Quasiclassical Trajectory Study of Mg(3s3pP₁) + H₂ Reaction on Fitted ab Initio Surfaces

Abstract

Quasi-classical trajectory calculations for the reaction of Mg(3s3p 1 P 1 ) with H 2 are performed on two potential energy surfaces (PES), the excited state 1 A′ (or 1 B 2 in the C 2V symmetry) in the entrance channel and the ground state 1 A′ (or 1 A 1 ) in the exit channel. A many-body expansion procedure is adopted for the construction of the analytical fit functions from the ab initio results. The title reaction involves a nonadiabatic transition between the two potential surfaces. For simplicity, the transition probability is assumed to be unity when the trajectory goes through the region of surface crossing and changes to the lower surface. The calculated total collisional deactivation and reaction cross sections decrease with the increase of translational collision energy. The calculated rotational product distributions are characterized by a bimodal feature both for the MgH V ) 0 and 1 states. The trend of bimodality is consistent with the observation reported in experimental studies. Our inspection of individual trajectories reveals that the low-rotational and high-rotational populations are caused by two distinct reaction pathways. This observation supports our previous expectation for the microscopic branching via the PES anisotropy. The angular product distribution indicates that the reaction proceeds predominantly via a linear collision complex. An increase of the collision energy from 2.026 to 8.104 kcal/ mol has resulted in a shift of the distribution toward forward direction. The vibrational product distribution tends to decrease with the quantum numbers. The ratio of MgH(V ) 1) to MgH(V ) 0) yields a value of ∼0.3, which is nevertheless underestimated as compared with the observation of 0.7 ( 0.2. The reasons for the discrepancy are also discussed.

Reaction dynamics of Mg(4 1S, 3 1D2) with H2: Harpoon-type mechanism for highly excited states

“…Therefore, the Mg(3 1 P 1 ) -H 2 radius for an effective collisional deactivation can be reasonably estimated to be ϳ1.7 Å from the crossing region between the 1 1 B 2 and the ground surface. 9,12 The Mg(4 1 S 0 ) -H 2 and Mg(5 1 S 0 ) -H 2 radii, estimated from the ionic-covalent crossings, are 2.75 and 3.5 Å, the same values as in the model ͑II͒. The ratio of the collisional deactivation cross sections gives rise to 1:2.62:4.24, which is consistent with our observation, although their absolute values are underestimated.…”

“…According to our ab initio calculations reported elsewhere, 9,12 the Mg(3 1 P 1 ) -H 2 collision species follows the attractive 1 1 B 2 surface and then crosses to the lower ground 1 1 A 1 surface before any chemical and physical quenching can take place. Therefore, the Mg(3 1 P 1 ) -H 2 radius for an effective collisional deactivation can be reasonably estimated to be ϳ1.7 Å from the crossing region between the 1 1 B 2 and the ground surface.…”

“…The maximum N is nevertheless expected to reach Ͼ45 for the MgH(vϭ0) product from the 3 1 P 1 state reaction. 12 Therefore, if the gained internal energy is predominantly partitioned in these higher N values ͑Ͼ30͒, the obtained portion of the MgH distributions cannot reflect the difference. Second, the rotational lines around the bandhead (Nϭ11-20, vϭ0) are resolved by computer simulation, which should cause some error due to the sensitivity.…”

“…The corresponding microscopic reaction pathways may be well characterized by invoking quasiclassical trajectory ͑QCT͒ calculations. 12 When individual low-N and high-N trajectories are sampled, the following distinct aspects are found. For the high-N trajectories, the initially bent collision complex changes to a relatively long-lived linear geometry rapidly with a short bond distance between H and Mg.…”

Reaction pathway, energy barrier, and rotational state distribution for Li (2 2PJ)+H2→LiH (X 1Σ+)+H

“…For instance, Ca( 1 S 0 ) in a reaction with H 2 O 2 proceeds via a long-lived neutral complex, whereas the Ca( 3 P j ) reaction is dominated by a process of electron transfer. 39 The Mg (3 1 P 1 ) -H 2 reaction favors an insertion mechanism, [40][41][42][43] whereas the Mg (4 1 S 0 ,3 1 D 2 ) -H 2 reactions are believed to proceed via a harpoon-type process. 44,45 Such a competition between insertion and harpoon mechanisms has also been found in other alkaline earth atoms with OH-containing molecules.…”