Excitation pulse

In practice, modem NMR instruments are designed to deliver high-power 90 pulses closer to 10 ps, rather than the hundreds predicted from the above arguments. This is to suppress the undesirable effects that arise when the pulse rf frequency is off-resonance, that is, when the transmitter frequency does not exactly match the nuclear Larmor frequency, a situation of considerable practical significance that has been ignored thus far. 

As shown in Chapter 2, spins that are on-resonance have their magnetisation vector driven about the rf Bi field towards the x-y plane during the pulse. Those spins that are off-resonance will, in addition to this Bi field, experience a residual component AB of the static Bo field along the z axis of the rotating frame for which  

For those spins further from resonance, the angle 9 becomes greater and the net rotation towards the x-y plane diminishes until, in the limit, 9 becomes 90°. In this case, the bulk magnetisation vector simply remains along the +z axis and thus experiences no excitation at all. In other words, the nuclei resonate outside the excitation bandwidth of the pulse. Since an off-resonance vector is driven away from the y axis during the pulse it also acquires a (frequency-dependent) phase difference relative to the on-resonance vector (Fig. 3.6). This is usually small and an approximately linear function of frequency, so can be corrected by phase adjustment of the final spectrum (Section 3.2.8). 

It is widely appreciated that modem NMR spectrometers use a short pulse of radiofrequency energy to excite nuclear resonances over a range of frequencies. This pulse is supplied as monochromatic radiation from the transmitter, yet the nuclear spin transitions giving rise to our spectra vary in energy according to their differing Larmor frequencies and so it would appear that the pulse will be unable to excite all resonances in the spectmm simultaneously. However, Heisenberg s Uncertainty principle tells us that an excitation pulse of duration At has associated with it a frequency uncertainty or spread of around 1/At Hz 

However, for nuclei that display a much greater frequency dispersion, such as C or F, this is often not the case, and resonance distortion and/or attenuation can occur, and spurious signals may arise in multipulse experiments as a result. One approach to overcoming these limitations is the use of clusters of pulses known as composite-pulses which aim to compensate for these (and other) defects see Chapter 9. 

The molecules are excited by a light pulse with a trailing edge short compared to the mean lifetime t of the excited level. The subsequent decay of the level population, monitored by the decay of the fluorescence intensity, is either viewed directly on a scope or is monitored with a boxcar integrator or a transient recorder (see Sect.4.5.10). This method does not suffer from the influence of induced emission since the exciting light is already switched off when the fluorescence is observed. It is especially adapted to the use of pulsed or mode-locked lasers as excitation sources. From the decay curve the mean lifetime can be derived directly. Deviations from exponential decays, caused for instance by cascade effects, can be seen immediately. The accuracy is comparable to that of the phase shift method. 

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This section begins with a brief description of the basic light-molecule interaction. As already indicated, coherent light pulses excite coherent superpositions of molecular eigenstates, known as wavepackets , and we will give a description of their motion, their coherence properties, and their interplay with the light. Then we will turn to linear and nonlinear spectroscopy, and, finally, to a brief account of coherent control of molecular motion. 

The pioneering use of wavepackets for describing absorption, photodissociation and resonance Raman spectra is due to Heller [12, 13,14,15 and 16]- The application to pulsed excitation, coherent control and nonlinear spectroscopy was initiated by Taimor and Rice ([17] and references therein). 

For an understandmg of pulsed excitation of spin ensembles it is of fiindamental importance to realize that radiation pulses actually contain ranges of frequencies A burst of monocln-omatic microwaves at frequency... 

Katzenellenbogen N and Grischkowsky D 1991 Efficient generation of 380 fs pulses of THz radiation by ultrafast laser pulse excitation of a biased metal-semiconductor interface Appl. Phys. Lett. 58 222-4... 

Bardeen C J, Wang Q and Shank C V 1998 Femtosecond chirped pulse excitation of vibrational wave packets in bacteriorhodopsin J. Phys. Chem. A 102 2759-66... 

With broad-band pulses, pumping and probing processes become more complicated. With a broad-bandwidth pulse it is easy to drive fundamental and overtone transitions simultaneously, generating a complicated population distribution which depends on details of pulse stmcture [75], Broad-band probe pulses may be unable to distinguish between fundamental and overtone transitions. For example in IR-Raman experiments with broad-band probe pulses, excitation of the first overtone of a transition appears as a fundamental excitation with twice the intensity, and excitation of a combination band Q -t or appears as excitation of the two fundamentals 1761. 

An ultrashort mid-IR pulse excited a C-H stretching vibration (-3000 cm ) of neat acetonitrile at 300 K. The loss of C-H stretching energy occurred in 3 ps. Only 1% of that energy was transferred to the C N stretch (2250 cm ), where it remained for -80 ps. Most of the energy was lost from the C-H stretch by the process,... 

Excitation of an aqueous solution of poly(A/St/Phen) with a 355-nm, 22-ps laser pulse in the presence of MV2+ generated a transient absorption band peaking at about 600 nm due to MV + [120]. As shown in Fig. 16, the buildup of the 600-nm band completes immediately after the pulse excitation, indicating that the photoinduced ET from the singlet-excited Phen residue ( Phen ) to MV2 + occurs on a time scale comparable to or shorter than the duration of the laser pulse (ca. 22 ps) [120], Figure 16 also shows that a fast decay of the absorbance at 600 nm owing to the back ET from MV + to the Phen cation radical (Phen+ )... 

For APh-2, on the other hand, the forward ET from Phen to MV2+ was a little slower than that for the poly(A/St/Phen)-MV2+ system i.e., the intensity of the S <- Sj band for the Phen moiety at 510 nm still remained significant for 27 ps after the pulse excitation (Fig. 17) [120]. In striking contrast to the poly(A/St/Phen)-MV2 + system, the APh-2-MV2+ system showed an extremely fast decay in the transient absorbance at 600 nm over the picosecond regime and no subsequent slower decay. The transient absorbance almost completely decayed in 200 ps after the pulse. 

How can such problems be counterbalanced Since a large capacitance of a semiconductor/electrolyte junction will not negatively affect the PMC transient measurement, a large area electrode (nanostructured materials) should be selected to decrease the effective excess charge carrier concentration (excess carriers per surface area) in the interface. PMC transient measurements have been performed at a sensitized nanostructured Ti02 liquidjunction solar cell.40 With a 10-ns laser pulse excitation, only the slow decay processes can be studied. The very fast rise time cannot be resolved, but this should be the aim of picosecond studies. Such experiments are being prepared in our laboratory, but using nanostructured... 

We use a7r/2 — vr — vr/2 pulse sequence to coherently divide, deflect and finally recombine an atomic wavepacket. The first vr/2 pulse excites an atom initially in the l,p) state into a coherent superposition of states l,p) and 2,p + hkeff). If state 2) is stable against spontaneous decay, the two parts of the wavepacket will drift apart by a distance hkT/m in time T. Each partial wavepacket is redirected by a vr pulse which induces the transitions... 

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. 

In principle, pulsed excitation measurements can provide direct observation of time-resolved polarization decays and permit the single-exponential or multiexponential nature of the decay curves to be measured. In practice, however, accurate quantification of a multiexponential curve often requires that the emission decay be measured down to low intensity values, where obtaining a satisfactory signal -to-noise ratio can be a time-consuming process. In addition, the accuracy of rotational rate measurements close to a nanosecond or less are severely limited by tbe pulse width of the flash lamps. As a result, pulsed-excitation polarization measurements are not commonly used for short rotational periods or for careful measurements of rotational anisotropy. 

Luminescence lifetimes are measured by analyzing the rate of emission decay after pulsed excitation or by analyzing the phase shift and demodulation of emission from chromophores excited by an amplitude-modulated light source. Improvements in this type of instrumentation now allow luminescence lifetimes to be routinely measured accurately to nanosecond resolution, and there are increasing reports of picosecond resolution. In addition, several individual lifetimes can be resolved from a mixture of chromophores, allowing identification of different components that might have almost identical absorption and emission features. 

Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

Figure 7.15 Pulse sequence for the semisoft TOCSY experiment. Purged half-Gaussian pulse excitation sequence and trim pulses (TR) are used. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

Gregor, 1., Patra, D. and Enderlein, J. (2005) Optical saturation in fluorescence correlation spectroscopy under continuous-wave and pulsed excitation. 

Figure 3.2. Single-pulse Raman spectra of the acetonitrile (a), low laser power DMABN in acetonitrile (b) high laser power DMABN in acetonitrile (c) and (d) = (c)-(b)-(a) obtained using 300 nm 10 ns laser pulse excitation.

An important consideration when employing high-energy laser systems to do pulsed TR spectroscopy experiments is to make sure the molecule or state is not perturbed. For pulsed excitation, the fraction of molecules photolyzed is described by a photoaltemation parameter F that can be expressed as... 

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. 

One day in July, 1949 a strange signal appeared on my oscilloscope display without any corresponding pulse excitation. So I kicked the apparatus and breathed... 

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