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.
Highlights
Inner-shell ionization by extreme-ultraviolet (XUV) or soft-xray pulses and the subsequent Auger relaxation is an efficient way of creating multiply-charged molecular ions, which often fragment extensively via a process known as Coulomb explosion [1,2,3,4, 6]
Spin-orbit-excited iodine atoms I∗(2 p1/2) are produced on the 4A and 4A states via an avoided crossing from the 5A state. Their production ratio was measured to be 4:3 at an excitation wavelength of 248 nm [14], and the I∗(2 p1/2) yield was shown to be relatively independent of the excitation wavelength between 222 nm and 280 nm for CH2ICl [21]. In molecules such as CH3I [8, 9, 22] or C6H3F2I [23], in which we have studied ultrafast charge transfer following inner-shell ionization in the past, the center of mass of the molecule lies along the C–I bond, almost all available energy is transferred to dissociation along this bond
The experiment was carried out using the CFEL-ASG MultiPurpose (CAMP) instrument [24] installed permanently at beamline BL1 of the free-electron laser in Hamburg (FLASH) [12]
Summary
Inner-shell ionization by extreme-ultraviolet (XUV) or soft-xray pulses and the subsequent Auger relaxation is an efficient way of creating multiply-charged molecular ions, which often fragment extensively via a process known as Coulomb explosion [1,2,3,4, 6]. When performing Coulomb explosion experiments with femtosecond pulses from free-electron lasers (FELs), molecular dynamics and fragmentation processes of gas-phase samples can be studied in a time-resolved fashion [5, 7,8,9,10,11]. We focus on the ultraviolet (UV)-induced photodissociation of bromoiodomethane, CH2BrI, with the intent to investigate charge transfer after XUV inner-shell ionization at different molecular sites. A recent experiment employing 8 eV probe pulses compared time-resolved ion yields of CH2BrI and
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