Ph.D. thesis (pdf) - dirac

96 High Q collective modes
this section we briefly describe the technique and give details regarding the specific
experiments in the following section.
The study of phonons by inelastic X-ray scattering (IXS) requires resolution of
the energy transfer in the meV range while the incoming photon has an energy in
the 10 keV range. This means that the relative resolution has to be in the 10 −7
range. This is obtained by using higher order reflections of silicon crystals in a
backscattering geometry. The scan in energy is done by changing the temperature
of the crystal steps of ∼ 5 mK and hereby varying the lattice spacing .
The temperature changes are made at the monochromator, while the analyzers are
kept at a constant temperature, meaning that the scan in energy is done by changing
the incoming energy and keeping the outgoing energy constant. The (11 11 11)
reflection of the Si monochromator and analyzer crystals was used for the reported
experiments yielding an energy resolution of FMHW=1.6 meV.
There are 5 analyzers on a moving arm, with a fixed position with respect to each
other. This allows spectra to be recorded at 5 Q values simultaneously with a
difference of 3 nm −1 between them.
6.1.2 The experiment
The dynamical structure factor of cumene was recorded at different temperatures in
the glass and in the liquid at atmospheric pressure and at 300 MPa. See appendix
A for details on the sample. The analyzers were set to give the Q values 2 nm −1 , 1
nm −1 , 4 nm −1 7 nm −1 and 10 nm −1 . The integration time per point was minimum
60 s per point and was increased by a factor 2 or 3 at lower temperatures where the
inelastic intensity is lower.
We measured the dynamical structure factor as a function of temperature at atmospheric
pressure for four PIB samples, with different molecular weight: PIB680,
PIB1100, PIB3580 and PIB500k. See appendix A for details on the samples. The
PIB680 and PIB3580 samples were moreover studied at 300 MPa. The Q setting 2
nm −1 , 5 nm −1 , 8 nm −1 11 nm −1 and 14 nm −1 was used for all the samples under
all P-T conditions. Several additional Q settings were used at some conditions. The
integration time per point was 70 s.
The samples were in both experiments placed in a 10mm (or 20mm) long cylindric
pressure cell, which was sealed at both ends by 1 mm thick diamond windows. The
pressure was applied using a piston-and-cylinder device. The pressure was always
imposed above the (pressure dependent) glass transition temperature, and cooling
was done isobarically by adjusting the imposed pressure upon cooling.