Excitation pulse, shaped

The rf transmitter amplifies an rf pulse signal of about 1 mW up to several W or up to several kW. The amplifier should work in a linear mode (class AB) because excitation pulse shape for slice selection must be reproduced. Class AB rf transmitters such as these with blanking gates are widely available commercially. 

An important general point to emerge from these experiments is that tailored excitation-pulse shapes can significantly alter the mass spectrum produced by laser radiation. This approach could, therefore, provide a method for the multidimensional analysis of complex molecules, as varying ion distributions, each with different information content, can be obtained as a function of pulse shape. The use of tailored femtosecond laser pulses may, therefore, open new avenues for mass spectromettic analysis of large and biologically relevant molecules. 

The Champ-Sons model is a most effieient tool allowing quantitative predictions of the field radiated by arbitrary transducers and possibly complex interfaces. It allows one to easily define the complete set of transducer characteristics (shape of the piezoelectric element, planar or focused lens, contact or immersion, single or multi-element), the excitation pulse (possibly an experimentally measured signal), to define the characteristics of the testing configuration (geometry of the piece, transducer position relatively to the piece, characteristics of both the coupling medium and the piece), and finally to define the calculation to run (field-points position, acoustical quantity considered). 

The integral describes the spatial amplitude modulation of the excited magnetization. It represents the excitation or slice profile, g(z), of the pulse in real space. As drops to zero for t outside the pulse, the integration limits can be extended to infinity whereupon it is seen that the excitation profile is the Fourier transfonn of the pulse shape envelope ... 

Fluorescence correlation spectroscopy can be used to measure the binding dynamics of host-guest complexes when the fluorescence quantum yields for the free and bound hosts are different. Analysis of fluorescence correlation spectra depends on the profile for the excitation pulse, which impacts the shape of the emission profile and mechanistic assumptions are made with respect to the diffusion of the various species in solution.58 For each chemical system different assumptions are made. 

The idea of using the linear phase increments to achieve frequency-shifted excitation can be adopted almost to any pulses, such as hard (amplitude fixed) pulses, shaped pulses, and even adiabatic inversion pulses. Unlike any other pulses, the adiabatic pulses have already used non-linear phase increments for tilting the effective RF field slowly compared with the Larmor frequency of the spins in the rotating frame in order to fulfill the adiabatic condition. 

As evidenced from the above discussion, vibrational line shapes provide information mostly about intermolecular structure. Transient hole burning and more recently echo experiments, on the other hand, can provide information about the dynamics of spectral diffusion. The first echo experiments on the HOD/ D2O system involved two excitation pulses, and the signal was detected either by integrating the intensity [20] or by heterodyning [22]. The experiments were analyzed with the standard model assuming Gaussian frequency fluctuations. The data were consistent with a spectral diffusion TCF that was bi-exponential, involving fast and slow times of about 100 fs and 1 ps, respectively. 

The emission spectra can also be recorded at different times after the excitation pulse has been absorbed. This experimental procedure is called time-resolved luminescence and may prove to be of great utility in the understanding of complicated emitting systems. The basic idea of this technique is to record the emission spectrum at a certain delay time, t, in respect to the excitation pulse and within a temporal gate. At, as schematically shown in Figure 1.12. Thus, for different delay times different spectral shapes are obtained. 

Similarly to non-selective experiments, the first operation needed to perform experiments involving selective pulses is the transformation of longitudinal order (Zeeman polarization 1 ) into transverse magnetization or ly). This can be achieved by a selective excitation pulse. The first successful shaped pulse described in the literature is the Gaussian 90° pulse [1]. This analytical function has been chosen because its Fourier transform is also a Gaussian. In a first order approximation, the Fourier transform of a time-domain envelope can be considered to describe the frequency response of the shaped pulse. This amounts to say that the response of the spin system to a radio-frequency (rf) pulse is linear. An exact description of the... 

The peak rf amplitude required to achieve optimum excitation with a selective excitation pulse is given in comparison to the rf amplitude required to achieve an on-resonance 90° flip-angle with a selective rectangular pulse, the simplest conceivable shape. 

Fig. 1. Computer simulations of four selective excitation pulses. (Top) Pulse shapes. From left to right 90° rectangular pulse, 270° Gaussian truncated at 2.5%, Quaternion cascade Q, and E-BURP-1. The vertical axis shows the relative rf amplitudes, whereas the horizontal axis shows the time. (Middle) Trajectories of Cartesian operators in the rotating frame...

The potential of broadband laser excitation and fs-pulse shaping for different microspectroscopy techniques ranges from pure dispersion compensation (in the case of SHG, THG), to highly functional pulse shaping (in the case of CARS), as summarized in Table 7.2. It is worth mentioning again that all techniques can be implemented in the same approach—broadband laser and pulse shaper. The detection technique of choice is just selected by the corresponding pulse shapes. 

The added benefit of intrinsic interferometric detection is only a further example of the great flexibility of using the pulse shaper in single-beam nonlinear microspectroscopy. The setup used here for CARS in its variants is, of course, also capable of immediately performing all the other nonlinear microspectroscopies simply by changing the shape of the excitation pulses with computer control. This is shown in the next section, where we discuss a broadband TPF application. 

Lozovoy, V. V., Xu, B. W., Shane, J. C., and Danms, M. 2006. Selective nonlinear optical excitation with pulses shaped by pseudorandom Galois fields. Phys. Rev. A 74(4) 041805. 

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