Ultrafast Structural Relaxation

It is shown here that such photodetachment experiments are a powerful tool to observe and charaterize properties of the transition state of molecules and clusters. Neumark and coworkers nicely demonstrated this in cw experiments [414-417]. The approach using ultrashort laser pulse excitation enables the preparation of the neutral in a superposition of eigenstates. Hence, wave packet dynamics can take place. The corresponding wave packet propagation is probed by the ionization process, which results in the production of the cation and a further photoelectron. Here it is shown that the real-time detection of the cation signal yields new information about the ultrafast nuclear dynamics of the neutral molecules or clusters. 

The first studies have been done with molecules and clusters of silver atoms [418], because there is considerable information about them from experiment and from theory. Only the clusters Aga, Ag5, Agy, and Agg have been looked at, because their ionization potentials are low enough to ionize them by two-photon absorption with the laser sources available (see Sect. 2.1.2). However, these are in some respects an ideal series because they provide several different structural relationships among the negative, neutral, and positive species [84, 419]. As is shown in Sect. 5.1, the silver trimer can 

In this section the ultrafast structural redistribution of Aga, initiated by a 100fs laser pulse, is presented. The experiment makes use of a high-intensity cluster anion source, an ion trap, a mass-analyzing detector for cluster cations, and a laser system which produces pairs of ultrashort laser pulses with an adjustable time delay between the pump and the probe laser pulse (for details see Sect. 2.1.2). 

Negatively charged, mass-selected silver trimers are caught in a linear quadrupole trap. There, their excess electrons are photodetached with a pulse of radiation in the range of 400 nm, with a duration of nearly 100 fs. The neutrals prepared in this way would remain in the trap for nanoseconds or even milliseconds. However, a second ultrashort pulse of the same radiation, intense enough to induce TPI of the clusters, is directed into the trap after a preselected delay, allowing the neutral to carry out internal motion, from a small fraction of a vibration to many vibrations. The positive ions generated in this way, are then mass-analyzed and collected. Thus, the yield of 

The first experiments were carried out using the silver trimer anion. Mass-selected Ag3 ions were produced with an intensity of about 2 nA and stored in the ion trap. The detachment was performed at wavelengths of 420 nm, 415 nm, 400 nm, and 390 nm, so that one-photon detachment of the anions was possible. The ionization was done nonresonantly using two photons of the same wavelength. The energy of two photons of 420 nm is only slightly above the ionization potential of the silver trimer, thus allowing a very soft ionization. 

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Kitagawa, T. Haruta, N. Mizutani, Y, Time-resolved resonance Raman study on ultrafast structural relaxation and vibrational cooling of photodissociated carhonmonoxy myoglobin. Biopolymers 2002, 61, 207-213. 

The first results are presented in Sect. 5.1, revealing information about ultrafast structural relaxation times of the prepared molecule or cluster. The... 

The experiments presented here, require in some cases sophisticated theoretical approaches. The calculations of the wave packet propagation (Sect. 2.2.1) observed for excited dimers and trimers (Chap. 3) are of this special kind. The photodissociation phenomena (Chap. 4), however, can be treated by two rather easy approaches, as described in Sect. 2.2.2. The essentials of thefirst theoretical approaches to interpretion of the results on ultrafast structural relaxation (Chap. 5) obtained by the NeNePo technique are summarized in Sect. 2.2.3. 

Fig. 5.3. Real-time spectra of the ultrafast structural relaxation of the silver trimer taken with wavelengths of (a) A = 390 nm, (b) A = 400 nm, (c) A = 415 nm,...

Fig. 5. 5. Ultrafast structural relaxation of the silver trimer s molecular configuration starting in the anion s linear geometry. Photodetachment by the pump pulse initiates the bending of the meanwhile neutral trimer. The probe pulse can most efficiently produce cations while the neutral s configuration is close to that of the cation, which is the equilateral geometry (taken from [223])...

Ultrafast Structural Relaxation in Small Silver Clusters... 

The time-resolved solvation of s-tetrazine in propylene carbonate is studied by ultrafast transient hole burning. In agreement with mode-coupling theory, the temperature dependence of the average relaxation dme follows a power law in which the critical temperature and exponent are the same as in other relaxation experiments. Our recent theory for solvation by mechanical relaxation provides a unified and quantitative explanation of both the subpicosecond phonon-induced relaxation and the slower structural relaxation. 

Combining a microscopic electronic theory with molecular dynamics simulations in the Born-Oppenheimer approximation, Bennemann, Garcia, and Jeschke presented the first theoretical results for the ultrafast structural changes in the silver trimer [135]. They determined the timescale for the relaxation from the linear to a triangular structure initiated by a photodetachment process and showed that the time-dependent change of the ionization potential (IP) reflects in detail the internal degrees of freedom. 

Vibrational spectroscopy can help us escape from this predicament due to the exquisite sensitivity of vibrational frequencies, particularly of the OH stretch, to local molecular environments. Thus, very roughly, one can think of the infrared or Raman spectrum of liquid water as reflecting the distribution of vibrational frequencies sampled by the ensemble of molecules, which reflects the distribution of local molecular environments. This picture is oversimplified, in part as a result of the phenomenon of motional narrowing The vibrational frequencies fluctuate in time (as local molecular environments rearrange), which causes the line shape to be narrower than the distribution of frequencies [3]. Thus in principle, in addition to information about liquid structure, one can obtain information about molecular dynamics from vibrational line shapes. In practice, however, it is often hard to extract this information. Recent and important advances in ultrafast vibrational spectroscopy provide much more useful methods for probing dynamic frequency fluctuations, a process often referred to as spectral diffusion. Ultrafast vibrational spectroscopy of water has also been used to probe molecular rotation and vibrational energy relaxation. The latter process, while fundamental and important, will not be discussed in this chapter, but instead will be covered in a separate review [4],... 

To conclude, the results presented in this section demonstrate that the semiclassical implementation of the mapping approach is able to describe rather well the ultrafast dynamics of the nonadiabatic systems considered. In particular, it is capable of describing the correct relaxation dynamics of the autocorrelation function as well as the structures of the absorption spectrum of... 

The ultrafast photoreactions in PNS of these proteins take place immediately after conversion from the FC state to vibrationally unrelaxed or only partially relaxed FI state [1-3]. For PYP [1] and Rh [3], the primary process is twisting of the chromophore, which causes the ultrafast fluorescence quenching, in the course of the isomerization, while the primary process for FP [2] is the ultrafast electron transfer leading to the fluorescence quenching reaction in PNS. Thus, in spite of the different molecular structures of PYP, Rh and FP chromophores and different kind of photoinduced reactions, these photoresponsive proteins show ultrafast and highly efficient photoreactions from FI state of similar nature (vibrationally unrelaxed or only partially relaxed), suggesting the supremely important role of the PNS controlling the reactions. 

Our objective is to understand how the noncovalent interactions responsible for nucleic acid secondary structure (i.e. base stacking and base pairing) affect the photophysics of these multichromophoric systems. Here we describe initial experimental results that demonstrate dramatic differences in excited-state dynamics of nucleic acid polymers compared to their constituent monomers. Although ultrafast internal conversion is the dominant relaxation pathway for single bases, electronic energy relaxation in single-stranded polynucleotides... 

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