Tel Aviv
University
School of Chemistry
Sergey Cheskis' group
Intracavity Laser Absorption Spectroscopy (ICLAS)
Fiber Laser Intracavity Absorption Spectroscopy (FLICAS)
Sulfur related light emission from flames
Flame assisted nanoparticle synthesis
ICLAS is a method in which absorbing species are placed inside the cavity of a broadband laser. Due to the positive feedback mechanism in lasers, even minute quantities of a narrow-line absorber will produce holes in the spectrum where the laser output is partially quenched. ICLAS was first proposed more than thirty years ago, and since then has been the subject of hundreds of papers and several review articles. Our own extensive experiments with ICLAS in flames have shown that this is probably the most sensitive of the absorption spectroscopy methods considered. ICLAS has several important advantages over CRDS, mainly:
The sensitivity of ICLAS is higher because it can attain longer optical paths.
In ICLAS, a CCD detector collects information from several thousand wavelength channels simultaneously during a single pulse. Thus, the time required to obtain a given amount of spectral information is several orders of magnitude shorter for ICLAS than for CRDS or LIF.
Because ICLAS tolerates relatively high broadband losses in the cavity, optical windows can be used to separate the cavity from the flame region. Experiments with sooty flames are also possible with ICLAS. We have used ICLAS extensively for concentration measurements in low-pressure flames.
More recently we have started using FLICAS, a modified ICLAS technique based on IR fiber lasers . In the proposed research, this variant will be used to monitor H2S, CO, and CO2molecules in the 1.5 μm range of the spectrum.
Various lasers, such as, dye, solid state and fiber lasers have been used successfully for intracavity absorption measurements: So far, the highest sensitivity to intracavity absorption has been achieved with a dye laser, where effective absorption path lengths of up to 70,000 km have been demonstrated in the visible spectral range.
However, doped fibre lasers are more suitable for the design of compact and inexpensive gas analyzers for practical field measurements, since they require substantially less pump power than other lasers and emit a broad spectrum in the near infrared range where many molecular species show strong absorption. Inhomogeneous broadening of the gain allows simultaneous measurements in a broad spectral range without tuning.
Intracavity absorption measurements in the combustion environment can be easily performed with an Er3+-doped fiber laser. Most of the components of this laser are very well developed for telecommunication and they are readily available. Tuning range extends from 1.527 µm to 1.613 µm and includes strong absorption of many atmospheric and combustion relevant molecules, such as CO, CO2, OH, HCN, C2H2, H2S, NH3, CH4, H2O and HI and it is almost free from strong water vapor absorption lines, typical for other lasers.
A low-pressure flat flame is very suitable for laser diagnostics due to its quasi-one-dimensional structure. This makes it rather simple to compare experimental results to computer models. In a low-pressure system, the flame front is also wide enough to recover detailed information on the concentration profiles of various species. In our laboratory we built a large vacuum chamber (40 cm in diameter) containing a porous plug McKenna burner 6 cm in diameter. The burner can be displaced in the vertical and horizontal directions with very high precision, allowing for tomographic reconstruction of the flame with line-of-sight spectroscopy Our research group has extensive experience with this kind of flame and with several different laser spectroscopy methods, including intracavity laser absorption spectroscopy (ICLAS), cavity ring down spectroscopy (CRDS), and laser-induced fluorescence (LIF)
Even a small amount of sulfur produces blue-violet chemiluminescence. A mechanism producing electronically excited S2 molecules is not fully understood. Two possibilities are normally considered: direct recombination of sulfur atoms
S + S +M = S2*+M (1)
and energy transfer to S2 molecules in the ground state, for example
H + H + S2 = H2 + S2* (2)
By studying laser-induced fluorescence (LIF) and chemiluminescence in a pulsed propagated flame doped with sulfur compounds, we have found that reaction (1) is probably the main
source of excited sulfur molecules. Unfortunately, this experiment failed to give an unambiguous proof.
Another important fact which demands explanation is the enhancement of emission obtained by introducing a cool body into the flame. Whatever mechanism is responsible for this effect may also be involved in generating the S2 spectrum
At the nanoscale, many materials display properties that differ from their corresponding bulk behavior, which makes them attractive in studies in chemistry and material science. There are many current studies on the properties of oxide nanocrystals, and therefore, an efficient method for their production is essential. One of the most common methods to get oxide nanoparticles is by flame pyrolysis. This study is an attempt to develop a robust method to produce oxide nanocrystals from liquid-phase precursor solutions in low-pressure premix flames. One of the goals of the study to be able to control the size of the particles produced. It can be possible by varying the flame parameters, the collection site location, and/or the precursor concentration. For this goal the special low pressure flame apparatus was designed and built. In this study, a special reaction chamber was build, and iron oxide nanocrystals were produced used the two methods.
