Sensing Signatures Mediated by Chemical Structure of Molecular Solids in Laser-Induced Plasmas

Abstract

Laser ablation of organic compounds has been investigated for almost 30 years now, either in the framework of pulse laser deposition for the assembling of new materials or in the context of chemical sensing. Various monitoring techniques such as atomic and molecular fluorescence, time-of-flight mass spectrometry, and optical emission spectroscopy have been used for plasma diagnostics in an attempt to understand the spectral signature and potential origin of gas-phase ions and fragments from organic plasmas. Photochemical and photophysical processes occurring within these systems are generally much more complex than those suggested by observation of optical emission features. Together with laser ablation parameters, the structural and chemical-physical properties of molecules seem to be closely tied to the observed phenomena. The present manuscript, for the first time, discusses the role of molecular structure in the optical emission of organic plasmas. Factors altering the electronic distribution within the organic molecule have been found to have a direct impact on its ensuing optical emissions. The electron structure of an organic molecule, resulting from the presence, nature, and position of its atoms, governs the breakage of the molecule and, as a result, determines the extent of atomization and fragmentation that has proved to directly impact the emissions of CN radicals and C2 dimers. Particular properties of the molecule respond more positively depending on the laser irradiation wavelength, thereby redirecting the ablation process through photochemical or photothermal decomposition pathways. It is of paramount significance for chemical identification purposes how, despite the large energy stored and dissipated by the plasma and the considerable number of transient species formed, the emissions observed never lose sight of the original molecule.

Insight into the Formation of Molecular Species in Laser-Induced Plasma of Isotopically Labeled Organic Samples

“…In a pure nitrogen atmosphere, less than 10% of the nitrogen in CN molecules originates from the sample itself but, rather, comes from the ambient gas. Despite the 15 N-to-C ratio of 1:2 in urea, 14 N of the ambient gas appears to be the preferential source for CN. This indicates a strong interaction of the ablated material with the ambient nitrogen.…”

Classification of plastic materials by imaging laser-induced ablation plumes

“…Moreover, it is capable to simultaneously provide elemental as well as molecular spectroscopic information, which is especially interesting for the analysis of organic materials such as plastics. Indeed, as an eminent elemental analysis technique, LIBS has been also demonstrated to be suitable to reveal molecular structure of organic materials [13][14][15]. The combination of elemental and molecular emissions, especially those of C 2 and CN radicals, offers an ensemble of spectral features to characterize plastics and organic materials in general [16][17][18][19][20][21][22].…”

Anions in laser-induced plasmas

“…Plasma modeling is a diverse subject [9,10] with variety of applications. Recently, much attention has been paid to formation and detection of molecules in LIPs of organic [11,12] and inorganic [13,14] origins. Molecular reactions occur at sufficiently low temperatures at which anions may form.…”