Point-resolved spectroscopy

Fig. 12. Sequences for volume selective single voxel spectroscopy. Both techniques work with three slice-selective RF-pulses. (a) The Point RESolved Spectroscopy (PRESS) sequence generates a volume selective double spin-echo. The entire time delay between the initial 90° excitation and the echo is sensitive to transverse relaxation, (b) The Stimulated Echo Acquisition Mode (STEAM) sequence generates a stimulated echo. Maximal signal intensity (without relaxation effects) is only half the signal intensity of PRESS under comparable conditions, but slice profiles are often better (only 90° pulses instead of 180° pulses) and the TM interval is not susceptible to transverse relaxation, (c) The recorded echo signal is only generated in a volume corresponding to the intersection of all three slices.

PRAWN pulsed rotating-frame transfer sequences with windows PRESS point-resolved spectroscopy... 

Point Resolved Spectroscopy and Stimulated-Echo Aoquisition Mode... 

Point resolved spectroscopy (PRESS) and stimulated-echo acquisition mode (STEAM) use three selective RE pulses in combination with linear field gradients to yield localization of the VOI in a single shot. Thus, the localization provided by these techniques is not degraded by subject motion or system instability. Generally, the use of three RF pulses leads to the formation of five coherent pathways for echo formation. Both PRESS and STEAM are designed to yield a particular echo that is localized to the VOI the other echoes are suppressed by the use of spoiler gradients and phase cycling of the RF pulses. The... 

Suggestive evidence for the protonation of diphenylcarbene was uncovered in 1963.10 Photolysis of diphenyldiazomethane in a methanolic solution of lithium azide produced benzhydryl methyl ether and benzhydryl azide in virtually the same ratio as that obtained by solvolysis of benzhydryl chloride. These results pointed to the diphenylcarbenium ion as an intermediate in the reaction of diphenylcarbene with methanol (Scheme 3). However, many researchers preferred to explain the O-H insertion reactions of diarylcarbenes in terms of electrophilic attack at oxygen (ylide mechanism),11 until the intervention of car-bocations was demonstrated by time-resolved spectroscopy (see Section III).12... 

The photoinduced reaction of chloranil with various 1,1-diarylethenes is another example of an intramoleclar [2 -I- 2] cycloaddition as reported by Xu and co-workers [86]. Although not interesting from the preparative point of view, the diverse reaction outcomes caused by parallel reaction pathways with and without single-electron transfer and various secondary reactions of the primary products show that the photochemistry involving haloquinones is far from being explored. Another interesting example in this context is the reaction of dichlorobenzoqui-none with various diarylacetylenes in the solid phase via photoinduced electron transfer as reported by Kochi and co-workers [87]. Here, time-resolved spectroscopy revealed the radical ion pair of the two reactants to be the first reactive intermediate that then underwent coupling. 

All hypothyroid and healthy neonates underwent cranial MRS study between days 5 and 7 of life, just before thyroxine therapy, and this procedure was repeated after 8 weeks of thyroxine therapy in the neonates with hypothyroidism and 8 weeks after the first scanning in the healthy neonates. Cranial MRS was performed on a 1.5T scanner. Single-voxel spectroscopy was performed in all the hypothyroidand healthy neonates by using a point-resolved... 

Time-resolved Spectroscopy Finally in this section, the advent of step-scan interferometers has opened up exciting opportunities to study fast, reversible surface processes. Details on step-scan interferometry may be foxmd elsewhere [144] briefly in conventional mode (see in previous text), the mirror moves essentially continuously, with intensity measurements taken at regular intervals (Fig. 12). In step-scan mode, the mirror is paused at each position (retardation), allowing the exploitation of the time-resolved spectroscopy option ofthe spectrometer. Once the mirror has settled at a particular position, a reference point can be taken, after which a reaction can be triggered, that is via a light pulse or potential step, and the intensity measured at regular intervals (Fig. 13). 

Fig. 13 Schematic representation of the data acquisition during the time-resolved spectroscopy operation of a step-scan FTIR spectrometer. S is the settlement time, and R, a reference data point.

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