Depopulation rate

Detailed information about collision-induced, vibrational, or rotational excitation and deactivation has been collected from much work on molecular lasers, expecially the COj laser 1. If the intensity of a laser line is sufficiently high, its gain in an amplifying medium saturates and is then determined by the number of molecules excited per second into the upper laser level and by the depopulation rate of the lower level. Measuring the saturation behaviour under different pressures and discharge currents allows the excitation and deactivating processes to be studied Observation of the... 

Kellert et al.M both measured the total depopulation rates of the initially populated Xe ni states and analyzed the final bound states resulting from these collisions. The total depopulation rates of the Xe ni states were measured by... 

Fig. 9. a Double-logarithmic power dependence of the population densities, Ni and N2, of levels 1 and 2 in Fig. 5 calculated from Eq. (10) using the same parameters as in Fig. 6 c, where power is taken as G. The dashed lines are the limiting slopes from Eqs. (14) and (15). b Double-logarithmic power dependence of the level 1 depopulation rate ratio between ETU (rate = 2 Wetu i) and linear downconversion (rate = kiNi). The horizontal dashed line indicates equal rates for the two. c Time-evolution of N2 following termination of a cw beam at f = 0 for the high, medium, and low powers indicated in (b), plotted on linear axes. The dashed lines are the limiting behaviors from Eqs. (22) and (24)... 

The rate constants for radiationless decay of the triplet state of mesoporphyrin IX dimethyl ester at 77 are 26 s-1 in EtOD and 57 s-1 in EtOH.352 The most probable cause is a decrease in the rate of tautomerism in the porphyrin due to deuteriation of the N—H hydrogen. However, for TPPH2 triplet state in n-octane matrices 363 such tautomerism does not appear to be an important mechanism for radiationless deactivation. Several recent reports deal with the low-temperature e.s.r. spectra of triplet states of porphyrins.364-368 The zero field splittings and depopulation rates of the various spin sub-levels of the triplet state of Zn-chlorophyll-a have been determined by an optically detected magnetic resonance method.359... 

The differences in the population and depopulation rate constants and the phosphorescence probabilities of the three components of the triplet states form the basis of all the methods for Optical Detection of Magnetic Resonance in triplet states of jr-electron systems. These methods were developed after the discovery of optical spin polarisation and extended to inorganic solids. The essential physical difference from the optical double resonance in atoms developed by Alfred Kastler is to be found in the selection mechanism in optical double resonance, the polarisation of the resonant UV light, i.e. the symmetry of an applied field, is responsible for the selection. In optical spin polarisation, the selection is due to the spin-orbit coupling, and thus to an internal field. 

If the repopulation rates Ci, C2 are small compared to the depopulation rates Ri, R2 the saturated population density N may become quite small. 

First of all, the lifetime of the unquenched charge transfer state is roughly 2.5 ns, which corresponds to a depopulation rate of the charge transfer state of about 4 X 10 s . Second, the depopulation is strongly accelerated by the application of an electric field, increasing the total rate to above 1 x 10 s at an electric field of... 

The decay of the correlation function for a single terrylene molecule in p-terphenyl due to photon bunching was already presented in Fig. 16. In this case a biexponential decay - not easily visible in Fig. 16 - is observed because two of the sublevels are sufficiently distinct with regard to their kinetics (see Section 1.2.4.2). The measurements and the analysis of the experimental data to determine the population and depopulation rates were done in analogy to the previous description for pentacene in p-terphenyl. The ISC rates were in fairly good agreement with those determined from quantum jump measurements discussed in Section 1.2.4.2. In contrast to pentacene in p-terphenyl, the terrylene molecules studied so far did not exhibit a large variation of the ISC rates between different molecules [6]. 

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. 

Here denotes the difference between the frequency of the optical field and the resonance frequency of the Si So transition and the difference between the frequency of the microwave field and the resonance frequency of the A >- Z> transition. Ti2 and Txz are the dephasing rates of the optical and the microwave transitions, Ic2 is the total decay rate of Si. k x, k y, k z are the depopulating rates of the sublevels of Ti and kx2, ky2, k 2 their populating rates. 

Fig. 14 compares the decay of the photon correlation function of a single pentacene molecule with and without a microwave field resonant with the Z>- Z> transition. It is obvious that the contrast, C, is strongly influenced by the presence of the micro-wave field. It is defined in Eq. (15) as the ratio of population versus depopulation rates of the triplet state. The increase of the contrast in the presence of the resonant microwaves results from the accompanied reduction of the triplet depopulation... 

Since magnetic resonance techniques - in special NMR - are very versatile methods, where are the limitations with respect to single molecule spectroscopy Probably the most severe limitation is the applicability of single molecule spectroscopy itself, which is presently limited to a small number of chromophores. The magnetic resonance experiment is even more demanding since strict requirements with respect to the population and depopulation rates of the triplet sublevels have to be fulfilled. Presently only the system presented in this chapter has been successfully studied by single molecule ESR. A major issue of future investigations will thus be to increase the number of systems accessible by the technique. 

Clarke and Hofeldt determined the depopulation rates for the individual triplet state spin sublevels of chlorophyll a and chlorophyll b by microwave-modulated fluorescence intensity measurements. The species was dissolved in n-octane at a temperature of 2 K. The solvent n-octane is a low-temperature host matrix which allows high-resolution spectroscopy in the chlorophyll triplet state. Triplet absorption detection of magnetic resonance as well as fluorescence-microwave double resonance techniques were applied. The experimental arrangement was described in Ref. 167. In the case of fluorescence detection, chlorophyll b was irradiated with the 457.9-nm single-mode line of an Ar" laser. Microwave transitions were... 

J. Farley, W.H. Wing Accurate calculation of dynamic Stark shifts and depopulation rates of Rydberg energy levels induced by blackbody radiation. Hydrogen, helium and alkali-metal atoms. Phys. Rev. A23, 2397 (1981)... 

In some active media of pulsed lasers the lower laser level has a longer lifetime than the upper level. After a time T the increasing induced emission populates the lower level faster than it is depopulated by spontaneous emission and the inversion drops below threshold. In such "self-terminating" lasers the duration of the laser pulse is limited by the slow depopulation rate of the lower level and the laser emission may stop even before the pulsed pump drops below the initial threshold. One example of this laser type is the N2 laser [11,4] which is pumped by a fast transverse gas discharge ( 100 ns duration) and delivers output pulses of a few ns (1-10 ns) with typical peak powers of 0.1-1 MW. 

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