We report the time-resolved observation of the photodissociation of ${\mathrm{Br}}_2$ using high-harmonic generation (HHG) as a probe. The simultaneous measurement of the high-harmonic and ion yields shows that high harmonics generated by the electronically excited state interfere with harmonics generated by the ground state. The resulting homodyne effect provides a high sensitivity to the excited state dynamics. We present a simple theoretical model that accounts for the main observations. Our experiment paves the way towards the dynamic imaging of molecules using HHG.

• Received 29 April 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.103002