Direct Experimental Evidence for Dissociative Photoionization of Oxygen Molecule via ²Σ_u^– Ionic “Optical Dark” State

Abstract

Direct experimental evidence for dissociative photoionization of oxygen molecule via the (2)Σ(u)(-) ionic optical dark state is presented by an investigation using the method of threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. Besides vibrational progress of the B(2)Σ(g)(-) state, several weak vibrational bands of the (2)Σ(u)(-) ionic optical dark state are observed concomitantly in an excitation energy range of 20.2-21.1 eV. Only O(+) fragments are detected in the whole excitation energy range; therefore, all vibrational bands are completely predissociative. TPEPICO three-dimensional time-sliced velocity images of O(+) fragments dissociated from vibrational state-selected O(2)(+)((2)Σ(u)(-),v(+)) ions are recorded. For the (2)Σ(u)(-)(v(+)=0-3) vibrational states, only the lowest dissociation channel of O(+)((4)S) + O((3)P) is observed. Once the photon energy is slightly increased to the (2)Σ(u)(-)(v(+)=4) level, a new concentric doughnut appears in the image, indicating that the second dissociation channel of O(+)((4)S) + O((1)D) is identified indeed. With the aid of potential energy curves, the dissociative mechanism of O(2)(+) in the (2)Σ(u)(-)(v(+)) state is proposed.

Ro-vibrational Distribution of NO+ Dissociated from NO2+ Ions in the a3B2 and b3A2 States: A Slow “Impulsive” Dissociation Example Revealed from Threshold Photoelectron–Photoion Coincidence Imaging

“…Moreover, according to the perpendicular configuration between the molecular beam (MB) and the ion flight direction, the velocity spread of MB contaminates images along its direction. Consequently, a multistep data reduction process of subtracting, deconvolution, and quadrant symmetrization is necessary to obtain the better-resolution velocity map images. − …”

“…Additionally, the agreement between the experimental and theoretical ⟨ f T ⟩ values of the classical “impulsive” model has always been used to judge if the dissociation occurs on a repulsive potential energy surface. ,− ,, However, the dissociation of NO 2 + in the a 3 B 2 and b 3 A 2 states provides an atypical example of a slow “impulsive” dissociation process. Thus, this study provides valuable insights into the application of the classical “impulsive” model on an overall slow dissociation process of a polyatomic molecule.…”

Advances in threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion coincidence (TPEPICO)

“…The example in Fig. 14 is for the case of two O 2 + ion states that by chance overlap at an energy of 10.817 eV. 119 These are the optically active B 2  g -(v=4) and the 2  u -(v=5) states, the latter being optically inaccessible and thus much weaker. This result, as well as data at other energies, show that the two electronic states dissociate to the ground state, O + ( 4 S) + O( 3 P), while the ungerade state dissociates only to the excited products.…”

Cl-Loss dynamics in the dissociative photoionization of CF3Cl with threshold photoelectron–photoion coincidence imaging

“…[31][32][33] Compared with the method of fitting TOF profiles to obtain the KERD, the TPEPICO velocity map imaging can provide a more exact KERD and the angular distribution of fragments simultaneously. [34][35][36][37][38] Here, the dissociative photoionization of CF 3 Cl in a photon energy range of 12.30-18.50 eV has been carefully reinvestigated. At the specific photon energy, the KERD and angular distribution of CF…”