Spectral and Temporal Properties of 1ωo, 2ωo and 3/2ωo Emission from Laser-Produced Plasmas
This paper summarises the results of an extensive study of the spectral and temporal properties of emission from laser-produced plasmas at the fundamental (1ωo), second harmonic (2ωo), and three-halves harmonic (3/2ωo). Some of the results have been presented in detail in recent publications1,2. In the experiments short pulses (20–400psec) of 1.053 μm radiation from a Nd:glass laser were focussed to high intensity (1014 – 1017W/cm2) using an F=l lens onto planar targets of glass, aluminium or lead. The observation of two common features of the spectra at all frequencies motivated this work. Firstly it was found that all time integrated spectra contain apparently random spectral modulations with the spectral width of these modulations simply reflecting the resolution of the recording system. Fourier analysis of the spectral envelope demonstrated that the modulations had the characteristic of “pink noise”, that is the power spectrum of the modulations decayed smoothly to high modulation frequencies. A consequence of this characteristic is that if the recording system has poor spectral resolution, there will be a distinct impression that the spectra consists of a few well defined spectral peaks. However, as the resolution is improved, it will be found that these apparently well defined peaks develop more and more substructure and eventually become unrecognisable.
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