Abstract

Trace concentrations of nitrogen dioxide (NO₂), nitromethane (CH₃NO₂), and 2,4,6-trinitrotoluene (TNT) are detected by both one- and two-color laser photofragmentation/fragment detection (PF/FD) spectrometry using one or two lasers. The PF/FD methods studied are (1) one-laser, one-color photofragmentation of the analyte molecule at 227 or 454 nm with subsequent detection of the characteristic nitric oxide (NO) photofragment by one- or two-photon laser-induced fluorescence using its A²Σ⁺ - Χ²Π(0,0) transitions near 227 nm; (2) one-laser, two-color PF/FD, where a 355 nm laser beam is used for additional analyte photofragmentation and NO is detected by both one- and two-photon LIF as in the previous case; (3) two-laser, two-color PF/FD, where the pump and probe beams are time delayed; and (4) one-laser, one-color PF/FD at 355 nm, where the 355 nm beam photofragments the target molecule and the prompt emission from electronically excited NO (A²Σ⁺) is monitored in the range of 200-300 nm. PF/FD excitation and emission spectra are recorded and also simulated with the use of a computer program based on a Boltzmann distribution analysis with transition probabilities, rotational energies, and rovibrational temperatures as input parameters. The effects of laser wavelength, laser pump energy, time delay between pump and probe beams, and analyte concentration on PF/FD signal are investigated and reported. Limits of detection [signal-to-noise (S/N) = 3] for the nitrocompounds range from low ppb_v to ppm_v for 10 s integration time and laser energies of ~ 5 mJ and 100 μJ for the pump and probe beams, respectively. These results are presented and compared to other PF/FD methods for nitrocompound monitoring.

Laser photofragmentation–fragment detection and pyrolysis–laser-induced fluorescence studies on energetic materials

Vaidhianat Swayambunathan, Gurbax Singh, and Rosario C. Sausa
Appl. Opt. 38(30) 6447-6454 (1999)

Trace detection of explosives with low vapor emissions by laser surface photofragmentation–fragment detection spectroscopy with an improved ionization probe

Jerry Cabalo and Rosario Sausa
Appl. Opt. 44(6) 1084-1091 (2005)

Photofragmentation-laser induced fluorescence: a new method for detecting atmospheric trace gases

M. O. Rodgers, K. Asai, and D. D. Davis
Appl. Opt. 19(21) 3597-3605 (1980)

Spectral differentiation of trace concentrations of NO₂ from NO by laser photofragmentation with fragment ionization at 226 and 452 nm: quantitative analysis of NO–NO₂ mixtures

Robert L. Pastel and Rosario C. Sausa
Appl. Opt. 39(15) 2487-2495 (2000)

Application of laser photofragmentation-resonance enhanced multiphoton ionization to ion mobility spectrometry

Jeffrey M. Headrick, Thomas A. Reichardt, Thomas B. Settersten, Ray P. Bambha, and Dahv A. V. Kliner
Appl. Opt. 49(11) 2204-2214 (2010)