Atomic fluorescence flame thermometry
(Iain Burns and Johan Hult)
The use of diode lasers to make spatially-resolved measurement of flame temperature has been demonstrated. The technique involves the use of two blue extended-cavity diode lasers to probe indium atoms seeded to the flame at trace levels. The atoms emit strong fluorescence signals despite the use of low power excitation sources. The narrow line-widths of the diode lasers allows high-resolution atomic spectra to be obtained by scanning the wavelength across the atomic line.
 Our Research
Two alternative approaches may be followed in evaluating the temperature. In the first of these we use two diode lasers, emitting at 410 nm and 451 nm respectively, to probe the 5²P1/2→5²S1/2 and 5²P3/2→5²S1/2 transitions of atomic indium. The fluorescence emitted by atoms in the flame is imaged through a pin-hole onto a filtered photomultiplier tube, and the ratio of the integrated fluorescence line-strengths is related to the relative populations in the two lower levels, and thus to flame temperature. In a second, more recent, approach, it has been possible to evaluate the temperature using a single diode laser, by analysing the line-shape of the high-temperature indium fluorescence spectrum. Through the effects of Doppler and pressure broadening, the spectral shape is highly sensitive to temperature. The diode laser flame thermometry experiments were originally done in laminar flames at atmospheric pressure, see figure below. Current themes are the implementation of the technique in low pressure flames and in sooting flames, and the increase of measurement repetition rate to enable the study of dynamic systems.
J. Hult, I.S. Burns, and C.F. Kaminski, Two-line atomic fluorescence flame thermometry using diode lasers, Proc. Combust. Inst. 30, 1535-1543 (2005).
I.S. Burns, J. Hult, G. Hartung, and C.F. Kaminski, A thermometry technique based on atomic lineshapes using diode laser LIF in flames, Proc. Combust. Inst. 31, 775-782 (2007).