Photothermal 2D Spectroscopy: A Different Type of Action.

Advances in multidimensional spectroscopy have seen the rise of action detection, where the coherent response is encoded into incoherent signals. These are typically proportional to the excited-state population. Examples include fluorescence-detected 2D electronic spectroscopy (F-2DES) and tag-loss 2D infrared spectroscopy (TL-2DIR). Very recently, a new type of photothermal action signal has been introduced in 2DIR, detecting the generated heat by atomic force microscopy-based 2DIR (AFM-2DIR). We present a unified theoretical framework for population- and heat-based 2D spectroscopy, highlighting their complementary features. In the infrared, TL-2DIR reflects the system's linear response, whereas AFM-2DIR produces spectra resembling conventional 2DIR, with sensitivity to anharmonicity and mode coupling. Our model reproduces key experimental AFM-2DIR features, confirming measurement in a highly nonlinear regime. Extending photothermal detection to electronic spectroscopy, we compare F-2DES with proposed photothermal 2DES (PT-2DES). PT-2DES closely resembles conventional 2DES, while enjoying the advantages of action detection.