Multipulse

Time-resolved fluorescence is perhaps the most direct experunent in the ultrafast spectroscopist s palette. Because only one laser pulse interacts with the sample, the mediod is essentially free of the problems with field-matter time orderings that arise in all of the subsequently discussed multipulse methods. The signal... 

Phase cycling is widely employed in multipulse NMR experiments. It is also required in quadrature detection. Phase cycling is used to prevent the introduction of constant voltage generated by the electronics into the signal of the sample, to suppress artifact peaks, to correct pulse imperfections, and to select particular responses in 2D or multiple-quantum spectra. 

Gorer S, Penner RM (1999) Multipulse electrochemical/chemical synthesis of CdS/S core/sheU nanocrystals exhibiting ultranarrow photoluminescence emission lines. J Phys Chem B 103 5750-5753... 

Figure 4. dj-vinyl acetate (MW=89) response curves derived from a long term multipulse sequence with a premixed dj-acetic acid + ethylene + oxygen feed. 

A wide variety of ID and wD NMR techniques are available. In many applications of ID NMR spectroscopy, the modification of the spin Hamiltonian plays an essential role. Standard techniques are double resonance for spin decoupling, multipulse techniques, pulsed-field gradients, selective pulsing, sample spinning, etc. Manipulation of the Hamiltonian requires an external perturbation of the system, which may either be time-independent or time-dependent. Time-independent... 

One-dimensional111 and 13C NMR experiments usually provide sufficient information for the assignment and identification of additives. Multidimensional NMR techniques and other multipulse techniques (e.g. distortionless enhancement of polarisation transfer, DEPT) can be used, mainly to analyse complicated structures [186]. 

Mach-Zehnder interferometer, 144 Medical applications, 153 Metal-insulator transitions, 52 Monte Carlo procedure, 135 Multi-energy X-ray imaging, 131 Multilayer targets, 131 Multiphoton absorption, 85 Multiphoton ionization, 82 Multiple filamentation, 91, 92 Multipulse techniques, 152... 

First, various advanced multipulse techniques have been developed since the mid-1970s, and nowadays are routinely applicable on spectrometers of the latest generation. Particularly innovative and ingenious among these methods are two-dimensional NMR techniques (506-508) and double quantum transition measurements (INADEQUATE) (507-509), which allow one to determine connectivities between carbon atoms within a molecule. 

Lesage et al. have shown that the resolution of the proton NMR spectroscopy of powdered solids can be improved significantly when multi-pulse sequences are employed [44a]. In the approach based on combined rotation and multipulse spectroscopy (CRAMPS) (Figure 7.9) the problem of dipolar line broadening is usually overcome. 

There are many advanced multipulse two-dimensional (2D) techniques which offer new solutions to the problems outlined above so that H-NMR spectroscopy of complex molecules, especially in natural products chemistry, has progressed remarkably. 13C Signal assignments, which were often a matter of empirical correlation and experience, now have a sound experimental basis. 

Since the advent of multipulse-NMR techniques, more detailed 1H-NMR studies on methylcyclo-hexanes547, phenylcyclohexanes548, neomenthyl halides549 and bicydo[4.4.0]decanes 550 551 have been undertaken. The two diastereomeric 4-fm-butyl-7,ll-diphenylspiro[5.5]undecane-1,9-diones (3) and (4) could be identified unambiguously552, and substituted spirodioxane cyclohexanes such as, 9- m-butyl-2-methyl-1.3-dioxaspiro[5.5]undecane (5), have also been investigated553,554. 

Figure 6.10 Ultrafast efficient switching in the five-state system via SPODS based on multipulse sequences from sinusoidal phase modulation (PL). The shaped laser pulse shown in (a) results from complete forward design of the control field. Frame (b) shows die induced bare state population dynamics. After preparation of the resonant subsystem in a state of maximum electronic coherence by the pre-pulse, the optical phase jump of = —7r/2 shifts die main pulse in-phase with the induced charge oscillation. Therefore, the interaction energy is minimized, resulting in the selective population of the lower dressed state /), as seen in the dressed state population dynamics in (d) around t = —50 fs. Due to the efficient energy splitting of the dressed states, induced in the resonant subsystem by the main pulse, the lower dressed state is shifted into resonance widi die lower target state 3) (see frame (c) around t = 0). As a result, 100% of the population is transferred nonadiabatically to this particular target state, which is selectively populated by the end of the pulse.

The multipulse excitation is generated by a modulation of the spectral phases,... 

Fourier transformation of Eq. (1) shows that the higher the value of a, where 0 a re, the more intensity is found in replica pulses of an equispaced multipulse sequence with At = > (Fig. 2a). Parameter c controls the relative carrier phases between successive pulses of the sequence. 

The mechanism of control with multipulse excitation is likely due to dynamics of the carotenoid donor. The presumably incoherent EET process [1] would not support the observed dependence on the carrier phase via the parameter c. Furthermore, the control effect does not suffer from annihilation at higher excitation intensities [2], as would be characteristic for the delocalised excitons in the B850 ring [1], However, it is well known that femtosecond pulses populate higher ground state vibrational levels by impulsive Raman scattering (IRS) [4], and that the periodic phase modulation (Eq. 1) makes IRS selective for specific vibrations... 

Fig. 2. Optimisation of internal conversion with multipulse excitation, a) Experimental setup Coherent multipulse sequences are generated by periodic modulation of the spectral phases. The multipulse spacing b is optimised for maximal ratio IC/EET by an evolutionary algorithm, b) Histogram of optimal modulation 2re/viightfe (i.e. multipulse separation) by successive optimisations.

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