Velocity imaging of the photodissociation and photoionization dynamics of hydrogen chloride and hydrogen bromide.

摘要

The multiphoton excitation of two hydrogen halides, HCl and HBr, was studied by using (2 + 1) Resonance Enhanced Multi-Photon Ionization and photoelectron and photoion Velocity-Map Imaging techniques in order to obtain information on the dynamics of the low-lying Rydberg and ion-pair states as well as the superexcited states localized in the Franck-Condon gap between the lowest bound ionic states.; The photoexcitation of the vibrationally excited ion-pair state V( 1Sigma+), [v = 9-13, 15 for HCl and v = m+3, m+5 -- m+8 for HBr] is characterized by the formation of a long vibrational progression of HCl+/HBr +, as well as the formation of electronically excited hydrogen and halogen atoms and, in the case of HCl, the formation of a H+ + Cl- ion-pair. The analysis of the branching ratios and angular distributions of the photoelectrons formed in the ionization allows us to conclude that the photoionization dynamics of the ion-pair state is dominated by the excitation of superexcited states and their subsequent decay by dissociation and electronic autoionization.; The formation of the electronically excited photofragments is explained as direct excitation of unbound superexcited states while the ion-pair dissociation of HCl is explained by excitation of 1Sigma+ bound superexcited states followed by predissociation by the V(1Sigma +) ion-pair state. The angular distributions of the protons formed in the ion-pair dissociation were also used to probe the rotational alignment of the O=0 two-photon intermediate states and the results are consistent with a O = 0 → O =1 → O = 0 → O = 0 three-photon excitation pathway.; The photoexcitation dynamics of the low-lying Rydberg states were found to depend on their interactions with the ion-pair state. The "unperturbed" Rydberg states, such as the F(1Delta2) state of HCl and the F(1Delta2) and i(3Delta 2) states of HBr, ionize directly into the ground vibrational state of the molecular ion.; On the other hand, the dynamics of the "perturbed" O = 0 + Rydberg states, such as the g(3Sigma-) and E(1Sigma+) states of HCl and the g( 3Sigma-), E(1Sigma+) and H(1Sigma+) states of HBr resembles the dynamics of the V(1Sigma+) as a result of strong Rydberg--valence electrostatic interactions. The Rydberg states of HBr retain more Rydberg character and, consequently, show a strong participation to the direct ionization process similar to the dynamics of the "unperturbed" Rydberg states.; Finally the dissociation of vibrationally excited HX+ ions and formation of the H+ ions is explained using bound-free transitions.