Laser-induced fluorescence of formaldehyde in combustion using third harmonic Nd:YAG laser excitation Christian Brackmann a, *, Jenny Nygren a , Xiao Bai a , Zhongshan Li a , Henrik Bladh a , Boman Axelsson a , Ingemar Denbratt b , Lucien Koopmans b , Per-Erik Bengtsson a , Marcus Alde ´n a a Division of Combustion Physics, Lund Institute of Technology, P.O. Box 118, SE-221 00 Lund, Sweden b Department of Thermo and Fluid dynamics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden Received 9 August 2002; received in revised form 27 March 2003; accepted 27 March 2003 Abstract Formaldehyde (CH 2 O) is an important intermediate species in combustion processes and it can through laser- induced fluorescence measurements be used for instantaneous flame front detection. The present study has focussed on the use of the third harmonic of a Nd:YAG laser at 355 nm as excitation wavelength for formaldehyde, and different dimethyl ether (C 2 H 6 O) flames were used as sources of formaldehyde in the experiments. The investigations included studies of the overlap between the laser profile and the absorption lines of formaldehyde, saturation effects and the potential occurrence of laser-induced photochemistry. The technique was applied for detection of formaldehyde in an internal combustion engine operated both as a spark ignition engine and as a homogenous charge compression ignition engine. # 2003 Elsevier B.V. All rights reserved. Keywords: Combustion diagnostics; Laser-induced fluorescence; Spectroscopy; Formaldehyde 1. Introduction Formaldehyde (CH 2 O) is formed as an inter- mediate species in the combustion of hydrocar- bons [1]. It is produced in the low temperature oxidation processes of hydrocarbon fuels and then consumed in the continuing flame chemistry processes. Formaldehyde is also associated with the ‘cool flame’ phenomenon, which appears when many hydrocarbon fuel  /air mixtures approach the explosion limit. Excited formaldehyde is the origin of the blue chemiluminescence that can be ob- served from a cool flame. Hence formaldehyde is an indicator of cool-flame regions and the first stage of combustion processes, and the detection of this molecule is a method to study the initiation and progression of combustion. A theoretical description of the spectroscopy of formaldehyde is rather complex due to the six * Corresponding author. Tel.:  /46-46-222-3150; fax:  /46- 46-222-4542. E-mail address: christian.brackmann@forbrf.lth.se (C. Brackmann). Spectrochimica Acta Part A 59 (2003) 3347  /3356 www.elsevier.com/locate/saa 1386-1425/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1386-1425(03)00163-X