Mid-infrared laser-induced grating experiments of C2H4 and NH3 from 0.1–2 MPa and 300–800 K

• Published in: Applied physics. B, Lasers and optics Lasers and optics, 2000-11, Vol.71 (5), p.673-680
• Main Authors: Gutfleisch, M., Shin, D.I., Dreier, T., Danehy, P.M.
• Format: Article
• Language: English
• Subjects: Beams (radiation); Coincidences; Ethylene; Lasers; Mathematical models; Molecular absorption; Pulse width; Tuning
• ISSN: 0946-2171; 1432-0649
• DOI: 10.1007/s003400000408

Laser-induced thermal gratings (LITG) were generated in mixtures of ethylene and ammonia in nitrogen using mid-infrared laser radiation from a grating-tuned, low-pressure, pulsed (5 ms pulse width) CO2 laser, and read out with a continuous wave Nd:YLF laser. The LITG signal intensity was measured as a function of pressure (0.1--2 MPa) and temperature (300--800 K, at 0.1 and 1 MPa) by tuning the laser to the accidental coincidences of the 10P(14) and 10R(6) emission lines with molecular absorption transitions of C2H4 and NH3, respectively. Comparisons are made with theoretical predictions for the grating efficiency from a simple thermalization model. A theoretical comparison of the temporal LITG signal response for three excitation pulse shapes -- a delta function, a realistic pulse, and a square wave is presented. Furthermore, it is shown that for NH3, most of the decrease of the LITG signal intensity with increasing temperature is due to the corresponding decrease in fractional molecular absorption of the pump beam radiation. The diagnostic capabilities of the mid-infrared LITG experiment is demonstrated for spatially resolved ethylene measurements with long laser pulses in a premixed stoichiometric CH4--air flame at atmospheric pressure.