Femtosecond observations

Mohammed OF, Adamczyk K, Banerji N, Dreyer J, Lang B, Nibbering ETJ, Vauthey E (2008) Direct femtosecond observation of tight and loose ion pairs upon photoinduced bimolecular electron transfer. Angew Chem Int Ed 47 9044... 

Similar femtosecond observations of the dynamics of 1,2- and 1,3-dehydroben-zene molecules starting from 1,2- and 1,3-dibromobenzene show that each dibromide gives rise to 76 amu intermediates within 100 fs, and that the 76 amu species have lifetimes of 400 fs Whether o- and m-benzyne intermediates isomerize top-benzyne and then decay, or just how such isomerizations might occur, remain open to further inquiry. 

M. Quack Prof. Zewail and Gerber, when you make an interpretation of your femtosecond observations (detection signal as a function of excitation), would it not be necessary to try a full quantum dynamical simulation of your experiment in order to obtain a match with your molecular, mechanistic picture of the dynamics or the detailed wavepacket evolution Agreement between experimental observation and theoretical simulation would then support the validity of the underlying interpretation (but it would not prove it). The scheme is of the following kind ... 

In the first place, we shall take a look at the recent advances in fast reaction photochemical kinetics and spectroscopy, in particular at picosecond laser flash photolysis and femtosecond observations. Next, photophysics and photochemistry in molecular beams will be considered. Here observations are made under single molecule-single photon conditions, and these experiments provide insight into the most fundamental unimolecular gas phase reactions. 

Direct Femtosecond Observation of the Transient Intermediate in the a-Cleavage Reaction of (CH3)2CO to 2CH3 + CO Resolving the Issue of Concertedness. 

Marvet, U. Dantus, M. Femtosecond observation of a concerted chemical reaction. Chem. Phys. Lett. 1996,256,57-62. 

Apparent non-RRKM dynamics has also been observed in time-resolved femtosecond (fs) experiments in a collision-free enviromnent [117], An experimental study of acetone illustrates this work. Acetone is dissociated to the CH and CH CO (acetyl) radicals by a fs laser pulse. The latter which dissociates by the channel... 

Zhu L, Li P, Huang M, Sage J T and Champion P M 1994 Real time observation of low frequency heme protein vibrations using femtosecond coherence spectroscopy Phys. Rev. Lett. 72 301-4... 

With tlie development of femtosecond laser teclmology it has become possible to observe in resonance energy transfer some apparent manifestations of tire coupling between nuclear and electronic motions. For example in photosyntlietic preparations such as light-harvesting antennae and reaction centres [32, 46, 47 and 49] such observations are believed to result eitlier from oscillations between tire coupled excitonic levels of dimers (generally multimers), or tire nuclear motions of tire cliromophores. This is a subject tliat is still very much open to debate, and for extensive discussion we refer tire reader for example to [46, 47, 50, 51 and 55]. A simplified view of tire subject can nonetlieless be obtained from tire following semiclassical picture. 

We shall consider just two examples of the use of femtosecond lasers in spectroscopy. One is an investigation of the transition state in the dissociation of Nal and the other concerns the direct, time-based observation of vibrational energy levels in an excited electronic state of I2. 

These two examples show how femtosecond spectroscopy allows the observation, in real time, of processes whose real-time behaviour could only be inferred, previously, from experimental observations made on a much longer time scale. 

Figure 10-4. Temporal behavior of the pholoinduccd transmission changes in LPPP alter excitation with a femtosecond pump pulse at 400 nnt. The two curves correspond to probe photon eneigies of 2.48 eV (dotted line) and 1.91 eV (solid line). At 2.48 eV the transmission change is positive due to stimulated emission (SE) while a photoin-dueed absorption (PIA) is observed at 1.91 eV (according to Ref.(24J).

Figure 10-7. (a) Absorption spectrum of 3 LPPP. The arrow indicates the spectral po-.oj, silion of the excitation pulse in the time-re- i solved measurements, (b) PL spectrum for LPPP for low excitation pulse energies, (c) Differential transmission spectrum observed in LPPP after photoexcitation with a femtosecond pulse having a pulse energy of 80 uJ at a wavelength of 400 nm. The arrow indicates the spectral position of the probe pulses used for a more detailed investigation of the gain dynamics. 

The femtosecond spectrum of the gas-phase Nal molecule as it dissociates into the separate atoms. A peak in the lower spectrum is observed whenever the bond distance in Nal reaches a certain value. 

Singlet diradicals are usually extremely short-lived intermediates. For example, trimethylene (TM, 2) was observed to have a fast decay time of 120 fs by femtosecond spectroscopy [84, 85]. Since the localized 1,3-cyclopentanediyl diradical (62) was characterized by Buchwalter and Closs in 1975 [81, 82], experimental efforts have been made to prepare and characterize the persistent, localized singlet 1,3-diradicals. Some experimental achievements of the localized diradicals are collected in Fig. 25 and Table 3. It should be mentioned that the literature of experimental studies selected here is not exhaustive and more related references can be found in [83-115] and others. 

The last problem of this series concerns femtosecond laser ablation from gold nanoparticles [87]. In this process, solid material transforms into a volatile phase initiated by rapid deposition of energy. This ablation is nonthermal in nature. Material ejection is induced by the enhancement of the electric field close to the curved nanoparticle surface. This ablation is achievable for laser excitation powers far below the onset of general catastrophic material deterioration, such as plasma formation or laser-induced explosive boiling. Anisotropy in the ablation pattern was observed. It coincides with a reduction of the surface barrier from water vaporization and particle melting. This effect limits any high-power manipulation of nanostructured surfaces such as surface-enhanced Raman measurements or plasmonics with femtosecond pulses. 

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... 

In the time-domain detection of the vibrational coherence, the high-wavenumber limit of the spectral range is determined by the time width of the pump and probe pulses. Actually, the highest-wavenumber band identified in the time-domain fourth-order coherent Raman spectrum is the phonon band of Ti02 at 826 cm. Direct observation of a frequency-domain spectrum is free from the high-wavenum-ber limit. On the other hand, the narrow-bandwidth, picosecond light pulse will be less intense than the femtosecond pulse that is used in the time-domain method and may cause a problem in detecting weak fourth-order responses. 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

In order to directly probe the dynamics of CT between Et and ZG, and to understand how the intervening DNA base stack regulates CT rate constants and efficiencies, we examined this reaction on the femtosecond time scale [96]. These investigations revealed not only the unique ability of the DNA n-stack to mediate CT, but also the remarkable capacity of dynamical motions to modulate CT efficiency. Ultrafast CT between tethered, intercalated Et and ZG was observed with two time constants, 5 and 75 ps, both of which were essentially independent of distance over the 10-17 A examined. Significantly, both time constants correspond to CT reactions, as these fast decay components were not detected in analogous duplexes where the ZG was re-... 

Schwartz BJ, Peteanu LA, Harris CB (1992) Direct observation of fast proton transfer femtosecond photophysics of 3-hydroxyflavone. J Phys Chem 96 3591-3598... 

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