Transient vibration and product formation of photoexcited CS2 measured by time-resolved x-ray scattering

Abstract

We have observed details of the internal motion and dissociation channels in photoexcited carbon disulfide (CS2) using time-resolved X-ray scattering (TRXS). Photoexcitation of gas-phase CS2 with a 200 nm laser pulse launches oscillatory bending and stretching motion leading to dissociation of atomic sulfur in under a picosecond. During the first 300 fs following excitation we observe significant changes in the vibrational frequency as well as some dissociation of the C-S bond leading to atomic sulfur in the both 1D and 3P states. Beyond 1400 fs the dissociation is consistent with primarily 3P atomic sulfur dissociation. This channel-resolved measurement of the dissociation time is based on our analysis of the time-windowed dissociation radial velocity distribution, which is measured using the temporal Fourier transform of the TRXS data aided by a Hough transform that extracts the slopes of linear features in an image. The relative strength of the two dissociation channels reflects both their branching ratio and differences in the spread of their dissociation times. Measuring the time-resolved dissociation radial velocity distribution aids the resolution of discrepancies between models for dissociation proposed by prior photoelectron spectroscopy work.

Mapping the photochemistry of cyclopentadiene: from theory to ultrafast X-ray scattering

“…Generally, we note that the overall intensities in (b) are greater than in (c), reecting that the BP photoproduct has a geometry which differs more from the initial t < 0 molecular reference geometry than the hot CP. The information on the structural dynamics contained in the signal can be retrieved for instance via either forward optimisation trajectory-tting 24,[75][76][77][78] or million-structure type analysis. 25,79,80 At longer times, 2-10 ps, the signatures of the dynamics in the scattering patterns are less distinct due to more subtle changes in intensity indicative of the dispersion of the wave packet 65,76,81 and increasingly statistical (thermalised) dynamics.…”

Robust Inversion of Time-Resolved Data via Forward-Optimization in a Trajectory Basis

“…The photodissociation of CS 2 , shown schematically in Figure b, has been the subject of numerous time-resolved experiments. ,,− Upon excitation to the 1 B 2 ( 1 Σ u + false) state, rapid bending and stretching vibrational motion is observed. More complex excited state dynamics ensues, exhibiting a striking competition between internal conversion (nonadiabatic couplings) and intersystem crossing (spin–orbit coupling), resulting in two dissociation channels that yield either S( 1 D ) or S( 3 P ) sulfur atoms, with the triplet product dominant.…”

“…In recent years, ultrafast imaging of photoexcited molecules have developed rapidly, with experiments increasingly capable of tracking molecular dynamics on fundamental time scales, observing phenomena such as vibrations, bond breaking, or charge transfer. − The techniques include spectroscopy (e.g., TRPES), − ultrafast electron diffraction (UED), , ultrafast X-ray scattering (UXS), , Coulomb explosion imaging (CEI), and others. Extracting a detailed time-dependent molecular model from this data is extremely challenging.…”

Robust inversion of time-resolved data via forward-optimisation in a trajectory basis

“…The photodissociation of CS 2 , shown schematically in Figure 1b, has been the subject of numerous time-resolved experiments. 11,43,[69][70][71][72][73][74][75][76][77][78][79] Upon excitation to the 1 B 2 ( 1 Σ + u ) state rapid bending and stretching vibrational motion is observed. More complex excited state dynamics ensues, exhibiting a striking competition between internal conversion (nonadiabatic couplings) and intersystem crossing (spinorbit coupling), resulting in two dissociation channels that yield either S( 1 D) or S( 3 P ) sulphur atoms, with the triplet product dominant.…”

“…9 In recent years, ultrafast imaging of photoexcited molecules have developed rapidly, with experiments increasingly capable of tracking molecular dynamics on fundamental timescales, observing phenomena such as vibrations, bond breaking, or charge transfer. [10][11][12] The techniques include spectroscopy (e.g. TRPES), [13][14][15][16] ultrafast electron diffraction (UED), 17,18 ultrafast x-ray scattering (UXS), 19,20 Coulomb explosion imaging (CEI), 21 and others.…”