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Mass removed from single crystal silicon samples by high irradiance (1×109 to 1×1011 W/cm2) single pulse laser ablation was studied by measuring the resulting crater morphology with a white light interferometric microscope. The craters show a strong nonlinear change in both the volume and depth when the laser irradiance is less than or greater than ≈2.2×1010 W/cm2. Time-resolved shadowgraph images of the ablated silicon plume were obtained over this irradiance range. The images show that the increase in crater volume and depth at the threshold of 2.2×1010 W/cm2 is accompanied by large size droplets leaving the silicon surface, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of the layer heated to approximately the critical temperature. The model includes transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency). In this case, the estimated thickness of the superheated layer at a delay time of 200–300 ns shows a close agreement with measured crater depths. Induced transparency is demonstrated to play an important role in the formation of a deep superheated liquid layer, with subsequent explosive boiling responsible for large-particulate ejection.

The craters resulting from high-irradiance (1×109–1×1011 W/cm2) single-pulse laser ablation of single-crystal silicon show a dramatic increase in volume at a threshold irradiance of 2.2×1010 W/CM2. Time-resolved shadowgraph images show ejection of large particulates from the sample above this threshold irradiance, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of a superheated layer near the critical state. Considering the transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency), the calculated thickness of the superheated layer at a delay time of 200–300 ns agreed with the measured crater depths. This agreement suggests that induced transparency promotes the formation of a deep superheated layer, and explosive boiling within this layer leads to particulate ejection from the sample.

Femtosecond and nanosecond lasers were compared for ablating brass alloys. All operating parameters from both lasers were equal except for the pulse duration. The ablated aerosol vapor was collected on silicon substrates for particle size measurements or sent into an inductively coupled plasma mass spectrometer. The diameters and size distribution of particulates were measured from scanning electron microscope (SEM) images of the collected ablated aerosol. SEM measurements showed that particles ablated using nanosecond pulses were single spherical entities ranging in diameter from several micrometers to several hundred nanometers. Primary particles ablated using femtosecond ablation were ∼100 nm in diameter but formed large agglomerates. ICPMS showed enhanced signal intensity and stability using femtosecond compared to nanosecond laser ablation.Laser ablation combined with inductively coupled plasma mass spectrometry (ICPMS) is a practical method for direct solid sample chemical analysis. [1][2][3][4][5] Significant improvements in this technology have led to numerous routine applications, especially in geochemistry. Efforts are still underway to study parameters such as wavelength, 6;7 gas ambient, 8 and energy fluence 9-11 for further improving accuracy and precision of analysis. The ablated aerosol particle sizes are believed to significantly influence analytical performance using ICPMS detection. 12-14 Chemical composition, entrainment, transport, and decomposition in the ICP all are related to the size of the aerosol particles. [15][16][17] For ablation, the laser wavelength and pulse duration play a dominant role in defining the size, size distribution, and chemistry of the ablated particulates. The goal of this work was to measure particles using femtosecond and nanosecond laser ablation and establish correlations with ICPMS performance.The use of femtosecond ablation to reduce thermal effects and minimize fractionation for chemical analysis has been tested, using both IR and UV pulses. [18][19][20][21][22] By using the same laser energy and spot size (same fluence), ICPMS performance with femtosecond laser ablation showed improvements in intensity, precision, and accuracy. To further investigate these improvements, the basis of this work was to examine the relationship between the particle size distribution and ICPMS response using UV femtosecond and nanosecond laser pulses. Brass alloys were ablated with fixed laser parameters of fluence, energy, spot size, and wavelength; pulse duration was the only difference. Brass alloys are commonly chosen as samples due to the thermal volatility difference of copper and zinc. [18][19][20][21][22][23][24] These alloys are ideal for studying effects of pulse duration on fractionation and signal stability using ICPMS. The ablated aerosols also were collected on silicon substrates for scanning electron microscopic (SEM) measurements of particle sizes. EXPERIMENTAL SECTIONThe experimental configuration is shown in Figure 1. Two lasers were used; a Nd:YAG laser with 6-n...

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Laser ablation of copper with a 4ns laser pulse at 1064nm was studied with a series of synchronized shadowgraph (100fs laser pulses at 400nm) and emission images (spectral line at 515nm). Data were obtained at two laser pulse energies (10 and 30mJ) and in three background gases (He, Ne, and Ar) at atmospheric pressure. The laser energy conversion ratio and the amount of sample vaporized for ablation in each condition were obtained by the theoretical analysis reported in paper I from trajectories of the external shock wave, internal shock wave, and contact surface between the Cu vapor and the background gas. All three quantities were measured from shadowgraph and emission images. The results showed that E, the amount of energy that is absorbed by the copper vapor, decreases as the atomic mass of the background gas increases; and M, the mass of the sample converted into vapor, is almost independent of the background gas [Horn et al., Appl. Surf. Sci. 182, 91 (2001)]. A physical interpretation is given based on the phenomena observed in shadowgraph and emission images during the first tens of nanoseconds after the beginning of the laser pulse for ablation in different background gases. In addition, an internal shock wave was observed in the emission images during the first tens of nanoseconds after the laser pulse, which strikes the surface and should be one of the mechanisms inducing the liquid sample ejection. Also, a significant vortex ring near the target was observed in emission images at longer times after the laser pulse (>100ns) which distorts the otherwise hemispherical expansion of the vapor plume.

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A study of the gas dynamics of the vapor plume generated during laser ablation was conducted including a counterpropagating internal shock wave. The density, pressure, and temperature distributions between the external shock wave front and the sample surface were determined by solving the integrated conservation equations of mass, momentum, and energy. The positions of the shock waves and the contact surface (boundary that separates the compressed ambient gas and the vapor plume) were obtained when the incident laser energy that is transferred to the vapor plume and to the background gas, E, and the vaporized sample mass, M, are specified. The values for E and M were obtained from a comparison of the calculated trajectories of the external shock wave and the contact surface with experimental results for a copper sample under different laser fluences. Thus E and M, which are the two dominant parameters for laser ablation and which cannot be measured directly, can be determined. In addition, the internal shock wave propagation within the vapor plume was determined; the interaction of the internal shock wave with the sample may be one of the mechanisms inducing liquid sample ejection during laser ablation.