Multiple excitation delay

Fixed spatial phase in the grating pattern also facilitates experiments with multiple excitation pulses (20). A second, delayed pulse incident on the diffractive optic is split in the same manner as the first and results in a second excitation pattern with the same peak and null positions. Thus, multiple excitation gratings, delayed temporally and shifted spatially if desired, can be used for excitation of phonon-polaritons whose coherent superposition is well controlled. A preliminary experiment of this type has been reported (21). 

Figure Al.6,8 shows the experimental results of Scherer et al of excitation of I2 using pairs of phase locked pulses. By the use of heterodyne detection, those authors were able to measure just the mterference contribution to the total excited-state fluorescence (i.e. the difference in excited-state population from the two units of population which would be prepared if there were no interference). The basic qualitative dependence on time delay and phase is the same as that predicted by the hannonic model significant interference is observed only at multiples of the excited-state vibrational frequency, and the relative phase of the two pulses detennines whether that interference is constructive or destructive.

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

The excitation profile of multiple bands was also known for periodic RF pulses, such as the DANTE (delays alternating with nutation for tailored excitation) sequence.26 Similar to the PIP, all the phases and strengths of the effective RF fields can be obtained by expanding the periodic pulse into a Fourier series and properly rearranging the terms afterwards.27 Detailed calculation, comparison with the PIP, and the excitation profiles by a periodic pulse of fix sin(7tt/T) Ix and a DANTE sequence are presented in Section 3. 

It is well-known that the excitation profile by a periodic pulse also has a pattern of multiple bands in response to the multiple effective RF fields. The DANTE sequence,26 for instance, was one of the most frequently used periodic pulse in the past for selective excitation of a narrow centre band. It is constructed by a long train of hard pulses with a certain delay between two adjacent pulses. The advantage of using the DANTE sequence over the weak, soft RF pulses relies on that it is not necessary to change the RF power level in the pulse sequence. Consequently, phase distortions and certain delays accompanied by the abrupt changes of the RF power level are avoided. 

On the other hand, additional spectroscopic information can be obtained by making use of this technique The Fourier transform of the frequency-filtered transient (inset in Fig. 8) shows that the time-dependent modulations occur with the vibrational frequencies of the A E and the 2 IIg state. In the averaged Na2+ transient there was only a vanishingly small contribution from the 2 IIg state, because in the absence of interference at the inner turning point ionization out of the 2 IIg state is independent of intemuclear distance, and this wavepacket motion was more difficult to detect. In addition, by filtering the Na2+ signal obtained for a slowly varying pump-probe delay with different multiples of the laser frequency, excitation processes of different order may be resolved. This application is, however, outside the scope of this contribution and will be published elsewhere. 

Figure 8.2.16 COSY spectra acquired with the four-coil probe, where the compounds and concentrations were the same as those of the one-dimensional spectra. Data acquisition parameters spectral width, 2000 Hz data matrix, 512 x 128 (complex) 16 signal averages delay between successive coil excitations, 400 ms effective recycle delay for each sample, ca. 1.7 s. Data were processed by using shifted sine-bell multiplication in both dimensions and displayed in magnitude mode. Reprinted with permission from Li, Y., Walters, A., Malaway, P., Sweedlar, J. V. and Webb, A. G., Anal. Chem., 71, 4815M820 (1999). Copyright (1999) American Chemical Society...

Figure 6.10 Frequency response of a SAW delay line, showing both the magnitude (solid curve, left ordinate) and phase (dotted curve, right ordinate) of the transmitted signal as a function of excitation frequency. The upper horizontal line represents a gain of 2S dB from a hypothetical amplifier used in constructing an oscillator loop the lower horizontal line represents zero phase shift (or any integral multiple of 2ir). The three points indicated on the frequency curve by filled circles indicate the frequencies at which the two conditions necessary for loop oscillation are satisfied less insertion loss than the gain of the amplifier, and a round-trip phase shift of zero.

The DANTE pulse sequence is shown in Figure 20b. A peak that is exactly on-resonance or an integral multiple of l/d2 Hz off-resonance, experiences the cumulative effect of the n pi pulses. Other resonances see a random excitation that effectively sums to zero. In exchange experiments, a variable delay (vd) is inserted between the end of the DANTE pulse train and the read pulse, p2. During vd, magnetization is transferred between exchanging sites. 

Choose d2 such that sideband excitation (which occurs at integral multiples of l/d2), does not interfere with other resonances in the spectrum. When the variable delay is very short, the inverted resonance appears in emission and all other signals should be in absorption. 

Figure 1. Multiple Quantum NMR pulse. Growth curves are obtained by incrementing either the interpulse delays or the number of multiple pulse excitation trains in the conversion and reconversion sequences. (Reproduced from reference 13. Copyright 2005 American Chemical Society.)...

Multiple parameters can be measured for fluorescence photons count rate (or intensity), wavelength (X), polarization (p), arrival time (to), time delay after excitation (td), and location x,y on an imaging detector). These parameters carry information about the fluorophore that includes the nature of its environment, its interactions with other molecules, and its motions. Fluorescence techniques that exploit each of these parameters are described below. 

The experimental setup for the broadband CARS is rather simple because only two pulses are needed for three-color CARS emission, as shown in Fig. 5.4a a broadband first pulse impulsively promotes molecules to vibrationally excited states through a two-photon Raman process, and a delayed narrowband second pulse induces anti-Stokes Raman emission from coherent superpositions to the ground state [29]. By changing the delay time for the second pulse, therefore, one can expect to probe dynamical behaviors of multiple RS-active modes. Such a two-dimensional observation in the time-frequency domains should be effective for detailed analysis of nanomaterials. 

Another possible process is internal conversion, radiationless transition to an electronic state of the same spin multiplicity, for example, S - 82, involving excited singlet states. This can be followed by delayed fluorescence. Chemical bonds in electronically excited molecules can also dissociate or rearrange themselves, thereby taking part in photochemical reactions. 

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