Echo time-resolved

The infrared echo is also used to measure vibrational dynamics but in the standard implementation involves a further reduction in dimension (35,36,41,42). The excitation interactions I and II are strictly analogous to those in the Raman echo the Raman interaction is simply replaced by a direct absorption (Fig. 3, dashed arrows). However, whereas the Raman echo time resolves the signal during r3, the infrared echo integrates the signal during this time period. In this way, the infrared echo reduces the correlation function to one dimension. The standard, two-pulse photon echo is reduced to one dimension in much the same way. Because the infrared echo derives from the same basic correlation function as the Raman echo,... 

Pshenichnikov M S, Duppen K and Wiersma D A 1995 Time-resolved femtosecond photon echo probes bimodal solvent dynamics Phys. Rev. Lett. 74 674-7... 

We can perform spatially resolved Carr-Purcell-Meiboom-Gill (CPMG) experiments, and then, for each voxel, use magnetization intensities at the echo times to estimate the corresponding number density function, P(t), which represents the amount of fluid associated with the characteristic relaxation time t. The corresponding intrinsic magnetization for the voxel, M0, is calculated by... 

Fig. 8.10. Short pulses for time-resolved measurements the usable bandwidth of the pulses above noise level is about 0.5 GHz, and they were digitized with an overall timing precision of 0.15ps. (a) Reference signal reflected from a glass slide at focus with no specimen (b) reflected signal from a cell on the glass slide, with echoes from the top of the cell and from the interface between the cell and the substrate (Wang et al.

Polymer coatings on stiffer substrates can be measured by time-resolved techniques (Sinton et al. 1989). Often in these cases it is not convenient to measure a direct reflection from an uncoated part of the substrate at more or less the same time, and anyway the substrate may not be flat, but this may not matter if it can be assumed that either the thickness or the longitudinal velocity of the coating does not vary. The time interval between the echoes from the top and bottom surfaces of the coating can then be used to determine the unknown quantity. An example of the kind of signal that can be obtained is shown in Fig. 10.5. The specimen was a coating of PET (polyethylene terephthalate) 15 m thick on a stone-finish rolled steel substrate. Although there is some overlap of the two echoes, there is no difficulty in... 

Fig. 12.10. Time-resolved S(f, y) along a line perpendicular to a crack in glass, scanning across the crack (a) some distance from the end of the crack (b) 75 //m from the end of the crack. As in Fig. 9.3(b), the horizontal axis is time t the vertical axis is y, and the value of S(t, y) is indicated by the intensity, with mid-grey as zero and dark and light as negative and positive values of S. In both figures, the first echo (seen as the first stripy vertical bar) is the geometric reflection from the surface of the specimen, and the second echo (seen as the second stripy vertical bar) is the Rayleigh reflection ( 7.2). The patterns forming a x are the reflections from the near and the far sides of the crack, which cross over when the lens is directly above the crack. In (b), where the scan passes quite near to the tip of the crack, the hyperbolic pattern is due to the crack-tip-diffracted wave (Weaver et al. 1989).

In this paper we report the use of spectrally resolved two-colour three-pulse photon echoes to expand the information that can be obtained from time-resolved vibrational spectroscopy. The experiments allow the study of intramolecular dynamics and vibrational structure in both the ground and excited electronic states and demonstrate the potential of the technique for studying structural dynamics. 

When deciding to study the dynamics of electronic excitation energy transfer in molecular systems by conventional spectroscopic techniques (in contrast to those based on non-linear properties such as photon echo spectroscopy) one has the choice between time-resolved fluorescence and transient absorption. This choice is not inconsequential because the two techniques do not necessarily monitor the same populations. Fluorescence is a very sensitive technique, in the sense that single photons can be detected. In contrast to transient absorption, it monitors solely excited state populations this is the reason for our choice. But, when dealing with DNA components whose quantum yield is as low as 10-4, [3,30] such experiments are far from trivial. 

While the first experiments of time-resolved IR spectroscopy were conducted with pulse durations exceeding 10 ps, the improved performance of laser systems now offers subpicosecond (12) to femtosecond (13-15) pulses in the infrared spectral region. In addition, the pump-probe techniques have been supplemented by applications of higher-order methods, e.g., IR photon echo observations (16). 

Firstly, the essential correctness of the tube picture has only recently been established in a remarkable series of experiments. The complex monomer diffusive self-correlation predicted has now been seen in field-gradient NMR. Reptative motion across an interface was the only successful explanation of time-resolved neutron reflectivity. Neutron Spin Echo (NSE) can now be extended in time sufficiently to identify the tube diameter directly. A series of massive many-chain numerical simulations have shown tube-like constraints with sizes identical to those obtained by rheology via the plateau modulus Go and NSE). 

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