We probe the dynamics of dissociating 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.
Femtosecond pump-probe photoelectron spectroscopy measurements using an extreme ultravioletprobe have been made on the photodissociation dynamics of UV (269 nm) excited CH3I. TheUV excitation leads to population of the 3Q0...
The products formed following the photodissociation of UV (200 nm) excited CS 2 are monitored in a time resolved photoelectron spectroscopy experiment using femtosecond XUV (21.5 eV) photons. By spectrally resolving the electrons we identify separate photoelectron bands related to the CS 2 + hν −→ S( 1 D) + CS and, CS 2 + hν −→ S( 3 P) + CS dissociation channels which show different appearance and rise times. The measurements show that there is no delay in the appearance of the S( 1 D) product contrary to the results of J. Chem. Phys. 147, 013932 (2017). Analysis of the photoelectron yield associated with the atomic products allows us to obtain a S( 3 P)/S( 1 D) branching ratio and the rate constants associated with dissociation and intersystem crossing rather than the effective lifetime observed through the measurement of excited state populations alone.
The dissociation dynamics of CH 3 I at three UV pump wavelengths (279 nm, 254 nm, 243 nm) are measured using an extreme ultraviolet probe in a timeresolved photoelectron spectroscopy experiment. The results are compared with previously published data at a pump wavelength of 269 nm, [Phys. Chem. Chem. Phys., 2020, 22, 25695], with complementary photoelectron spectroscopy experiments performed using a multiphoton ionization probe [Phys. Chem. Chem. Phys., 2019, 21, 11142] and with the recent action spectroscopy measurements of Murillo-Sánchez et al. [J. Chem. Phys., 2020, 152, 014304]. The measurements at 279 nm and 243 nm show signals that are consistent with rapid dissociation along the C-I bond occurring on timescales that are consistent with previous measurements. The measurements at 254 nm show a significantly longer excited state lifetime with a secondary feature appearing after 100 fs is indicative of more complex dynamics in the excited state. The timedependence of the changes are consistent with the previously measured multiphoton ionization photoelectron spectroscopy measurements of Warne et al., [Phys. Chem. Chem. Phys., 2019, 21, 11142]. The consistency of the signal appearance across ionization processes suggests that the extended observation time at 254 nm is not an artefact of the previously used multiphoton ionization process but is caused by more complex dynamics on the excited state potential. Whether this is caused by complex vibrational dynamics on the dominant 3 Q 0 state or due to enhanced population and dynamics on the 1 Q 1 state remains an open question.
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