RESUMEN
To clarify the contentions about dissociative photoionization mechanism of nitrogen dioxide via the a3B2 and b3A2 ionic states, a new threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging has been conducted in the 12.8-14.0 eV energy range at the Hefei Light Source. The fine vibrational-resolved threshold photoelectron spectrum agrees well with the previous measurements. The ro-vibrational distributions of NO+, as the unique fragment ion in the dissociation of NO2+ in specific vibronic levels of a3B2 and b3A2 states, are derived from the recorded TPEPICO velocity images. A "cold" vibrational (v+ = 0) and "hot" rotational population is observed at the a3B2(0,3,0) and (0,4,0) vibronic levels, while the dissociation of NO2+ in b3A2(0,0,0) leads to the NO+ fragment with both hot vibrational and rotational populations. With the aid of the quantum chemical calculations at the time-dependent B3LYP level, minimum energy paths on the potential energy surfaces of the a3B2 and b3A2 states clarify their adiabatic dissociation mechanisms near the thresholds, and this study proposes reliable explanations for the observed internal energy distributions of fragment ions. Additionally, this study provides valuable insights into the application of the classical "impulsive" model on an overall slow dissociation process.
RESUMEN
Dissociative ionization of trifluoromethane (CHF3) is investigated in the 13.9-18.0 eV energy range using the threshold photoelectron-photoion coincidence (TPEPICO) technique coupled to a vacuum ultraviolet synchrotron radiation source. Four electronic states of CHF3+, i.e., the X2A1, A2A2, B2E, and C2E states, are populated upon ionization. In this energy range, the parent CHF3+ ions fully dissociate. For the CHF3+ ions in the ground state, the analysis of the time-of-flight profile of the unique CF3+ fragment ions suggests statistical dissociation. For the electronically excited CHF3+ ions, the C-F bond cleavage preferentially occurs to predominantly produce CHF2+ + F. Moreover, all TPEPICO images of the CHF2+ ions exhibit identical patterns, with a weak central spot revealing a previously unobserved statistical decomposition pathway, and the predominant ring in the images documents a fast nonstatistical dissociation channel. The unimolecular decomposition mechanisms of the CHF3+ ions are illuminated with the aid of the one-dimensional potential energy curves along the C-H and C-F coordinates calculated using the time-dependent density-functional theory. Moreover, a comparison of the dissociation dynamics of CHF3+ in these low-lying states with those of CF3Cl+ strongly suggests a substituent effect of chlorine atoms on the binding structure.
RESUMEN
The dissociative photoionization of CF3Cl was investigated using threshold photoelectron photoion coincidence (TPEPICO) imaging in the energy range of 12.30-18.50 eV. The coincident time-of-flight mass spectra and three-dimensional time-sliced images of the CF2Cl+ fragment were recorded at a few specific photon energies. Two fragmentation pathways were observed that led to the breakage of the C-Cl and C-F bonds, while the branching ratio elicited an energy-dependent relationship. The CF3+ fragment was dominant in the dissociation of the X2E, A2A' and B2A'' states, while CF2Cl+ became the predominant fragment and its branching ratio remained constant in the energy range associated with the C2E and D2E states. Based on the inflection point in the energy-dependent curve of the fragment branching ratios, the adiabatic ionization energy (IEa) of C2E is suggested to be 15.46 eV. Although the excess energy increased considerably from C2E to D2E, the kinetic energy release distributions (KERDs) of CF2Cl+ were similar. Moreover, the anisotropy parameters ß for the F-loss channel were positive and larger than those for the Cl-loss channel. The calculated F-loss potential energy curves of CF3Cl+ suggested that for the C2E and D2E states, the C-F bond rupture occurred via the internal conversion to the A2A' state followed by the dissociation attributed to the crossing of the barrier along the C-F coordinate. Based on the experimental and theoretical conclusions, the internal conversion is the rate-determining step in the dissociative photoionization of CF3Cl via the C2E and D2E ionic states, irrespective of whether the C-F and C-Cl bonds rupture.
RESUMEN
Time-sliced velocity map imaging (VMI) has extensively been applied in photodissociation dynamics studies, thanks to its unique advantages, such as high energy resolution and no requirement of inverse Abel or Hankel transformations. However, its time resolution is generally insufficient for distinguishing adjacent m/z ions with a certain kinetic energy due to the overlapping of time-of-flight distributions. Herein, we have made a novel and convenient switch design for the common ion optics in three-dimensional (3D) VMI. By simply introducing two additional resistors out of the vacuum chamber, the strength ratio of the extraction and acceleration fields is easily changed from 3D VMI to two-dimensional (2D) VMI under optimized conditions, as well as a significant extension of free drift length, leading to a higher time resolution while maintaining the high energy resolution. As a result, 2D and 3D VMI can be quickly switched without breaking the vacuum and replacing the electrostatic plates.