Holldack, K.; Ovsyannikov, R.; Kuske, P.; Müller, R.; Schälicke, A.; Scheer, M.; Gorgoi, M.; Kühn, D.; Leitner, T.; Svensson, S.; Martensson, N.; Föhlisch, A.: Single bunch X-ray pulses on demand from a multi-bunch synchrotron radiation source. Nature Communications 5 (2014), p. 4010/1-7

Synchrotron radiation facilities routinely operate in a multi-bunch regime, but applications relying on time-of-flight schemes require single bunch operation. Here we show that pulse picking by resonant excitation in a storage ring creates in addition to the multi-bunch operation a distinct and separable single bunch soft X-ray source. It has variable polarization, a photon flux of up to 10(7)-10(9) ph s(-1)/0.1%BW at purity values of 10(4)-10(2) and a repetition rate of 1.25 MHz. The quasi-resonant excitation of incoherent betatron oscillations of electrons allows horizontal pulse separation at variable (also circular) polarization accessible for both, regular 30 ps pulses and ultrashort pulses of 2-3 ps duration. Combined with a new generation of angularly resolving electron spectrometers this creates unique opportunities for time-resolved photoemission studies as confirmed by time-of-flight spectra. Our pulse picking scheme is particularly suited for surface physics at diffraction-limited light sources promising ultimate spectral resolution.