Femtosecond laser observations of molecular vibration and rotation

Abstract

ULTRAFAST molecular vibrations and rotations are the fundamental motions that characterize chemical bonding and determine reaction dynamics at the molecular level. The timescales for these motions are typically 10⁻¹⁰ s for vibrations and 10⁻¹³ s for rotations. For decades, time-integrated (frequency-resolved) spectros-copy has provided a powerful tool for probing the dynamics of motion, but the motions themselves are not 'seen' directly in real-time. With femtosecond laser techniques^1–4 it is now possible to follow the motions of isolated molecular systems as they occur. The requirement is that the system is excited (for vibration) and aligned (for rotation) on a timescale shorter than the vibrational and rotational periods. Here we report real-time observations of these molecular motions. The system—in this case, molecular iodine—is prepared in the particular state(s) of interest by coherent excitation with an initial femtosecond laser pulse, and the subsequent motions are probed with successive femtosecond pulses. The probe monitors changes in the interatomic distance (vibration) or molecular orientation (rotation), so that the measured signal provides direct 'snapshots' of the molecular motions.