Molecular Light Control in Rare-Gases Matrices: The HCl Case

6.1. Introduction

The light-matter interactions in spectroscopically studied processes such as crystallization, excitation, dissociation, pre-dissociation, and decay should be unitarily induced and observed for open systems. Nevertheless, these processes follow the basic scenarios that an entrapped (guest) molecule in a host lattice/cage may undergo through photonic and potential surface competition, as illustrated in Figure 1. Accordingly, the elementary guest-host phenomena may be listed as follows (Schwentner & Chergui 1992; Apkarian & Schwentner 1999):

Figure 1.

Illustrations of various molecular spectroscopic evolution channels from/within a matrix cage: I) exiting the cage by overcoming its barrier; II) tunneling through the cage barrier; III) fragment stabilization within the cage; IV) recombination in an excited molecular state; V) recombination in high vibrational ground states; adapted and redrawn from Schwentner & Chergui (1992)

• 1.

  Exiting of the guest molecule fragment or ions by overcoming the cage’s barrier, which may occur directly and impulsively or indirectly after scattering at the cage walls (delayed exit);

• 2.

  Guest exiting by tunneling through the cage barrier;

• 3.

  The stabilization of guest-host fragments at an equilibrium position defined by the junction between the repulsive surface and the barrier: this process may eventually be followed by fluorescence;

• 4.

  The cage (germinate) recombines through the excited electronic-vibrational state;

• 5.

  The cage (germinate) recombines through the vibrational-rotational states of the ground state; in both of the latter two cases, chemiluminescence is to be expected from the near UV to the mid–IR range.

This phenomenology is unfolded here for studying halogen acids (XH, X=Cl, F, I) in rare gas matrices (RGM=Ar, Xe, or Kr) due to the peculiar properties that these chemical-physical systems display through the chemically reactive halogens and chemical-inerting noble elements. Especially, the systems Ar/HCl and Xe/HCl will be under focus with both experimentally observed and quantum mechanical models. In addition to the physical-chemical interaction that is revealed, the van der Waals potential behavior and extension is also revealed and explicated for further paradigmatic use of charge-transfer processes and reactions, which will eventually occur at the chemical–biological level.