Three-body decay of the $3s{}^2{A}_1^{\prime }(N=1,$ $K=0)$ and $3d{}^2{E}^{″}(N=1,$ $G=0,$ $R=1)$ Rydberg states of the triatomic hydrogen molecule ${\mathrm{H}}_3$

The breakup of $n=3\mathrm{}$ Rydberg states of the triatomic hydrogen molecule into three $\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ atoms was studied using SRI’s fast neutral beam photofragment spectrometer. A beam of fast metastable ${\mathrm{H}}_3$ $2p{}^2{A}_2^{″}\mathrm{}(N=K=0\mathrm{})\mathrm{}$ molecules was generated by charge transfer of ${\mathrm{H}}_3^{+}$ ions in Cs vapor. The metastable beam was crossed by an intracavity dye laser beam in order to selectively prepare the $3s{}^2{A}_1^{\prime }(N=1,$ $K=0)$ and $3d{}^2{E}^{″}(N=1,$ $G=0,$ $R=1)$ states of ${\mathrm{H}}_3.$ Correlated fragment pairs were detected by a time- and position-sensitive detector. For both excited states, the two-body decay into ${\mathrm{H}+\mathrm{H}}_2$ fragment pairs as well as the three-body breakup into three correlated $\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ fragments are open channels. The two processes produce distinguishable events on the time- and position-sensitive detector. For three-body decay, only two of the three products are detected. An extensive model using physically reasonable assumptions was developed in order to explain the observed spectra and to gain insight into the kinematics of the three-body breakup. Branching ratios between three- and two-body decay are estimated for the breakup of the ${\mathrm{H}}_3$ $3s{}^2{A}_1^{\prime }(N=1,K=0)$ and $3d{}^2{E}^{″}(N=1,G=0,R=1)$ states, and compared to results of a two-dimensional wave-packet calculation.