Dimitra N. Stratis

Dual-Pulse LIBS Using a Pre-Ablation Spark for Enhanced Ablation and Emission

In this paper we report the first observations of dual-pulse laser-induced breakdown spectroscopy (LIBS) signal enhancements by using a pre-ablation spark. In this technique a laser pulse is brought in parallel to the sample surface and focused a few millimeters above it to form an air plasma or air spark. A few microseconds later a second laser pulse, which is focused on the sample, ablates sample material and forms the LIBS plasma from which analyte emission occurs. In this way, large LIBS signal enhancements, 11-to 33-fold, are observed for copper and lead, respectively, relative to the signal in the absence of the air spark. In all cases where enhanced LIBS signals are seen, greatly enhanced sample ablation also occurs.

Effect of Pulse Delay Time on a Pre-ablation Dual-Pulse LIBS Plasma

In this paper, we investigate the effect of dual-pulse timing on material ablation, plasma temperature, and plasma size for pre-ablation spark dual-pulse laser-induced breakdown spectroscopy (LIBS). Although the plasma temperature increases for dual-pulse excitation, the signal enhancement is most easily attributed to increased sample ablation. Plasma images show that the magnitude of the enhancement can be affected by the collection optic and by the collection geometry. Enhancements calculated using the total integrated intensity of the plasma are comparable to those measured using fiber-optic collection.

Enhancement of Aluminum, Titanium, and Iron in Glass Using Pre-Ablation Spark Dual-Pulse LIBS

In this paper, we report the first enhanced emission for elements in a nonmetal or nonconducting matrix, glass, with the use of a pre-ablation spark. The glass samples used in this work are prototypes of samples used to immobilize inorganic waste at the Savannah River Site Vitrification Facility. We have found that using a pre-ablation spark results in larger signal enhancements, 11- to 20-fold for titanium, aluminum, and iron in glass compared to the metal under the same experimental conditions. We also demonstrate that this method is more sensitive than single-pulse LIBS experiments for the direct solid sampling of vitrified waste glass.

Energy Dependence of Emission Intensity and Temperature in a LIBS Plasma Using Femtosecond Excitation

In this paper, we investigate the effect of laser energy on laser-induced breakdown emission intensity and average temperature in a short-pulse plasma generated by using 140 fs laser excitation. Both line emission and continuum background intensity and plasma temperature decrease very rapidly after excitation compared to the more conventional nanosecond pulse excitation. Both emission intensity and plasma temperature increase with increasing laser energy. However, the intensity increase appears to be mostly related to the amount of material ablated. Also, nongated laser-induced breakdown spectroscopy (LIBS) is demonstrated using a high-pulse (1 kHz) pulse repetition rate.

Novel Probe for Laser-Induced Breakdown Spectroscopy and Raman Measurements Using an Imaging Optical Fiber

A fiber-optic probe designed for remote laser-induced breakdown spectroscopy (LIBS), Raman spectroscopy, and Raman imaging has been developed for the microanalysis of solid samples. The probe incorporates both single-strand optical fibers and an image guide and allows atomic emission and Raman analysis of any spot on a solid sample within a 5 mm diameter field of view. The real-time sample imaging aspects of the probe are demonstrated by measuring LIBS spectra from different regions of a granite sample and by measuring the Raman spectra of individual TiO2 and Sr(NO3)2 particles on a soil substrate. The ability to obtain remote Raman images of the TiO2 and Sr(NO3)2 particles on the soil substrate is also demonstrated. In this paper we discuss the design and implementation of the fiber-optic probe for obtaining LIBS spectra, Raman spectra, and Raman images.

Some Comparisons of LIBS Measurements using Nanosecond and Picosecond Laser Pulses

Laser-induced breakdown spectra were measured by using a 1.3 ps laser pulse on glass, steel, and copper. Material ablation with the use of picosecond excitation is very precise with well-formed sharp-edged craters. The spectra obtained with 570 nm, 1.3 ps excitation decay more quickly and show significantly lower background emission than those that use 1064 nm, ∼ 7 ns excitation. The background was low enough that excellent laser-induced spectroscopy (LIBS) spectra were obtained on the three samples by using a single 1.3 ps laser pulse and a nongated detector. Similar results were obtained by using nanosecond excitation but with higher relative background signals. The radiance was similar with the use of pico- or nanosecond excitation; however, the radiant intensity was larger with nanosecond excitation because of the larger plasma.

Comparison of Acousto-Optic and Liquid Crystal Tunable Filters for Laser-Induced Breakdown Spectroscopy

In this paper, we report the first time-resolved laser-induced plasma images acquired using a liquid crystal tunable filter (LCTF). We also compare the use of LCTFs and acousto-optic tunable filters (AOTFs) for time-resolved plasma imaging applications in terms of resolution, out-of-band rejection, and image quality. Application of tunable filter technologies to plasma imaging is unlike other spectroscopic imaging methods because of the intense and spectrally broad background generated by a laser-induced plasma. High quality images of the distribution of atomic emission within a laser-induced plasma can be achieved using both AOTFs and LCTFs. However, additional filters are needed for rejection of wavelengths outside the tuning ranges of the devices. Both devices exhibited superior resolution in the lower working range of the filters (∼500 nm) with the LCTF exhibiting superior spectral resolution to the AOTF.

Development of a dual-pulse fiber optic LIBS probe for in-situ elemental analyses

One of the strengths of laser-induced breakdown spectroscopy (LIBS) is the ability to acquire atomic emission spectra for a wide variety of samples non-invasively, with only optical access being required. The use of optical fibers makes the technique ideal for applications where the measurement area of interest is either not accessible or where it is not safe to take a sample. Fiber-optic LIBS probes have been described where a single laser pulse is delivered to the sampling region by one optical fiber and the emission is collected by another. One of the problems with this approach is fiber degradation from the high power laser pulses. To minimize this problem, we are investigating dual-pulse LIBS where the laser power is split between 2 different laser pulses that are separated by a short delay time. We have found in related studies that the use of dual laser pulses to obtain LIBS signals can lead to enhanced intensity and reproducibility for some types of samples. A natural extension of this result is to make dual- pulse measurements using optical fibers. Thus far, we have seen 1.5 to 2 fold enhancements for copper and lead using fiber-optics in various geometries to both deliver the dual laser pulses and collect the emission.

Feasibility of remote Raman imaging using tunable filters

We are investigating the use of small, transportable, Raman systems for remote Raman measurements at intermediate ranges. Previous work focused on the use of an imaging spectrograph and a fiber-optic coupled probe for making single point measurements. More recently we have considered the use of tunable filters for remote Raman imaging. For this work, acousto-optical and liquid crystal tunable filters are being used both with, an in place of dispersive spectrometers and fixed filtering devices. In addition, we have improved the system by the use of a modified holographic fiber-optic Raman probe for light and image collection. In these experiments, a 100 micron collection optical fiber is replaced with a small diameter image guide for light collection and imaging. The feasibility of tunable filter technology for remote Raman imagin will be discussed along with the merits of image transfer devices using small- diameter image guides.

Remote Raman using polymer mirrors

Recent work performed in this laboratory has demonstrated the feasibility of using tunable filter technologies in place of dispersive spectrometers and fixed filtering devices for the purpose of creating field transportable standoff Raman imaging systems. Recently, a development in the area of polymer science has led to the production of polymer mirrors which are lightweight compared to glass mirrors of similar size. In addition, the techniques used to produce these polymer mirrors make it easy to design low f/pound optical devices, with much higher optical speeds than identically sized glass mirrors. The performance of a low f/pound polymer mirror system in combination with a liquid crystal tunable filter for standoff Raman chemical imaging is demonstrated and evaluated.