NOO Peroxy Isomer Exposed with Velocity-Map Imaging

The chemistry of NO₂, a key atmospheric trace gas, has historically been interpreted in terms of the C_2v isomer ONO, with the peroxy isomer NOO only postulated to be stable. In this work, a velocity-map-imaged photoelectron spectrum of the nitrite anion, NO₂^-, reveals energetic-electron structure that may only occur by photodetachment from the NOO^-(X̃¹A') isomer. This measurement defines NOO(X̃²A') bond frequencies and an electron affinity of only 335(30) cm^-1, which, supported by ab initio calculations, confirm the first observation of this important reactive species.

Magnetic trapping of SH radicals

Magnetic trapping of SH radicals, produced via the photostop technique, has been demonstrated. H₂S in a skimmed, supersonic molecular beam was photodissociated at 212.8 nm to produce SH inside a 330 mK deep static magnetic trap. The molecular-beam speed was controlled by the mixing ratio of H₂S in Kr to match the recoil velocity of the SH photofragments such that some SH radicals were produced with near-zero laboratory-frame velocity. The density of SH radicals in the ²Π_3/2, v = 0, J = 3/2 state was followed by (2 + 1) REMPI over seven orders of magnitude of signal intensity. 5 ms after photodissociation, SH radicals moving faster than the capture velocity of 13 m s^-1 had left the trap. The 1/e trap lifetime of the remaining SH radicals was 40 ± 10 ms at an estimated density of 5 × 10⁴ molecules per cm³. Photostop offers a simple and direct way to accumulate absolute ground state molecules in a variety of traps.

Femtosecond time-resolved photoelectron-photoion coincidence imaging of multiphoton multichannel photodynamics in NO2

The multiphoton multichannel photodynamics of NO(2) has been studied using femtosecond time-resolved coincidence imaging. A novel photoelectron-photoion coincidence imaging machine was developed at the laboratory in Amsterdam employing velocity map imaging and "slow" charged particle extraction using additional electron and ion optics. The NO(2) photodynamics was studied using a two color pump-probe scheme with femtosecond pulses at 400 and 266 nm. The multiphoton excitation produces both NO(2) (+) parent ions and NO(+) fragment ions. Here we mainly present the time dependent photoelectron images in coincidence with NO(2) (+) or NO(+) and the (NO(+),e) photoelectron versus fragment ion kinetic energy correlations. The coincidence photoelectron spectra and the correlated energy distributions make it possible to assign the different dissociation pathways involved. Nonadiabatic dynamics between the ground state and the A (2)B(2) state after absorption of a 400 nm photon is reflected in the transient photoelectron spectrum of the NO(2) (+) parent ion. Furthermore, Rydberg states are believed to be used as "stepping" states responsible for the rather narrow and well-separated photoelectron spectra in the NO(2) (+) parent ion. Slow statistical and fast direct fragmentation of NO(2) (+) after prompt photoelectron ejection is observed leading to formation of NO(+)+O. Fragmentation from both the ground state and the electronically excited a (3)B(2) and b (3)A(2) states of NO(2) (+) is observed. At short pump probe delay times, the dominant multiphoton pathway for NO(+) formation is a 3x400 nm+1x266 nm excitation. At long delay times (>500 fs) two multiphoton pathways are observed. The dominant pathway is a 1x400 nm+2x266 nm photon excitation giving rise to very slow electrons and ions. A second pathway is a 3x400 nm photon absorption to NO(2) Rydberg states followed by dissociation toward neutral electronically and vibrationally excited NO(A (2)Sigma,v=1) fragments, ionized by one 266 nm photon absorption. As is shown in the present study, even though the pump-probe transients are rather featureless the photoelectron-photoion coincidence images show a complex time varying dynamics in NO(2). We present the potential of our novel coincidence imaging machine to unravel in unprecedented detail the various competing pathways in femtosecond time-resolved multichannel multiphoton dynamics of molecules.