. This correction factor alters the amplitude of the vibrations without affecting the frequencies. We carried out the discrete Fourier transformations of the ACFs of μ µ µ or α α α employing an interface to the FFTW 3.3 library. Prior to conducting the Fourier transformation, we applied so-called "data mirroring" to the ACFs. That means, we added the data set in reverse order to the original autocorrelation data. Data mirroring does not change the integral of the spectrum, however it is useful to increase the resolution of the spectrum since it doubles the data points. To further enhance the resolution of the spectra, we employed window functions. More specifically, we multiplied the autocorrelation data with the Hann window function cos [π/(2τ max -2)] before performing the Fourier transformations. τ max is the correlation depth defined in Equation (2).