High-order multiphoton ionization photoelectron spectroscopy of nitric oxide

Resonance-enhanced and nonresonant five- and six-photon ionization of NO was studied using angle resolved photoelectron spectroscopy. The (3+3) resonance-enhanced multiphoton ionization photoelectron spectrum (REMPI-PES) of NO via the A 2 Σ+ (v=0) level yielded a distribution of electron energies corresponding to all accessible vibrational levels (v+ =0–6) of the nascent ion. The observed energy distributions suggest near resonant enhancement due to vibrational levels of the D 2 Σ+ and C 2 Π states at the fourth photon level. Angular distributions of photoelectrons corresponding to v+ =0 and v+ =3 (rotationally unresolved) were significantly different, perhaps reflecting these different pathways. The (3+2) REMPI via the A 2 Σ+ (v=1) level produced only one low-energy electron peak corresponding to v+ =1. Nonresonant MPI at 532 nm yielded a distribution of photoelectron energies corresponding to both four- and five-photon ionization. Prominent peaks in the five-photon ionization photoelectron spectrum show that near resonant enhancement occurs via the A 2 Σ+ (v=5), the C 2 Π(v=2) and the D 2 Σ+ (v=1) states at the three-photon level. This study amply demonstrates the utility of angle resolved MPI-PES in understanding high-order multiphoton ionization processes. Finally, measurements of the ratio of NO+ ion signal for circular and linear polarized light when tuning near the A 2 Σ+ (v=0) and A 2 Σ+ (v=1) levels are reported and compared with theory.