Direct geometry spectrometer

Fig. 3.26 Trajectories in (Q,a>) space for a direct geometry spectrometer with detectors at angles 3, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120 and 135° and with an incident energy of 4000 cm. The dashed lines are the trajectories of an indirect geometry instrument (low-bandpass) using scattering angles of 45 (long dashes, forward scattering)) and 135° (short dashes, backscattering) and a final energy of 28 cm". ...

Fig. 3.30 Schematic of the direct geometry spectrometer IN4 at the ILL. Reproduced from [32] with permission from the Institut Laue Langevin.

The ratio of the intensity of an overtone to its fundamental is given by the effective mass, men, which increases as extra vibrational contributions increase the total displacements of the scattering atoms. Ultimately, the effective mass, Weff, may be quite different from the oscillator mass, /Xy. This is related to, but subtly different from, effects observed on direct geometry spectrometers ( 5.3.2). 

Molecular systems using a direct geometry spectrometer... 

The INS spectrum of rubidium hexahydridoplatinate(IV), Rb2[PtH6], obtained on the direct geometry spectrometer MARI, is given in Fig. 5.10. As anticipated from the considerations presented above the spectrum consists of a series of ridges, all relatively well-defined in energy but broad as a function of Q. The point where the scattering reaches a maximum (as a function of Q) defines the mass-line that joins that point to the origin, for an effective mass, /Weff. 

Fig. 5.10 The INS spectrum of Rb2[PtH]j, taken on a direct geometry spectrometer (MARI, ISIS) [20]. Showing the ridges that are due to the variation of band intensities with momentum and energy transfer.

It is stressed that spectral contamination of transition intensities is a common feature of INS data at all energy transfers and on all types of instrument. It arises from the spectral congestion typical of molecules (even those as simple as benzene) and the effects of phonon wings. The effects of congestion can be exacerbated on direct geometry spectrometers if their energy resolution is not good. 

The impact of phonon wings on the spectra observed on direct geometry spectrometers can be as taxing as their effects on indirect geometry spectrometers at the same Q values. The spectrum of Rb2[PtH6]... 

Direct geometry spectrometers have better access to low Q regimes at higher energy transfers than low-bandpass spectrometers and this is a considerable advantage. At low Q the phonon wing has not yet developed and its contaminating effects are much reduced. 

The observed transitions can be used to parameterise the potential, either according to simple models or, in terms of the symmetry adapted spherical harmonics for the most general potential [51]. The advantage of this approach is that the wavefimctions of the hydrogen in its excited states can be extracted. Further the full form of the Scattering Law (see Fig. 2.4) for any transition can be calculated. These can be compared with those observed on direct geometry spectrometers ( 5.3.2.1) [64]. 

AE/Ei 3.22 relative incident energy uncertainty (instrumental resolution of direct geometry spectrometers) ... 

Figure 10 Schematic diagram of the direct-geometry spectrometer MARI at ISIS. Ml, M2 and M3 are incident beam monitors.

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