Femtosecond rise time

The mass spectrum of diazabicyclo[2.2.1]hept-2-ene shows only a weak molecular ion and a very strong fragment at 68 amu. The femtosecond studies found that the 68 amu easily ionized transient profile could be modeled with a rise time of 30 10 fs and decay time of 190 10 fs, a value comparable to the decay time of trimethylene. 

On the Osaka University thermionic cathode L-band linac, a time resolution of two picoseconds was achieved using magnetic pulse compression and time jitter compensation systems (Fig. 13). The time jitter between the Cerenkov light from the electron beam and the laser pulse was measured shot-by-shot with a femtosecond streak camera to accurately determine the relative time of each measurement in the kinetic trace. In this way, the time jitter that would otherwise degrade the time resolution was corrected, and the remaining factor dominating the rise time was the electron-light velocity difference over the 2-mm sample depth. 

Fig. 4. Rise time of a 4 GPa shock in a thin film of A1 generated by a femtosecond laser pulse. The open squares are the experimentally measured phase shift of interference fringes generated by a pair of femtosecond probe pulses monitoring shock breakout at the free A1 surface. The solid circles are the data corrected for changes in the optical properties of Al. The shock front rise time tr = 6.25 ps. Reproduced with permission from ref. [32].

Figure 10. Influence of counterion valence (X , n = i, 2) on the time dependence offemtosecoridy photoinduced, electron-transfer trajectories in aqueous ionic solutions (X , nCl, X = Na , Mg ). The upper part represents the absorption signal rise time at 1.77 eV following the femtosecond UV excitation of aqueous Cl (2X4 eV). The difference in the signal rise times of Na and Mg is due to the balance between two electronic transitions e iR e hyd fCl e X ...

During recent years the development of fast photodetectors has made impressive progress. For example, PIN photodiodes (Sect. 4.5) are available with a rise time of 20 ps [11.100]. However, until now the only detector that reaches a time resolution slightly below Ips is the streak camera [11.101]. Femtosecond pulses can be measured with optical correlation techniques, even if the detector itself is much slower. Since such correlation methods represent the standard technique for measuring of ultrashort pulses, we will discuss them in more detail. 

The time resolution of the pump-and-probe technique is not limited by the rise time of the detectors. It can therefore be used in the pico- and femtosecond range (Sect. 11.4) and is particularly advantageous for the investigation of ultrafast relaxation phenomena, such as collisional relaxation in liquids and solids [13.84,13.85]. It is also useful for the detailed real-time study of the formation and dissociation of molecules where the collision partners are observed during the short time interval when forming or breaking a chemical bond [13.86]. 

The cis-trans isomerisation of retinal is concerned also in photochemical change in some bacteria where the membrane chromatophore is the protein bacteriorhodopsin. The chain of reactions following absorption of a photon has been studied by time-resolved laser flash spectrometry. The overall reaction has a half-life of about 10 ms, but there are several intermediate reactions with rise times of femtoseconds to milliseconds. In one of these a proton is transferred across bacteriorhodopsin this leads to an electrochemical proton-concentration gradient across the membrane, which is used by the bacterium as a driving force for ATP synthesis [57]. Since the structure of bacteriorhodopsin is known, it is a useful system for the study of membrane transport. 

Most of these processes are very fast. Ionization happens on the low femtosecond timescale, direct bond cleavages require between some picoseconds to several tens of nanoseconds, and rearrangement fragmentations usually proceed in much less than a microsecond (Fig. 5.3 and Chap. 2.7). Finally, some fragment ions may even be formed after the excited species has left the ion source giving rise to metastable ion dissociation (Chap. 2.7). The ion residence time within an electron ionization ion source is about 1 ps. [9]... 

Excitation of acetone with two photons at X = 307 nm delivers 186 kcal/mol of energy, more than enough to break both C—C bonds and give carbon monoxide and two methyl radicals. Following the process with femtosecond mass spectrometry shows that the excited state of acetone of 58 amu rises and falls very quickly, in a spike-like fashion. It is formed and decays with a time constant of 50 fs. For acetone-iig, a similar time dependence is seen The rise and fall of the 64 amu excited species take place in 80 and 80 fs (Fig. 20.5). 

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