Dissociative excitation

Second, in the collision term only electron-neutral collisions are considered, because RF plasmas are weakly ionized. The inelastic collisions considered are ionization, dissociation, excitation, and attachment (see also Table II). The crude... 

Merged-beam measurements 23-26 have consistently shown that the measured recombination cross section depends on conditions in the ion source. The authors have ascribed the effect to differing vibrational distributions. In one of the later measurements,16 the Hj vibrational state was inferred from the threshold energy for electron-ion dissociative excitation,... 

J. Troe Prof. Neumark, how well under control do you have energy and angular momentum of the dissociating excited species ... 

Both VUV light and ionizing radiation can lead to the formation of dissociative excited states, e.g. 

Absorption and Emission Spectra of Small Molecules. In diatomic molecules the number of vibrational and rotational levels is small, so that their energy spacing remains relatively large. Their absorption spectra are therefore line spectra which correspond to transitions to stable , associative excited states, but if a dissociative excited state is reached then the absorption spectrum becomes a continuum since such states have no vibrational levels. 

Figure 3.11 Potential energy diagram of a diatomic molecule. E is the potential energy and r the internuclear distance. The theoretical bond dissociation energy is in the ground state and E in the associative excited state Si, while T] is a dissociative excited state...

H2+(He.H.He)H + Collision-induced dissociation excitation functions measured for different v values 85a. 9... 

Because of the rapidity with which the first-formed excited states are usually converted to the Sx or Tx state, most photochemical reactions start from these states. There are exceptions. An obvious one is when an upper dissociative excited state, in which the molecule immediately fragments, is populated.49 Other exceptions occur when a molecule contains two different chromophores. For example, 20 reacts analogously to franj-stilbene (21) from its state but also undergoes photoreduction, a process typical of an n,n state (see p. 719).60... 

D. M. Fedor, R. B. Cody, D. J. Burinsky, B. S. Freiser, and R. G. Cooks, "Dissociative Excitation of Gas-Phase Ions. A. Comparison of Techniques Utilizing Ion Cyclotron Resonance Spectroscopy and Angle-Resolved Mass Spectrometry," Int. J. Mass Spectrom. Ion Proc., 39., 55-64 (1981). 

From inspecting the atomic database of the EIRENE code [31], which is used in many applications to a large number of different tokamaks, including for the ITER design, in particular its collisional-radiative models for molecules, it was clear that matters can be more complicated. The relaxation time for establishing a vibrational distribution of the ground state molecule is comparable to the transport time of the molecule, hence the applicability of local collisional-radiative approximations is questionable. Furthermore, one of the two atoms created in dissociative recombination is electronically excited, and, hence, can be ionized very effectively even at low divertor plasma temperatures (instead of radiative decay). In this case, the whole chain of reactions would be just an enhanced ( molecular activated ) dissociation (MAD, i.e., dissociative excitation of those H]]", which have been produced... 

The rotational temperatures which are obtained from the Q—branch of the v = 0 — v" = 0 transition in the hydrogen spectra of Fig. 4.1 are Trot (X) = 6000 K and 4500 K for H2 and D2, respectively. Evaluation of the v = 1 — v" = 1 transition yields lower temperatures Trot(X) = 2000 K (H2) and Trot(X) = 1500K (D2) typically decreasing with increasing vibrational quantum number. Detailed experimental investigations of laboratory plasmas have shown that the v = 2 — v" = 2 transition should be used preferentially for gas temperature determination. For CH and CD molecules, the dissociative excitation mechanism contributes to the rotational population and therefore, Trot represents the temperature of the excited state only. Calculated spectra of CH and CD bands fit best to the measurements shown in Fig. 4.1 for Trot = 3000 K. 

The upper states of both emission bands can be populated by either direct excitation from the ground state of the corresponding molecule or by dissociative excitation from methane or higher hydrocarbons. Figure 4.9 shows a compilation of emission rate coefficients for the A2A — X 277 CH band (v = 0,1,2, 3 — v" = 0,1, 2, 3) and the C2 Swan band (v = 0 — v" = 0) [41]. There is experimental evidence that for CD the same rate coefficients than for CH can be applied [42]. The emission rate coefficient for direct excitation is several orders of magnitude higher than the emission rate coefficient for dissociative excitation. The dominant excitation mechanism depends on the... 

Fig. 4.9. Emission rate coefficients for direct and dissociative excitation of CH and C2...

The factor r] accounts for the number of emitted Balmer-a photons per molecule, which involves the type of dissociation process, t] = 1 holds for dissociative excitation with the products D°(n = 3)+D° (Is), i.e., only one atom will directly emit Balmer-a radiation whereas the other is set free already as a proton. In the worst case of a pure molecular flux the total deuterium flux would be underestimated by a factor of 2 by determining it from the Balmer-line emissions solely. More details concerning the energies of the dissociated atoms and accompanying heating mechanisms can be found in [46] and below. 

For the dissociative excitation process (19.32), proceeding via purely dissociative (N A )d>ss states, only the cross-sections for X E (v) —> 63F + transitions are available. The purely dissociative states of H2 having N > 4 are not well known from molecular structure calculations, and they energetically lie far away from the ground X1 Xg state. The transitions to them from X1 Sg should be, therefore, much weaker. They can be nevertheless important in producing two excited atomic products at high collision energies. However, the process (19.32) may become important for AT > 2 initial states, and it should be included in the kinetic scheme of H2 CR models. 

---