This paper reviews the theory and applications of photoacoustic (also called optoacoustic) methods belonging to the more general area of photothermal measurement techniques. The theory covers excitation of gaseous or condensed samples with modulated continuous light beams or pulsed light beams. The applications of photoacoustic methods include spectroscopy, monitoring deexcitation processes, probing physical properties of materials, and generating mechanical motions. Several other related photothermal methods, as well as particle-acoustics and wave-acoustics methods are also described. This review complements an earlier and narrower review [Rev. Mod. Phys. 53, 517 (1981)] that is mainly concerned with sensitive detection by pulsed photoacoustic spectroscopy in condensed matter.

Applications of photoacoustic sensing techniques

The first part of this two-part review focused on the fundamental and diagnostics aspects of laser-induced plasmas, only touching briefly upon concepts such as sensitivity and detection limits and largely omitting any discussion of the vast panorama of the practical applications of the technique. Clearly a true LIBS community has emerged, which promises to quicken the pace of LIBS developments, applications, and implementations. With this second part, a more applied flavor is taken, and its intended goal is summarizing the current state-of-the-art of analytical LIBS, providing a contemporary snapshot of LIBS applications, and highlighting new directions in laser-induced breakdown spectroscopy, such as novel approaches, instrumental developments, and advanced use of chemometric tools. More specifically, we discuss instrumental and analytical approaches (e.g., double- and multi-pulse LIBS to improve the sensitivity), calibration-free approaches, hyphenated approaches in which techniques such as Raman and fluorescence are coupled with LIBS to increase sensitivity and information power, resonantly enhanced LIBS approaches, signal processing and optimization (e.g., signal-to-noise analysis), and finally applications. An attempt is made to provide an updated view of the role played by LIBS in the various fields, with emphasis on applications considered to be unique. We finally try to assess where LIBS is going as an analytical field, where in our opinion it should go, and what should still be done for consolidating the technique as a mature method of chemical analysis.

https://journals.sagepub.com/doi/pdf/10.1366/11-06574

Laser-induced breakdown spectroscopy (LIBS), part II: review of instrumental and methodological approaches to material analysis and applications to different fields.

An approach is proposed which combines simultaneously advantages both of pulse (PT) and modulated infrared thermography. In a nondestructive evaluation perspective, the specimen is pulse heated as in PT and the mix of frequencies of the thermal waves launched into the specimen is unscrambled by performing the Fourier transform of the temperature evolution over the field of view. Of interest is the maximum phase image with many attractive features: deeper probing, less influence of surface infrared and optical characteristics, rapid image recording (pulse heating, surface‐wide inspection), and the possibility to inspect high thermal conductivity specimens. Several results are presented and the theory is discussed as well.

Pulse phase infrared thermography

Laser-induced breakdown spectroscopy (LIBS) has become a very popular analytical method in the last decade in view of some of its unique features such as applicability to any type of sample, practically no sample preparation, remote sensing capability, and speed of analysis The technique has a remarkably wide applicability in many fields, and the number of applications is still growing From an analytical point of view, the quantitative aspects of LIBS may be considered its Achilles' heel, first due to the complex nature of the laser–sample interaction processes, which depend upon both the laser characteristics and the sample material properties, and second due to the plasma–particle interaction processes, which are space and time dependent Together, these may cause undesirable matrix effects Ways of alleviating these problems rely upon the description of the plasma excitation-ionization processes through the use of classical equilibrium relations and therefore on the assumption that the laser-induced 

Laser-Induced Breakdown Spectroscopy (LIBS), Part I: Review of Basic Diagnostics and Plasma–Particle Interactions: Still-Challenging Issues Within the Analytical Plasma Community

A new procedure, based on the laser-induced plasma spectroscopy (LIPS) technique, is proposed for calibration-free quantitative elemental analysis of materials. The method here presented, based on an algorithm developed and patented by IFAM-CNR, allows the matrix effects to be overcome, yielding precise and accurate quantitative results on elemental composition of materials without use of calibration curves. Some applications of the method are illustrated, for quantitative analysis of the composition of metallic alloys and quantitative determination of the composition of the atmosphere.

New Procedure for Quantitative Elemental Analysis by Laser-Induced Plasma Spectroscopy

Laser-induced breakdown spectroscopy has been applied to perform elemental analysis of aluminum alloy targets. The plasma is generated by focusing a pulsed Nd:YAG laser on the target in air at atmospheric pressure. Such a plasma was characterized in terms of its appearance, emission spectrum, space-integrated excitation temperature, and electron density. The electron density is inferred from the Stark broadening of the profiles of ionized aluminum lines. The temperature is obtained by using Boltzmann plots of the neutral iron lines. Calibration curves for magnesium, manganese, copper, and silicon were produced. The detection limits are element-dependent but are on the order of 10 ppm.

http://journals.sagepub.com/doi/pdf/10.1366/0003702953964408

Quantitative Analysis of Aluminum Alloys by Laser-Induced Breakdown Spectroscopy and Plasma Characterization

Analysis of solids using laser-induced plasma spectroscopy in double-pulse mode

Laser generation of convergent acoustic waves for materials inspection

A fast two-color pyrometer is used to monitor the temperature evolution of individual plasma- sprayed niobium particles after their impact on a substrate. The experimental setup permits rejection of thermal events corresponding to in-flight particles intersecting the pyrometer field of view, ensuring that only particles impacting on the substrate surface are examined. Cooling rates as high as 10 8 Ks -1 are measured when particles are sprayed on steel and alumina substrates. An interruption of the rapid cooling process and a transient temperature increase are observed near the niobium melting point. Experimental results are interpreted with the help of a numerical thermal flow model taking into account undercooling and the recalescence effects.

Temperature evolution of plasma-sprayed niobium particles impacting on a substrate

In this paper the use of laser-induced plasma spectrometry (LIPS) for
the quantitative analysis of Al, Cu, Fe, Pb and Sn components in solid
zinc alloys is evaluated. Laser-induced plasmas are characterized using
spectroscopic diagnostic techniques that yield the excitation temperature
and electron density. Optimal experimental conditions for analysis are
evaluated, including time gating parameters and distance from focusing
lens to target where it is found that the focus of the laser beam should
be positioned behind the target in order to prevent secondary air plasmas
from forming in front of the target. Calibration curves are produced for
several analytical lines, and the analytical performance of the technique
is assessed. While low detection limits (<60 ppm) are found, the
precision of measurement could be improved.

Paolo Cielo

Papers

Quantitative Analysis of Aluminum Alloys by Laser-Induced Breakdown Spectroscopy and Plasma Characterization

Analysis of solids using laser-induced plasma spectroscopy in double-pulse mode

Laser generation of convergent acoustic waves for materials inspection

Temperature evolution of plasma-sprayed niobium particles impacting on a substrate

Quantitative analysis of additives in solid zinc alloys by laser-induced plasma spectrometry