Quenching cross section

A S state of BaO by seven visible Ar laser lines, Sakurai etal. studied rotational and vibrational transitions induced by collisions with He, Ar and Nj. From measurements of the pressure-dependent lifetime (r = o = 12 3 /tsec), the authors found for the electronic quenching cross section of the A 2 state by He the value 6 = 0.25. ... 

The relative reactivities of various substances toward mercury triplets are expressed as quenching cross sections. These have been determined both by measuring the efficiency of quenching the 2537 A mercury phosphorescence494 and by Cvetanovic s chemical method employing nitrous oxide.495 In general, quenching cross sections are... 

The quenching cross section for cyclopropane is considerably smaller than those for olefins.497 However, the fact that cyclopropane-de is, if anything, a more effective quencher of triplet mercury than cyclopropane-/ 6497 favors an energy transfer process over a hydrogen abstraction process. Recently Gunning has reported that methylene-cyclobutane is the major high-pressure product in the mercury-sensitized decomposition of spiropentane.509 This result provides very compelling support for the intermediacy of a biradical species. 

Blickensderfer et al. (119) have recently extended the Samson theory to include various vessel geometries under conditions of pure Doppler, pure Lorentz, and Voigt (a combination of Doppler, Lorentz, and natural broadening) line profiles. The quenching cross section can be obtained also by a dynamic method in which the decay of the fluorescence intensity is measured after the exciting light is cut off. If the initial fluorescence intensity is / and the intensity after a lime f is If, we obtain... 

The excited C2H4 further decomposes or is quenched by C2H4. Quenching cross sections of Hg(3P,) atoms by olefins are usually much larger than those by paraffins (256). Primary products and yields for some quenching reactions are given in Table IV 1. 

Lifetimes and quenching cross sections of rotational levels in the B3fl (Ou+) state have been measured by Broyer et al. (153) and by Ornstein and Derr (781). The production of iodine atoms 2P1/2, 2P3/2 was observed by absorption in the vacuum ultraviolet following the flash photolysis of l2 above 2000 A (296). While 2P1/21 atoms are produced it is not certain whether the ratio of the metastable to the ground state population is 1 1. 

The threshold wavelength of incident photons to produce the electronically excited Na(2P) is 2440 A. The quenching cross sections by H2. HC1, C02, and H20 for the sodium D lines have been measured as a function of exciting wavelength above 500"C. The quenching cross sections by HCI and C 02 decrease with an increase of relative velocities between the excited Na atoms and quenching molecules [Hanson (441), Earl et al. (332, 333)]. 

IV-l. Fluorescence Quenching in Atoms Quenching Cross Sections 141... 

Hence quenching collisions per excited atom by the foreign gas M per second, Z, can be expressed by the quenching cross section a1 (cm2 molec ) in analogy with the gas kinetic collision cross section (Note that a is used before to designate the absorption cross section see Section I —8.1.)... 

Ma and Mq arc masses of the atom A and the quenching gas, respectively. Some workers define the quenching cross section as... 

The quenching cross sections for N2, CO, and NO for processes (IV-14) and (IV-15) have been obtained by Horiguchi and Tsuchiya (481) by measuring the steady stale concentrations of Hg(3P,) and (3P0) atoms by optical absorption at 4358 and 4047 A, respectively in the presence of quenching gases. 

Capelle et al. (186) have measured fluorescence lifetimes and quenching cross sections of vibrational levels from v = 1 to 31 of the B3n(0 +) state of Br2 molecules. The lifetimes vary from 1.3 (v = 27) to 0.14 psec (i> = 17). Lifetimes must be much shorter than the radiative life since in this absorption region (5130 to 6260 A) photodissociation is predominant (571). 

The lifetimes and quenching cross sections of rotational levels in the B3n(0u+) state have been measured near the dissociation limit by McAfee and Hozack (671). The observed lifetime is on the order of 3 psec. 

The fluorescence lifetime and quenching cross section of the B3fl(0 + ) state of BrCl near the lowest vibrational level have recently been measured by Wright et al. (1055). The observed lifetime is 18.5 + 3 //sec and the quenching cross section by BrCl is 0.1 x 10 16 cm2. 

This process is well known as quenching of resonance radiation, and the first measurements of quenching cross sections date back to 1926.1 Besides the fundamental importance outlined in the preceding paragraph, quenching processes invoke interest for a number of more practical reasons ... 

A common result of all the experiments is that most molecules quench the alkali resonance radiation very effectively with total cross sections ranging from 10 A2 to over 200 A2. However, if the molecule BC is replaced by a rare-gas atom, the quenching cross sections become very small at thermal energies. They are probably below 10 2 A2 for quenching by helium, neon, argon, krypton, and xenon.55 The latter result is easily understood in terms of Massey s adiabatic criterion.67 If Ar is a characteristic interaction range, v the impact velocity, and AE the energy difference between initial and final electronic states E(3p) and E(3s), respectively, then we must have a Massey parameter... 

Figure 7. Total quenching cross section as function of incident kinetic energy from different experiments and theoretical predictions.

We first discuss some simple diatomic molecules H2, D2, N2, and CO, which are known to be good quenching agents, having total quenching cross sections of around 10 to 40 A2 at the incident energies to... 

Figure 22. Partial quenching cross sections for Na + H2 and Na + D2 (from Figs. 20 and 21) plotted as function of energy transfer A vib rol and alternatively as function of final vibrational state v. ...

Before turning to more complicated systems, we just wish to note that another simple model also yields curiously good agreements with the experimental differential quenching cross sections. Following Wilson and Levine, 105 the vibrational excitation is achieved by a sudden change in the equilibrium molecular distance at the crossing from intermediate ionic... 

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