Depopulating pulse

After about 25 fs, the Pg (t) gradually decreases due to predominance of the radiationless transitions into the manifold of states over the laser excitation process. The decay rates are different in different groups of states, with some states over 4.83 eV being more slowly depopulated due to IC than states with lower energies [42]. This feature of the IC decay in pyrazine is manifest here in more pronounced way than in the case of ultrashort pulse excitation. Here, it is clear... 

Magnetic quantum beats in the transient process after pulsed depopulation of the ground state may be observed not only in fluorescence, but also in a more direct way, namely in absorption. In connection with what was discussed in Section 3.5, one must expect maximum sensitivity if the experiment is conducted according to the laser interrogated dichroism method see Fig. 3.17. To this end it is convenient to direct the external magnetic field B along the 2-axis as shown in Fig. 4.21 where the probe beam E-vector can be either in the xy plane (Em) or in the yz plane (Epr2). 

Radiationless Processes. There has been little work on the photochemistry of propynal. It is only recently that a systematic study of the radiationless transitions which depopulate the low-lying -A" state has been undertaken. Thayer et al. (240, 241,242) have used a pulsed dye laser to excite emission from various vibrational levels of the i-A" state. By measuring the... 

Fluorescence and stimulated emission on transitions (p = 0 -> p" > 0) lead to a fast rise of the population densities A (p") of high vibrational levels in the Sq state. This would result in a self-termination of the laser oscillation if these levels were not depopulated quickly enough by collisions. The relaxation of N(v") toward the thermal equilibrium population No(v") can again be probed by a weak visible probe laser. Polarization spectroscopy (Sect. 2.4) with femtosecond pulses allows one to separately determine the decay times tvib of population redistribution and the dephasing times [814]. 

F ure 2. Schematic representation of the ODMR effect on a single molecule. Part (a) of the figure shows the five-level scheme relevant for the experiment. k 2 denote the population rates of the sublevels Uy U = X. Y. Z) and represent the depopulation rates. The relative magnitudes of the rates with respect to each other are indicated by the thickness of the arrows. Part (b) depicts the temporal evolution of the photon electron pulses as they are created by the detection system. Upon irradiation with microwaves in resonance with the T>- Z> transition the dark periods between the photon bunches are lengthened. 

In some pulsed lasers (for example, the N2 laser) the lower laser level has a longer effective lifetime than the upper level [11.14]. The increasing laser power Pl(0 decreases the population inversion by stimulated emission. Since the lower level is not sufficiently quickly depopulated, it forms a bottleneck for maintaining threshold inversion. The laser pulse itself limits its duration and it ends before the pump pulse ceases (self-terminating laser. Fig. 11.1b). 

---