Tailored excitation pulse

FT-ICR mass spectrometers belong to the tandem-in-time category of instruments. The stage of precursor ion selection (MSI) is accomplished by selectively storing the ions of interest, whereas all others are ejected by means of a suitably tailored excitation pulse, e.g., using the SWIFT technique. [206] FT-ICR mass spectrometers are also capable of MS . 

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. 

Selective experiments can also be performed by the tailored excitation method of Tomlinson and Hill. The selective pulse is frequency-modulated with a function designed to yield zero effective field at the resonance offset of the neighboring nuclei. Although this technique is especially promising for studies of more-complex spin systems, its use is as yet very limited, in part because the instrumentation needed is not yet commercially available. 

Fig. 3.11. Left spectrum of the tailored pump beam for different spectral separations Aw CC2 — oji between two spectral packets (oJi and Lof). The bottom case is resonant to the E2 phonon frequency l o- Right transient transmittance of GaN excited with the tailored pump pulses, showing the enhancement of coherent oscillation of the E2 phonon for u) — u)2 — Oo- From [28]...

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. 

In the ICR cell, there is a stringent correlation of cyclotron frequency/c and m/z value. For simplicity, the very first FT-ICR experiment was therefore performed with an excitation pulse of a fixed/c tailored to fit the model analyte, methane molecular ion. [185] However, useful measurements require the simultaneous excitation of all ions in the cell, and this in turn demands for a large RF bandwidth. 

Here we present the latest results from our group focused on the design of tailored femtosecond pulses to achieve control of nonlinear optical excitation in large molecules based on the concept of multiphoton intrapulse interference (Mil) [1-4]. Our goal is to elucidate well-defined and reproducible pulse shapes that can be used to enhance or suppress particular nonlinear optical transitions in large molecules such as laser dyes and proteins in solution. We demonstrate the use of Mil to probe the local and microscopic environment of molecules by selective two-photon laser induced fluorescence (LIF). 

Coherent control Control of the motion of a microscopic object by using the coherent properties of an electromagnetic held. Coherent phase control uses a pair of lasers with long pulse durations and a well-defined relative phase to excite the target by two independent paths. Wave packet control uses tailored ultrashort pulses to prepare a wave packet at a desired position at a given time. 

Kleinian VD, Arrivo SM, Melinger JS, Heilweil EJ. Controlling condensed-phase vibrational excitation with tailored infrared pulses. Chem Phys 1998 233 207-216. 

In some spectrometers it is not feasible to generate BURP or related tailored soft pulses. A useful alternative is the DANTE pulse sequence (delays alternating with nutation for tailored excitation). Normal high excitation power is used, but rather than apply a single 90° pulse, the DANTE method uses a sequence of small angle pulses that sum to 90° but with a short time between pulses during which nuclei... 

An alternative solution to the problem of selectively observing, for example, the H NMR spectrum of a metal-hydride in protio-, rather than deutero-solvent is to excite just that region of the NMR spectrum that contains signals of interest. This can be done using a shaped soft pulse, using a DANTE (delays alternating with nutation for tailored excitation) type sequence or, on a modem... 

An early alternative to soft pulses was the DANTE Delays Alternating with Nutation for Tailored Excitation) experiment, which used a sequence of short, hard pulses of angle a <3C 90°, followed by a fixed delay t to achieve selective excitation. Thus, the pulse sequence is (a-T), ]. Nuclei that are on resonance are eventually driven to the y axis and hence are selected, whereas those more removed from the frequency range are not affected. The sequence of hard pulses can achieve a result similar to that of soft pulses and even can be shaped by modulating the duration of the pulse lengths, but DANTE pulses lead to spectral artifacts not created by soft pulses, such as unwanted sidebands. 

For the coherent control of reactions at surfaces, the manipulation of adsorbate motion is essential. Cs/Pt(lll) is a suitable system which provides us with a good opportunity to test whether or not we can excite preferentially one of the two modes whose frequencies are very close to each other by using tailored laser pulses. We have demonstrated the mode-selective excitation of coherent surface phonon modes on Cs/Pt(lll) by synthesized femtosecond pulse trains. 

In the early days of selective excitation, spectrometers were not equipped to generate amplitude modulated rf pulses and the DANTE method (Delays Alternating with Nutation for Tailored Excitation) was devised, [49] requiring only short, hard pulses. Although largely superseded by the amplitude modulated soft pulses, DANTE may still be the method of choice on older instramentation or on those newer instmments which lack waveform generators. 

Spin pinging [5.86] has been proposed as a method of using rectangular pulses in 2D homonuclear COSY experiments for tailored excitation. Using spin pinging it would be possible to select regions of overlapping multiplets which are not defined sufficiently in a standard 2D COSY spectrum because of the restricted number of data points. Probably... 

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