Phonon ringing and anharmonicity effects in core spectra

We show that the soft x-ray emission spectrum of the boron K core exciton in B₂{O}₃$ displays the largest phonon coupling effects yet observed in core spectroscopies. We interpret the observed double-peaked spectrum as the first clear example of phonon ringing. It is necessary to modify the existing theory of this effect by including anharmonicity in order to obtain an accurate description of the experimental results.