Imaging transient molecular configurations in UV-excited diiodomethane

TL;DR

This work investigates how UV excitation of $CH_{2}I_{2}$ drives multiple fragmentation channels and ultrafast transient geometries. It applies time-resolved three-body Coulomb Explosion Imaging with UV pump at 290/330 nm and an $810$ nm near-infrared probe to map kinetic energies, momentum correlations, and channel assignments. A key finding is the observation of transient iso-$CH_{2}I_{2}$-like geometries formed within ~100 fs and decaying within ~100 fs, alongside the dominant $CH_{2}I$ + $I$, $I_{2}$ formation, and CH$_{2}$ + $I$ + $I$ channels. The results demonstrate CEI's ability to disentangle competing photochemical pathways and reveal ultrafast structural dynamics in a polyhalogenated alkane, with similar behavior across the two excitation wavelengths.

Abstract

Femtosecond structural dynamics of diiodomethane ($\mathrm{CH_2I_2}$) triggered by ultraviolet (UV) photoabsorption at 290 nm and 330 nm are studied using time-resolved coincident Coulomb explosion imaging driven by a near-infrared probe pulse. We map the dominant single-photon process, the cleavage of the carbon-iodine bond producing rotationally excited $\mathrm{CH_2I}$ radical, identify the contributions of the three-body ($\mathrm{CH_2} + \mathrm{I} + \mathrm{I}$) dissociation and molecular iodine formation channels, which are primarily driven by the absorption of more than one UV photon, and demonstrate the existence of a weak reaction pathway involving the formation of short-lived transient species resembling iso-$\mathrm{CH_2I{-}I}$ geometries with a slightly shorter I-I separation compared to the ground-state $\mathrm{CH_2I_2}$. These transient molecular configurations, which can be separated from the other channels by applying a set of conditions on the correlated momenta of three ionic fragments, are formed within approximately 100 fs after the initial photoexcitation and decay within the next 100 fs.

Figures (8)

• Figure 1: Sketch depicting the pump-probe process and the different reaction pathways after photoexcitation of $\mathrm{CH_{2}I_{2}}$ (left) by the ultraviolet (UV) pump pulse. The products and possible intermediates (middle) are ionized to five-fold final charge state by the intense near-infrared (NIR) probe pulse, and the ionic fragments are detected in coincidence.
• Figure 2: Schematic of the pump-probe setup and the coincident ion momentum imaging system. The diagram depicts the propagation of the ultraviolet (UV) pump and near-infrared (NIR) probe pulses into the experimental chamber. Both pulses are colinearly directed and focused onto a cold supersonic molecular jet containing diiodomethane. The setup is integrated with a double-sided velocity map imaging (VMI) spectrometer, operated in multi-ion coincidence mode. Ions are detected in coincidence using a time and position-sensitive detector comprising a microchannel plate (MCP) and delay line anode assembly. It should be noted that in this particular experiment, the electron detector is not utilized.
• Figure 3: (a) $\mathrm{CH_{2}^{+} + I^{2+} + I^{2+}}$ ion coincidence yield as a function of pump-probe delay and KER. Positive delays correspond to the NIR probe pulse arriving after the UV pump pulse. A step size of 15 fs and 100 fs was used for delays up to and beyond 800 fs, respectively. (b) KER spectra of the same coincidence channel recorded at a fixed delay of 13.5 ps. The data shown here were recorded at a pump wavelength of 290 nm. The equivalent plots for this and all the following figures with the data recorded at 330 nm are shown in Section II of the SM.
• Figure 4: (a) $\mathrm{CH_{2}^{+} + I^{2+} + I^{2+}}$ ion coincidence yield as a function of KER and angle between the momentum vectors of the two iodine dications for a fixed pump-probe delay of 13.5 ps. Four distinct regions are marked by colored ovals corresponding to different contributions to the coincidence ion yield: Black: Coulomb explosion of bound molecules in or near the equilibrium geometry. Red: C-I cleavage and dissociation to $\mathrm{CH_{2}I}$ + $\mathrm{I}$. Green: Molecular iodine ($\mathrm{I_2}$) formation after UV absorption. Blue: UV-induced three-body dissociation into $\mathrm{CH_{2}}$ + I + I. (b) Coulomb explosion simulations of $\mathrm{CH_{2}I_{2}}$ for the same $\mathrm{CH_{2}^{+} + I^{2+} + I^{2+}}$ channel observed in the experiment, for different molecular geometries: the ground-state equilibrium geometry, the iso-$\mathrm{CH_{2}I_{2}}$ geometry according to Borin et al.Borin2016DirectStudy, and for the three dissociation processes that are identified and marked in (a).
• Figure 5: Coulomb explosion simulations of $\mathrm{CH_{2}I_{2}}$ for the $\mathrm{CH_{2}^{+} + I^{2+} + I^{2+}}$ breakup channel for different molecular geometries: the ground-state equilibrium geometry, the iso-$\mathrm{CH_{2}I_{2}}$ geometry according to Borin et al.Borin2016DirectStudy, and for a specific geometry during the C-I dissociation process when the $\mathrm{CH_{2}}$-I-I angle in the dissociating molecule is the same as in iso-$\mathrm{CH_{2}I_{2}}$ due to the rotation of the $\mathrm{CH_{2}I}$ radical. The two panels show the distribution of these events (a) as a function of the sum KE of the two iodine fragments and angle between the two $\mathrm{I^{2+}}$ momentum vectors, and (b) normalized yield of the KE sum of the two $\mathrm{I^{2+}}$ ions integrated for all angles.