The authors report the observation of very-high-order odd harmonics of Nd:YAG laser radiation in rare gases at an intensity of about 1013 W cm-2. Harmonic light as high as the 33rd harmonic in the XUV range (32.2 nm) is generated in argon. The key point is that the harmonic intensity falls slowly beyond the fifth harmonic as the order increases. Finally, a UV continuum, beginning at 350 nm and extending down towards the short wavelength region is apparent in xenon.

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We have studied highly excited autoionizing states in atomic sodium, using two laser beams and synchrotron radiation. We have observed the electron spectra resulting from the decay of these 2p 5 3s4p, 2p 5 3s4d, and 2p 5 3s5s states to the 2p 6 l So ground state of Na + . The excitation energies and the relative oscillator strengths of inner-shell transitions in the excited atoms have been measured.PACS numbers: 32.80.Fb, 32.80.Dz, 32.80.Hd, 32.80.Wr In this Letter, we present the first experimental evidence that it is possible to produce and study highly excited atomic states, with use of stepwise excitation by several laser beams to excite one outer electron and synchrotron radiation to excite an inner electron. In our experiment, we used two dye lasers to promote the 3s electron of atomic Na to either the 4d or the 5s excited state via the resonant 3p state, and monochromatized synchrotron radiation to excite one of the 2p 6 electrons to 2p 5 3s core-excited states. With the help of electron spectrometry, we have been able to observe electrons produced in the autoionization of several of these 2p 5 3snl (n > 3) states and to measure the energies and the excitation functions of a number of them. Our results demonstrate a new capability of studying photoexcitation and photoionization processes in a widely increased number of excited atomic states over a broad range of photon energies. In particular, this new step may constitute an alternative approach to the current studies 1 of highly excited atomic states in which electron-electron correlations as well as screening are expected to produce large effects on the resulting autoionization spectrum. 2 Two-electron excited atomic states, involving the simultaneous excitation of one inner-and one outer-shell electron, have been the subject of a large number of studies, since the earlier experiments in the rare gases. 3 ' 4 However, these excited atomic states were produced exclusively via correlation effects in two-electron, onephoton transitions. The first experiment involving innershell excitation in an excited atom used a pulsed laser and a discharge lamp to measure, in photoabsorption, the energy of two-electron states in alkali metals. 5,6 During the last few years, the combination of a laser beam with a synchrotron radiation (SR) beam as the two sources of photons was successfully achieved 7 " 10 and used 11 ' 12 to start studies of inner-shell photoionization processes in excited atoms over a wide range of photon energies. However, all these studies involved the excitation of the outer electron to only the first empty orbital. It was thus felt important to test if the broad spectral range accessible with SR could be efficiently combined with several lasers to study, at will, atoms in a widely extended number of highly excited states. Na was chosen for this feasibility experiment, because of the high intensity of SR available in the energy range of inner-shell excitation of a 2p electron. Also, Na is a good candidate to study the variation of electrostatic in...

We report experimental studies of harmonic generation in Xe using a 1064 nm Nd-YAG laser. Third-harmonic conversion is observed in a positively dispersive medium, using a tight-focusing geometry. The intensity of third-harmonic generation in a gas cell varies much more steeply with the laser intensity than a cubic law. The reason for the breaking of the phase matching condition is discussed.