Locking pulse

Figure Al.6,8 shows the experimental results of Scherer et al of excitation of I2 using pairs of phase locked pulses. By the use of heterodyne detection, those authors were able to measure just the mterference contribution to the total excited-state fluorescence (i.e. the difference in excited-state population from the two units of population which would be prepared if there were no interference). The basic qualitative dependence on time delay and phase is the same as that predicted by the hannonic model significant interference is observed only at multiples of the excited-state vibrational frequency, and the relative phase of the two pulses detennines whether that interference is constructive or destructive.

Scherer N F, Carlson R J, Matro A, Du M, Ruggiero A J, Romero-Rochin V, Cina J A, Fleming G R and Rice S A 1991 Fluorescence-detected wave packet interferometry time resolved molecular spectroscopy with sequences of femtosecond phase-locked pulses J. Chem. Rhys. 95 1487... 

It is also possible to switch a single picosecond pulse out of the train of mode-locked pulses using an electrooptic switch. It is possible to obtain a single pulse having duration in the picosecond regime or even less. Pulses with durations in the regime of a few hundred femtoseconds (10 s) are also available (Fig. 4e). 

Linearly polarized, near-diffraction-hmited, mode-locked 1319 and 1064 nm pulse trains are generated in separate dual-head, diode-pumped resonators. Each 2-rod resonator incorporates fiber-coupled diode lasers to end-pump the rods, and features intracavity birefringence compensation. The pulses are stabilized to a 1 GHz bandwidth. Timing jitter is actively controlled to < 150 ps. Models indicate that for the mode-locked pulses, relative timing jitter of 200 ps between the lasers causes <5% reduction in SFG conversion efficiency. 

The ability to create and observe coherent dynamics in heterostructures offers the intriguing possibility to control the dynamics of the charge carriers. Recent experiments have shown that control in such systems is indeed possible. For example, phase-locked laser pulses can be used to coherently amplify or suppress THz radiation in a coupled quantum well [5]. The direction of a photocurrent can be controlled by exciting a structure with a laser field and its second harmonic, and then varying the phase difference between the two fields [8,9]. Phase-locked pulses tuned to excitonic resonances allow population control and coherent destruction of heavy hole wave packets [10]. Complex filters can be designed to enhance specific characteristics of the THz emission [11,12]. These experiments are impressive demonstrations of the ability to control the microscopic and macroscopic dynamics of solid-state systems. 

In our tip-enhanced near-field CARS microscopy, two mode-locked pulsed lasers (pulse duration 5ps, spectral width 4cm ) were used for excitation of CARS polarization [21]. The sample was a DNA network nanostructure of poly(dA-dT)-poly(dA-dT) [24]. The frequency difference of the two excitation lasers (cOi — CO2) was set at 1337 cm, corresponding to the ring stretching mode of diazole. After the on-resonant imaging, CO2 was changed such that the frequency difference corresponded to none of the Raman-active vibration of the sample ( off-resonant ). The CARS images at the on- and off- resonant frequencies are illustrated in Figure 2.8a and b, respectively. 

Figure 6 X-half filters used for filtering or selecting 13C and 15N-attached protons. Thick and thin closed rectangles are 180° and 90° pulses, respectively, open rectangles are spin lock pulses. (A) A simple X-half filter (2). The delay t is equal to 0/(2[1JXH]) where 1JXH is the one-bond coupling between proton and either 13C (120 to 140 Hz) or 15N (95 Hz). The second 90° pulse is the editing...

Total correlation spectroscopy (TOCSY) is similar to the COSY sequence in that it allows observation of contiguous spin systems [35]. However, the TOCSY experiment additionally will allow observation of up to about six coupled spins simultaneously (contiguous spin system). The basic sequence is similar to the COSY sequence with the exception of the last pulse, which is a spin-lock pulse train. The spin lock can be thought of as a number of homonuclear spin echoes placed very close to one another. The number of spin echoes is dependent on the amount of time one wants to apply the spin lock (typically 60 msec for small molecules). This sequence is extremely useful in the identification of spin systems. The TOCSY sequence can also be coupled to a hetero-nuclear correlation experiment as described later in this chapter. 

Fig. 10.23. Cross-polarization pulse sequence. The high abundance nuclei, such as protons, are first irradiated with a standard 90° pulse to create the initial magnetization. A special pair of spin-locking pulses is applied during a period called the contact time in order to transfer the magnetization from the protons to the low abundance nuclei, such as carbons. Protons are then decoupled from carbons during the acquisition of the carbon signal. In the case of protons and carbons, cross-polarization can enhance the observed carbon signal by as much as four-fold.

The light source can be a xenon lamp associated with a monochromator. The optical configuration should be carefully optimized because the electro-optic modulator (usually a Pockel s cell) must work with a parallel light beam. The advantages are the low cost of the system and the wide availability of excitation wavelengths. In terms of light intensity and modulation, it is preferable to use a cw laser, which costs less than mode-locked pulsed lasers. 

Fig. 8. ID ROESY-TOCSY. (a) H spectrum of the oligosaccharide 3 (5 mg/0.5 ml D2O). (b) ID ROESY spectrum of 3 acquired using the pulse sequence of fig. 7(a) with selective excitation of the H-lb proton. Duration of the 270° Gaussian pulse and the spin-lock pulse ( yBi/ K = 2.8 kHz) was 49.2 ms and 0.5 s, respectively. The spin-lock pulse was applied 333.3 Hz downfield from the H-lb resonance. The time used for the frequency change was 3 ms. (c) ID ROESY-TOCSY spectrum acquired using the pulse sequence of fig. 7(c) and the selective ROESY transfer from H-lb followed by a selective TOCSY transfer from H-4c. Parameters for the ROESY part were the same as in (b). A 49.2 ms Gaussian pulse was used at the beginning of the 29.07 ms TOCSY spin lock. 256 scans were accumulated. A partial structure of 3 is given in the inset. Solid and dotted lines represent TOCSY and ROESY...

Fig. 3. Long range and one-bond carbon-13 satellite spectrum of a 5% w/w solution of ethanediol in D2O at 94°C. 16 transients were measured on a Varian Associates Unity 500 spectrometer using the sequence of fig. 1, with 2.5 s presaturation, a t value of 100 ms, spin lock pulses of 450 ps, no homospoil pulse, and no homodecoupling during acquisition.

Fig. 4. (a) 300 MHz proton spectrum and (b)-(e) selective reverse INEPT spectra of 28% menthone (Aldrich) in acetone-ds, measured using a 5 mm sample in the 10 mm broadband probe of a Varian Associates XL300 spectrometer using the sequence of fig. 1. The sample contains substantial quantities of isomenthone, seen clearly in the methyl region of trace (a). Spectra (b) to (e) used selective excitation of carbon sites 6, 7, 2 and 8, respectively, with delays 2r of 3.85, 3.85, 1.92 and 1.54 ms. 32 transients were used for each trace no spin lock pulses or 180° pulses were used. Traces (b) to (e) have a vertical scale lOOOx that of trace (a). No homodecoupling was used during acquisition. 

In the following, three different experiments are discussed, where short, high-power spin-lock pulses are used to purge the spectrum from undesired resonances. The experiments are (i) the HSQC experiment [5], (ii) experiments with C half-filter elements [6], and (iii) NOESY and ROESY experiments for the observation of water-protein NOEs [7]. In the first two experiments, spin-lock purge pulses are used to suppress the signals from... 

Figure 1 shows the pulse sequence of the C HSQC experiment supplemented by a spin-lock pulse to suppress the signals from C-bound protons. The experiment is readily described in terms of Cartesian product operators [9]. For a two spin system consisting of a proton spin H coupled to a C spin C, the relevant coherence transfer pathway is... 

Fig. 1. Pulse sequence of the C HSQC experiment with a spin-lock pulse for the suppression of signals from protons not bound to C. Narrow and wide bars denote 90° and 180° pulses, respectively. The spin-lock pulse is labeled SL. r is set to 1/[2J( C, H)]. The detection period is symbolized by a triangle. Phase cycle ] = 8(y) 4>2 = 2 x,x,y,y) 03 = 4 = 4n = 8(x) 05 =4(x,—x) 05 = 4(x),4(—x) acquisition = 2(x,—x,—x,x). The phases of the C pulses before U (03 and 0.5) are subjected to the States-TPPI scheme [38].

Averaging over all possible flip angles / which result from the spin-lock pulse, all signals from eq. (2) cancel. It can be shown that this holds for... 

Because of the favorable cross-peak multiplet fine-structure, the HSQC experiment offers superior spectral resolution over the HMQC (heteronuclear multiple quantum coherence) experiment [13, 14], On the other hand, the HMQC experiment works with fewer pulses and is thus less prone to pulse imperfections. The real advantage of the HSQC experiment is for measurements of samples at natural isotopic abundance and without the use of pulsed field gradients, since the HSQC experiment lends itself to purging with a spin-lock pulse. Spin-lock purging in the HMQC experiment... 

Fig. 2. N HSQC spectrum of a 75 mM solution of Pro -cyclosporin in CDCI3 at natural isotope abundance using the pulse sequence of fig. 1 without N decoupling during acquisition, t = 5.7 ms, SL = 2.5 ms. An additional, short spin-lock pulse was used right before signal detection [8]. The projections are shown at the top and on the left. (Reproduced by permission of Academic Press from...

High Power Spin-Lock Purge Pulses 11. Solvent signal suppression by spin-lock pulses... 

The use of spin-lock pulses for water suppression is illustrated with the NOESY and ROESY pulse sequences (fig. 5). Using the Cartesian product operator description [9], the effect of the NOESY pulse sequence of fig. 5(A) is readily illustrated ... 

The first spin-lock pulse in equation 4 apparently does not alter the magnetization. Its function is to ensure the desired coherence transfer pathway by defocusing any magnetization which is not aligned along the y axis. 

Fig. 5. Pulse sequences of NOESY and ROESY with spin-lock purge pulses for water suppression. (A) NOESY pulse sequence. The spin-lock pulses are typically of length 0.5 ms and 2 ms, and r = 1/SW, where SW is the spectral width in the acquisition dimension. Phase cycle (pi = x,—x) 4>2 = 4 x,x,—x,—x) ...

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