Stereocontrol of Reactive Encounters Using Polarized Light

Abstract

The reaction geometry of selected species can be controlled by using polarized light even in bulk experiments. One reactant A is generated in a photodissociation process. Its spatial distribution is completely described by the anisotropy parameter β. The other molecular reactant B is excited in a specific rovibrational state. Its spatial distribution is given by the j-and branch-dependent alignment parameter A 0 (2) . The unnormalized probability of an attack of A on B under an angle γ is then given by the simple expression P(γ) ∝ [1 + 1 / 5 βA 0 (2) P 2 (cos γ) P 2 (cos δ)], where δ is the angle between the E B-vectors of the dissociating and of the exciting laser beams. P 2 (x) represents the second Legendre polynom. We have studied the reaction of X + HCN f HX + CN with X ) H, Cl. The attacking H atom is generated in the photodissociation of CH 3 SH at 266 nm, and the chlorine atom is formed in the photolysis of Cl 2 at 355 nm. In both cases the β-parameter is close to -1. In order to align the HCN partner reactant, the first and third vibrational overtone of the CH stretch vibration was excited via the R and P branches. The nascent CN(V)0) product molecules were observed by laser induced fluorescence (LIF). The experimental results prove a preferred linear reaction geometry, i.e. an end-on attack of the X atom on the terminating hydrogen atom of the HCN reactant. However, the angle of acceptance is higher for the Cl + HCN reaction than for the H + HCN one.

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“…see Refs. [5] and [6]. Figure 2 and Table 1 provide examples of energy levels in a typical di-atomic molecule, for a perfect harmonic oscillator, and for an anharmonic oscillator following the Morse potential [5].…”

Polarization properties of coherent VUV light at 125 nm generated by sum-frequency four-wave mixing in mercury

“…Energetic polarized light is also convenient to obtain vectorial information on photodissociative excitation processes through measurement of fluorescence anisotropy of excited fragments [5]. It can be used to control the reaction geometry of small encounters when one of the reactants is generated by a photodissociation process [6]. With circularly polarized radiation, fundamental information about electronic structures are accessible.…”