Translational energy (Et ) spectra of Cl and HCl fragments from vinylchloride, trans-dichloroethylene, cis-dichloroethylene, and 1,1′-dichloroethylene have been measured for the π*←π excitation at 193 nm. Et distribution and angular dependence of the Cl fragment indicate that the two-center dissociation occurs in a time faster than a rotation period and the recoiling organic radical (the counter fragment) is highly vibrationally excited. In dichloroethylenes, the presence of a second channel producing Cl atoms has been confirmed and attributed to the dissociation from the lower (n,σ*) state through a (π,σ*) state. The Et distribution of HCl fragments is nonstatistical and found to converge to null population at an energy less than half of the total available energy. This convergence point coincides with the value of the local available energy for the elimination reaction (activation energy —ΔH0reaction ). The yield of HCl molecules relative to Cl atoms is estimated to be ∼1.1 for vinylchloride. High efficiency of the HCl elimination is attributed to a rapid internal conversion from the (π,π*) state to the lowest (π,σ*) state from which the pathway to the transition state for HCl elimination is opened in ground electronic manifolds.
The quantum yield (QY) of diacetylene in the 193.3-nm photolysis of acetylene has been measured as a function of pressure, decomposition, and added gases. The QY of diacetylene is near unity (0.9 f 0.1) when the decomposition is 1% or less. The QY of CZHZ + hv -CZH + H is 0.3 f 0.1, which is determined by the C2HD yield from the photolysis of C2H2 + C2D6 (or Dz) mixtures. The yield is in good agreement with the QY of H atoms photodissociated from C2Hz; C2H reacts with C2H2 to produce an equivalent amount of diacetylene. The remaining 60-70% diacetylene arises from the metastable acetylene (C2H2**) reacting with ground-state acetylene at 193.3 nm and above 0.1 Torr of acetylene. The quenching of diacetylene formed via C2H2** by various foreign gases at various wavelengths is compared. The quenching order at 193.3 nm is N2 C D2 C H2 C CzD6 C n-C4Hlo. It is postulated that an adduct from CzH2** + C2H2 is initially formed before it dissociates into C~H Z + Hz or C2H + C2H3. Yields of C4H2, CZHD, and C2H2 were measured by FT-IR calibrated with pure samples. The C4H2 yield at 193 nmdoes not change with the addition of N2 up to 600 Torr. The absorption cross section of C2H2 has been measured in the 190-230-nm region. The production of C4H2 was examined at wavelengths below and above thedissociation threshold. The metastable acetylene reactions may be important in haze formation in Titan's atmosphere.
Electron paramagnetic resonance (EPR) on the lowest excited triplet (T1) states of polyphenyl [diphenyl(polyphenylene)] molecules were studied in rigid organic glasses at 77 K. We observed the very interesting differences among their three groups of o-, m-, and p-polyphenyls [diphenyl(poly(1,2-, 1,3-, and 1,4-phenylene)), respectively]. For p-polyphenyls, the |D| value decreases with the increase of the number of the composed benzene rings, whereas it is scarcely changed at about 0.109 cm-1 for m-polyphenyls and at about 0.086 cm-1 for o-polyphenyls except for o-quaterphenyl. Because there are several conformers for o- and m-polyphenyls composed of more than three benzene rings, the |E| values obtained are distinguishable in some cases of m-polyphenyls with planar conformations but could not generally be separable for o-polyphenyls with nonplanar conformations which have changeable twist angles around the C−C bond connecting the adjacent benzene rings. For the quaterphenyls having two different groups, the EPR spectra of o,p- and m,p-quaterphenyls [C6H5−(1,2-C6H4)−(1,4-C6H4)−C6H5 and C6H5−(1,3-C6H4)−(1,4-C6H4)−C6H5, respectively] are relatively close to that of p-terphenyl, whereas that of o,m-quaterphenyl [C6H5−(1,2-C6H4)−(1,3-C6H4)−C6H5] appears approximately to be a superposition of those of o- and m-terphenyls. These relations can be elucidated from the viewpoints of the geometrical and electronic structures. The lifetimes of their T1 states (τ p's) were measured from the decay curves of their EPR B min signals. For p-polyphenyls, the τ p decreases with the increase of the number of the composed benzene rings, whereas it is scarcely changed at about 5.0 s for m-polyphenyls and at about 2.2 s for o-polyphenyls except for o-terphenyl. These trends are generally similar to those of the |D| values.
Cyanoacetylene (CA) is an important minor constituent in the Titan atmosphere and is present in the interstellar medium. The absorption cross section of CA has been measured in the region from 190 to 255 nm with a resolution of 1 nm. The photochemistry of CA at 193.3 nm has been studied using a quadrupole mass spectrometer and a Fourier transform infrared spectrometer for product analysis. From the photolysis of HC3N−D2 and HC3N−CD4 mixtures and a plateau value of 0.3 for the quantum yield (QY) of DC3N (C3N + D2 → DC3N + D), it is concluded that the main dissociation process is HC3N + hν → H + C3N with a QY of 0.30 ± 0.05 and a minor process is HC3N + hν → C2H + CN with a QY equal to or less than 0.02. The remaining process is the formation of metastable CA (a triplet or carbene). The photolysis of CA induces a noticeable pressure decrease and a concomitant formation of a mist. The QY of CA disappearance is 4.5 ± 0.5, which is much higher than that of diacetylene (QY = 2.0 ± 0.5) and of acetylene (QY = 2.3). The rapid mist formation in CA may explain a haze observed in the Titan atmosphere. A detailed mist formation process is not known. The C3N radical disappears partially by C3N + HC3N → C6N2 + H and 2C3N → C6N2. To explain the formation of minor products, HCN, C2H2, HC5N, and C4N2, two processes involving an unspecified CA metastable state or states may be proposed: HC3N* + HC3N → HC5N + HCN and C4N2 + C2H2.
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