Large molecule excitation source

The vast majority of single-molecule optical experiments employ one-photon excited spontaneous fluorescence as the spectroscopic observable because of its relative simplicity and inlierently high sensitivity. Many molecules fluoresce with quantum yields near unity, and spontaneous fluorescence lifetimes for chromophores with large oscillator strengths are a few nanoseconds, implying that with a sufficiently intense excitation source a single... 

The dipolar or induced dipolar natime of molecules means that the impacting electron can cause rotational excitation but, because of conservation of momentum, very little of the kinetic energy of the electron can be imparted and little direct vibrational excitation can occur (Cottrell, 1965). Further, although ion-sources frequently operate at fairly high temperatures, the population of vibrationally excited states of molecules even at 500°K is very low and the source of the large vibrational excitation of ions must be sought elsewhere. For illustrative... 

It will be shown that this terminology is critically dependent on the way one performs experiments. Properties that are taken as characteristic for large systems appear in photodynamics when one uses a broad laser as the excitation source, while properties usually taken as characteristic for small molecules are obtained when one uses a narrow laser. In this chapter we will define relatively large molecules as those molecules in which, with existing optical sources, both situations can be realized. We can then study the transition from large to small in one molecule just by changing the excitation source. 

Photo-excitation of gas-phase ions may result in the photodetachment of an electron rather than photo-fragmentation. Coulombic considerations dictate that this process is more prevalent for anions than for cations. Electron photodetachment action spectroscopy of trapped anions has proved also to be a valuable source of molecular information. In some systems, electron photodetachment and PD compete. The mechanisms for these two processes in large molecules are yet to be understood fully consequently, their branching ratios in specific experimental conditions cannot be predicted as yet. One exciting possibility is the idea of using frequency and phase-shaped pulses to promote selected photochemical pathways. 

In the previous discussion our attention was focused on the behavior of isolated molecules. However, in most photochemical and photophysical problems, the interaction of the excited molecule with its environment plays an essential role. On the other hand, even in a study of an isolated molecule, environment effects serve as a supplementary source of information. Finally, a close analogy between a simple molecule, collisionally perturbed or weakly coupled to the crystal lattice, and isolated, medium-sized or large molecules, is of the greatest importance for a better understanding of decay mechanisms. 

Since the probability per cycle of detecting a fluorescence photon has to be smaller than unity, the cycle frequency should be large and is only limited by the requirement that the time between successive excitation pulses should be larger than the lifetime of the excited molecules. Therefore, mode-locked synchronously pumped cavity-dumped CW dye lasers are ideal excitation sources. For the excitation of higher electronic states, the visible output pulses can be converted into ultraviolet pulses by optical frequency doubling in optically nonlinear crystals. 

Here t. is the intrinsic lifetime of tire excitation residing on molecule (i.e. tire fluorescence lifetime one would observe for tire isolated molecule), is tire pairwise energy transfer rate and F. is tire rate of excitation of tire molecule by the external source (tire photon flux multiplied by tire absorjDtion cross section). The master equation system (C3.4.4) allows one to calculate tire complete dynamics of energy migration between all molecules in an ensemble, but tire computation can become quite complicated if tire number of molecules is large. Moreover, it is commonly tire case that tire ensemble contains molecules of two, tliree or more spectral types, and experimentally it is practically impossible to distinguish tire contributions of individual molecules from each spectral pool. 

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