Carbon Isotope Separation and Molecular Formation in Laser- Induced Plasmas by Laser Ablation Molecular Isotopic Spectrometry Meirong Dong, †,‡ Xianglei Mao, ‡ Jhanis J. Gonzalez, ‡ Jidong Lu, † and Richard E. Russo* ,‡ † School of Electric Power, South China University of Technology, Guangzhou, Guangdong 510640, China ‡ Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, United States ABSTRACT: Laser ablation molecular isotopic spectrometry (LAMIS) recently was reported for rapid isotopic analysis by measuring molecular emission from laser-induced plasmas at atmospheric pressure. This research utilized the LAMIS approach to study C 2 molecular formation from laser ablation of carbon isotopic samples in a neon gas environment at 0.1 MPa. The isotopic shift for the Swan system of the C 2 Δν = 1 band was chosen for carbon isotope analysis. Temporal and spatial resolved measurements of 12 C 2 , 12 C 13 C, and 13 C 2 show that C 2 forms from recombination reactions in the plasma. A theoretical simulation was used to determine the temperature from the molecular bands and to extract the isotopic ratio of 12 C/ 13 C derived from 12 C 2 , 12 C 13 C, and 13 C 2 . Our data show that the ratio of 12 C/ 13 C varies with time after the laser pulse and with distance above the sample. 12 C/ 13 C deviates from the nominal ratio (2:1) at early times and closest to the sample surface. These measurements provide understanding of the chemical processes in the laser plasma and analytical improvement using LAMIS. I sotope measurements in laser plasmas are of current interest for both fundamental understanding of plasma properties and for applications. Isotope separation in laser-induced plasmas has been reported under different ablation conditions and using different methods of detection. 1-5 Several studies showed isotope separation directly in the laser plume and by depositing films. 6-8 Mechanisms for isotope fractionation are under investigation and not well established. Atomic emission from laser-induced plasmas has been demonstrated as an approach for the analysis of isotope ratios, with studies at reduced and atmospheric pressure. 9-12 Laser ablation molec- ular isotopic spectrometry (LAMIS) is a promising approach for real-time isotopic analysis of samples at ambient pressure. 13-16 The technology exploits the measurement of optical spectra from molecular species that are produced during plasma expansion into air. Optical isotope shifts in molecular spectra are significantly larger than those in atomic spectra; differences in isotopic masses have only a small effect on electronic transitions (atoms) but appreciably affect the vibrational and rotational energy levels in molecules. 13 Large shifts in the isotope molecular bands of several elements in laser ablation plasmas were presented; boron and strontium isotopes were demonstrated using the LAMIS technology. 13-15 Carbon is one of the most abundant elements in geological and natural materials, and it is present in all known life forms. There are two stable isotopes, 12 C and 13 C. The study of 13 C is related with research areas including climate change, 17 coal combustion, 18 human diet, 19 plants, 20 and others. 21,22 Carbon molecular emission has been measured in sources like flames, electric arcs, and furnaces, and it has also been detected in a number of astrophysical environments including the sun, interstellar gas clouds, late-type stars, and comets; 23 there is abundant literature of its spectroscopy. 24-26 Pesic et al. 27 observed the swan bands of 13 C 2 and 12 C 13 C in a low pressure hollow discharge spectrum and derived the molecular constants for several bands. A series of 12 C 2 , 13 C 2 , and 12 C 13 C were made by Amiot and co-workers; Amiot and Verge 28,29 recorded the (0-0) band of the Swan system of 12 C 2 , 12 C 13 C, and 13 C 2 by Fourier spectroscopy and made a complete analysis for the a 3 Π and d 3 Π (v = 0) levels. Perturbations were observed in the d 3 Π g (v = 0) level, and their origins were discussed. The (0-0) and (1-0) bands of the Phillips system and the (0-0) band of the Ballik-Ramsay system of 13 C 2 and 12 C 13 C were observed and analyzed using a microwave electromagnetic field with the high accuracy of the Fourier transform interferometer. Curtis and Sarre 30 analyzed the nuclear hyperfine structure of 13 C 2 by laser-induced fluorescence to yield Fermi-contact parameters. Optical emission in the laser ablation plasma, known as laser- induced breakdown spectroscopy (LIBS), offers an ideal characteristic for real-time elemental analysis and in situ plasma characterization at atmospheric pressure. 31-36 In a number of previous studies, carbon molecular emission was used for laser- induced plasma diagnostics, 37,38 thin film preparation, 39,40 plasma fluctuations correction, 41 and as additional spectral Received: December 5, 2012 Accepted: February 1, 2013 Published: February 1, 2013 Article pubs.acs.org/ac © 2013 American Chemical Society 2899 dx.doi.org/10.1021/ac303524d | Anal. Chem. 2013, 85, 2899-2906