Coherent phonon dynamics: Br2 in solid Ar.

A long lasting coherent oscillation with a sharp frequency of f(p) = 2 THz is observed in fs pump probe spectra for B <-- X excitation of Br2 in solid argon. It exactly matches the frequency of a coherent zone boundary phonon (ZBP) of the Ar environment. The ZBPs have a vanishing group velocity v(g), thus they stay in the vicinity of the chromophore. They originate from a displacive excitation of coherent phonons (DECP) initiated in the electronic B <-- X transition, because neither f(p) nor the phase of the oscillation do depend on the B state vibrational dynamics. A model calculation shows that an expansion of the electronic density in going from the electronic ground state X to the B state kicks the Ar atoms in the Br2 vicinity. In addition, a group of Ar atoms in the (100) plane is decoupled from the intramolecular dynamics afterwards. The ZBP modulates the solvation energy of the terminal charge transfer states used in the probe transition from the B state and thus the detection sensitivity. The contrast is enhanced by probing the B state wave packet with the cutting edge of the probe window. This is in full accordance with a study for I2 : Kr.