Thermal decomposition of t-butyl hydroperoxide and di-t-butyl peroxide was investigated using flash pyrolysis (in a short reaction time of <100 μs) and vacuum-ultraviolet (λ = 118.2 nm) single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS) at temperatures up to 1120 K and quantum computational methods. Acetone and methyl radical were detected as the predominant products in the initial decomposition of di-t-butyl peroxide via O–O bond fission. In the initial dissociation of t-butyl hydroperoxide, acetone, methyl radical, isobutylene, and isobutylene oxide products were identified. The novel detection of the unimolecular formation of isobutylene oxide, as supported by the computational study, was found to proceed via a roaming hydroxyl radical facilitated by a hydrogen-bonded intermediate. This new pathway could provide a new class of reactions to consider in the modeling of the low temperature oxidation of alkanes.

Ultraviolet (UV) photodissociation dynamics of jet-cooled 1-propenyl radical (CHCHCH3) were investigated at the photolysis wavelengths from 224 to 248 nm using high-n Rydberg atom time-of-flight (HRTOF) technique. The 1-propenyl radicals were produced from 193 nm photolysis of 1-chloropropene and 1-bromopropene precursors. The photofragment yield (PFY) spectra of the H atom product have a broad peak centered at 230 nm. The H + C3H4 product translational energy P(ET) distribution’s peak near ∼8 kcal/mol, and the fraction of average translational energy in the total available energy, ⟨fT⟩, is nearly a constant of ∼0.12 from 224 to 248 nm. The H atom product has an isotropic angular distribution with the anisotropy parameter β ≈ 0. Quasiclassical trajectory calculations were also carried out using an ab initio ground-state potential energy surface for dissociation of 1-propenyl at the excitation energy of 124 kcal/mol (230 nm). The calculated branching ratios are 60% to the methyl + acetylene products, 16% to H + propyne, 4% to H + allene, and 1% to H + cyclopropene. The experimental and calculated P(ET) distributions of the H + C3H4 products at 230 nm are in a qualitative agreement, suggesting that the H + propyne dissociation is the main H atom product channel. The calculated dissociation time scale on the ground electronic state is ∼1 ps, shorter than but close to the time scale of >10 ps for the overall UV photodissociation implied by the isotropic H atom product angular distribution. The UV photodissociation mechanism of 1-propenyl can be described as unimolecular decomposition of hot 1-propenyl radical on the ground electronic state following internal conversion from the electronically excited states of 1-propenyl.

Ultraviolet (UV) photodissociation dynamics of jet-cooled allyl radical via the B̃2A1(3s), C̃2B2(3py), and Ẽ2B1(3px) electronically excited states are studied at the photolysis wavelengths from 249 to 216 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance-enhanced multiphoton ionization (REMPI) techniques. The photofragment yield (PFY) spectra of the H atom products are measured using both allyl chloride and 1,5-hexadiene as precursors of the allyl radical and show a broad peak centered near 228 nm, whereas the previous UV absorption spectra of the allyl radical peak around 222 nm. This difference suggests that, in addition to the H + C3H4 product channel, another dissociation channel (likely CH3 + C2H2) becomes significant with increasing excitation energy. The product translational energy release of the H + C3H4 products is modest, with the P(ET) distributions peaking near 8.5 kcal/mol and the fraction of the average translational energy in the total excess energy, ⟨fT⟩, in the range 0.22–0.18 from 249 to 216 nm. The P(ET)’s are consistent with production of H + allene and H + propyne, as suggested by previous experimental and theoretical studies. The angular distributions of the H atom products are isotropic, with the anisotropy parameter β ≈ 0. The H atom dissociation rate constant from the pump–probe study gives a lower limit of 1 × 108/s. The dissociation mechanism is consistent with unimolecular decomposition of the hot allyl radical on the ground electronic state after internal conversion of the electronically excited state.

Peroxy (HO2 and RO2) radicals are important intermediates in tropospheric oxidation of hydrocarbons, and their accurate atmospheric measurements remain challenging. In this work, the peroxy radical chemical amplification (PERCA) method was combined with cavity ringdown spectroscopy (CRDS) to develop a dual-channel instrument for measurements of atmospheric peroxy radicals. In the amplification channel, the peroxy radicals were converted in an excess amount of NO and CO into a higher level of NO2 and measured along with the background NO2, while in the reference channel, only the background NO2 (ambient NO2 and NO2 converted from O3 reaction with NO) was monitored. The NO2 levels from both channels were measured simultaneously at a high time resolution (∼1 s) using two identical CRDS systems with one 408.5-nm diode laser, and their difference gave the amplified NO2 from PERCA. The peroxy radical concentration was obtained from the amplified NO2 and the calibrated amplification factor or chain length (CL). The optimized CL was 190 ± 20 (1σ) using laboratory-generated HO2 and CH3O2 radical sources. The detection sensitivity was 4 ppt/10 s (3σ). Ambient measurements in Riverside, CA were carried out. This dual-channel diode-laser PERCA-CRDS instrument was compact and capable of providing real-time, in situ, and sensitive measurements of atmospheric peroxy radicals with fast time response.

•The initial decomposition step of TMS and TMG is loss of a methyl radical to form Si(CH3)3 and Ge(CH3)3, respectively.•While Si(CH3)3 can lose a second methyl radical to form :Si(CH3)2, it could also undergo sequential H loss, which is reported for the first time in this study.•H loss and CH4 loss from :Si(CH3)2 are more competitive than the methyl loss.•Ge(CH3)3 undergoes sequential methyl loss to :Ge(CH3)2, Ge(CH3), and Ge.Thermal decomposition of tetramethylsilane (TMS) and tetramethylgermane (TMG) was studied on a short time scale of 20–100 μs using flash pyrolysis vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). Primary decomposition of TMS and TMG occurred via loss of a methyl radical to form Si(CH3)3 and Ge(CH3)3, respectively. Both the Si(CH3)3 and Ge(CH3)3 radicals underwent secondary loss of a second methyl radical to form :Si(CH3)2 and :Ge(CH3)2, respectively. A previously unobserved secondary decomposition process in TMS involving loss of H atom from Si(CH3)3 followed by elimination of H2 to form SiC3H8, SiC3H6, and SiC3H4 was also identified. Sequential loss of the third and fourth methyl radical with significant formation of Ge and Ge2 was observed in the TMG pyrolysis. Loss of a third methyl radical in the TMS pyrolysis was not significant, while Si and SiC products were possibly produced. Secondary reactions of methyl to form unsaturated CxHy species, particularly in the TMG decomposition, were also observed.

Accurate measurement of optical extinction of atmospheric aerosols is important for quantifying the direct climate effects of aerosols. A portable cavity ringdown spectrometer utilizing a modulated multimode blue diode laser (linewidth ∼0.2 nm) is developed to measure the aerosol optical extinction. Laboratory generated ammonia sulfate particles (<1 μm in diameter) are characterized, with good agreements between the experimental measurements and Mie theory calculations. An optical extinction detection sensitivity of 0.24 Mm−1 (1σ) is achieved. Measurements of ambient aerosols are also carried out. This study demonstrates the feasibility of a compact, multimode diode laser cavity ringdown spectrometer for sensitive measurements of the optical extinction of atmospheric aerosols.

Ultraviolet (UV) photodissociation dynamics of jet-cooled o-pyridyl radical (o-C5H4N) was studied in the photolysis region 224–246 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The o-pyridyl radicals were produced from 193 nm photolysis of 2-chloropyridine and 2-bromopyridine precursors. The H-atom photofragment yield (PFY) spectrum contains a broad peak in this wavelength region and reveals the UV absorption feature of o-pyridyl for the first time. The translational energy distributions of the H-atom loss product channel, P(ET)’s, peak at ∼7 kcal/mol, and the fraction of average translational energy in the total excess energy, ⟨fT⟩, is nearly constant at ∼0.18 in the region 224–242 nm. The P(ET) distribution indicates the production of the lowest energy dissociation products, H + cyanovinylacetylene. The H-atom product angular distribution is isotropic. The dissociation mechanism is consistent with unimolecular dissociation of the hot o-pyridyl radical to H + cyanovinylacetylene after internal conversion from the electronically excited state.

Sulfur dioxide (SO2) is a major air pollutant that can contribute to the production of particulate sulfate and increase the acidity in the environment. SO2 is detected by cavity ring-down spectroscopy (CRDS) utilizing the SO2 absorption in the 308 nm region. A ferrous sulfate scrubber and a sodium carbonate annular denuder are used to reduce background interferences and to obtain quantitative values of SO2. The method is characterized using SO2 standards in the laboratory and compared to a commercial pulsed fluorescence analyzer (PFA). A limit of detection of 3.5 ppb/10 s (S/N = 2) is demonstrated. Ambient measurements are attempted to demonstrate this technique.

Ultraviolet (UV) photodissociation dynamics of jet-cooled benzyl radical via the 42B2 electronically excited state is studied in the photolysis wavelength region of 228 to 270 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, H-atom photofragment yield (PFY) spectra are obtained using ethylbenzene and benzyl chloride as the precursors of benzyl radical, and they have a broad peak centered around 254 nm and are in a good agreement with the previous UV absorption spectra of benzyl. The H + C7H6 product translational energy distributions, P(ET)s, are derived from the H-atom TOF spectra. The P(ET) distributions peak near 5.5 kcal mol−1, and the fraction of average translational energy in the total excess energy, 〈fT〉, is ∼0.3. The P(ET)s indicate the production of fulvenallene + H, which was suggested by recent theoretical studies. The H-atom product angular distribution is isotropic, with the anisotropy parameter β ≈ 0. The H/D product ratios from isotope labeling studies using C6H5CD2 and C6D5CH2 are reasonably close to the statistical H/D ratios, suggesting that the H/D atoms are scrambled in the photodissociation of benzyl. The dissociation mechanism is consistent with internal conversion of the electronically excited benzyl followed by unimolecular decomposition of the hot benzyl radical on the ground state.

Thermal decomposition of the oxygenated fuel component tert-amyl methyl ether (TAME) has been studied by flash pyrolysis up to 1250 K in a 20–100 μs time scale. Pyrolysis was followed by supersonic expansion to isolate intermediates and products, which are monitored by vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). The species detected, such as CH3, C2H4, C2H5, C4H8, C5H10, C3H6O, and C4H8O, show competition between molecular elimination and bond fission pathways. The alkenes 2-methyl-1-butene (1) and 2-methyl-2-butene (2), the primary molecular elimination products of TAME, were separately pyrolyzed to evaluate the extent of secondary decompositions, as were the ketones (acetone and 2-butanone) produced by losses of two alkyl radicals. While vicinal elimination of methanol from TAME to form 1 and 2 in an approximate 3:1 ratio begins around 600 K and continues to dominate at higher temperatures, homolysis of TAME to form radicals onsets >825 K, yielding more acetone than 2-butanone. Contributions from secondary dissociations of the ketone and alkene products are evaluated.Graphical abstractResearch highlights▶ Molecular elimination is the predominant unimolecular dissociation of neutral TAME below 1000 K. ▶ MeOH elimination from neutral TAME gives 2-methyl-1-butene and 2-methyl-2-butene in a ≈3:1 ratio. ▶ 118.2 nm photoionization shows that neutral TAME begins to pyrolyze to CH3 and C2H5 radicals >850 K. ▶ Ketones from successive loss of 2 radicals from neutral TAME are detected at the onset of homolysis. ▶ Acetone and butanone (ratio ≈2:1) give photoion intensities less than those of C2H5 or CH3 radicals.

The peroxy radical chemical amplification (PERCA) method is combined with cavity ringdown spectroscopy (CRDS) to detect peroxy radicals (HO2 and RO2). In PERCA, HO2 and RO2 are first converted to NO2 via reactions with NO, and the OH and RO coproducts are recycled back to HO2 in subsequent reactions with CO and O2; the chain reactions of HO2 are repeated and amplify the level of NO2. The amplified NO2 is then monitored by CRDS, a sensitive absorption technique. The PERCA−CRDS method is calibrated using a HO2 radical source (0.5−3 ppbv), which is generated by thermal decomposition of H2O2 vapor (permeated from 2% H2O2 solution through a porous Teflon tubing) up to 600 °C. Using a 2-m long 6.35-mm o.d. Teflon tubing as the flow reactor and 2.5 ppmv NO and 2.5−10% vol/vol CO, the PERCA amplification factor or chain length, Δ[NO2]/([HO2]+[RO2]), is determined to be 150 ± 50 (90% confidence limit) in this study. The peroxy radical detection sensitivity by PERCA−CRDS is estimated to be ∼10 pptv/60 s (3σ). Ambient measurements of the peroxy radicals are carried out at Riverside, California in 2007 to demonstrate the PERCA−CRDS technique.

Ultraviolet (UV) photodissociation dynamics of the SD radical in vibrationally ground and excited states (X2Π3/2, v″ = 0–5) are investigated in the photolysis wavelength region of 220 to 244 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The UV photodissociation dynamics of SD (X2Π3/2) from v″ = 0–5 are similar to each other and to that of SH studied previously. The anisotropy parameter of the D-atom product is ∼−1; the spin–orbit branching fractions of the S(3PJ) products are essentially constant, with an average S(3P2):S(3P1):S(3P0) = 0.51:0.37:0.12. The UV photolysis of SD is a direct dissociation from the repulsive 2Σ− state following the perpendicular 2Σ−–X2Π excitation. The S(3PJ) product fine-structure state distributions approach that in the sudden limit dissociation on the single repulsive 2Σ− curve, but they are also affected by nonadiabatic couplings among the repulsive 4Σ−, 2Σ−, and 4Π states. A bond dissociation energy D0(S–D) = 29660 ± 25 cm−1 is obtained.

The thermal decomposition of ethyl and propyl iodides, along with select isotopomers, up to 1300 K was performed by flash pyrolysis with a 20−100 μs time scale. The pyrolysis was followed by supersonic expansion to isolate the reactive intermediates and initial products, and detection was accomplished by vacuum ultraviolet single photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). The products monitored, such as CH3, CH3I, C2H5, C2H4, HI, I, C3H7, C3H6, and I2, provide for the simultaneous and direct observation of molecular elimination and bond fission pathways in ethyl and propyl iodides. In the pyrolysis of ethyl iodide, both C−I bond fission and HI molecular elimination pathways are competitive at the elevated temperatures, with C−I bond fission being preferred; at temperatures ≥1000 K, the ethyl radical products further dissociate to ethene + H atoms. In the pyrolysis of isopropyl iodide, both HI molecular elimination and C−I bond fission are observed and the molecular elimination channel is more important at all the elevated temperatures; the isopropyl radicals produced in the C−I fission channel undergo further decomposition to propene + H at temperatures ≥850 K. In contrast, bond fission is found to dominate the n-propyl iodide pyrolysis; at temperatures ≥950 K the n-propyl radicals produced decompose into methyl radical + ethene and propene + H atom. Isotopomer experiments characterize the extent of surface reactions and verify that the HI molecular eliminations in ethyl and propyl iodides proceed by a C1, C2 elimination mechanism (the 1,2 intramolecular elimination).

Ultraviolet (UV) photodissociation dynamics of jet-cooled propargyl (C3H3) radical is studied in the photolysis wavelength region of 230 to 250 nm with high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, the photofragment yield (PFY) spectra of the H + C3H2 product channel are obtained by using propargyl chloride, allene, and propyne as precursors of the C3H3 radicals, and they have a broad peak centered near 240 nm and are in good agreement with the previous UV absorption spectrum of C3H3 by Fahr et al. The H + C3H2 product translational energy distributions, P(ET)’s, are obtained from all three precursors and are essentially the same. The P(ET) distributions peak at ∼5 kcal/mol, and the fraction of average translational energy in the total excess energy, ⟨fT⟩, is ∼0.3. The H-atom product angular distribution is isotropic, with the anisotropy parameter β ≈ 0. The dissociation mechanism is consistent with internal conversion of the electronically excited propargyl followed by unimolecular decomposition on the ground state. Our study supports the previously observed UV absorption spectrum of propargyl near 240 nm by Fahr et al. and is in general agreement with the results in the UV photodissociation of propargyl by the groups of Chen and Neumark, but disagrees with the recent theoretical calculations by Eisfeld.

Ultraviolet (UV) photodissociation dynamics of jet-cooled benzyl radical via the 42B2 electronically excited state is studied in the photolysis wavelength region of 228 to 270 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, H-atom photofragment yield (PFY) spectra are obtained using ethylbenzene and benzyl chloride as the precursors of benzyl radical, and they have a broad peak centered around 254 nm and are in a good agreement with the previous UV absorption spectra of benzyl. The H + C7H6 product translational energy distributions, P(ET)s, are derived from the H-atom TOF spectra. The P(ET) distributions peak near 5.5 kcal mol−1, and the fraction of average translational energy in the total excess energy, 〈fT〉, is ∼0.3. The P(ET)s indicate the production of fulvenallene + H, which was suggested by recent theoretical studies. The H-atom product angular distribution is isotropic, with the anisotropy parameter β ≈ 0. The H/D product ratios from isotope labeling studies using C6H5CD2 and C6D5CH2 are reasonably close to the statistical H/D ratios, suggesting that the H/D atoms are scrambled in the photodissociation of benzyl. The dissociation mechanism is consistent with internal conversion of the electronically excited benzyl followed by unimolecular decomposition of the hot benzyl radical on the ground state.

Ultraviolet (UV) photodissociation dynamics of the SD radical in vibrationally ground and excited states (X2Π3/2, v″ = 0–5) are investigated in the photolysis wavelength region of 220 to 244 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The UV photodissociation dynamics of SD (X2Π3/2) from v″ = 0–5 are similar to each other and to that of SH studied previously. The anisotropy parameter of the D-atom product is ∼−1; the spin–orbit branching fractions of the S(3PJ) products are essentially constant, with an average S(3P2):S(3P1):S(3P0) = 0.51:0.37:0.12. The UV photolysis of SD is a direct dissociation from the repulsive 2Σ− state following the perpendicular 2Σ−–X2Π excitation. The S(3PJ) product fine-structure state distributions approach that in the sudden limit dissociation on the single repulsive 2Σ− curve, but they are also affected by nonadiabatic couplings among the repulsive 4Σ−, 2Σ−, and 4Π states. A bond dissociation energy D0(S–D) = 29660 ± 25 cm−1 is obtained.