PHOTODISSOCIATION SPECTROSCOPY OF MG2+ (1(2)SIGMA(U)(+)-1(2)SIGMA(G)(+))

We have observed the photodissociation of Mg2+ via the repulsive 1 2SIGMA(g)+ state in a reflection time-of-flight mass spectrometer. The photodissociation spectrum consists of a broad structureless continuum ranging from almost-equal-to 580 to almost-equal-to 690 nm and peaking near 640 nm. A spectral simulation based on theoretical ab initio potential-energy curves agrees well with the experimental profile. We have observed a triple-vector (E-mu-v) correlation in the photofragment time-of-flight spectrum. Analysis of the correlation allows a determination of the anisotropy parameter beta and the recoil velocity as a function of the laser wavelength. In the long- and short-wavelength limits of the photodissociation spectrum, the anisotropy parameter attains its maximum value, beta = 2, characterizing the process as a parallel (1 2SIGMA(u)+ --> 1 2SIGMA(g)+) transition followed by a rapid, recoil-like fragmentation. However, excitation in the middle of the spectrum (almost-equal-to 620-660 nm) results in an anisotropy parameter that varies smoothly and significantly from the maximum value, indicating a second dissociation pathway for excitation in this region. Several alternatives for the competing channel are discussed. The measured recoil energy varies from E almost-equal-to 4100 to 7100 cm-1 as the dissociating wavelength is tuned from lambda=680 to 590 nm. This allows us to obtain an estimate for the ground-state binding energy D0'' almost-equal-to 10 200 +/- 300 cm - 1, a result in very good agreement with recent theoretical predictions [G. Durand, J.-P. Daudey, and J.-P. Malrieu, J. Phys. (Paris) 47, 1335 (1986); M. Sodupe, C. W. Bauschlicher, Jr., and H. Partridge, Chem. Phys. Lett. 192,185 (1992)].