Picosecond experiments

With the advent of short pulsed lasers, investigators were able to perfonn time resolved coherent Raman scattering. In contrast to using femtosecond pulses whose spectral widtii provides the two colours needed to produce Raman coherences, discussed above, here we consider pulses having two distinct centre frequencies whose difference drives the coherence. Since the 1970s, picosecond lasers have been employed for this purpose [113. 114], and since the late 1980s femtosecond pulses have also been used [115]. Flere we shall briefly focus on the two-colour femtosecond pulsed experiments since they and the picosecond experiments are very similar in concept. 

An approximate method, described in detail in Ref. (15), was applied to simulate a complementary pump-probe experiment performed with picosecond laser pulses. In this method the interaction with the probe laser pulse is approximated. A complete three-dimensional ab initio simulation, as carried out for the femtosecond experiment, is hardly possible for the picosecond experiment with the computers available today. The free laser pulse parameters were taken from the picosecond experiment duration AffWhm = 1.5 ps, intensity I = 300 MW/cm2, and central wavenumber v = O.QTiEh/h = 16021 cm 1. The dynamics induced by such a laser pulse are illustrated by... 

G. Gerber In our time-resolved experiments on the NaafZ ) state we observe the symmetric stretch even for long delay times. From nanosecond laser and CW laser spectroscopy it is well known that the B state does not decay on femtosecond or picosecond time scales. So I do not see how the decay in the picosecond experiment by Prof. Woste can be understood and how the evolution of the B state symmetric stretch into the pseudorotation and the radial motion can occur. 

Our review emphasizes three molecular systems 9,9 -bianthryl (BA) [30, 82, 88, 113-121], 4-(9-anthryl)-N,N-dimethylaminoaniline (ADMA) [122-130], and to a lesser extent the well known p-JV,IV-dimethylaminobenzonitrile (DMABN), a compound in the para-cyano-N,JV-dimethylaminoaniline class [1-5, 75-81, 131]. We only briefly mention the arylaminonaph-thalenesulfonates, which have recently been reviewed by Kosower and Huppert [4]. Brief mention is also made of picosecond experiments on diaminophenylsulphones and a number of other systems (see Section III.F). 

If 1 1 complexes are formed (1) in the ground state, then two types of photoreactions will occur parallel to each other, that of the bare and that of the complexed solute, each in its own type of cage. Mixed alcohol/ alkane systems, for example, show an indication of preformed solute-solvent complexes as evidenced by the picosecond experiments of Wang and Eisenthal80 81 on DMABN. Planar model systems like the indolines 3 and 5 (Sec. II.A.l) indicate that an additional channel opens for the B state in alcoholic solvents which increases the nonradiative decay path. This can explain the observed reduction of the fluorescence quantum yield in jirotic solvents about 0.1 in the aprotic polar solvent n-butyl chloride, about 0.01 in 1,2-propanediol, and about 0.001 in water.228... 

The first picosecond experiments investigating bathorhodopsin were carried out... 

It is clear that a number of questions need to be answered. Why, in the condensed phase, is the intersystem crossing between two nn states so efficient What is the explanation of the conflict between the linewidth studies of Dym and Hochstrasser and the lifetime studies of Rentzepis and Busch, with respect to the vibrationally excited levels It was in an attempt to provide some answers to these questions that Hochstrasser, Lutz and Scott 43 carried out picosecond experiments on the dynamics of triplet state formation. In benzene solution the build up of the triplet state had a lifetime of 30 5 psec, but this could only be considered as a lower limit of the intersystem crossing rate since vibrational relaxation also contributed to the radiationless transition to the triplet state. The rate of triplet state build-up was found to be solvent-dependent. 

The pioneering work of A. Migus et alP in water showed that the solvation process is complete in a few hundreds of femtoseconds and hinted at the existence of short-lived precursors of the solvated electron, absorbing in the infrared spectral domain (Fig. 10). Such precursors had already been suggested from picosecond experiments... 

The structure of the intermediate states in Rh7 and Rh8 has been studied recently by theoretical investigation [42], Regarding the proton translocation model, it should be noted that the excitation photon density was extremely high in the low-temperature picosecond experiments [10,35]. Therefore, the non-Arrhenius dependence of the formation rate of bathorhodopsin on temperature and the deuterium isotope effect may be results which could be detected only under intense excitation conditions. In fact, a deuterium isotope effect was not observed in the process from photorhodopsin to bathorhodopsin under weak excitation conditions [43],... 

Fourth, what is the nature of IVR near chemical activation energies Pump-probe picosecond experiments at these energies on ground-state... 

These results provided the first experimental evidence for the existence of bathorhodopsin at physiological temperatures and also permitted conclusions to be drawn and models of the intermediate to be postulated that would be consistent with this time scale for formation. When later the complete difference spectra for bathorhodopsin at room temperature were obtained in the time range of 6-300 ps (Fig. 5), it was possible to measure (Fig. 6) the formation kinetics of a band located between 530 and 680 nm that corresponds to bathorhodopsin having its maximum absorption at 580 nm and its isosbestic point at approximately 525 nm. These values correspond quite well with previously published data. Picosecond experiments performed by the same method revealed that rhodopsin also bleached with the same time constant as bathorhodopsin was formed, namely, < 6 ps. 

Taken together, the relaxation data demonstrate that there is a hierarchy of dynamic processes. The spin probe undergoes fast intramolecular librations on the time scale of a few picoseconds, experiences local rearrangement of the cage on the time scale of about hundreds of nanoseconds, and performs cooperative reorientation over time scales comparable to or longer than several microseconds in the vicinity of the glass transition. 

With the advent of femtosecond laser technology [72], it is now possible to study, in real time, the dynamics of elementary chemical reactions. The first example of an application of this technique to a chemical dynamical problem comes from sub-picosecond experiments [73] on the photodissociation of ICN. In the most recent work [73b], a 40-fs pulse (k = 307 nm) excited the ICN molecule to a repulsive state and a delayed, second femtosecond pulse probed the CN fragment (via LIF) as a function of time. By varying the probe wavelength to the red of the free CN bandhead, they were able to observe fluorescence from the CN in the I-CN molecule in the process of falling apart. They found that the transition state lives for about four times the vibrational period of the I-CN bond. In this time interval there is negligible rotation of the parent ICN molecule. 

This means, in our picosecond experiments we have observed for the first time the formation process of triplets in the T series. 

The five-membered ring is a completely planar and rigid ring, so that C11-C12 double bond cannot be distorted any more. The picosecond experiments of the five-membered rhodopsin (Rh5) showed that... 

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