Broad-band 90° pulse

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. 

Conoerning frequency, the two most common ways of interrogating a sample by US are to apply a disorete US frequency or a broad-band pulse. There are single and dual transduoer versions for both modes. Discrete US frequencies use a tone-burst, the duration of whioh is made long enough to obtain a central frequency that is well defined, but short enough to avoid interference from multiple refleotions within the sample. 

In relation to applioation duration, most ultrasonio instruments utilize either pulsed (tone-burst or broad-band pulses) or continuous-wave US (Fig. 9.4). Ultrasound pulses are by far the most widely used choice as pulses are easily applied, measurements are rapid and non-invasive, and the ultrasound device can be readily automated. Continuous-wave teohniques have been traditionally used for highly aoourate measurements in speoialized research laboratories. 

General ultrasonic spectrometry relies on direct measurements of the physical changes caused by the US-sample interaction, namely velocity changes and attenuation of the radiation. The different modes of this technique arise from factors such as (1) the way US is applied e.g. as a single frequency, broad-band pulses, scanning frequency), after which modes are named (2) the way US impinges on the sample (normal, parallel, oblique), after which the waves produced in the material (longitudinal, shear, oblique) are named (3) the way the experimental data provided are used viz. amplitude or phase spectra) or processed viz. frequency or time domain). 

The uitrasonic properties of emuisions are usuaiiy frequency dependent, as are the effects of diffraction and transmission-refiection at muitiiayer boundaries. Thus, the frequency content of a US pulse used in an experiment is important. If a pulse contains a wide range of frequencies, then each frequency component will travel through a material at a different velocity and will be attenuated to a different extent, so only average values will be measured. This problem can be overcome by conducting experiments at a particular frequency or by using Fourier analysis to examine the frequency content of broad-band pulses. 

Frequency-modulation spectroscopy can also be used for sensitive absorption measurements using pulsed lasers [9.184]. The sidebands are then generated by microwaves ( 10GHz) to separate them from the rather broad-band pulsed radiation. Small differential absorptions from, e.g., pressure-broadened molecular transitions, can be detected. Other extremely sensitive laser absorption spectroscopy techniques have been described in [9.185,186]. 

Fig.11.23a,b. Quantum beat spectroscopy, (a) Level scheme illustrating coherent excitation of levels 1 and 2 with a short broad-band pulse, (b) Fluorescence intensity showing a modulation of the exponential decay... 

The preparation period is a relaxation delay to ensure that the spin system is in equilibrium. Following a 90° pulse, the magnetizations from each of the lines of the doublet are colinear. If the carrier frequency is equal to the frequency midway between the two lines, then during the delay period immediately following the 90° pulse, the higher-frequency component of the doublet wiU move ahead of the carrier frequency, and the lower-frequency component will faU behind. At time te, a broad-band 180° pulse is applied to reverse the orientation of the methyl protons. Because it is a broad-band pulse, however, it simultaneously reverses the orientation of the methine proton. Thus, those methyl protons that were originally coupled to... 

Fiq. 20a. The pulsed Raman spectrum of Mn-doped ZnSe single crystal using a detection interval of 200 nsec. Broad band fluorescence superimposed on a large instrumental scattered light component was observed. Recordings taken with ratemeter time constants (TC) of 1 sec and 10 sec are shown (37). 

The pulse sequence used in the reverse DEPT experiment is shown in Fig. 2.16. Presaturation of the protons removes all H magnetization and pumps up the C population difference due to nOe. Broad-band decoupling of the C nuclei may be carried out. The final spectrum obtained is a one-dimensional H-NMR plot that contains only the H signals to which polarization has been transferred—for instance, from the enriched C nucleus. 

The basic INEPT spectrum cannot be recorded with broad-band proton decoupling, since the components of multiplets have antiphase disposition. With an appropriate increase in delay time, the antiphase components of the multiplets appear in phase. In the refocussed INEPT experiment, a suitable refocusing delay is therefore introduced that allows the C spin multiplet components to get back into phase. The pulse sequences and the resulting spectra of podophyllotoxin (Problem 2.21) from the two experiments are given below ... 

Laser Flash Photolysis at 248 nm of TDI-PU. MDI-PUE. and Model Compounds. Figures 1 and 2 show the transient absorption spectra of MDI-PUE (5.5 X lO-3 g/dL) and TDI-PU (2.3 X 10 3 g/dL) in THF at a 2.0 ns delay after pulsing with a krypton fluoride excimer laser (Xex=248 nm) in air and nitrogen saturated samples. Both spectra have common peaks in nitrogen saturated solutions (shown by arrows) at 310 nm, 330-360 nm (broad), and above 400 nm (broad, diffuse absorbance).. The MDI-PUE sample has an additional and quite distinctive peak at 370 nm. In the presence of air, the peak at 370 nm for MDI-PUE is completely extinguished, while the sharp peaks at 310 nm for TDI-PU and MDI-PUE and the broad band above 400 nm are only marginally quenched by oxygen. 

A broad band emission for the probe pulse is obtained by focussing part of the monochromatic laser pulse into a cell containing a suitable liquid, such as water or tetrachloromethane. The liquid converts the laser beam into a polychromatic pulse covering a wide range of wavelengths through much of the UV, visible and IR regions. 

Figure 10. Selective irradiation of linear PE (2 X 10 mol wt, 1 — K 0.5). Spectral details are 35°C 67.9 MHz sweep width 5 KHz (quadrature detection) line broadening 9.7 Hz pulse width 35 jisec (90°C = 48 /jsec) delay = 1.0 sec, 4K data points 1024 scans accumulated 10-mm sample tube. Decoupling 7W (forward), 0.4W (reflected), broad band noise modulated decoupling.

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