Ab initio analysis of autoionization of ${\mathrm{H}}_3$ molecules using multichannel quantum-defect theory and new quantum-defect surfaces

Highly accurate Born-Oppenheimer potential-energy surfaces of ${\mathrm{H}}_3$ were calculated and combined with multichannel quantum-defect theory (MQDT) to predict with high precision the photoionization cross section of laser-excited triatomic hydrogen recently measured in this laboratory. The experiment first prepares ${\mathrm{H}}_3$ in stepwise excitation in a single rotational level of the symmetric stretch excited $3s$ Rydberg state. One-photon ionization from this state populates the continuum and $\mathrm{np}$ Rydberg states which autoionize into ${\mathrm{H}}_3^{+}{+e}^{-}.$ In the vicinity of the first symmetric stretch excited level of ${\mathrm{H}}_3^{+}$ the ionization spectrum shows features similar to those observed at the lowest ionization threshold: a quasidiscrete region below the first symmetric-stretch excited threshold, a Beutler-Fano region of rotational autoionization and interlopers of low-$n$ Rydberg states belonging to high vibrationally excited cores dispersed over the continuum. The MQDT calculations include rotational, vibrational, and Jahn-Teller interactions, and permit the assignment of most of the spectral features.