PHOTOFRAGMENTATION SPECTROSCOPY OF MGD2+

MgD2+ ion-molecule complexes, produced in a liquid-nitrogen-cooled laser vaporization source, are mass-selected and studied by laser photofragmentation spectroscopy in a reflection time-of-flight mass spectrometer. We have observed molecular absorption to the red of the Mg+(3S-2(1/2)-3(2)P(J)) atomic resonance lines. Notably, the reactive daughter ion (MgD+) is the only fragmentation product clearly observed in this spectral region. The molecular absorption profile consists of a seven-membered progression of discrete vibrational transitions with decreasing vibrational spacings from 489 to 314 cm-1, superimposed on a broad continuum background. The observed spectrum can be tentatively assigned to the perpendicular transitions 1B-2(2), 1B-2(1) <-- 1(2)A1 in C2v geometry. We assign the continuum background to the 1B-2(2) <-- 1(2)A1 transition. It is likely that reaction on this surface, possibly through insertion into a near linear D-Mg+-D intermediate, is direct and rapid. The presence of sharp, discrete vibrational absorption bands in the reactive fragmentation channel suggests a second, indirect reaction mechanism through a relatively long-lived excited-state complex. We assign this structure to the 1B-2(1) <-- 1(2)A1 transition. Presumably, reaction occurs indirectly through coupling to the 1B-2(2) state. The discrete structure within a given vibrational band is assigned to K subband rotational transitions (K(a'') = 0 --> K(a') = 1 and K(a'') = 1 --> K(a') = 0, 2) in o- and p-MgD2+, respectively. Additional unassigned lines may be due to overlapping hot band absorptions or transitions from higher K(a'') states. An alternative spectral assignment based on excited-state spin-orbit fine structure is also noted.