Femtosecond time-resolved photoelectron spectroscopy of nitric oxide

Femtosecond pump-probe time-resolved photoelectron spectroscopy (TRPES) is used to study the nitric oxide molecule at high laser intensities. A brief description is given of two processes, above-threshold ionization and the AC Stark effect, that play important roles in the photoionization of molecules at the high laser intensities available with femtosecond pulses. Next, an overview of the laser system used is provided, followed by a detailed account of the equipment constructed to perform TRPES and also time-resolved mass spectrometry (TRMS) on species in molecular beams via time-of-flight (TOF) analysis. The spectrometer consists of a microchannel plate detector in a magnetically shielded flight tube, with removable acceleration grids for mass spectrometry, housed in a three-stage differentially pumped vacuum chamber. The data acquisition system allows the collection of TOF spectra at the full 3kHz repetition rate of the laser, and includes a software-based discrimination scheme that greatly enhances the signal-to-noise ratio of the spectra. The mass and energy resolution of the system are discussed; the latter is found to be limited by either the laser's spectral bandwidth or the duration of the detector pulses, depending on the energy range considered.; Nitric oxide is the first molecule to be studied with this new system. Long-lived pump-probe peaks are observed in the TRPES spectrum, and are attributed to a 2+1 process involving the Rydberg A (v = 3) and C (v = 0) states, Stark-tuned into resonance with the laser frequency by the intense light field. It is shown that by varying the intensity of the laser, either of these states can be selectively populated. Transient peaks seen in the TRPES spectra are assigned to the Rydberg F, H, and H{dollar}spprime{dollar} states, which are populated via a 1+1 process and ionized by either pump or probe. As with the A and C states, the populations of these states can be selectively controlled by varying the laser power; spectra taken at various wavelengths support these assignments.