Laser UV photofragmentation of halogenated molecules. Selective bond dissociation and wavelength-specific quantum yields for excited iodine (2P1/2) and bromine (2P1/2) atoms

Pence, William H.; Baughcum, Steven L.; Leone, Stephen R.

doi:10.1021/j150625a027

Cited by 126 publications

(77 citation statements)

References 5 publications

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“…The rapid and intense spike in Figure 9a at short times (within the 15 µs detector time constant) arises from CH 2 I 2 photolysis and is identified as I 2 (B 3 Π u f X 1 Σ 0g ) emission. 19 Figure 9b shows the I( 2 P 1/2 ) emission time profile under the same photolysis conditions but with the addition of 5.7 × 10 15 molecules/cm 3 of ClN 3 . The time profile shows a prompt I 2 (B 3 Π u ) spike followed by a rise and a slow decay of significantly enhanced I( 2 P 1/2 ) emission over several hundred microseconds due to the collisional pumping by NCl(a 1 ∆).…”

Section: Resultsmentioning

confidence: 99%

Kinetics of NCl(aΔ) via Photodissociation of ClN₃

Henshaw¹,

Herrera²,

Haggquist³

et al. 1997

J. Phys. Chem. A

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Kinetic measurements of NCl(a 1 ∆) metastables produced by the photolysis of chlorine azide (ClN 3 ) at 193 nm are reported. High NCl(a 1 ∆) density (>10 15 molecules/cm 3 ) production facilitates measurements of the NCl(a 1 ∆) + NCl(a 1 ∆) self-annihilation rate. A gas phase transient chemical titration scheme is used to calibrate absolute NCl(a 1 ∆) density. The NCl(a 1 ∆) bimolecular rate coefficient at room temperature is (7.2 ( 0.9 × 10 -12 cm 3 /(molecule s) and has a magnitude very critical to the development of an NCl(a 1 ∆) + I( 2 P 3/2 ) f I*( 2 P 1/2 ) + NCl(X 3 Σ -) chemical laser. Despite the large value of the self-annihilation rate, intense I*( 2 P 1/2 ) emission via transfer from NCl(a 1 ∆) is strongly observed when mixtures of CH 2 I 2 and ClN 3 are photolyzed at 193 nm. Unlike the analogous O 2 (a 1 ∆) + I( 2 P 3/2 ) energy transfer system, the relatively large NCl(a 1 ∆) bimolecular rate constant suggests that NCl(a 1 ∆) transport times must be short and will require small chemical mixing regions for efficient lasing. In addition, the 300 K collisional quenching rate constants of NCl(a 1 ∆) by F 2 , Cl 2 , and Br 2 are (2.5 ( 1.1) × 10 -13 , (2.9 ( 0.6) × 10 -11 , and (1.4 ( 0.2) × 10 -10 cm 3 /(molecule s), respectively.

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Section: Resultsmentioning

confidence: 99%

Kinetics of NCl(aΔ) via Photodissociation of ClN₃

Henshaw¹,

Herrera²,

Haggquist³

et al. 1997

J. Phys. Chem. A

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“…12 In addition, the position and shape of the C-I absorption continuum in CH s CH 2 1 is virtually unchanged when a ,a-hydrogen is substituted with a CI of Br. Further evidence for localized transitions comes from Butler,et al,8,9,26 who showed that excitation near the peak of the supposed C-Br absorption continuum in CH 2 BrI resulted in simple bond fission of only the stronger C-Br bond.…”

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confidence: 99%

Photodissociation of CF2BrCH2I at 248, 266, and 308 nm

Minton

Nathanson

Lee

1987

The Journal of Chemical Physics

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The technique of photofragmentation translational spectroscopy has been used to study the photodissociation of CF2BrCH2I at excitation wavelengths of 248, 266, and 308 nm. The primary photofragments are CF2BrCH2 and either I(2P1/2) or I(2P3/2), although some C–Br bond fission does occur at 248 and 266 nm. A large fraction of the CF2BrCH2 radical product contains enough internal excitation after the primary process to undergo secondary dissociation into CF2CH2 and Br. Secondary dissociation is also observed to take place at 248 and 266 nm via absorption of a photon by the CF2BrCH2 photofragment. By observing the threshold for the spontaneous secondary dissociation process, the reaction enthalpy for CF2BrCH2I→CF2CH2+Br+I, was determined to be 67.5±2 kcal/mol, which leads to: Δ H○f,0 (CF2BrCH2I)=−92.6±2 kcal/mol. The c.m. translational energy distributions were derived for both the I(2P1/2) and I(2P3/2) dissociation channels resulting from primary C–I bond breakage. The I(2P1/2)/I(2P3/2) branching ratios are 3.3, 9.0, and 0.5 for excitation wavelengths of 248, 266, and 308 nm, respectively. The translational energy distributions also reveal that a major fraction of the CF2BrCH2 product radicals are formed with high internal energies, averaging around 50% of the excess energy. The angular distributions of dissociation products with respect to the laser polarization indicate that the primary photodissociation process for the ground and excited state channels at both wavelengths proceeds via a parallel transition—i.e., the transition moment must be nearly parallel to the C–I bond.

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“…Similarly, one could excite the short wavelength band and look for preferential dissociation of the C-Br bond. Earlier, Pence et al (195) had suggested that excitation in the respective wavelength regions did lead to independent dissociation of the molecule. Krajnovich et al (193), using the TOF method and each of the monohalides as a control, were able to show that this was not the case.…”

Section: E Halogenated Ethanesmentioning

confidence: 99%