Spontaneous Raman Scattering as a High Resolution Source in the Extreme Ultraviolet.

A new radiation source and its application to high resolution spectroscopy in the extreme ultraviolet is described. The radiation source is based upon spontaneous Raman scattering of incident laser photons off atoms excited to an energetic metastable state. The photons are scattered into two very narrow energy sidebands having energy equal to the metastable state storage energy plus or minus the energy of the incident laser photons. Thus, by tuning the energy of the incident laser photons, the energy of the photons scattered into the extreme ultraviolet is also tuned. A theoretical description of the characteristics of the spontaneous Raman source is presented. Experiments are then described which were performed to demonstrate spontaneous Raman scattering as a high resolution source in the extreme ultraviolet. In these experiments helium atoms excited to the first singlet state were used for the spontaneous Raman process. The radiation scattered into the higher energy sideband, near five-hundred angstroms, was used to probe the inner shell absorption spectrum of potassium. The resolution with which the absorption spectra was taken was greater than that achieved by previous workers by a factor of approximately thirty. As a result, four previously unobserved narrow absorption features of potassium were detected in these absorption measurements. The narrowest absorption feature observed has a measured linewidth of 1.9 cm('-1). This sets an experimental upper bound on the linewidth of the source which is theoretically predicted to be 1.3 cm('-1). Also, a sharp interference between two nearby autoionizing states of potassium was observed and is described theoretically. The spectral region of potassium examined extended over more than 7500 cm('-1), from 536.8 (ANGSTROM) to 558.4 (ANGSTROM). The extension of the radiation source to other spectral regions requires one to use a medium other than helium atoms for the Raman process. The other noble gases as well as the alkali ions are suggested for this purpose, and the atomic parameters of these systems are presented. With the additional spectral coverage supplied by these systems, it should be possible to access approximately 75% of the frequency range between five- and fifteen-hundred angstroms using the spontaneous Raman source.