Two-photon ionization processes of PO in a C2H2/air flame

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Abstract
Laser induced two‐photon ionization processes are observed for PO in a premixed C2H2/air flame in the 302–334 nm wavelength region. The ionization signal obtained as a function of energy shows distinct band heads, which correspond to one‐photon absorption from the ground state to the resonant intermediate B 2Σ+ state. A second photon of the same energy then either (1) excites PO to high‐lying electronic states from which collisional ionization occurs or (2) directly photoionizes the PO molecule, depending upon the total excitation energy available. Several new bands are assigned on the basis of calculated spectra. The intensities of the observed ion signals vary in an unexpected fashion for transitions which reach the same vibrational level of the resonant intermediate state. Further analysis indicates that absorption of the second photon makes a significant contribution to the overall intensity of some of the observed bandheads. This result is attributed to the presence of short‐lived autoionizing states which lie close to the ionization limit, despite some uncertainties encountered in calculating Franck–Condon factors for absorption of the first photon. It is estimated that PO can be easily detected at a mole fraction of 1×10−8 with a laser pulse energy of 100 μJ.