Cavity Ringdown Spectroscopy of Ambient
Peroxy Radicals
The low temperature oxidation of hydrocarbons is a critically important process both in nature and technology. Not only does it directly affect the environmental quality of our atmosphere, but it is also critical to the efficiency and fuel economy of internal combustion engines. While there are many elementary steps in the overall mechanism of this oxidation, arguably the most important reaction is the production of peroxy radicals (RO2.) from alkyl radicals (R.).
While our
group has successfully studied many reactive chemical intermediates by means of
fluorescence-based spectroscopic techniques like Laser Induced Fluorescence
(LIF), these peroxy radicals are different in that they have a very small
quantum yield of emission. To study alkoxy radicals, we typically excite them to their second
excited electronic state (
state) and then monitor their fluorescence back down to the
ground state (
state). While the absorption
cross-section of the same excitation is strong for peroxy radicals, in most cases
the state is repulsive and
the absorption frequency (
) is similar for many peroxy radicals giving a lack of
selectivity studying this excitation.
Therefore, we have adapted the highly sensitive absorption technique,
Cavity Ringdown Spectroscopy (CRDS), to study the 
transition in peroxy
radicals, which has an absorption cross-section that is approximately four
orders of magnitude weaker than the .
A detailed discussion of CRDS, as well as our experimental setup and results, can be found below.
Cavity Ringdown Spectroscopy (CRDS)
CRDS is a conceptually simple, highly sensitive absorption technique. This technique involves the coupling of a pulse of laser light into an optical cavity made up of two highly reflective mirrors (R>99.99%). The extremely high reflectivity of these mirrors allows the laser pulse to travel between them many times. Each time the light pulse hits a mirror a small number of photons are transmitted. This transmitted packet of photons will be directly related to the number remaining in the cavity. Therefore, if a detector were placed behind one of these mirrors, an exponentially decaying signal would be observed. The decay constant for this process is called the ringdown time, τ (τ0 for the empty cavity).
If an absorbing species is placed in the cavity, it will absorb a number of photons on each pass of the light pulse. This would result in a shorter ringdown time compared to the empty cavity (τabsorber< τ0).
By measuring the ringdown time as a function of the laser frequency, an absorption spectrum of various molecules can be observed.
I.
Experimental Setup
II.
Radical Production
We produce our peroxy radicals at room temperature by first making the alkyl radical by photolysis, and then reacting that radical with molecular oxygen to form the desired alkyl peroxy radical. All reaction chemistry takes place in the ringdown cell, and we allow sufficient time for the desired chemistry to happen by delaying the probe beam from the photolysis beam on the order of 50-100μs. Typical precursors used to generate the alkyl radicals by direct photolysis are ketones (i.e. acetone) or halogenated alkanes (i.e. iodomethane). In some cases, we also perform a hydrogen abstraction reaction of the alkane precursor (i.e. propane) using oxalyl chloride (COCl2) to make the alkyl radical, and then react it with oxygen to produce the desired alkyl peroxy radical.
III.
Recent Experimental Results
Our first alkyl peroxy radical successfully studied using Cavity Ringdown Spectroscopy was methyl peroxy (CH3O2). When we record an absorption spectrum in real time, we record two traces simultaneously, excimer on and excimer off.
To visualize the true absorption spectrum, we then subtract these two traces to get the absorption due solely to the radical, and not that attributed to the precursor or water traces present in the cavity.
M. B. Pushkarsky, S. J. Zalyubovsky, and T. A. Miller, J. Chem. Phys.
112, 10695 (2000)
Since methyl peroxy, we have also studied trifluoromethyl peroxy (CF3O2), acetyl peroxy (CH3C(O)O2), and other alkyl peroxy radicals such as ethyl, n- and isopropyl, n-, sec-, t-, and isobutyl peroxies. Our latest completed project was on our first aromatic peroxy species ever studied with this apparatus, phenyl peroxy radical. And we are currently working on pentyl peroxy and its related isomers (C5H11O2).
