Advanced

Investigation of photochemical effects in flame diagnostics with picosecond photofragmentation laser-induced fluorescence

Jonsson, Malin LU ; Larsson, Kajsa LU ; Borggren, Jesper LU ; Aldén, Marcus LU and Bood, Joakim LU (2016) In Combustion and Flame 171. p.59-68
Abstract

Photofragmentation laser-induced fluorescence (PFLIF) is for the first time performed based on picosecond laser pulses for detection of hydroperoxyl radicals (HO2) in a stoichiometric laminar methane/air flame. Photofragmentation is performed with a pump laser pulse of 80 ps duration and a wavelength of 266 nm, whereupon the produced OH photofragments are detected by a second picosecond probe laser pulse, inducing fluorescence via excitation in the A2Σ+(v = 1) ← X2Π(v = 0) band of OH near 283 nm. Excitation spectra of the OH photofragments formed in the reaction zone were recorded for pump-probe delays ranging from 0 to 5 ns. The spectra suggest that the population distribution of the nascent... (More)

Photofragmentation laser-induced fluorescence (PFLIF) is for the first time performed based on picosecond laser pulses for detection of hydroperoxyl radicals (HO2) in a stoichiometric laminar methane/air flame. Photofragmentation is performed with a pump laser pulse of 80 ps duration and a wavelength of 266 nm, whereupon the produced OH photofragments are detected by a second picosecond probe laser pulse, inducing fluorescence via excitation in the A2Σ+(v = 1) ← X2Π(v = 0) band of OH near 283 nm. Excitation spectra of the OH photofragments formed in the reaction zone were recorded for pump-probe delays ranging from 0 to 5 ns. The spectra suggest that the population distribution of the nascent OH fragments is rotationally cold and that it takes on the order of 5 ns for the nascent non-equilibrium rotational distribution to relax into a thermal distribution. The radial OH-fragment distribution was extracted from spectral images (radial position versus emission wavelength) recorded at six different pump-probe delays. Photochemical OH production was observed both in the reaction zone and the product zone. Comparison with a kinetic model for OH production suggests that more than 20% of the oxygen fragments produced by photolysis in the reaction zone are formed in the excited 1D state, explaining a very rapid initial signal growth. The OH-production model was also compared with previous reaction-zone data, acquired with nanosecond laser pulses in the same flame, indicating that no O(1D), but only O(3P), is formed. A plausible explanation of the discrepancy between the two results is that the picosecond pulses, having more than two-orders of magnitude higher irradiance than the nanosecond pulses used in the previous study, might cause 2-photon photodissociation, allowing production of O(1D). In terms of flame diagnostics with PFLIF, it is concluded that a setup based on nanosecond laser pulses, rather than picosecond pulses, appears preferable since photochemical OH production in the reaction zone can be avoided while for short delay times the ratio between the photofragment signal and the photochemical interference in the product zone, stemming from CO2 photolysis, is sufficiently large to clearly visualize the photofragments.

(Less)
Please use this url to cite or link to this publication:
author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Combustion diagnostics, Comparison picosecond and nanosecond excitation, Hydroperoxyl radical, Laser-induced fluorescence, Photofragmentation
in
Combustion and Flame
volume
171
pages
10 pages
publisher
Elsevier
external identifiers
  • Scopus:84976495013
ISSN
0010-2180
DOI
10.1016/j.combustflame.2016.06.012
language
English
LU publication?
yes
id
3689d149-17c0-4b5c-bbd3-f36200148c15
date added to LUP
2016-07-26 12:07:51
date last changed
2016-07-26 12:07:51
@misc{3689d149-17c0-4b5c-bbd3-f36200148c15,
  abstract     = {<p>Photofragmentation laser-induced fluorescence (PFLIF) is for the first time performed based on picosecond laser pulses for detection of hydroperoxyl radicals (HO<sub>2</sub>) in a stoichiometric laminar methane/air flame. Photofragmentation is performed with a pump laser pulse of 80 ps duration and a wavelength of 266 nm, whereupon the produced OH photofragments are detected by a second picosecond probe laser pulse, inducing fluorescence via excitation in the A<sup>2</sup>Σ<sup>+</sup>(v = 1) ← X<sup>2</sup>Π(v = 0) band of OH near 283 nm. Excitation spectra of the OH photofragments formed in the reaction zone were recorded for pump-probe delays ranging from 0 to 5 ns. The spectra suggest that the population distribution of the nascent OH fragments is rotationally cold and that it takes on the order of 5 ns for the nascent non-equilibrium rotational distribution to relax into a thermal distribution. The radial OH-fragment distribution was extracted from spectral images (radial position versus emission wavelength) recorded at six different pump-probe delays. Photochemical OH production was observed both in the reaction zone and the product zone. Comparison with a kinetic model for OH production suggests that more than 20% of the oxygen fragments produced by photolysis in the reaction zone are formed in the excited <sup>1</sup>D state, explaining a very rapid initial signal growth. The OH-production model was also compared with previous reaction-zone data, acquired with nanosecond laser pulses in the same flame, indicating that no O(<sup>1</sup>D), but only O(<sup>3</sup>P), is formed. A plausible explanation of the discrepancy between the two results is that the picosecond pulses, having more than two-orders of magnitude higher irradiance than the nanosecond pulses used in the previous study, might cause 2-photon photodissociation, allowing production of O(<sup>1</sup>D). In terms of flame diagnostics with PFLIF, it is concluded that a setup based on nanosecond laser pulses, rather than picosecond pulses, appears preferable since photochemical OH production in the reaction zone can be avoided while for short delay times the ratio between the photofragment signal and the photochemical interference in the product zone, stemming from CO<sub>2</sub> photolysis, is sufficiently large to clearly visualize the photofragments.</p>},
  author       = {Jonsson, Malin and Larsson, Kajsa and Borggren, Jesper and Aldén, Marcus and Bood, Joakim},
  issn         = {0010-2180},
  keyword      = {Combustion diagnostics,Comparison picosecond and nanosecond excitation,Hydroperoxyl radical,Laser-induced fluorescence,Photofragmentation},
  language     = {eng},
  month        = {09},
  pages        = {59--68},
  publisher    = {ARRAY(0xce63d08)},
  series       = {Combustion and Flame},
  title        = {Investigation of photochemical effects in flame diagnostics with picosecond photofragmentation laser-induced fluorescence},
  url          = {http://dx.doi.org/10.1016/j.combustflame.2016.06.012},
  volume       = {171},
  year         = {2016},
}