Single-shot third-harmonic spectral diagnostics for tender X-ray FEL pulses using a bent-crystal spectrometer

The tender X-ray (2.0-4.0 keV) region provides access to the K-edges of light- and mid-Z elements that are central to catalysis, energy materials and soft condensed matter, but direct single-shot spectral diagnostics in this range are technically challenging. Ruled gratings typically offer limited resolving power, while scattering-based approaches suffer from low efficiency in an on-line geometry. We demonstrate that a bent-crystal spectrometer on the SwissFEL Aramis beamline can be adapted to provide indirect single-shot spectral diagnostics for tender X-ray operation by measuring the natural third harmonic of 2.1 and 2.5 keV self-amplified spontaneous emission pulses at 6.3 and 7.5 keV, respectively. The scaled third-harmonic spectra reproduce the bandwidth obtained from independent monochromator scans of the fundamental, yielding a relative bandwidth of ΔE/E ≃ 3.1‰ at 2.5 keV. An autocorrelation analysis of 2400 shots gives an upper limit on the instrumental resolution better than 6 × 10^-5 at 7.5 keV. The two-dimensional spectral images further reveal the transverse-mode structure and provide sensitive real-time feedback for optimizing free-electron laser performance in the tender X-ray regime.