Pulsed Excitation Methods

Benton et al. extended this technique to the decay of He(2 5) in the presence 

Bour ne and Le Calvd used pulse radiolysis to study the quenching of metastable Ar states by a wide range of reagents these workers monitored the excited atoms by tracer emission from added Na, 

More recently, excited atomic states have been generated by impact of high energy (typically 200 keV to 2 MeV) electrons or protons on the noble gas at pressures up to 30 atm. The decay profiles in the pure gases show complex behaviour and it has been established only recently that such systems can 

Pulsed excitation methods are the main source of rate data for the excited noble gas atoms in the parent gas and have also provided limited data on quenching of metastable states by foreign gases. Almost all the studies of the Pi excited states have used these techniques, but photolysis as a means of populating resonant states has not been widely applied,partly because of severe problems due to radiation imprisonment.  

In these and later studies, the decay of excited atoms along the flow tube in the presence of a known concentration of reagent was measured deviations from plug flow of the carrier gas were corrected for. This system has also yielded temperature dependences of quenching rate constants for He(2 5,2 5). 

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In this case, the K s are the intersystem crossing rate constants. A detailed description of the different experimental techniques used at the present time, together with equations derived for molecules whose emission results from two zf levels, is given in a recent review of the subject (87). In this section we will give only a brief description of these PMDR methods. In all but the pulsed-excitation method, these assumptions are made (a) the spin-lattice relaxation is absent and (b) the microwave radiation either saturates or inverts the population of... 

Figure 1 shows the basic diagram of this single pulse excitation method. In the figure, small letters represent the transfer functions in time domain and capital letters represent the frequency transfer function. As shown in Fig. 1, the observed function x(t) may be described as the following convolution chain ... 

To carry out this procedure, a wide frequency characteristic of the signal is required. In this pulse excitation method, a rectangular pulse like the 5-function is used. When height and duration of the pulse are H and T respectively, the spectrum of the pulse (Fig. 2) is... 

As described above, the single pulse excitation method is very simple and powerful, but to put this technique into practice we have had to wait for the development of the wide-band transducer and the digital electric instruments described below. 

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

Pulsed method. Using a pulsed or modulated excitation light source instead of constant illumination allows investigation of the time dependence of emission polarization. In the case of pulsed excitation, the measured quantity is the time decay of fluorescent emission polarized parallel and perpendicular to the excitation plane of polarization. Emitted light polarized parallel to the excitation plane decays faster than the excited state lifetime because the molecule is rotating its emission dipole away from the polarization plane of measurement. Emitted light polarized perpendicular to the excitation plane decays more slowly because the emission dipole moment is rotating towards the plane of measurement. 

Selective experiments can also be performed by the tailored excitation method of Tomlinson and Hill. The selective pulse is frequency-modulated with a function designed to yield zero effective field at the resonance offset of the neighboring nuclei. Although this technique is especially promising for studies of more-complex spin systems, its use is as yet very limited, in part because the instrumentation needed is not yet commercially available. 

Another approach to obtain spatially selective chemical shift information is, instead of obtaining the entire image, to select only the voxel of interest of the sample and record a spectrum. This method called Volume Selective spectroscopY (VOSY) is a ID NMR method and is accordingly fast compared with a 3D sequence such as the CSI method displayed in Figure 1.25(a). In Figure 1.25(b), a VOSY sequence based on a stimulated echo sequence is displayed, where three slice selective pulses excite coherences only inside the voxel of interest. The offset frequency of the slice selective pulse defines the location of the voxel. Along the receiver axis (rx) all echoes created by a stimulated echo sequence are displayed. The echoes V2, VI, L2 and L3 can be utilized, where such multiple echoes can be employed for signal accumulation. 

We consider a model for the pump-probe stimulated emission measurement in which a pumping laser pulse excites molecules in a ground vibronic manifold g to an excited vibronic manifold 11 and a probing pulse applied to the system after the excitation. The probing laser induces stimulated emission in which transitions from the manifold 11 to the ground-state manifold m take place. We assume that there is no overlap between the two optical processes and that they are separated by a time interval x. On the basis of the perturbative density operator method, we can derive an expression for the time-resolved profiles, which are associated with the imaginary part of the transient linear susceptibility, that is,... 

The lifetime of the singlet excited state (the fluorescence lifetime TF) is of the order of picoseconds to 100 nanoseconds (10—12 - 10-7 seconds) and can now be measured accurately using pulsed laser excitation methods and other techniques. Since the radiative transition from the lowest triplet state to the ground state is formally forbidden by selection rules, the phosphorescence lifetimes can be longer, of the order of seconds. 

Time-resolved method 1 decay of the donor fluorescence If the fluorescence decay of the donor following pulse excitation is a single exponential, the measurement of the decay time in the presence (td) and absence (t ) of transfer is a straightforward method of determining the transfer rate constant, the transfer efficiency and the donor-acceptor distance, by using the following relations ... 

Time-resolved method 2 increase in the acceptor fluorescence The transfer rate constant can also be determined from the increase in the acceptor fluorescence following pulse excitation of the donor. The concentration of excited acceptors following (5-pulse excitation of the donor obeys the following differential equation ... 

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