Imaging the Photodissociation Dynamics and Fragment Alignment of CH2BrI at 193 nm

Abstract

The photodissociation dynamics and photofragment alignment of bromoiodomethane (CH₂BrI) have been studied at 193 nm using a double experimental and theoretical approach. In addition, the ultraviolet (UV)-vacuum ultraviolet (VUV) absorption spectrum of gas phase CH₂BrI has been measured in the photon energy range of 5-11 eV using the VUV Fourier transform spectrometer (FTS) at the VUV beamline DESIRS of the synchrotron SOLEIL facility. The slice imaging technique in combination with resonance enhanced multiphoton ionization (REMPI) detection of the Br(²P_J) and I(²P_J) (with J = 3/2 and 1/2 for Br/I and Br*/I*, respectively) atomic photofragments have been used to produce experimental translational energy and angular distributions, which were analyzed to deliver, on one hand, the partitioning of the available energy among the different degrees-of-freedom of the photofragments and, on the other, the photofragment polarization in terms of a_q^k(p) alignment parameters. The experimental measurements were rationalized in terms of high-level ab initio calculations of vertical excitation energies, transition dipole moments and potential energy curves (PECs) along different reaction coordinates to provide a complete picture of the photodissociation dynamics. The results indicate that for excitation at 193 nm, prompt C-X cleavage (with X being either halogen atom, Br or I) competes with fast internal conversion and consequent stochastic dissociation in lower electronic states. In the case of the CH₂Br + I(²P_3/2)/I*(²P_1/2) channels, the dynamics are greatly biased toward the stochastic dissociation process due to both the particular PECs landscape and the unfavored excitation of the CH₂BrI ensemble with respect to the C-I molecular axis at this excitation energy. The ab initio PECs provide a tentative path for the fast dissociation process in either case. For the C-Br bond breakage, excitation to the 13A' electronic state and predissociation through the 11A'/11A^″ or 12A'/12A^″ states, leading to direct dissociation through the 10A'/9A″ states, appear as the most consistent dynamics. For the C-I channel, predissociation does not become a reliable possibility and a fast internal conversion may precede dissociation through the repulsive 6A'/6A^″ and 4A'/4A^″ states. The large content of rotational and vibrational excitation of the polyatomic cofragments is justified through the soft impulsive model and the geometrical changes produced along the dissociation pathway. Strikingly, the a_q^k(p) alignment parameters obtained for the Br(²P_3/2) and I(²P_3/2) photoproducts indicate that the rotational angular momentum of the CH₂X (X = I or Br) cofragment appears highly constrained along the recoil direction. Finally, this work presents a highly plausible explanation for the branching ratio of secondary dissociation processes in the photodynamics of CH₂BrI at 193 nm.