Low-bandpass spectrometer

Solving the vibrational problem, as presented in this chapter, provides the eigenvalues (vibrational frequencies) and the eigenvectors (atomic displacements) that are needed to calculate 5 (g,o)). Recently we have developed and released ACLIMAX [39] as fi-eeware . This takes ab initio outputs to generate a spectrum fi-om a low-bandpass spectrometer ( 3.4.2.3). The program can be downloaded from ... 

The observed INS spectrum of NEUBr is given in Fig. 5.2. The spectrum was taken on the indirect geometry low-bandpass spectrometer TOSCA at ISIS ( 3.4.2.2.2). The spectrum consists of a series of features across the whole spectrum, which sits on a gently rising background. However, bands appear between 500 and 1400 cm, where none were expected and there are no bands about 3000 cm, where the stretches... 

On low-bandpass spectrometers, when the energy transferred is doubled then the momentum is also approximately doubled such that the momentum transferred to an overtone, Qqi, is twice that transferred at its fundamental, Substituting this into the scattering law expressions for overtone intensities, Eq (2.36) gives ... 

Fig. 5.4 A schematic diagram showing how different systems respond on a low-bandpass spectrometer under recoil. The trajectories are shown in the uppermost diagram. The spectrometer s trajectory (i), the recoil of a heavy mass (ii), the recoil of an intermediate mass (iii) and the recoil of a unit mass (iv). Below are shown the corresponding INS spectra associated with the heavy mass, above, and intermediate mass, below. Reproduced from [14] with permission from Elsevier.

Recalling Eq. (5.9), on low-bandpass spectrometers the intensity is a slowly varying function of the energy transferred (°cg ). Therefore, small but seemingly arbitrary changes to the ab initio frequencies will have little impact on the calculated ab initio intensities. 

This is related to similar but subtly different effects observed on indirect geometry spectrometers ( 5.2.1.2). The INS spectrum of Rb2[PtH6], obtained on the low-bandpass spectrometer TOSCA, is given... 

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. 

As solid dihydrogen is warmed the effects of recoil become more marked near the melting point, 15 K, the transition line is reduced to an edge. Fig 6.3. In the liquid even the edge disappears and the onset of recoil occurs below the rotational transition. In the solid the effects of recoil are eomplete at higher energy transfers (which on low-bandpass spectrometers have higher Q values) and we then see broad bands centred about 1850 and 4000 cm". These bands correspond to the recoiled J(3<-0), 1428 cm", and y(5<-0), 3570 cm", rotational transitions, see Fig. 6.2(c). 

The INS spectra of methane, ethane, propane and butane are shown in Fig. 8.3 and the reason for the lack of INS spectra of methane is evident. The spectrum is dominated by its molecular recoil ( 2.6.5) and all spectroscopic information is washed out. This is a consequence of its light mass and the very weak intermolecular interactions methane boils at 112K. As the molecular weight of the alkane increases, it becomes possible to distinguish features at increasing wavenumber. Also modes measured at low wavenumber on a low-bandpass spectrometer, are measured at low Q and since recoil is proportional to spectral quality improves ( 5.2.2). 

The INS spectra of ethene [23,24] and propene [24] are discussed in 7.3.2.3 and shown in Fig. 7.16. The spectra are dominated by the effects of molecular recoil. This is less of a problem for propene because it has internal vibrations at lower energy (and hence on low-bandpass spectrometers, lower Q) than ethene. With the much heavier tetrabromoethene [25] this does not occur but the small cross section means that a large (8 g) sample was needed. Tetracyanoethene has been studied by coherent INS [26]. The bicyclic alkene norbomene [27] has been studied by INS because it is the parent compound for a class of advanced composites. 

Fig. 8.10 INS spectra of isotopomers of formic acid recorded with a low-bandpass spectrometer (now defunct), (a) HCOOH, (b) HCOOD, and (c) DCOOH. Reproduced from [54] with permission from the American Chemical Society.

The INS spectrum of [Co(CO)4H] [72] Fig. 11.20a illustrates this point. The intense band at 696 cm is the doubly degenerate Co-H bend and the features at 330 and 430 cm are M-C=0 bending modes that involve the axial CO. The assignments are confirmed by a DFT calculation of the isolated molecule [73], Fig. 11.20b. Fig. 11.20c is an estimate of the spectrum that would be observed on TOSCA and demonstrates the huge strides in instrument performance that have occurred in the 30 years since Fig. 11.20a was recorded on a low-bandpass spectrometer at a reactor source. 

In this Appendix we derive the analytical expression for the energy resolution of a low-bandpass spectrometer like TOSCA ( 3.1) (also known as crystal analyser spectrometers) and describe two key features of the design ( 3.2), time focussing ( 3.2.1) and the Marx principle ( 3.2.2) that improve the resolution at high and low energy transfer respectively. 

We show in the column headed H/D where isotopomers were studied (Y). The spectrometer on which a spectrum was recorded is also reported most of these are described in Table 3.2. If the system has been studied by coherent INS, the references are also included and TAS (triple axis spectroscopy) appears in the instrument column. Much of the early work was carried out on low-bandpass spectrometers that are now defunct, these were generally of the beryllium filter type and are indicated as LBS . 

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