Excited state dynamics of CH2I2 and CH2BrI studied with UV pump VUV probe photoelectron spectroscopy

Abstract

We compare the excited state dynamics of diiodomethane (CH2I2) and bromoiodomethane (CH2BrI) using time resolved photoelectron spectroscopy. A 4.65 eV UV pump pulse launches a dissociative wave packet on excited states of both molecules and the ensuing dynamics are probed via photoionization using a 7.75 eV probe pulse. The resulting photoelectrons are measured with the velocity map imaging technique for each pump-probe delay. Our measurements highlight differences in the dynamics for the two molecules, which are interpreted with high-level ab initio molecular dynamics (trajectory surface hopping) calculations. Our analysis allows us to associate features in the photoelectron spectrum with different portions of the excited state wave packet represented by different trajectories. The excited state dynamics in bromoiodomethane are simple and can be described in terms of direct dissociation along the C–I coordinate, whereas the dynamics in diiodomethane involve internal conversion and motion along multiple dimensions.

Spectroscopic and Structural Probing of Excited-State Molecular Dynamics with Time-Resolved Photoelectron Spectroscopy and Ultrafast Electron Diffraction

“…In the molecular ground state, CH 2 I 2 absorbs strongly at wavelengths corresponding to both our pump and probe laser pulses, which means that near time zero, when the pump and probe pulses overlap, the signal has contributions from both UV and VUV driven dynamics. Figure 2(a) shows the experimental data [73]. In order to highlight the UV driven dynamics, we multiply the signal at positive time delays outside the temporal overlap of pump and probe pulses by a factor of 4 since we estimate that the VUV absorption cross section is about 4 times that of the UV absorption cross section [66].…”

“…In performing a more detailed analysis of the TRPES, a natural question is whether different trajectories are responsible for the different features in the photoelectron spectrum. In order to address this question, our previous work looked for and found a correlation between trajectories that led to symmetric C-I bond stretching and produced a low-energy tail in the photoelectron spectrum versus trajectories that featured asymmetric C-I stretching and did not contain a low-energy tail [73]. We thus sorted the trajectories into two groups based on whether they displayed symmetric or asymmetric stretching of the C-I bonds.…”

“…A detailed description of the photoelectron VMI image processing is provided in Ref. [73]. Most of the details of the UED data processing can be found in the Supplemental Material for Ref.…”

“…More details are available in Ref. [73]. For the case of UED, the time evolution of the molecular geometry along the trajectory was used to calculate the electron diffraction pattern at each delay.…”

Molekulare Photochemie: Moderne Entwicklungen in der theoretischen Chemie

“…[172][173][174] Ausd iesem Grund wurde im letzten Jahrzehnt ein großer Entwicklungsaufwand hin zu genaueren und flexibleren Tr ajektorien-basierten Verfahren fürn ichtadiabatische Dynamik betrieben, wie in mehreren aktuellen Übersichtsartikeln dargelegt ist. [141,[175][176][177][178][179][180] Die große Popularitätd es TSH-Verfahrens aufgrund seiner Einfachheit und rechnerischen Effizienz lässt sich aus der großen Zahl seiner Anwendungen ablesen, welche von der Chemie [181][182][183][184][185] zur Biologie, [186][187][188][189][190] von Überflächenchemie [191][192][193] bis zu konjugierten Polyenen [194] und von molekularen Rotationsmotoren [116,195] bis zu Übergangsmetallkomplexen [11,196] reichen.…”