Photodissociation dynamics of bromoiodomethane from the first and second absorption bands. A combined velocity map and slice imaging study

Abstract

The photodissociation dynamics of bromoiodomethane (CHBrI) have been investigated at the maximum of the first A and second A' absorption bands, at 266 and 210 nm excitation wavelengths, respectively, using velocity map and slice imaging techniques in combination with a probe detection of both iodine and bromine fragments, I(P), I*(P), Br(P) and Br*(P) via (2 + 1) resonance enhanced multiphoton ionization. Experimental results, i.e. translational energy and angular distributions, are reported and discussed in conjunction with high level ab initio calculations of potential energy curves and absorption spectra. The results indicate that in the A-band, direct dissociation through the 5A' excited state leads to the I(P) channel while I*(P) atoms are produced via the 5A' → 4A'/4A'' nonadiabatic crossing. The presence of Br and Br* fragments upon excitation to the A-band is attributed to indirect dissociation via a curve crossing between the 5A' with upper excited states such as the 9A'. The A'-band is characterized by a strong photoselectivity leading exclusively to the Br(P) and Br*(P) channels, which are likely produced by dissociation through the 9A' excited state. Avoided crossings between several excited states from both the A and A' bands entangle however the possible reaction pathways.

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

“…The most remarkable information provided by the present work is the similarity between the experimental results obtained at 193 nm (excitation energy of 6.424 eV, assigned to the 2 0 1 vibronic band of the 13 A ′ ← X̃ 1 A ′ transition–see above), with those previously obtained at 266 and 210 nm for dissociation into Br( 2 P J ) from the A and A ′ bands, respectively …”

“…The deconvolution analysis of the Br* and I* distributions into three different components, as described above, is amply justified by three relevant arguments. First, the results depicted in Figure are similarly reproduced when certain experimental conditions, such as the laser power and the temperature of the molecular beam, were varied; second, these three contributions have been previously reported at other excitation wavelengths by different authors, including our previous work on the CH 2 BrI photodissociation from the A and A ′ bands; third, the measured photofragment polarization discussed hereafter supports the existence of the three contributions.…”

“…e ., 9 A ′ ← X̃ 1 A ′ based on the high-level ab initio calculations carried out in the present worksee below). As in our previous work, this band is referred to as the A ′ band. This band has been previously reported ,,,− where a general agreement is noted for its maximum position, yet the absolute cross-section values in the wavelength range 215–250 nm (5.767–4.959 eV) of Mössinger et al are 9–35% lower than the present high-resolution data (see Figure ).…”

“…A low-recoil, Boltzmann-like contribution was assigned to REMPI-assisted multiphoton dissociative ionization. The middle contribution (translational energy of 0.25 eV at 266 nm and 0.5 eV at 210 nm), which constituted the main result, was assigned, in the case of excitation at 266 nm ( A band), to indirect dissociation via absorption to the 5 A ′ state and subsequent curve crossing with upper excited dissociative states such as the 9 A ′, while upon excitation at 210 nm ( A ′ band), it was attributed to direct dissociation in the 9 A ′ excited state. A third broad and faster contribution was tentatively interpreted in terms of secondary dissociation of the CH 2 Br fragment upon an additional one-photon absorption.…”

Coulomb Explosion Imaging for the Visualization of a Conical Intersection

“…Velocity map images of the charged fragments were obtained and Abel inverted using the pBasex procedure . High level ab initio calculations in combination with surface hopping semiclassical dynamics − were performed to describe the effect of CE during the photodissociation dynamics. Details about both the experimental and computational procedures can be found in the Supporting Information.…”