Reduction photon beam

Advanced Photon Source at Argonne National Lab shown in Figure 17.10. The problem with conventional X-ray sources is the drastic reduction in beam intensity when time-resolved measurements are required. Using tomographic analysis, three-dimensional reconstruction of the dynamics of the spray can be obtained (Cai et al. 2003). 

Because the excitation intensity varies as the square of the distance from the focal plane, the probability of two-photon absorption outside the focal region falls off with the fourth power of the distance along the z optical axis. Excitation of fluorophores can occur only at the point of focus. Using an objective with a numerical aperture of 1.25 and an excitation beam at 780 nm, over 80% of total fluorescence intensity is confined to within 1 pm of the focal plane. The excitation volume is of the order of 0.1-1 femtoliter. Compared to conventional fluorometers, this represents a reduction by a factor of 1010 of the excitation volume. 

Even in a molecule the size of benzene the resolution achieved in this way is sufficient to investigate the dynamic behavior of individual rotational states. For this it is necessary to eliminate the Doppler broadening of the rovibronic transitions. Two methods have been applied (i) the elimination of Doppler broadening in a Doppler-free two-photon-transition and (ii) the reduction of Doppler broadening in a molecular beam. Measurements of the dynamic behavior have been performed in the frequency [3] and time domain [4]. We will briefly summarize the results from high-resolution measurements and discuss the conclusions on the intramolecular decay mechanism. Then it will be discussed how the intramolecular dynamics is influenced by the attachment of an Ar or Kr atom to the benzene molecule, leading to a weakly bound van der Waals complex. 

Two-photon excitation is preferable in 3D optical memory because the crosstalk between two adjacent layers is much reduced. Another advantage of two-photon excitation is reduction in multiple scattering. This reduction occurs because of the use of an illumination beam at infrared wavelength. 

The brilliance of a photon-source can be optimized by a reduction of the emittance of the electron beam and the use of periodic magnetic structures (insertion devices) as radiation sources. 

The second method is the laser hole-burning method. By firing the excimer laser at the parent molecular beam, the intensity of the parent molecules along the detector axis is reduced for a time equal to the pulse width (full width = 16 ns) of the laser. The reduction of the parent beam signal is due to the dissociation of the parent molecule induced by the absorption of a 193-nm photon. The laser burn hole recorded by the MCS gives an accurate measure of the velocity spread and the most probable speed (vq) of the parent molecular beam traveling from the photodissociation region to the ionizer. 

By studying the shape of the whole time-dispersed photon distribution from a scattering medium the absorption and scattering coefficients of the medium can be evaluated. The same information can be obtained by employing a sinusoidally modulated CW laser beam and studying the phase-shift and the modulation contrast reduction in the scattered Hght [10.225]. This is basically the phase-shift method discussed previously in Sect. 9.4.4. Several modulation frequencies are needed to obtain the same information as in the conceptually simpler pulsed techniques. 

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