DescriptionFundamentals of Excitation Emission Spectra of Flames
Description
An excitation emission spectrum (EES) displays laser-induced emission intensities as a function of excitation wavelengths and emission wavelengths. For the measurements we use wavelength for laser excitation and not wave numbers.
An useful example
In the figure (left side) an excitation-emission spectrum (EES) of a methane/air flame is shown. The flame is investigated in the excitation wavelength range from 321,0 nm to 302,0 nm (31.153 cm-1 to 33.113 cm-1). The laser beam is focused with a symmetric-convex lens (Suprasil 1, Heraeus), focal length 300 mm. The focus of the laser beam is about 4 centimetres outside the region investigated. Fluorescence and Rayleigh signals are averaged in the flame front/reaction zone of the Bunsen burner.
The excitation wavelength is tuned from 321,0 nm down to 302,0 nm. The emission wavelength is detected from 300 nm to 327 nm. The EES is recorded with a 1200 grooves/mm grating. In the excitation-emission spectrum signals of Rayleigh scattering and laser-induced fluorescence are detected. The line of the elastic Rayleigh scattering signal starts at an emission wavelength of 321,0 nm at the upper side of the figure and ends at an emission wavelength of 302 nm at the bottom respectively to the excitation wavelength of the laser. In order to reduce the signal intensity of Rayleigh scattering, the laser's E-vector was turned 90° into the horizontal plane by a polarization rotator. Signals of Rayleigh scattering interfere with fluorescence signals. The signals in the excitation-emission spectrum have been identified as laser-induced fluorescence of hydroxyl radicals. In the tuned excitation range electronic transitions of hydroxyl are A2S+ (v' = 0) ¬ X2P (v'' = 0), A2S+ (v' = 1) ¬ X2P (v'' = 1) and A2S+ (v' = 2) ¬ X2P (v'' = 2) (Herzberg, 1971). The excitation of OH in the (0,0) band starts at an excitation wavlength of 302,2 nm. Rotational lines of the OH radical are found in the whole range. Transitions in the (1,1) band start at an excitation wavelength of 308,4 nm and at an excitation wavelenth of 315,1 nm the transitions in the (2,2) band start. In comparison to the rotational lines of the (0,0) band, transitions in the (1,1) and (2,2) bands have a low intensity.
In the middle part of the figure (emission wavelength from about 305 nm to 320 nm) superposition of Rayleigh scattering (slope) and OH fluorescence occurs. The OH emissions are within the (0 ® 0), (1 ® 1) and (2 ® 2) bands (Vibrational energy transfer, VET).
