ABSTRACT
We did not observe the broadening and spectral shift of harmonics at Io<5xl014 W cm"2. However, at higher intensity the 2 nm red-shift with comparing to low-intensity case was registered for 3rd harmonic generation. We did not observe any variation of transmitted spectra of 795 nm radiation at different delays. Both the 5th, 7th, and 9th harmonics were considerably depended on vacuum conditions inside the vacuum chamber. We investigated the influence of air pressure on harmonic generation and found that it cannot be explained by a decrease of registered VUV radiation due to simple consideration of absorption by air in the chamber and monochromator. It means that both nitrogen and oxygen can play a significant role in the disruption of optimal conditions for high-order harmonic generation in Mo plasma. The growth of harmonic generation in the case of low-density, low-excited plasma can be caused by the conditions of closeness of multi-photon transitions with quasi-resonance transitions of molybdenum plasma. The population of these transitions can be increased at observed conditions, thus leading to the growth of high-order nonlinear susceptibilities responsible for frequency conversion. The ionization energy is around of 7 eV for neutral Mo atoms and of 16 eV for single-ionized Mo. So there should be an abundance of population of excited states of Mo even at average plasma temperatures. The degree of excitation of the surface plasma affected significantly the nonlinear optical processes occurring in the plasma, which was particularly marked in the case of high (7th and 9th) harmonics. The intensity of the 9th harmonic was two to four time higher comparing with the intensity of the plasma radiation in a nearby range.
