Probing ultrafast C–Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy

Abstract

UV pump–extreme UV (XUV) probe femtosecond transient absorption spectroscopy is used to study the 268 nm induced photodissociation dynamics of bromoform (CHBr3). Core-to-valence transitions at the Br(3d) absorption edge (∼70 eV) provide an atomic scale perspective of the reaction, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. The XUV spectra track how the singly occupied molecular orbitals of transient electronic states develop throughout the C–Br bond fission, eventually forming radical Br and CHBr2 products. Complementary ab initio calculations of XUV spectral fingerprints are performed for transient atomic arrangements obtained from sampling excited-state molecular dynamics simulations. C–Br fission along an approximately CS symmetrical reaction pathway leads to a continuous change of electronic orbital characters and atomic arrangements. Two timescales dominate changes in the transient absorption spectra, reflecting the different characteristic motions of the light C and H atoms and the heavy Br atoms. Within the first 40 fs, distortion from C3v symmetry to form a quasiplanar CHBr2 by the displacement of the (light) CH moiety causes significant changes to the valence electronic structure. Displacement of the (heavy) Br atoms is delayed and requires up to ∼300 fs to form separate Br + CHBr2 products. We demonstrate that transitions between the valence-excited (initial) and valence + core-excited (final) state electronic configurations produced by XUV absorption are sensitive to the localization of valence orbitals during bond fission. The change in valence electron-core hole interaction provides a physical explanation for spectral shifts during the process of bond cleavage.

Ultrashort XUV pulse absorption spectroscopy of partially oxidized cobalt nanoparticles

“…The ultrafast XUV setup has been described elsewhere. 36,37 HHG is performed by focusing 50% of the output of a 785 nm, 3.3 mJ, 3 kHz, 40 fs full-width-at-half-maximum (FWHM) Ti:sapphire laser system into a semi-infinite gas cell filled with about 150 Torr of neon. A quasi-continuous XUV spectrum consisting of overlapping harmonics is generated, spanning from 48 to 72 eV photon energy.…”

Photoinduced Charge Transfer and Bimetallic Bond Dissociation of a Bi–W Complex in Solution

“…Notably, the ultrafast bimetallic bond dissociation within the instrument response function (Supporting Information) is consistent with the stepwise dissociative electron transfer (DET) theory, wherein simultaneous electron transfer and bond dissociation occur with a dominant effect in the solvent reorganization energy term, which allows for distinction between inertial and diffusive solvent molecular motions. ,− Nondiffusive hindered solvent rotation in MeOH (EtOH) was reported to be 210 fs (290 fs), , closely matching our experimental values of 250 fs (430 fs) in MeOH (EtOH). A similar heavy-atom displacement was observed during the initial separation between Br and CHBr 2 moieties after the 268 nm laser-induced C–Br bond fission of bromoform (CHBr 3 ) in the gas phase, which occurs after ∼300 fs . Next, the retrieved 10 ps (15 ps) component largely matches the solvent longitudinal relaxation time within the first solvation shell. − ,, To rationalize the interesting observation that the ESA peak blue-shifts from ca.…”

“…A similar heavy-atom displacement was observed during the initial separation between Br and CHBr 2 moieties after the 268 nm laser-induced C−Br bond fission of bromoform (CHBr 3 ) in the gas phase, which occurs after ∼300 fs. 56 Next, the retrieved 10 ps (15 ps) component largely matches the solvent longitudinal relaxation time within the first solvation shell. [44][45][46]55,57 To rationalize the interesting observation that the ESA peak blue-shifts from ca.…”

Minimizing the Time-Dependent Density Functional Error in Ehrenfest Dynamics

“…Our results imply that when using TDDFT with adiabatic functionals, LR-Ehrenfest calculations are prefer- able to RT ones. As a last, dramatic, illustration of this, consider the application to femtosecond pump-probe spectroscopy set-ups, where the prospect of probing nuclear motion through electronic spectra has been raised [26][27][28]. The idea is that by comparing the measured time-resolved absorption spectrum of a molecule with the calculated spectrum at different nuclear configurations one can track the nuclear geometries as a function of time.…”

“…As a last, dramatic illustration of this, consider the application to femtosecond pump–probe spectroscopy set-ups, where the prospect of probing nuclear motion through electronic spectra has been raised. − The idea is that by comparing the measured time-resolved absorption spectrum of a molecule with the calculated spectrum at different nuclear configurations one can track the nuclear geometries as a function of time. If LR TDDFT is used to calculate the spectrum then Figure plots the deduced nuclear separation R LRTDDFT (ω) where ω = ω­( t ) is the time-resolved excitation energy of the molecule at time t .…”