Femtosecond time resolution

With the short pulses available from modem lasers, femtosecond time resolution has become possible [7, 71, 72 and 73], Producing accurate time delays between pump and probe pulses on this time scale represents a... 

In addition to the described above methods, there are computational QM-MM (quantum mechanics-classic mechanics) methods in progress of development. They allow prediction and understanding of solvatochromism and fluorescence characteristics of dyes that are situated in various molecular structures changing electrical properties on nanoscale. Their electronic transitions and according microscopic structures are calculated using QM coupled to the point charges with Coulombic potentials. It is very important that in typical QM-MM simulations, no dielectric constant is involved Orientational dielectric effects come naturally from reorientation and translation of the elements of the system on the pathway of attaining the equilibrium. Dynamics of such complex systems as proteins embedded in natural environment may be revealed with femtosecond time resolution. In more detail, this topic is analyzed in this volume [76]. 

Contrary to the above-described detection methods, fluorescence up-conversion and optical Kerr gate techniques readily achieve picosecond/femtosecond time resolution (Ippen and Shank 1975 Shah 1988 Takeuchi and Tahara 1998), because they are in the pump-probe measurement, in principle. 

Here we first describe the ultrafast fluorescence microscope, which nses the fluorescence up-conversion method. This microscope simnltaneonsly achieves femtosecond time resolution and snbmicron space resolution (Fnjino and Tahara 2003, 2004). 

Experimentally it has proven very difficult to investigate bimolecular reaction kinetics. Although optical techniques have been developed with femtosecond time resolution, the bimolecular nature of the reactions precludes standard femtosecond pump-probe experiments, as a common starting time for the reaction is not readily established. Here we initiate the fast bimolecular acid-base reaction by converting a very weak base, NO3 to a weak base ONOO" and follow the reaction ONOO + H+ as a function of [ft]. 

Detectors for electron microscopy are required to improve spatial and temporal sensitivity. Improved detectors will enable femtosecond time resolution and higher sensitivity and will reduce the number of electrons needed. 

The electron dynamics in metals have been investigated with femtosecond time resolution in two photon photoemission experiments [3,16,17], where the idea that the signal can be described by a double convolution of an exponential decay with the laser pulse shape is commonly... 

Earlier studies on dye-sensitized Ti02 reported nanosecond time constants for the injection kinetics [16, 40-42]. These results were obtained indirectly from the measurement of the injection quantum yield and implicitly assumed that the interfacial electron transfer reaction was competing only with the decay of the dye excited state. Other studies were based on the same assumption but used measurements of the dye fluorescence lifetime, which provided picosecond-femtosecond time resolution [43-45]. Direct time-resolved observation of the buildup of the optical absorption due to the oxidized dye species S+ has been employed in more recent studies [46-51]. This appears to be a more reliable way of monitoring the charge injection process as it does not require any initial assumption on the sensitizing mechanism. 

We remark that much progress in our understanding of the very first thermalization step in large molecules such as dyes is exp>ected to follow the development of subpicosecond laser techniques. Unimolecular thermalization times in the 100-fs range have already been estimated. Femtosecond time resolution could also shed new light on the possible many-body contribution to near-resonant V-V transfer in simple dense systems. 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

Lasers with ultrashort pulse lengths down to a few femtoseconds (1 fs = 10 15 s) are now available commercially. However, photomultipliers and the associated electronic digitizers are not sufficiently fast to follow waveforms much below 1 ns. Therefore, devices with pico- and femtosecond time resolution use optical delay lines to define the time delay between the excitation pulse and the probe pulse (Figure 3.16). By focusing part of the... 

There is an interesting connection between the energy needed to destroy entanglement, as indicated by this experiment, and the entropy production when quantum information is erased [30], Further experiments with femtosecond time resolution will probably help to clarify this connection and its possible influence on future quantum computer and communication systems. 

In this section, spectroscopies that use higher energy electrons and photons than in standard optics are reviewed. Techniques that are directly related to the results presented below in Sections 2.4—2.6 are described, and also a short overview is given of X-ray analyses that have been selected because of their application in the analysis of biomolecules including the energetics of the metallic centers, buried interfaces in solar cells, and femtosecond time resolution. 

The study of chemical dynamics concerned with the ultrashort time interval when a chemical bond is formed or broken may be called real-time femtochemistry [1412], It relies on ultrafast laser techniques with femtosecond time resolution [1413]. 

Using a high repetition rate (8 kHz) data acquisition astern, we have been able to measure time resolved spectra of Induced transmittance changes to an accuracy of one part in 10 with better than 100 femtosecond time resolution. 

With the development of fluorescence upconversion techniques, which nowadays provide femtosecond time resolution, it is also possible to directly measure the time evolution of the spontaneous emission following the excitation of the sample by the pirnip pulse. In this method, the fluorescence is collected and focused onto a nonlinear crystal, where it is superposed with the probe beam in order to perform upconversion. Time resolution is achieved because the probe pulse creates a time gate for the spontaneous emission, i.e. the fluorescence is only measured within the duration of the probe. Frequency resolution is achieved by subsequently dispersing the upconverted signal in a monochromator. Although fluorescence detection provides less photon yield than stimulated techniques, it has the desirable feature to exclusively monitor the time evolution in the initially excited electronic states (cf. the discussion above). 

In the limiting case of Ek 0, Eq. (19) describes the zero-kinetic-energy (ZEKE) photoelectron signal. ZEKE photoelectron spectroscop is a background-free technique with high frequency-resolution, which recently has been combined with PP techniques with picosecond and femtosecond " time resolution. 

Here, the principal features and characteristics of the ultrafast laser systems used are briefly summarized. Besides the titanium sapphire laser which acts as the workhorse in nearly all of the discussed experiments, a synchronously pumped dye laser is employed to study the ultrafast dynamics of Nas on a picosecond timescale (see Sect. 3.2.2). For measurements with femtosecond time resolution and wavelengths located between 600 and 625 nm a synchronously titanium sapphire pumped optical parametric oscillator followed by frequency doubling is used. To investigate the Nas C state, two mode-locked titanium sapphire lasers have been synchronized. In all cases the essential parameter of the generated laser pulses, the pulse width, has to be determined. This problem is solved by an autocorrelation technique. Hence, the principles of an autocorrelator are briefly described at the end of this section. 

Extended Energy-Level Model. The real-time spectra - performed with femtosecond time resolution - of the Naa system, as well as of the larger sodium and potassium clusters (see Sects. 4.2, 4.3 and 4.4), reveal a nonexponential decay which cannot be explained within the simple energy-level model introduced above. It seems reasonable that this different behavior is caused by clusters in the beam which are larger than those of interest. Therefore, the simple model has to be slightly extended. This extended energy-level model has the following features. 

In the first pump probe experiments with picosecond time resolution the real-time spectra of the Nas E) state (Fig. 4.8) reveal a fast exponential decay caused by ultrafast photo-induced dissociation [306]. This behavior could be well explained with the simple fragmentation model described in Sect. 2.2.2. With femtosecond time resolution it was expected to observe the photodissociation process with more detail in the recorded transients. Especially, the energy dependence of the ultrashort fragmentation process should allow deeper insight into the fragmentation dynamics within a photoexcited molecular beam. 

---