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Chemical vapor detection in ambient air via REMPI

The 1+1 and 2+2 resonance-enhanced multiphoton ionization (REMPI) spectra of gas phase indene near the origin of the first electronic transition at 287.9 nm are reported. This work relates to chemically-specific measurements of trace organic constituents in ambiant air. The spectra were acquired to help assess options for on- and off-resonance REMPI, similar in concept to differential absorbance LIDAR (DIAL) and differential optical absorbance spectroscopy (DOAS). The 1+1 REMPI spectrum for indene at 50 ppbv concentration in air compares well with published high resolution absorbance spectra obtained for much longer pathlength and higher sample concentration. Extensive sequence band structure is observed near the origin. The rotational contours of the origin and sequence bands consist of narrow, nearly symmetric features, accompanied by a lower intensity broad sideband shading to the red. The narrow feature is not as narrow as in the highest resolution absorbance spectrum and cannot be explained in terms of the laser linewidth or intensity (saturation) broadening at high laser power; pressure broadening may be the source. We also report styrene 1+1 REMPI spectrum to document instrument improvements made since our previous study.