Ultrafast spin-flip is used to monitor the subpicosecond intersystem crossing dynamics from the 1Π to the 3Π state following photodissociation of ClF isolated in an Ar matrix by means of pump–probe spectroscopy. After photoexcitation of the 1Π state analysis of the populations of triplet states shows that about 50 percent of the spin-flip occurs during the first bond stretch which takes about 250 fs. The early time dynamics of the Cl–F bond in an Ar matrix is investigated theoretically by selecting representative singlet and triplet excited states from a diatomics-in-molecules Hamiltonian. In a one-dimensional model, wave-packet simulations for the first excursion are performed which give a lower limit of about 60 fs for the spin-flip process. The ultrafast spin flip is supported by the caging of the wave packet by the neighboring Ar atoms. Already before collision of the F and Ar atoms the rather large energy gap between the 1Π and 3Π states in the Franck–Condon region is reduced rapidly to near degeneracy. As a consequence the spin–orbit interaction becomes dominant, inducing more than 40% admixture of the triplet character in the 1Π state. Subsequent kinetic energy transfer from ClF to Ar, not yet included in the model, should slow down the Cl and F atoms on their way back toward shorter bond distances, implying stabilization of the wave packet in the 3Π state, where it is monitored by the probe laser pulse.
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