Two- and three-body photodissociation dynamics of diiodobromide (I2Br−) anion

Self-assembling of the neutral intermediate with chemically bound argon in photoexcited van der Waals complex Ar-I2

Photodissociation of the van der Waals complex Ar-I₂ after excitation into the Rydberg states of I₂ has been investigated with velocity map imaging of photofragments. Formation of the translationally hot ions of argon Ar⁺ with three modes in kinetic energy distribution has been revealed. The measured dependence of the kinetic energy of Ar⁺ on the pumping photon energy indicates the appearance of Ar⁺ from three channels of the photodissociation of the linear intermediate Ar⁺-I-I^- containing chemically bound argon. These channels are (1) dissociation into Ar⁺+ I₂ ^-; (2) three-body dissociation into (Ar⁺)* + I* + I^-, with (Ar⁺)* and I* being the ²P_1/2 states of the species; and (3) two-body electron photodetachment, giving rise to Ar⁺ + I₂ + e. Three indicated channels are similar to those established for the photodissociation of trihalide anions. This similarity confirms the conclusion on the formation of the Ar⁺-I-I^- intermediate, which is isoelectronic to the trihalide anion Cl-I-I^-. The mechanism of the Ar⁺-I-I^- formation involves two-photon excitation of the complex Ar-I₂ into the Rydberg state of I₂ converted into the ion-pair state and further electron transfer from Ar to I⁺ of the ion-pair state. The self-assembling of the structure making the formation of the Ar⁺-I-I^- intermediate energetically accessible is confirmed by modeling the dynamics in the excited linear complex Ar-I₂. Photoexcitation of the van der Waals complexes of noble gases with halogens into the ion-pair states of halogen is supposed to be a promising approach for generating the new chemical compounds of noble gas atoms.

Photoelectron-photofragment coincidence spectroscopy of the mixed trihalides

Photoelectron-photofragment coincidence (PPC) spectroscopy is used to study the photodetachment, photodissociation, and dissociative photodetachment (DPD) of I₂Br^-, IBr₂ ^-, I₂Cl^-, and ICl₂ ^- at 266 nm. The mixed trihalides are asymmetric analogs of the well-studied I₃ ^- anion, with distinguishable dissociation asymptotes and the potential for selective bond breaking. The high beam energy PPC spectrometer used in this study couples an electrospray ionization source, a hexapole accumulation ion trap, and a linear accelerator to produce a 21 keV beam of a particular trihalide. Total, stable, and dissociative photoelectron spectra have been recorded for all the anions, except ICl₂ ^- that does not photodetach at 266 nm. A bound ground state (X) is observed for all the anions, and a dissociative first excited (A) state is also seen for I₂Br^- and I₂Cl^- at low electron kinetic energies (eKE). A 258 nm photoelectron spectrum recorded for I₂Br^- and I₂Cl^- rules out autodetachment of a dipole-bound state as the origin of the low eKE feature. The threshold detachment energy (TDE) of I₂X^- to the X state of the radical is similar to I₃ ^-, whereas the TDE to the radical A state increases with substitution of iodine for a lighter halogen. Two-body DPD is observed for I₂Br^- and I₂Cl^-, resulting in IBr/ICl + I + e^-. For IBr₂ ^- and ICl₂ ^-, the charge symmetric three-body photodissociation of [Br-I-Br]^- and [Cl-I-Cl]^- is seen yielding Br + Br and Br + Br^*, and Cl + Cl and Cl + Cl^* neutral fragments. Evidence for the minimum energy anion structure is observed in all cases, where the iodine atom is located at the center of the trihalide.

Spectroscopic characterisation of radical polyinterhalogen molecules

Spectroscopic characterisations of the radical polyinterhalogen molecules IF2 and I2F are reported using anion photoelectron spectroscopy. The corresponding parent anions, IF2- and I2F-, are common products formed in hard Ar-CF3I plasmas and are relevant in the semiconductor manufacture industry. The I2F- species, which is present as the [I-I-F]- isomer, is a "non-classical" polyinterhalogen.

Photodissociation of iso-propoxy (i-C3H7O) radical at 248 nm

Photodissociation of the i-C₃H₇O radical is investigated using fast beam photofragment translational spectroscopy.

Photodissociation dynamics of the simplest alkyl peroxy radicals, CH3OO and C2H5OO, at 248 nm

The photodissociation dynamics of the simplest alkyl peroxy radicals, methyl peroxy (CH₃OO) and ethyl peroxy (C₂H₅OO), are investigated using fast beam photofragment translational spectroscopy. A fast beam of CH₃OO^- or C₂H₅OO^- anions is photodetached to generate neutral radicals that are subsequently dissociated using 248 nm photons. The coincident detection of the photofragment positions and arrival times allows for the determination of mass, translational energy, and angular distributions for both two-body and three-body dissociation events. CH₃OO exhibits repulsive O loss resulting in the formation of O(¹D) + CH₃O with high translational energy release. Minor two-body channels leading to OH + CH₂O and CH₃O + O(³P) formation are also detected. In addition, small amounts of H + O(³P) + CH₂O are observed and attributed to O loss followed by CH₃O dissociation. C₂H₅OO exhibits more complex dissociation dynamics, in which O loss and OH loss occur in roughly equivalent amounts with O(¹D) formed as the dominant O atom electronic state via dissociation on a repulsive surface. Minor two-body channels leading to the formation of O₂ + C₂H₅ and HO₂ + C₂H₄ are also observed and attributed to a ground state dissociation pathway following internal conversion. Additionally, C₂H₅OO dissociation yields a three-body product channel, CH₃ + O(³P) + CH₂O, for which the proposed mechanism is repulsive O loss followed by the dissociation of C₂H₅O over a barrier. These results are compared to a recent study of tert-butyl peroxy (t-BuOO) in which 248 nm excitation results in three-body dissociation and ground state two-body dissociation but no O(¹D) production.

Identification of photofragmentation patterns in trihalide anions by global analysis of vibrational wavepacket dynamics in broadband transient absorption data

Photodissociation pathways of a trihalide series are systematically investigated by globally fitting vibrational wavepacket signals in broadband transient absorption spectra.

Investigation of 3-fragment photodissociation of O3 at 193.4 and 157.6 nm by coincident measurements

Photodissociation of the ozone molecule at 193.4 nm (6.41 eV) and 157.6 nm (7.87 eV) is studied by fast-beam translational spectroscopy. Coincident detection of the dissociation products allows direct observation of the 3-fragment channel and determination of its kinematic parameters. The results indicate that at each wavelength, 3-fragment dissociation proceeds through synchronous concerted bond breaking, but the energy partitioning among the fragments is different. The branching fraction of the 3-fragment channel increases from 5.2(6)% at 193.4 nm to 26(4)% at 157.6 nm, in agreement with previous studies. It is shown that vibrational excitation of the symmetric stretch mode in O3 molecules created by photodetachment of O(3)(-) anion enhances the absorption efficiency, especially at 193.4 nm, but does not have a strong effect on the 3-fragment dissociation.

Photodissociation dynamics of the thiophenoxy radical at 248, 193, and 157 nm

The photodissociation dynamics of the thiophenoxy radical (C6H5S) have been investigated using fast beam coincidence translational spectroscopy. Thiophenoxy radicals were produced by photodetachment of the thiophenoxide anion followed by photodissociation at 248 nm (5.0 eV), 193 nm (6.4 eV), and 157 nm (7.9 eV). Experimental results indicate two major competing dissociation channels leading to SH + C6H4 (o-benzyne) and CS + C5H5 (cyclopentadienyl) with a minor contribution of S + C6H5 (phenyl). Photofragment mass distributions and translational energy distributions were measured at each dissociation wavelength. Transition states and minima for each reaction pathway were calculated using density functional theory to facilitate experimental interpretation. The proposed dissociation mechanism involves internal conversion from the initially prepared electronic excited state to the ground electronic state followed by statistical dissociation. Calculations show that SH loss involves a single isomerization step followed by simple bond fission. For both SH and S loss, C-S bond cleavage proceeds without an exit barrier. By contrast, the CS loss pathway entails multiple transition states and minima as it undergoes five membered ring formation and presents a small barrier with respect to products. The calculated reaction pathway is consistent with the experimental translational energy distributions in which the CS loss channel has a broader distribution peaking farther away from zero than the corresponding distributions for SH loss.