Time-resolved high-harmonic spectroscopy of the photodissociation of CH3I and CF3I

The photodissociation dynamics of iodomethane (CH3I) and trifluoroiodomethane (CF3I) have been studied using time-resolved high-harmonic spectroscopy. The molecules were photoexcited in a transient grating geometry to their respective A bands by laser pulses centred at 267 nm and were probed through high-harmonic generation driven by a delayed 800 nm laser pulse. Both molecules display a fast buildup of the diffracted high harmonics (H9-H15) followed by a slower decay, while the undiffracted harmonics exhibit an opposite modulation. The time scales on which the signals reach their asymptotic values were found to be slower in CF3I compared to CH3I and to weakly increase with high-harmonic order in both molecules. In the case of CH3I the obtained time scales are in agreement with dissociation times measured by other techniques. Our results on CF3I constitute the first time-resolved measurements of the A-band dissociation. A simple theoretical model for the variation of the harmonic phase induced by the changing vertical ionisation potential is combined with one-dimensional nuclear wave-packet propagation and shown to qualitatively account for the main observations.