Mapping the Complete Reaction Path of a Complex Photochemical Reaction

We probe the dynamics of dissociating ${\mathrm{CS}}_2$ molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms.

Figure 1

Overview of the potential energy surfaces of ${\mathrm{CS}}_2$ leading to dissociation, with a 3D rendering of one singlet and one triplet potential energy surface, and potential energy curves of the relevant electronic states shown at linear and bent geometry along the C–S bond stretching coordinate (with the second C–S bond kept at equilibrium length $R_{\mathrm{CS}}=1.569\AA$). Solid lines represent states of $A^{\prime }$ symmetry, and dashed lines represent states of $A^{\prime \prime }$ symmetry. From top to bottom: linear (178°) singlets; linear (178°) triplets; bent (120°) singlets; and bent (120°) triplets. Several states are (near) degenerate at linear geometry. The potential energies are calculated at the MRCI(14,10)/aug-cc-pVTZ level (further details and angular potentials in Supplemental Material [24]).

Figure 2

Pump-probe photoelectron spectra. (a) Ground state photoelectron spectrum (black line, left axis). Average background-subtracted spectra obtained at early (40–540 fs, red line, right axis) and late (4.04–10.54 ps, blue line, right axis) times following a 6.2 eV pump. Negative features correspond to ground state bleaching, while positive features correspond to new bands in the photoelectron spectrum. The combs mark assignments based on known ionization limits of the produced fragments. (b) False color surface map showing the changes in the background-subtracted photoelectron spectrum as a function of the pump-probe time delay.

Figure 3

Pump-probe kinetics. (a) Time-dependent photoelectron intensity plots corresponding to the ground state, ${\mathrm{CS}}_2(\tilde{X})$; the excited singlet states, ${\mathrm{CS}}_2(S)$; the excited triplet states ${\mathrm{CS}}_2(T)$; the triplet dissociation products, $\mathrm{S}({}^{3}P)$; and the singlet dissociation products, $\mathrm{S}({}^{1}D)$. For $\mathrm{S}({}^{1}D)$, solid points are used where the data are reliable and hollow symbols once the $\mathrm{S}({}^{3}P)$ state contributes to the measured intensity. Dashed blue lines are the results of fits to the kinetic model. The dashed orange line in the $\mathrm{S}({}^{1}D)$ is the kinetic fit associated with triplet state formation and confirms the origin of the delayed rise in this region. (b) An expanded view of the photoelectron spectrum in the region covered by the $\mathrm{S}({}^{1}D)$ and $\mathrm{S}({}^{3}P)$ dissociation products. Each spectrum is the average of three time delays with the average delay provided in the legend. The highlighted regions cover those used for the $\mathrm{S}({}^{1}D)$ and $\mathrm{S}({}^{3}P)$ plots in (a).