Knowledge Bank

Symmetry arguments reveal that the $B~2p2A2-$ state of $H3$ is metastable with respect to predissociation by the ground $X~2p2E$ state state for only the rotationless $N=0$ level, but for any amount and mode of vibrational excitation. We detect the existence of these levels in a fast neutral beam by laser photoabsorption, leading to photodissociation or to reionization. Since we start from only a single N, the resulting spectra are atomic-like in their simplicity. Vibrationally excited $B~2p2A2″$ levels in the beam are detected by ``hot bands'' in laser photodissociation in the $3s\leftarrow 2p$ and $3d\leftarrow 2p$ transitions and by autoionization in the $ns\leftarrow 2p$ and $nd\leftarrow 2p$ transitions for $n\ge 6$ or 7 for $\nu 1$ or $\nu 2$ initial vibrational excitation. Vibrationally off-diagonal transitions are observed to 3-7s and 3-7d electronic states accompanied by one quantum of $v1$ symmetric-stretch vibration [by two-photon ionization $(n=3)$, ionization-depletion double resonance $(n=3-5)$, and autoionization $(n=6,7)$]. Excitation of $v2$ asymmetric-stretch or bending vibration changes the electronic selection rules, allowing us to observe transitions to the $3pA+2″(v1=0,v2=1),3pE\prime (0,1),3pE\prime (1,1),3pE\prime (0,3)$, and 3dE'(0, 1) states, and tentatively to the 3pE'(0, 2), 4pE'(0, 1), and 5pE'(0, 1) states. Vibrationless ns; np; nd; and nf Rydberg series have been observed for $n=3-7,9-22;30-90;3-7,9-100$; and 26-70; respectively. Excitation of the np and nf field-ionizing Rydberg series is accomplished by two-photon excitation using the 3s and 3d states as intermediates. In the latter case, the existence of two intermediate states with different amounts of core rotation, $3dE″(N+\equiv 1)$ and $3dA1(N+\approxeq 3)$, allows us to distinguish the nf1 and nf3 Rydberg series, converging to the $N+=1$ and $N+=3$ ionization limits.