Figure 2.

Schematic of timing between optical heating and arrival of mid-IR probe. The first mid-IR probe shot arrives between 0 and 1 ms (${\tau }_{TJ}^1$) after optical heating is initiated, and subsequent probe shots arrive every 1 ms. ${\tau }_{TJ}^1$ can be electronically controlled or allowed to wander due to the mismatch in mid-IR probe and T-jump (1/τ_T) repetition rate. T_f is held for hundreds of milliseconds to seconds (τ_on) while probe shots are collected according to their time of arrival after heating is initiated (τ_TJ). At τ_on, the laser is switched off for another period of hundreds of milliseconds to seconds (τ_off), and probe shots are tracked as a function of time after the T-jump laser turns off. For detection of nonlinear IR probe signals, every other mid-IR shot is chopped (dashed line). T-jump events are collected in pairs where the second T-jump cycle is delayed 1 ms relative to the first, allowing for chopper subtraction between adjacent T-jump events.