Photodissociation spectroscopy of MgCH4+

The photodissociation spectroscopy of MgCH4+ has been studied in a reflectron time-of-flight mass spectrometer. MgCH4+ molecular absorption bands are observed to the red of the Mg+(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+), and two reactive fragmentation products (MgH+ and MgCH3+) are observed, all with similar action spectra. The product branching is independent of wavelength, Mg+:MgCH3+:MgH(+)similar to 60:33:7. The absorption is assigned to the transition (1(2)E<--1(2)A(1)) in C-3 nu symmetry (with eta(3) coordination), followed by a geometrical relaxation of the complex toward states of B-2(1) and B-2(2) symmetry in C-2 nu geometry (with eta(2) coordination). Dissociation requires a nonadiabatic transition to the ground electronic surface. Analysis of broadening in the photofragment flight time profile shows the nonreactive Mg+ product angular distribution to be isotropic, with an average translational energy release which increases slightly from E(t) similar to 370+/-150 cm(-1) at 332.5 nm to E(t) similar to 520+/-180 cm(-1) at 315 nm. These values are less than 2% of the available energy and are well below statistical expectations. Analogous experiments on MgCD4+ show the kinetic energy release in the nonreactive channel to be significantly larger for the CD4 case, ranging from E(t) similar to 540+/-180 cm(-1) at 332.5 nm to E(t) similar to 830+/-200 cm(-1). 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. (C) 1996 American Institute of Physics.