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Notes: The A 2A1−X 2E fluorescence of CH3O in solid Ar in the wavelength range 310–420 nm has been studied either by simultaneous laser photolysis and excitation of CH3ONO in the Ar matrix or by laser excitation of the products deposited from the reactions of microwave-discharged CF4 with CH3OH diluted by Ar. The spectrum showed an extensive progression in C–O stretching (ν3). The zero-phonon lines of 12CH3O and 13CH3O yielded unambiguous vibrational assignments with ν00=31 291, ω‘e=1051, and ω‘ex‘e=6.5 cm−1 for 12CH3O. Observation of several weak combination bands also yielded ν‘2=1356, ν‘4=2758, and ν‘5=1406 cm−1. The laser excitation spectra in the 273–322 nm region also exhibited an intense progression in C–O stretching with ω'e=657 and ω'ex'e=4.4 cm−1 for 12CH3O. Additional wave numbers ν'2=1308 and ν'5=1410 cm−1 were also obtained from the combination bands.

Notes: The dimeric ClO has been generated in a gaseous discharge–flow system at 220 K and 10–30 Torr by reacting atomic Cl with O3, ClOCl or OClO, and reacting atomic O with ClOCl or OClO, in a stream of Ar. The gas mixture in the flow tube was sampled through a pinhole and condensed on a CsI window maintained at 12 K. The infrared absorption spectrum of the matrix formed after 30–120 min deposition was then recorded. Absorption lines at 752.6, 649.8, and 647.6 cm−1 have been assigned to ClOOCl, and previous assignments for dimeric ClO seem to be in error. The results are in excellent agreement with recent ab initio calculations. Under our experimental conditions, no other isomer of Cl2O2 was observed.

Notes: The radiative lifetimes of the v(triple-prime)3 = 0–2 levels of the CH3S (A˜ 2A1) radical have been determined by means of the laser-induced fluorescence technique. CH3S was generated by either laser photolysis of CH3SSCH3 (both in cell and in supersonic expansion) or the reaction of F and CH3SH in a discharge-flow system. The radiative lifetimes for v(triple-prime)3 = 0, 1, and 2 levels are 1105±25, 910±20, and 250±10 ns, respectively, greater than previously reported values. The rate coefficients for the v(triple-prime)3 = 0, 1, and 2 levels for quenching by CH3SSCH3 have also been determined to be 7.8, 7.8, and 16.5×10−10 cm3 molecule−1 s−1, respectively.

Notes: The S2 emission in the red and near-infrared regions has been reinvestigated using the laser-induced emission technique. Four progressions of S2 in solid Ar were observed in the emission spectra following excitation in the UV region with a pulsed Nd-YAG laser system. 34S-isotopic shifts allowed the determination of spectroscopic parameters for these progressions. Among them, ν00=19 757 and 15 417 cm−1 for the two progressions with distinct zero-phonon lines (ZPL); ν00=19 384 and 15 003 cm−1 for the other two with no ZPL, respectively. The progressions are assigned as the c 1Σ−u→X 3Σ−g and the c 1Σ−u →a 1Δg transitions of S2 in two matrix sites, respectively. The data yield the T0 values for the c 1Σ−u and the a 1Δg states. The relative intensities of these four progressions varied with excitation wavelengths and isotopic species.

Notes: The laser-induced fluorescence technique has been employed to study the first excited electronic state A˜ 2A1 of CH3O. Vibrational levels v'3 =0–7 of the A˜ state were excited and fluorescence decay rates were measured. Compared to a value 2.2±0.2 μs at v3=0, a significant decrease of decay rate was found at 277.6 nm (0.89 μs, v'3=7). Excitation to higher levels further reduced the lifetime. The predissociation threshold was estimated to be slightly above 276 nm (36 220 cm−1). The electronic quenching rates of the A˜ state (v'=0–2) by CH3OH, O2, NO2, and N2 have also been determined and are compared with those of OH in the A˜ state.

Notes: Following photodissociation of vinyl chloride at 193 nm, fully resolved vibration-rotational emission spectra of HCl in the spectral region 2000–3310 cm−1 are temporally resolved with a step-scan Fourier-transform spectrometer. Under improved resolution and sensitivity, emission from HCl up to v=7 is observed, with J〉32 (limited by overlap at the band head) for v=1–3. All vibrational levels show bimodal rotational distribution with one component corresponding to ∼500 K and another corresponding to ∼9500 K for v≤4. Vibrational distributions of HCl for both components are determined; the low-J component exhibits inverted vibrational population of HCl. Statistical models are suitable for three-center (α, α) elimination of HCl because of the loose transition state and a small exit barrier for this channel; predicted internal energy distributions of HCl are consistent but slightly less than those observed for the high-J component. Impulse models considering geometries and displacement vectors of transition states during bond breaking predict substantial rotational excitation for three-center elimination of HCl but little rotational excitation for four-center (α, β) elimination; observed internal energy of the low-J component is consistent with that predicted for the four-center elimination channel. Rate coefficients 33.8 and 4.9×1011 s−1 for unimolecular decomposition predicted for three-center and four-center elimination channels, respectively, based on Rice-Ramsberger-Kassel-Marcus theory are consistent with the branching ratio of 0.81:0.19 determined by counting vibrational distribution of HCl to v≤6 for high-J and low-J components. Hence we conclude that observed high-J and low-J components correspond to HCl (v, J) produced from three-center and four-center elimination channels, respectively. © 2001 American Institute of Physics.

Notes: New species cis- and trans-OSNO, designated c-OSNO and t-OSNO, respectively, are produced and identified with infrared absorption spectra when an argon or nitrogen matrix containing OCS and NO2 is irradiated with laser emission at 248 nm. Lines at 1156.1 and 1454.4 cm−1 are assigned to c-OSNO and those at 1178.0 and 1459.0 cm−1 are assigned to t-OSNO in solid N2. Lines at 1154.9 and 1450.8 cm−1 are assigned to c-OSNO and those at 1181.2 and 1456.0 cm−1 are assigned to t-OSNO in solid Ar; further lines associated with minor matrix sites are identified. Assignments of spectral lines are based on results of both experiments with 15N- and 18O-isotopic substitution and theoretical calculations using density-functional theories, B3LYP with an aug-cc-pVTZ basis set; these calculations predict the geometry, energy, vibrational frequencies, and infrared intensities of SNO2 as four isomers: C2v-SNO2, t-SONO, t-OSNO, and c-OSNO, in increasing order of stability. Mechanisms are proposed to rationalize that c-OSNO and t-OSNO, rather than t-SONO or C2v-SNO2, are produced from irradiated matrices containing OCS and NO2, and that no reaction product is observed in an Ar matrix containing CS2 and NO2 after irradiation at 193 nm. © 2001 American Institute of Physics.

Notes: The reaction Cl(2P)+CH4 was initiated on laser irradiation of a flowing mixture containing Cl2, CH4, and Ar at 355 nm; reaction products were monitored with a step-scan time-resolved Fourier-transform absorption spectrometer coupled with a multipass absorption cell. Not only loss of CH4 but also production of HCl, CH3Cl, highly rotationally excited CH4 [designated as CH4(J*)], and vibrationally excited CH4 (v2=1 or v4=1), designated as CH4(v*), was observed after laser irradiation. Absorption lines of CH4(J*) and CH4(v*) are assigned according to published spectral parameters. Rates of formation and decay of CH4(v*) are derived on fitting observed temporal profiles with a simple kinetic model. A bimolecular rate coefficient for formation of CH4(v*) is determined to be (1.1±0.2)×10−14 cm3 molecule−1 s−1, nearly identical to that reported for the reaction Cl+CH4. Experimental evidence indicates that the reaction Cl+CH4 is rate determining to formation of CH4(v*). CH4(v*) is likely produced through energy transfer from vibrationally excited CH3Cl that is produced via secondary reactions. A rate coefficient for relaxation of CH4* by collision with Ar is determined to be (2.2±0.1)×10−15cm3 molecule−1 s−1, consistent with previous results. The proportion of CH4(v*) in the system is estimated to be ∼1.4% in CH4. According to theoretical calculations reported previously, the rate coefficient for the reaction Cl+CH4(v*) is much greater than that for Cl+CH4 at 298 K, especially at low temperatures (10–235 times at 200 K); formation of CH4(v*) in the Cl+CH4 system can thus explain why rate coefficients determined previously through flash photolysis near 220 K are ∼20% greater than those determined in a discharge-flow system. © 2001 American Institute of Physics.

Notes: Following photodissociation of vinyl fluoride (CH2CHF) and vinyl bromide (CH2CHBr) at 193 nm, fully resolved vibration–rotational emission spectra of HF and HBr in spectral regions 3050–4900 and 2000–2900 cm−1, respectively, are temporally resolved with a step–scan Fourier transform spectrometer. With a data acquisition window 0–5 μs suitable for spectra with satisfactory ratio of signal-to-noise, emission from HX (with X = F or Br) up to v=6 is observed. All vibrational levels show bimodal rotational distributions. For CH2CHF, these two components of HF have average rotational energies ∼2 and 23 kJ mol−1 and vibrational energies ∼83 and 78 kJ mol−1, respectively; the values are corrected for small quenching effects. For CH2CHBr, these two components of HBr correspond to average rotational energies ∼4 and 40 kJ mol−1, respectively, and similar vibrational energies ∼68 kJ mol−1. The separate statistical ensemble (SSE) model is suitable for three-center (α, α) elimination of HX because of the loose transition state and a small exit barrier for this channel; predicted vibrational energy distributions of HX are consistent with those observed for the high-J component. An impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts substantial rotational excitation for three-center elimination of HX but little rotational excitation for four-center (α, β) elimination; observed rotational energies of low-J and high-J components are consistent with those predicted for four-center and three-center elimination channels, respectively. The model also explains why observed rotational energy of HF produced via three-center elimination of CH2CHF is smaller than that of HCl from CH2CHCl. Ratios of rate coefficients (0.66:0.34 and 0.88:0.12) predicted for three-center or four-center elimination channels based on Rice–Ramsberger–Kassel–Marcus theory are consistent with estimated branching ratios ∼0.75:∼0.25 and ∼0.81:0.19 determined based on counting vibrational distribution of HF and HBr, respectively, to v≤5 for high-J and low-J components and considering possible quenching effects within 5 μs. Hence we conclude that, similar to photolysis of CH2CHCl, observed high-J and low-J components correspond to HX (v,J) produced from three-center and four-center elimination channels, respectively. The results are compared with those from photolysis of vinyl chloride at 193 nm. © 2001 American Institute of Physics.

Notes: Visible chemiluminescence in the 4300–6600 A(ring) region was observed from the Ca+N2O and Ca+O3 reactions in argon matrix. The broad bands at 6066 and 6246 A(ring) were ascribed to the d 3Δ (or D 1Δ)→a 3Π and the D 1Δ→A' 1Π transitions of CaO, respectively. A weaker band at 6416 A(ring) (15 586 cm−1) was assigned as emission from the previously unobserved c 3Σ+ state to the a 3Π state of CaO. Emission due to Ca atoms in the 1P(4s4p)→1S(4s2) and the 3P(4s4p)→1S(4s2) transitions was also observed as sharp peaks at 4231 and 6575 A(ring), respectively. The atomic emitters were excited by energy transfer from CaO*, most probably from the D, d, and c states. The Ca(3P) also receives population via fast relaxation from the 1P state.