Orbitally selective chemical reaction in Hg–H2 van der Waals complexes
Authors: Breckenridge W. H. , Jouvet Christophe , Soep Benoit 
Published: July 2002

A new technique is described for probing the reaction dynamics of ‘‘half-collisions’’ in systems where ‘‘full-collision’’ chemical dynamics can also be studied. By selective laser excitation of an atom–molecule van der Waals complex, an electronically excited atom can be created at a known distance from, and with a known orbital symmetry with respect to, the reactive molecule. From spectra of the complex and from detection of nascent products in a state-resolved fashion, not only can a great deal be learned about the dynamics of the half-collision, but comparison can also be made with analogous full-collision dynamical information. Reported here are initial results involving the Hg⋅H2 van der Waals complex. When the Hg (6s 1S0)⋅H2 ground-state complex is excited to the Hg(6p 3P1)⋅H2 complex with frequencies near that of the Hg(6 1S0–6 3P1) free atom transition, the molecular product HgH(X 2Σ+) is readily detected. No fluorescence of the Hg(6p 3P1)⋅H2 complex is observed, nor is Hg(6p 3P0) detected as a major product. The two electronic configurations of the excited Hg(6p 3P1)⋅H2 complex, 3Σ and 3Π (which correspond approximately to axial and perpendicular orientation, respectively, of the p orbital with respect to the freely rotating H2 molecule) exhibit different behavior. The reaction to form HgH (X 2Σ+) via the 3Π complex is ‘‘direct,’’ i.e., occurs within 0.1 ps, since the HgH(X 2Σ+) action spectrum for 3Π excitation is
continuous. In contrast, there is rovibrational structure in the HgH (X2 Σ+) action spectrum for 3Σ excitation, showing that HgH(X 2Σ+) formation in this case is ‘‘indirect,’’ i.e., occurs on a time scale between 2 ps and 1 ns. Furthermore, the HgH (X 2Σ+, v=0) rotational quantum-state distribution from 3Σ complex excitation is bimodal, with a major component quite similar to that resulting from excitation of the 3Π complex, but with a minor component present at low N. Possible explanations of these results, which definitely show orbitally selective chemical reactivity, are discussed. Because the initial total angular momentum of the Hg⋅H2 complex is approximately zero in the cold supersonic jet, the distribution of exit–channel impact parameters could be determined from HgH (X 2Σ+, v=0) rotational state distributions. For 3Π excitation the fairly narrow distribution is peaked at 1.2 Å, and geometrical considerations indicate that energy release into rotation most likely results from the angular dependence of the exit–channel
potential surface of an H–Hg–H species and not from H–H bond-breaking impulsion. The angular dependence could result from transitions from the bent excited triplet to the linear ground-state singlet surface of H–Hg–H. The HgH (X 2Σ+, v=0) initial rotational state distribution from the thermal reaction Hg(6p 3P1) +H2 → HgH(X 2Σ+)+H, measured independently at 300 K, was found to be similar to that for half-collision excitation of Hg⋅H2(3Π), but somewhat broadened. This was interpreted to mean that the thermal reaction proceeds via insertive Π attack of the H–H bond, and that exit-channel forces, rather than initial orbital angular momentum, play the dominant role in determining rotational energy disposal in this reaction. From preliminary measurements of HgH (X 2Σ+, v=1,2) rotational state distributions, it is also proposed that the known Hg(1S)+H+H product channel in the 300 K thermal reaction results merely from the known predissociation of highly rotationally excited HgH (X 2Σ+, v=1,2) produced initially.