  Single vs. multimode laser beams have been compared for laser ablation on
steel samples. Laser plasma properties and analytical capabilities (precision,
limit of detection) were used as key parameters for comparison. Peak fluence at
focal spot has been observed to be higher for Gaussian beam despite ~14-fold
lower pulse energy. A comparison of Gaussian and multimode beams with equal
energy was carried out in order to estimate influence of beam profile only.
Single mode lasing (Gaussian beam) results in better reproducibility of
analytical signals compared to multimode lasing while laser energy
reproducibility was the same for both cases. Precision improvements were
attributed to more stable laser ablation due to better reproducibility of beam
profile fluence at laser spot. Plasma temperature and electron density were
higher for Gaussian laser beam. Calibration curves were obtained for four
elements under study (Cr, Mn, Si, Cu). Two sampling (drilling and scanning
procedures) and two optical detection schemes (side-view and optical fiber)
were used to compare Gaussian and multimode beam profile influence on
analytical capabilities of LIBS. We have found that multimode beam sampling was
strongly influenced by surface effects (impurities, defects etc.). For all
sampling and detection schemes, better precision was obtained if Gaussian beam
was used for sampling. In case of single-spot sampling better limits of
detection were achieved for multimode beam. If laser sources have same
wavelength and equal energy than quality of laser beam became a crucial
parameter which determined plasma properties and analytical capabilities of
LIBS
