54
2 Experimental and Computational Methods
powder mixture, a phenomenon known as texturing or preferred orientation. This
is particularly problematic when powder samples are analysed on a flat plate as is
commonly done on the Bruker D2 phaser used in this work. If reliable peak intensities
are required, it is more common to collect diffraction data from powder samples held
within a capillary.
2.2.2 Inelastic Neutron Scattering Spectroscopy
Inelastic neutron scattering spectroscopy (INS) is used for studying the vibrational
properties of molecules and materials. An excellent text on its theory and applications can be found in Refs. [66] and [67]. In INS spectroscopy, a beam of neutrons is
incident on a sample. These neutrons scatter from the nuclei within the sample and
exchange energy, hence measurement of vibrational frequencies, (ω) and momentum
(q) with the sample. The INS experiment is often compared to the optical spectroscopies: Raman and infrared spectroscopy [68]. While there are many similarities between INS and optical probes, the nature of INS possesses several distinct
advantages, including:
1. Ease of modelling
The scattering function is purely dynamical (Sect. 2.2.2.2) and as such is easily
calculated within the framework of classical and quantum mechanics.
2. Broad spectral range
Typical INS spectrometers (e.g. TOSCA) extend from 0 to > 4000 cm
−1 [69]. This
is much broader than typical optical spectrometers, which often miss the far infrared
region 10–400 cm
−1 , and thus omit the lattice, or external, modes of vibration which
are critical in this work.
3. Sensitivity to normal modes involving hydrogen atoms
Optical probes are dominated by heavy atoms due in part to higher electron densities.
INS intensities are proportional to neutron cross sections (σ ), and that of hydrogen
is particularly high.
5
4. Lack of selection rules
Unlike optical probes, all vibrational modes (including fundamental, overtone and
combination modes) are in principle observed in INS spectroscopy. Group theory
5 No theoretical method is available to calculated neutron scattering cross sections σ , and all tabulated
values are directly obtained from experiment. Molecular vibrations are dominated by incoherent
neutron scattering, with σ inch for common nuclei 1 H, 2 D, 12 C, 13 C, 14 N and 16 O are; 80.27, 2.05,
0, 0.034, 0.5 and 0 barn [83], respectively. σ inch of 1 H clearly dominates.
2 Experimental and Computational Methods
powder mixture, a phenomenon known as texturing or preferred orientation. This
is particularly problematic when powder samples are analysed on a flat plate as is
commonly done on the Bruker D2 phaser used in this work. If reliable peak intensities
are required, it is more common to collect diffraction data from powder samples held
within a capillary.
2.2.2 Inelastic Neutron Scattering Spectroscopy
Inelastic neutron scattering spectroscopy (INS) is used for studying the vibrational
properties of molecules and materials. An excellent text on its theory and applications can be found in Refs. [66] and [67]. In INS spectroscopy, a beam of neutrons is
incident on a sample. These neutrons scatter from the nuclei within the sample and
exchange energy, hence measurement of vibrational frequencies, (ω) and momentum
(q) with the sample. The INS experiment is often compared to the optical spectroscopies: Raman and infrared spectroscopy [68]. While there are many similarities between INS and optical probes, the nature of INS possesses several distinct
advantages, including:
1. Ease of modelling
The scattering function is purely dynamical (Sect. 2.2.2.2) and as such is easily
calculated within the framework of classical and quantum mechanics.
2. Broad spectral range
Typical INS spectrometers (e.g. TOSCA) extend from 0 to > 4000 cm
−1 [69]. This
is much broader than typical optical spectrometers, which often miss the far infrared
region 10–400 cm
−1 , and thus omit the lattice, or external, modes of vibration which
are critical in this work.
3. Sensitivity to normal modes involving hydrogen atoms
Optical probes are dominated by heavy atoms due in part to higher electron densities.
INS intensities are proportional to neutron cross sections (σ ), and that of hydrogen
is particularly high.
5
4. Lack of selection rules
Unlike optical probes, all vibrational modes (including fundamental, overtone and
combination modes) are in principle observed in INS spectroscopy. Group theory
5 No theoretical method is available to calculated neutron scattering cross sections σ , and all tabulated
values are directly obtained from experiment. Molecular vibrations are dominated by incoherent
neutron scattering, with σ inch for common nuclei 1 H, 2 D, 12 C, 13 C, 14 N and 16 O are; 80.27, 2.05,
0, 0.034, 0.5 and 0 barn [83], respectively. σ inch of 1 H clearly dominates.