ABSTRACT
This chapter considers driving molecular vibrations through two different mechanisms: infrared (IR) or Raman transitions. It shows that direct IR excitation requires the laser frequency match the vibrational frequency of the molecule to generate significant motion. The chapter explains how light interacts with the quantum systems of interest. Interaction with electromagnetic radiation is the primary means for both initiating and probing dynamics. Since the mass of a proton is about 2,000 times the mass of an electron, the proton experiences roughly 2,000 times smaller acceleration than an electron in the same electric field. Given this large difference, the nuclei play a far less important role than the electrons in light–matter interactions. If the molecule has no permanent dipole moment, this interaction Hamiltonian vanishes, and there is no coupling of the vibrational states by the field. In two-photon absorption, the interaction can be thought of as “up-up”, whereas for Raman transitions, the interaction is “up-down”.
