page_title: Xenon monochloride
section_title: Mechanism
hierarchical_section_title: Xenon monochloride / XeCl exciplex / Synthetic pathways / Collision pathway / Harpoon collisions / Mechanism
caption_attribution_description: Français&#160;: cycleformationxecl
context_page_description: Xenon monochloride is an exciplex which is used in excimer lasers and excimer lamps emitting near ultraviolet light at 308 nm. It is most commonly used in medicine. Xenon monochloride was first synthesized in the 1960s. Its kinetic scheme is very complex and its state changes occur on a nanosecond timescale. In the gaseous state, at least two kinds of xenon monochloride are known: XeCl and Xe
  ₂Cl, whereas complex aggregates form in the solid state in noble gas matrices. The excited state of xenon resembles halogens and it reacts with them to form excited molecular compounds.
context_section_description: Generally, these reactions can describe the result of collisions of the noble gas atoms (Rg) and halogen donors (RX), where X is a halogen atom and R a radical molecule. The products of the reactions depend strongly on the type of rare gas and halogen donor. In our case where Rg = Xe and X = Cl, the nature of the products follows this rule. In some cases, this collision may not provide any halide rare gas.
  The atom Rg and the molecule RX follow when they approach the lowest adiabatic potential and the reaction proceeds by the orbital mechanism controlled at the crossover of the ionic-covalent. The reagents (Rg and RX) approach on a covalent diabatic surface. They then form a complex Rg
  *...RX at a fairly large internuclear distance. Its potential is V(Rg, RX). When the distance becomes sufficiently small, it may be that V(Rg, RX) intersects an ionic potential surface (Rg⁺
  ...RX⁻). The crossover can occur through the transfer of an electron from Rg to RX. This is known as a harpoon mechanism. In this case, the atoms continue on the new surface. This leads to a diffusion reaction and the formation of RgX*.
  Figure 3 shows the process of creating XeCl
  * which involves Rg=Xe and X=Cl. After its transfer, the electron occupies an antibonding orbital of RCl. In the presence of Xe⁺
  , RCl⁻
   splits into R and Cl⁻
  . Xe
  * ions and Cl⁻
   then recombine to form XeCl in states B, C and D because there is no new force between Cl⁻
   and R. The vibrational excitation of XeCl
  * is always important. In total, everything takes place according to the reaction equation:
  Xe* + RCl → XeCl
  *(B,C,D) + R with rate constant of kMX
  However, the competitive formation of XeCl
  * reactions occur before or after the intersection. They correspond to the interactions of the V potential (Rg
  *, RX*) and V (Rg + RX *).
  In general, this situation occurs when the ionic surface is intersected by covalent surfaces where RX is in its lowest excited state. The distribution of output depends on the number and nature of output channels that are possible following collisions. The most common occurs at the intersection of the potential surfaces by a transfer of electronic energy that can cause a dissociation of the excited acceptor:
  Rg
  * + RX → (Rg⁺...RX⁻) → Rg(B,C,D) + RX* with rate constant kET
  Rg
  * + RX → (Rg⁺...RX⁻) → Rg + R + X with rate constant kD
  This pathway tends to become less important as the complexity of RX increases
  It is also possible that the transfer took place in a state that is not correlated to the RX* ion but at very high Rydberg states in the neutral molecule and lying just below the limits of ionization. Critical factors regulating the branching ratios are the potential energies interrelated with the molecular ion (VI), the Rydberg group close to the ionization (VII) or an initial excited atom (VIII). The importance of these pathways increases with the depth of hole V (Rg
  *, RX*).
  When the highly separated asymptotic energy levels are in the order VI > VII > VIII and the potential energies (VII) are attractive, the first failed intersection is encountered when the approach of reacting atoms favors output of (VII) rather than the anionic (VI). Since (VII) has a cationic center that remains strongly linked, it preferentially leads to a transfer of excitation. This is the dissociative excitation reaction:
  Rg
  * + RX → Rg + R* + X or Rg + R + X* with rate constant kDE
  If VIII > VII at long distance, the Penning ionization pathway or associative ionization are possible:
  Penning ionization : Rg
  * + RX → Rg + RX⁺ + e⁻ with rate constant kPI
  Associative ionization: Rg
  * + RX → (RgRX)⁺ + e⁻ with rate constant kAI
  In (VI) bonding with an halogen atom is in principle, weak and atomic transfer is enhanced between Rg and R. This potential thus leads to the formation of the exciplex.
  There are therefore a priori five competitive ways of synthesizing RGX. For XeCl
  * an excited xenon atom collides with a chlorine donor. These five reactions were all observed for various chlorine donors. To quantify the proportion of produced exciplex, it is customary to define the 
caption: Figure 3. Schematic energy diagram of the interaction between Xe and an halogen donor, RX[110] (RX = RCl). The strongly excited state of Xe** highly designates a Rydberg state whose center is Xe+
   (2P1/2); the other with an Xe+
  (2P3/2) center. The dotted area represents a highly populated RX* area close to the ionization level of RX.
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