): This is where the magic happens. A cross peak means a molecule absorbed energy at ω1omega sub 1 , but after waiting a time , it emitted light at ω3omega sub 3
Time (Upward) | |g> | Arrow pointing right (Interaction on Ket) | | |g> ): This is where the magic happens
Different techniques filter out specific pathways using , a condition that selects signals based on the directions of their emitted light. The rotating wave approximation (RWA) then simplifies the treatment by ignoring terms that don't conserve energy, such as those that would create molecules in an excited state without an incoming photon. The result is a set of "Liouville pathways" that form the core of the calculation. The result is a set of "Liouville pathways"
Think of nonlinear spectroscopy as a guided by pulses of light. The "practical approach" (often called the diagrammatic approach) focuses on how the sample's density matrix interacts with these pulses. Pulse 1 ( Pulse 1 ( mean a photon is absorbed
mean a photon is absorbed. This pushes that side of the density matrix to a higher energy level.
The bottom of the diagram is the start of the experiment (usually the ground state population, ), and the top is the final signal emission. Arrows are Laser Interactions: An arrow pointing right ( →right arrow ) means a photon is interacting with the ket (left side). An arrow pointing left ( ←left arrow ) means a photon is interacting with the bra (right side). Arrow Direction Means Absorption or Emission: An arrow pointing into the diagram ( →right arrow ←left arrow ) represents absorbing a photon or adding a field.