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

Abstract

Using a pump–probe technique, the reactions of Mg(4 1S0 and 3 1D2) with H2 have been measured to yield similar rotational distributions of MgH(v=0 and 1) as that obtained for the reaction of the Mg(3 1P1) state with H2. A series of measurements is conducted to clarify that the reactions are initiated directly by these higher states, rather than occurring from the lower 3 1P1 state following radiative and collisional relaxation. The reactivity of the Mg 4 1S0 state with H2 is found to be comparable to that of the 3 1P1 state, but about three times larger than that of the 3 1D2 state. The Mg(4 1S0, 3 1D2)–H2 reactions proceed via a harpoon-type process, and are closely associated with the Mg(3 1P1)–H2 reaction coordinate through evolution of a series of surface crossings. To support our suggestion that the harpoon mechanism is involved, the cross sections of collisional deactivation by H2 for various excited states are measured. The ratios of cross sections observed for the 3 1P1, 4 1S0, and 5 1S0 state, equal to 1:2.85:4.3, are consistent with the calculated prediction of 1:2.62:4.24. The calculated cross sections are based on a simple hard sphere model with effective radii evaluated differently. Here, the effective radii for the higher states are determined from the crossing of ionic and covalent curves, while the Mg(3 1P1)–H2 radius is estimated from the nonadiabatic crossing between the reactive 1 1B2 state and the ground state. Consistency between observation and prediction confirms that the harpoon mechanism proposed in this work is plausible.

Dynamical and stereodynamical studies of alkaline-earth atom–molecule reactions

“…Furthermore, a harpoon mechanism was proposed for these high-lying state reactions to account for the distinct difference in the collisional deactivation crosssections measured for various Mg excited states. As Mg(4s 1 S 0 , 3d 1 D 2 ) approaches H 2 , the collision complex first crosses to the ion-pair reaction coordinate; a series of diabatic curve crossings are then found along the Mg þ H À 2 coordinate, and finally one to the reactive 1 B 2 surface, which correlates with the Mg(3p 1 P 1 ) state [49].…”

“…Nevertheless, energy deposited in various degrees of freedom may result in different dynamical complexity. For instance, the reaction of O( 1 D) and H 2 favours an insertion mechanism [49]. But when energy greater than 1.8 kcal mol À1 is deposited in translation, a new channel of H-abstraction reaction along a collinear configuration may be opened [50,51].…”

“…Meanwhile, an inherent difficulty linked to this type of experiment lies in the fact that the product contribution from those lower states, relaxed via collision or radiative cascade, may interfere with the state of interest. A quantitative measure of such product interference from other excited states was done in the Mg(4s 1 S 0 , 3d 1 D 2 ) þ H 2 reactions by looking at the influence of the lower 3p 1 P 1 state [47][48][49]. This was realized by use of a third pulsed laser beam to deplete the 3p ( 1 P 1 ) population by exciting the 3p ( 1 P 1 ) !…”