We present the first results using a new technique that combines femtosecond pump-probe methods with energy-and angle-resolved photoelectron-photoion coincidence imaging. The dominant dissociative multiphoton ionization ͑DMI͒ pathway for NO 2 at 375.3 nm is identified as three-photon excitation to a repulsive potential surface correlating to NO(C 2 ⌸)ϩO( 3 P) followed by one-photon ionization to NO ϩ (X 1 ⌺ ϩ ). Dissociation along this surface is followed on a femtosecond timescale.The capability for following photodissociation dynamics directly in time is an exciting development in chemical dynamics brought about by the availability of femtosecond lasers. Initially, femtosecond time-resolved laser-induced fluorescence 1 and absorption 2 techniques were applied as probes of dissociation, but subsequently time-resolved photoionization probes, using mass spectrometry and photoelectron spectroscopy, were developed and applied to a substantially wider range of molecules and more complicated molecular systems. 1,3-5 Time-resolved mass spectrometric techniques that provide one-dimensional photofragment energy and angular resolution have also been developed for studies of reactions with multiple dissociation pathways. 6 However, when using photoionization probes, it is advantageous to detect both the photoion and photoelectron in coincidence, 7 since both particles provide important information on the dissociation event. This communication presents a new femtosecond time-resolved photoionization probe technique that combines photoelectron-photoion coincidence ͑PEPICO͒ 8,9 detection with three-dimensional energyand angle-resolved imaging. 10,11 The present studies follow the time evolution of the dissociation process that occurs during dissociative multiphoton ionization ͑DMI͒ of NO 2 induced by femtosecond laser pulses centered at 375.3 nm ͑3.30 eV͒. This technique also provides time-resolved identification of the dissociation products and ionization pathway.There have been a tremendous number of previous studies on dissociation and ionization processes in NO 2 . 12-14 Recently, there has been increased interest in the femtosecond dynamics of these processes, 15 particularly multiphoton ionization. 16,17 Other work at 375 nm ͑Ref. 16͒ found that DMI strongly dominates over simple parent ionization. This is an interesting result since parent ionization ͑9.59 eV͒ is accessible by three-photon absorption, whereas four photons are required for DMI ͑12.37 eV͒. The DMI channel was postulated to be one-photon dissociation to ground-state NOϩO ͑3.12 eV͒, followed by three-photon ionization to NO ϩ . However, the present work demonstrates that a different DMI channel dominates.A schematic diagram of the experimental setup is shown in Fig. 1. Laser pulses of 100 fs duration ͑determined from the single-shot autocorrelation width of 160 fs͒ are produced from a regeneratively amplified titanium sapphire laser system ͑Clark MXR͒, operating at 2.2 kHz. The pulses are frequency doubled to a peak wavelength of 375.3 nm using a BBO cryst...
Vibrationally resolved PIRI spectra of the B̃ state of the fluorobenzene cation via the origin, 16a, 6b, and 11 vibrational modes in the ground ionic state require a reassignment of the accepted state symmetry. On the basis of lower resolution studies, the B̃ ← X̃ transition has been previously assigned as an electronically forbidden 2B2 ← 2B1 transition. Vibrational analyses of the spectra observed via various ground-state nonsymmetric vibrations, particularly from the 16a vibrational mode, unambiguously locate the origin of the transition at 21 075 cm-1, resulting in the reassignment of the ionic state as 2B1. Ab initio calculations, while not conclusive, also suggest that the B̃ ← X̃ transition is an allowed π to π transition.
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