UV-induced dissociation of CH₂BrI probed by intense femtosecond XUV pulses

Abstract

The ultraviolet (UV)-induced dissociation and photofragmentation of gas-phase CH2BrI molecules induced by intense femtosecond extreme ultraviolet (XUV) pulses at three different photon energies are studied by multi-mass ion imaging. Using a UV-pump — XUV-probe scheme, charge transfer between highly charged iodine ions and neutral CH2Br radicals produced by C—I bond cleavage is investigated. In earlier charge-transfer studies, the center of mass of the molecules was located along the axis of the bond cleaved by the pump pulse. In the present case of CH2BrI, this is not the case, thus inducing a rotation of the fragment. We discuss the influence of the rotation on the charge transfer process using a classical over-the-barrier model. Our modeling suggests that, despite the fact that the dissociation is slower due to the rotational excitation, the critical interatomic distance for charge transfer is reached faster. Furthermore, we suggest that charge transfer during molecular fragmentation may be modulated in a complex way.

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

“…The present study is as an extension of a previous publication, 21 where the photodissociation of bromoiodomethane was investigated around the maximum of the first two absorption bands, the A and A′ bands, at 266 and 210 nm, respectively. In a similar fashion than that of the present work, the formation of the I( 2 P 3/2 ), I*( 2 P 1/2 ), Br( 2 P 3/2 ), and Br*( 2 P 1/2 ) photoproducts was experimentally characterized by using the VMI and slice imaging (SI) techniques and 22 which was very much in agreement with the conclusions found by Kockert et al 18 The region of the VUV absorption spectrum of CH 2 BrI in which the present work focuses has not received as much attention as the A and A′ bands. As introduced in the previous paragraphs, only Butler et al 8 studied thoroughly the photodissociation of the title molecule at 193 nm using photofragment translational spectroscopy.…”

“…The main finding of this work was an anchor effect due to the substituent Br atom in the parent molecule, which implies significant rotational motion of the dissociating molecule, and leads to a remarkable rotational energy of the CH 2 Br radical cofragment. It was concluded that this energy flux into internal degrees of freedom of the molecule is the main key factor governing the real time photodissociation dynamics, 22 which was very much in agreement with the conclusions found by Kockert et al 18 The region of the VUV absorption spectrum of CH 2 BrI in which the present work focuses has not received as much attention as the A and A′ bands. As introduced in the previous paragraphs, only Butler et al 8 studied thoroughly the photodissociation of the title molecule at 193 nm using photofragment translational spectroscopy.…”

“…The main finding of this work was an anchor effect due to the substituent Br atom in the parent molecule, which implies significant rotational motion of the dissociating molecule, and leads to a remarkable rotational energy of the CH 2 Br radical cofragment. It was concluded that this energy flux into internal degrees of freedom of the molecule is the main key factor governing the real time photodissociation dynamics, which was very much in agreement with the conclusions found by Köckert et al…”

“…The results, consistent with previous results from Butler and co-workers, 8 revealed in particular secondary dissociation of CH 2 Br through a second UV-photon absorption. Kockert and co-workers 18 have recently published a work where UV pump−XUV probe pulses along with multimass ion imaging were used to study the photodissociation of CH 2 BrI at 271 nm in the A band yielding the CH 2 Br + I channel, where the influence of parent rotation on a charge transfer process was investigated. The results suggest that, in spite of the slow dissociation, the critical interatomic distance for charge transfer is rapidly reached.…”

Energy-dependent timescales in the dissociation of diiodothiophene dication

“…From the fundamental point of view, tracking photoinduced molecular dynamics allows stringent tests of quantum chemistry models over a broad range of conditions. Such experimental studies, many of which have been performed at synchrotron radiation sources, have gained momentum in the past decade owing to the new possibilities offered by free electron laser (FEL) radiation sources, 1–7 high-order harmonic generation (HHG) 8–15 and the development of various time-resolved techniques. 8,16–19 Consequently, there is a wealth of new information on time evolution of photoinduced molecular dynamics in small quantum systems.…”