Multiphoton ionization studies of NO: Spontaneous decay channels in the (4pπ)K 2Π(v=2) Rydberg state

Time-delayed optical double resonance multiphoton ionization (MPI) studies involving (4pπ)K ²Π(v=2)←(3sσ)A ²Σ⁺(v=1) ←←X ²Π(v=0) NO show dramatic loss of the ionization signal amplitudes on a time scale that is very short relative to the lifetime of the A state. From direct comparisons of MPI and laser induced fluorescence (LIF) dip temporal profiles, it is concluded that loss of the signal amplitude results from inefficient ionization of the K state. MPI temporal profiles measured in a supersonic jet are identical with those measured in a cell indicating that collisional effects are not important. For delay times τ≥20 ns, ionization signals can be observed only at high probe laser intensities. The resultant spectra exhibit marked power broadening and a pronounced dip appears in the center of each of the power broadened resonances. The observed line shapes are rationalized in terms of the spatial and temporal distributions of the probe laser field in conjunction with a dynamical competition between photoionization and spontaneous decay channels in the K state. Such arguments lead to an upper bound on the K ²Π(v=2) lifetime of ≊2 ns. The short lifetime of K ²Π(v=2) is attributed to indirect heterogeneous predissociation through the (4pσ)M ²Σ⁺ state and/or to homogeneous predissociation via the a ⁴Π continuum.