The Effect Of Sample Temperature On The Expansion Dynamics Of Laser Induced Plasma

Laser induced breakdown spectroscopy(LIBS),also known as laser induced plasma spectroscopy(LIPS),is an analytical technique based on the interaction between laser pulse and material.In order to create intense plasma,a high energy laser pulses is focused to the surface of the sample.We use he spectrometer to collect the plasma radiation spectrum,which can detect the composition and content of the samples.Compared with the traditional analytical spectral technique,LIBS technology has the following advantages: any physical state of the sample,without sample pre-treatment,simultaneous multi elemental monitoring,simplicity of operation,fast detection and remote analysis.Therefore,LIBS technology has received extensive attention in scientific investigations and technical applications in a broad range of topical areas.For example: real-time and on-line analysis in industrial environments,analysis of mineral composition,air quality inspection,biomedical science,deep-sea exploration,geological survey,jewelry and artwork detection.The core problem of LIBS technology is how to get better spectral intensity and increase the signal-to-noise ratio(SNR).The interaction of high energy laser pulse with matter is a nonlinear process.The result of this process produces a high temperature and high density plasma which is composed of electrons,ions and atoms.The emission spectrum is produced by these excited ions and atoms return to the low energy state from the high energy state.This process is mainly divided into three stages: the bond is broken in high energy laser pulse and preliminary formation of plasma,plasma expansion and cooling,ejection condensation.The result is the formation of the ablation crater on the sample surface.In this paper,we select germanium wafer and silicon wafer as the experimental samples,which is more than 99.99%.A Nd:YAG laser with the wavelength of 1064 nm is used to ablate sample in the experiment.We use spectral imaging system to collecte the laser induced plasma spectra,and use ICCD camera to shoot the plasma plume.Establish mathematical model through the analysis of experimental data,qualitative and semi-quantitative analyse the effects of sample temperature on laser induced plasma spectral.It provides an effective method to enhance the LIBS spectral intensity,increase the signal to noise ratio and improve the sensitivity of spectrum detection.The thesis is composed of six chapters.In the first chapter,we discuss the principle of laser induced breakdown spectroscopy and the physical mechanism of plasma.The development status at home and abroad is briefly introduced.In the second chapter,the interaction of pulse laser with materials and the expansion of plasma are discussed,and the mathematical method for calculating plasma parameters is introduced in detail.In the third chapter,we discuss the effect of sample temperature on the laser induced breakdown spectra from the detection spectral signal.In the fourth chapter,we use the ICCD camera system to capture the dependence of the plasma plume on the delay time.We discuss the expansion and the decay process of plasma plume in the different sample temperature,establishe a mathematical model and infer the changing trend of crater.In the fifth chapter,the RBF radial basis function neural network fitting method is firstly proposed to restore the LIBS spectral line and the signal to noise ratio is discussed.The experimental results and the significance of this thesis are summarized in the sixth chapter;at the same time suggest the direction to future research.