The photodissociation spectroscopy of MgCH 4 ϩ has been studied in a reflectron time-of-flight mass spectrometer. MgCH 4 ϩ molecular absorption bands are observed to the red of the Mg ϩ (3 2 P J ←3 2 S 1/2 ) atomic ion resonance lines. The photofragmentation action spectrum consists of a broad structureless continuum ranging from 310 nm to 342 nm, and peaking near 325 nm. In this spectral region, both the nonreactive ͑Mg ϩ ͒, and two reactive fragmentation products ͑MgH ϩ and MgCH 3 ϩ ͒ are observed, all with similar action spectra. The product branching is independent of wavelength, Mg ϩ :MgCH 3 ϩ :MgH ϩ ϳ60:33:7. The absorption is assigned to the transition (1 2 E←1 2 A 1 ) in C 3v symmetry ͑with 3 coordination͒, followed by a geometrical relaxation of the complex toward states of 2 B 1 and 2 B 2 symmetry in C 2v geometry ͑with 2 coordination͒.Dissociation requires a nonadiabatic transition to the ground electronic surface. Analysis of broadening in the photofragment flight time profile shows the nonreactive Mg ϩ product angular distribution to be isotropic, with an average translational energy release which increases slightly from E t ϳ370Ϯ150 cm Ϫ1 at 332.5 nm to E t ϳ520Ϯ180 cm Ϫ1 at 315 nm. These values are less than 2% of the available energy and are well below statistical expectations. Analogous experiments on MgCD 4 ϩ show the kinetic energy release in the nonreactive channel to be significantly larger for the CD 4 case, ranging from E t ϳ540Ϯ180 cm Ϫ1 at 332.5 nm to E t ϳ830Ϯ200 cm Ϫ1 . These results clearly demonstrate that the dissociation is nonstatistical. Preliminary ab initio potential surface calculations suggest a possible dynamical mechanism to explain these unusual results.

The population distributions of the rotational quantum states of the @ascent MgH(u" =0 and 1) produced in the reaction of Mg(3s3p P& ) with H2 are bimoda1. With the use of the surprisal method, the two components are separated. The minor low-N component of the distribution in the u"=0 state is found to be larger than that in the v" = 1 state, whereas, the major high-E component of the distribution in the v"=0 state becomes roughly equivalent to that in the v"=1 state. The two parallel low-N and high-N processes are expected to correspond to two distinct types of reaction dynamics. One (minor) type produces MgH in lower rotational levels and preferentially v"=0, and the other (major) type produces MgH in higher rotational levels with comparable v" =0 and v" =1 populations. Possible dynamical models are discussed. PACS number(s): 82.30.b, 34.50.s, 35.20.i

Two ab initio methods have been employed to calculate the dynamical potential energy surfaces (PES’s) for the excited (B21 or A'1) and the ground (A11 or A'1) states in the Mg(3s3p1P1)–H2 reaction. The obtained PES’s information reveals that the production of MgH in the Σ+2 state, as Mg(1P1) approaches H2 in a bent configuration, involves a nonadiabatic transition. The MgH2 intermediate around the surface crossing then elicits two distinct reaction pathways. In the first one, the bent intermediate, affected by a strong anisotropy of the interaction potential, decomposes via a linear HMgH geometry. The resulting MgH is anticipated to populate in the quantum states of rotational and vibrational excitation. In contrast, the second pathway produces MgH in the low rotational and vibrational states, as a result of the intermediate decomposition along the stretching coordinate of the Mg–H elongation. These two tracks may account for the previous experimental findings for the MgH distribution, which the impulsive model has failed to comprehend. By far, different interpretations have been proposed especially for the low-N MgH product. The supply of a detailed PES’s information in this work helps to clarify the ambiguity. It is also conducive to an interpretation of the isotope and temperature effects on the product rotational distribution.