The effect of pulse duration upon the line profile of Cu I emission observed by laser ablation of a copper metal plate immersed in water has been examined. By irradiating a pulse with the duration longer than 40 ns the spectral profile with clear narrow emission lines of Cu atoms is obtained, while the emission spectra always suffer from broadening and self-absorption by the irradiation of the 20 ns pulse for the ablation. The results show that the use of a long-duration pulse enables in situ elemental analysis of the solid surface in contact with a bulk liquid.

Reducing nanoparticles in metal ablation plumes produced by two delayed short laser pulses Appl. Phys. Lett. 94, 053120 (2009); Time-resolved images of an ablation plume obtained at different pulse durations have been measured for the liquid-phase laser ablation of a Cu plate, and the emission intensity and size of the plume have been discussed. In some plume images, high-density bright spots were observed. The size of the light-emitting region increases and the bright spots disappear for the irradiation with a 150-ns pulse. We conclude that consecutive excitation by the later part of a longer pulse expands the plume and reduces plume density. These results are consistent with our previous result that less-broadened atomic line spectra with high emission intensity were obtained with a 150-ns-pulse irradiation.

Emission spectra of the laser ablation plume formed by the irradiation of Cu65/Zn35 binary alloy in water at the room temperature with 150-ns pulsed laser were measured. The spectra were analyzed by comparing with the theoretical calculation based on the assumption that self-absorption effect is negligible and that the same temperature can be applied to Cu atoms and Zn atoms in the plume. The calculation reproduced the spectra very well, and gave reasonable temperature as a best-fit parameter. However, the best-fit value of the Cu atomic density relative to Zn is significantly low compared with the target composition. Care should be taken to perform in situ LIBS in liquid due to the complicated plume formation mechanism and dynamics of material intake into the plume.

Emission spectroscopy of laser ablation plume can be applied to in situ elemental analysis of solid surface in liquid, especially if the laser pulse with the pulse width of 150 ns is used. The long nanosecond pulse not only suppresses the disadvantageous broadening and deformation of spectral line profiles usually observed when short nanosecond pulses are used but also greatly suppresses the material removal from analyte surfaces. The latter is also an important advantage for the application to surface analysis.