Dynamics on the ground-state potential surfaces of H/sub 3/ and its isotopomeres from their uv spectra

The uv spectra of ${\mathrm{H}}_{3},$ ${\mathrm{D}}_{2}\mathrm{H},$ and ${\mathrm{D}}_{3}$ emitted by a neutralized ion beam have been measured in the wavelength range 200\char21{}400 nm. The spectra are continuous and show two maxima at 240 and 310 nm. Numerical simulations of the uv spectra, taking ab initio potential surfaces and transition moments as a starting point, explain their bimodal structure as arising from radiative decay into the two Jahn-Teller sheets of the ground-state potential surface. Two different lifetimes ${\ensuremath{\tau}}_{1}$ and ${\ensuremath{\tau}}_{2}$ for the lower sheet and the upper sheet of the Jahn-Teller adiabatic ground-state surface, respectively, were introduced and varied when fitting the simulated spectra to the experimental ones. These lifetimes have to be interpreted as average values for different vibrational states with angular momentum quantum numbers ranging from 0 to 7. The values are for ${\mathrm{H}}_{3}$ ${\ensuremath{\tau}}_{1}=3.5\mathrm{fs}$ and ${\ensuremath{\tau}}_{2}=6.7\mathrm{fs},$ for ${\mathrm{D}}_{2}\mathrm{H},$ ${\ensuremath{\tau}}_{1}=4.9\mathrm{fs}$ and ${\ensuremath{\tau}}_{2}=8.2\mathrm{fs},$ and for ${\mathrm{D}}_{3},$ ${\ensuremath{\tau}}_{1}=5.9\mathrm{fs}$ and ${\ensuremath{\tau}}_{2}=8.7\mathrm{fs}$ and the error is estimated to be 0.5 fs. These results are consistent among each other, i.e., they show the expected dependence on the square root of the reduced masses. Furthermore, lifetimes for the uv emitting Rydberg states have been measured. Since lifetimes of Rydberg states of ${\mathrm{H}}_{3}$ and its isotopomeres are already known from analogous measurements in the visible spectrum, the $3s$ and $3d$ states could be identified as the main emitters of the uv radiation. Their mean lifetimes range from about 4 ns for ${\mathrm{H}}_{3},$ 5 ns for ${\mathrm{H}}_{2}\mathrm{D},$ and 6 ns for ${\mathrm{D}}_{2}\mathrm{H}$ to 10 ns for ${\mathrm{D}}_{3}.$