Detection and characterization of alkyl peroxy radicals using cavity ringdown spectroscopy

Abstract

Cavity ringdown spectra of theà 2 A −X 2 A electronic transition in the IR are reported for the methyl and ethyl peroxy radicals. Analysis of partially resolved rotational structure for the origin band of the transition provides information about both theà andX states of CH3O2·. An estimate for the absorption cross-section is determined from the CRDS absorption and the rate of radical-radical recombination.Low temperature oxidation of hydrocarbons is a pervasive process in both nature and technology. It is critically important for the environmental quality of our atmosphere. [1][2][3][4][5] Similarly it is critical to the efficiency and fuel economy of internal combustion engines. 6-8 Arguably the most important reaction 7 for low temperature oxidation is the production of peroxy radicals (RO 2 ·) from alkyl radicals (R·), i.e.,There are a number of important alkyl peroxy radicals defined by the nature of the R group, ranging 1 from the simplest species, methyl (R=CH 3 ) peroxy to complex species with R containing at least 8-10 carbon atoms with correspondingly many structural isomers. Because of their significance, much effort 1 has been expended studying the mechanisms and kinetics of peroxy radical production and their subsequent reactions. This work has largely been based upon monitoring of the peroxy radicals via theirB 2 A −X 2 A UV absorption. This is a strong transition, common to all the peroxies, that is centered around 240nm.Unfortunately this transition is broad and unstructured with a half-width of ≈40nm. The quasi-continuum nature of this transition has at least two clear disadvantages. It is unsuitable for obtaining rotational or vibrational information about the radical. Additionally the overlapping of the UV spectra of different RO 2 · radicals makes the identification of a specific alkyl peroxy radical, particularly from a mixture, a significant challenge.Experiments involving the peroxyà 2 A −X 2 A transition in the IR are sparse. There was an early report, 9 using low resolution modulated absorption spectroscopy, of the observation of theÃ−X IR transition of several RO 2 · radicals and more recently a report 10 of the observation of fragmentary spectra of ethyl peroxy using a cw absorption technique. There was also a report 11 of the detection of theÃ−X transition of CH 3 O 2 · by intracavity laser absorption spectroscopy, but few spectroscopic details were given. Based upon the recent observation 12 and analysis 13 of a well resolved spectrum of the hydroperoxy radical, HO 2 ·, we expect thẽ A −X IR transitions of the alkyl peroxy radicals to be well structured and observable using cavity ringdown spectroscopy (CRDS).Historically this IR transition has been viewed as difficult to study because of its small oscillator strength (the cross-section σ is ≈ 2 × 10 −21 cm −2 for the corresponding transition 13 in HO 2 ·) and the near IR spectral region (≈ 7000 − 8000 cm −1 ) in which the transition is located. However CRDS is a powerful technique 14-16 for dealing with these difficulties. T...

Cavity Ringdown Spectroscopy of the à − X̃ Electronic Transition of the CH3C(O)O2Radical

“…Acetyl peroxy radical, To check the accuracy of G2 method, we have performed G2 calculations for numerous peroxy radicals, and the results are present in Table 2. Maximum disagreement between experimental observations 17,20,23 and theoretical predications is around 250 cm −1 , which is well within the error bars reported for the G2 method. 34 In the spectrum one should expect transitions to the origin and exited vibrational levels of the A state from the vibrationless level of the X state.…”

“…It has been observed for several peroxy radicals in early low resolution modulated absorption experiments 17 and intracavity laser absorption studies. 18 It has a relatively small (σ ≈ 10 −21 − 10 −20 cm 2 ) transition 19,20 moment, but cavity ringdown spectroscopy (CRDS) technique has evolved into a powerful tool for spectroscopic observations of weakly absorbing transient species. 21,22 Our group has reported application of the CRDS technique to study the A 2 A − X 2 A transition of peroxy radicals, and sharp, structured spectra for several alkyl peroxy and fluorinated methyl peroxy radicals have been reported.…”

“…21,22 Our group has reported application of the CRDS technique to study the A 2 A − X 2 A transition of peroxy radicals, and sharp, structured spectra for several alkyl peroxy and fluorinated methyl peroxy radicals have been reported. 20,23 Recently, a CW version of CRDS has been applied to study alkyl peroxy radical spectroscopy and kinetics. 24 In this paper we report observation of A 2 A −X 2 A transition of acetyl peroxy radical using our IR extended CRDS apparatus.…”

Near-IR Cavity Ringdown Spectroscopy and Kinetics of the Isomers and Conformers of the Butyl Peroxy Radical

“…The sharp structure that appears at 7382 cm −1 can be attributed to the methyl peroxy radical. 19 Transitions arising from methyl peroxy radical were observed to some degree using both methods of peroxy radical production and with all six of the precursors as well as in prior experiments for the propyl peroxy radicals. 6 Figure 2 shows the CRDS spectrum (top trace) that was recorded using the hydrogen abstraction method of butyl peroxy radical production.…”

“…5,6,8 Briefly the second harmonic light from an Nd:YAG laser pumped a dye laser (20 Hz QuantaRay PRO-270 Nd:YAG, Spectra Physics SIRAH dye laser). The dyes used were DCM, Rhodamine B, and Rhodamine 101.…”