Velocity Map Imaging Study of the O₂ Ion-Pair Production at 17.499 eV:  Simultaneous Parallel and Perpendicular Transitions

Abstract

The fine structure resolved photofragment O(-)((2)P(j)) image from the O(2) ion-pair production at 17.499 eV has been recorded. The branching ratio for producing the low energy spin-orbit O(-)((2)P(3/2)) component to the high energy spin-orbit O(-)((2)P(1/2)) component is 1:0.78 and the optical transitions for them correspond to perpendicular and parallel transitions, respectively. The anisotropy parameters, 1.64 for channel producing O(-)((2)P(1/2)) and -0.35 for O(-)((2)P(3/2)), suggest that the dissociation proceeds via the states with symmetry (3)Sigma(u)(-) and (3)Pi(u), respectively. Although the main mechanisms for the O(2) ion-pair production are the predissociation via the intermediate Rydberg states, the direct dissociation mechanism for the channel producing O(-)((2)P(1/2)) may also be involved.

Direct mapping of recoil in the ion-pair dissociation of molecular oxygen by a femtosecond depletion method

“…The 3 ⌺ u − and 3 ⌸ u ion-pair states were therefore concluded to be the precursor states giving rise to these channels. The closeness of ␤ = 1.64 to the limiting value of 2 for a parallel transition was interpreted by Hao et al 5 to be possibly due to the contribution of direct dissociation via the repulsive state responsible for a weak contribution to the continuum in the O − appearance spectrum observed by Dehmer and Chupka. 1 Hikosaka and Shigemasa 6 also used VMI, but for detection of O − arising from the photodissociation of superexcited states ͑O 2 * ͒ from higher-lying ion-pair states correlating to O + ͑ 2 D͒ +O − ͑ 2 P͒ ͑second dissociation limit͒ and O + ͑ 2 P͒ +O − ͑ 2 P͒ ͑third dissociation limit͒.…”

“…Excited ͑Rydberg͒ states of ⌬ u or ⌽ u symmetry can only correlate to higher-lying ion-pair states, and can dissociate only in a neutral path ͑O * +O͒ where O * is an electronically excited atom. ͑We have only taken into account the formation of 3 O * , not 5,1 O * states.͒ According to Hao et al, 5 the ͑lowest energy͒ ͑O − ͑ 2 P 3/2 ͒ +O + ͑ 4 S 3/2 ͒͒ dissociation channel is associated with the initial excitation of a ⌸ u state, and the ͑O − ͑ 2 P 1/2 ͒ ͑b͒ Reconstruction of the energy distribution with the p-Basex algorithm, with two different Basis sets: on the left P 0 , P 2 , P 4 , P 6 , P 8 , where P n is the Legendre polynomial function of nth order. In ͑c͒ a basis set is used that is restricted for a three-photon process to the functions cos 6 , sin 2 cos 4 , sin 4 cos 2 , and sin 6 , where only the cos 6 function is seen to reproduce the measurement.…”

“…Energetic thresholds for these two channels have been also observed by Martin and Hepburn 4 who detected O + ions after application of a delayed pulsed electric field to the highly vibrationally excited Rydberg states of O 2 . Recently, Hao et al 5 applied the velocity map imaging ͑VMI͒ technique to detect the O − partner of ion-pair dissociation after excitation of O 2 by extreme ultraviolet ͑XUV͒ radiation at 17.499 eV. They detected O − in both the 2 P 3/2 and 2 P 1/2 spin-orbit states with a yield ratio of 1:0.78, respectively.…”

Vibrational structure, spin-orbit splitting, and bond dissociation energy of Cl2+(X̃Πg2) studied by zero kinetic energy photoelectron spectroscopy and ion-pair formation imaging method

“…The details of our XUV/Vacuum ultraviolet ͑VUV͒ photoelectron and photoion spectrometer have been described previously, so only a brief summary is given here. 12,13,20,21 For ZEKE experiments, coherent XUV radiation was generated using the resonance enhanced four-wave sum mixing ͑2 1 + 2 ͒ in a pulsed Xe jet. A Nd:YAG ͑yttrium aluminum garnet͒ ͑20 Hz͒ pumped two dye laser system was used in the experiments.…”

Quantum state-to-state vacuum ultraviolet photodissociation dynamics of small molecules

“…The obtained anisotropy parameters showed that perpendicular transitions dominated the photoabsorption process at these energies, and the directly excited state should have Σ or ∆ symmetry. Mo and coworkers investigated the ion-pair dissociation dynamics of O 2 in the photon energy range of 17.20−17.50 eV by using the VMI setup with a TPR-FWM generated VUV source [53,54]. The branching ratios between the fine structure ion-pair dissociation [56], and that of HBr at a series of photolysis wavelengths in the wavelength ranges of 123.90−125.90 nm and around 137.0 nm [57].…”