ABSOLUTE PHOTON NUMBER MEASUREMENT OF HIGH-ORDER HARMONICS IN THE EXTREME UV

We have measured the absolute photon number of single-shot high-order harmonic spectra generated by focusing intense short pulsed laser radi ation into a gas target with a calibrated extreme ultraviolet (EUV) sp ectrometer. For the calibration, we used a compact hollow-cathode sour ce as a radiometric transfer standard providing radiation from rare ga s plasma emission lines of known radiant intensity in the spectral ran ge from 13.1 nm to 58.4 nm. The photon number measurement is based on the comparison of the detected signals of the high-order harmonics and the hollow-cathode emission lines with nearly identical spectrometer set-ups. This technique provides a measurement of the harmonics photon number with a relative uncertainty of less than 50% (1 sigma). (C) 19 98 Elsevier Science B.V.