zhu L, Wang W, Sage J T and Champion P M 1995 Femtosecond time-resolved vibrational spectroscopy of heme proteins J. Raman Spectrosc. 26 527-34... 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

Structure, then the time-resolved photoelectron spectra [20, 21] could reveal signatures of two different intermediate structures, representing two different pathways on the PES. Transient absorption spectroscopy and other femtosecond time-resolved techniques may also be applicable to this problem. 

Finally a few sentences are deserved for the vast area of DNA photochemistry. Thymine dimerization is the most common photochemical reaction with the quantum yield of formation in isolated DNA of all-thymine oligodeoxynucleotides 2-3% [3], Furthermore, a recent study based on femtosecond time-resolved transient absorption spectroscopy showed that thymine dimers are formed in less than 1 ps when the strand has an appropriate conformation [258], The low quantum yield of the reaction in regular DNA is suggested to be due to the infrequency of these appropriate reactive conformations. 

Strand cleavage studies have provided relative rate constants for hole transport versus the rate constant for the initial chemical event leading to strand cleavage [18-20]. However, they do not provide absolute rate constants for hole transport processes. Several years ago we introduced a method based on femtosecond time-resolved transient-absorption spectroscopy for investigating the dynamics of charge separation and charge recombination in synthetic DNA hairpins [21, 22]. Recently, we have found that extensions of this method into the nanosecond and microsecond time domains permit investigation of the dynamics of hole transport from a primary hole... 

The dynamics of photoinduced charge separation, kcs, and charge recombination, kcr (Fig. 2a), have been studied in several families of hairpins containing an Sa linker and a single G C base pair by means of femtosecond time-resolved transient absorption spectroscopy [27, 28]. Both the singlet state and anion radical of Sa have strong transient absorption centered at 575 nm. The difference in the independently determined band shapes for Sa ... 

Seel, M., and Domcke, W. (1991), Femtosecond Time-resolved Ionization Spectroscopy of Ultrafast Internal-Conversion Dynamics in Polyatomic Molecules Theory and Computational Studies, J. Chem. Phys. 95,7806. 

After the introduction of frequency resolved CARS by Maker and Terhune [1], time resolved experiments became possible with the invention of high power lasers with femtosecond resolution. Leonhardt [2] and for example Hayden [3] performed femtosecond CARS experiments in liquids. A first femtosecond time resolved CARS experiment in gas phase was performed by Motzkus et. al. [4] where the wave packet dynamics of the dissociation of Nal was monitored. The first observation of wave packet dynamics in gaseous iodine was reported by Schmitt et al. [5]. They were able to observe dynamics in both, the ground and excited state with the same experiment. A summary of high resolution spectroscopy in gas phase by nonlinear methods is given by Lang et al. [6]. 

D. R. Cyr and C. C. Hayden, /. Chem. Phys., 104,771 (1996). Femtosecond Time-Resolved Photoionization and Photoelectron-Spectroscopy Studies of Ultrafast Internal Conversion in 1,3,5-Hexatriene. 

Interfacial hole transfer dynamics from titanium dioxide (Degussa P 25) to SCN has been investigated by Colombo and Bowman using femtosecond time-resolved diffuse reflectance spectroscopy [6c]. A dramatic increase in the population of trapped electrons was observed within the first few picoseconds, demonstrating that interfacial charge transfer of an electron from the SCN" to a hole on the photoexcited titanium dioxide effectively competes with electron-hole recombination (reactions (7.12) - (7.15)) on an ultrafast time scale [6c]. 

Finally, time-resolved spectroscopy with femtosecond pulses was recently carried out by Gale and coworkers on a similar HD0 D20 sample (125). Due to the notably wider bandwidth of the applied IR pulses in the latter investigations, no details on reshaping of the transient spectra in dependence of the excitation frequency were accessible. A time-dependent position of the peak position of the induced sample bleaching was interpreted in terms of a shift within the statistical distribution of OH frequencies with a time constant of 1 ps. However, because only the parallel signal of the induced sample transmission was detected, the measured dynamics corresponds to a superposition of vibrational, reorientational, and structural relaxation. The data are interpreted by the help of a model of with random (bell-shaped) distribution of OH oscillators, quite different from the results of other groups. 

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