Photo-thermal 2D spectroscopy: a different type of action

Advances in multidimensional spectroscopy have seen the rise of action detection, where coherent response is encoded into incoherent signals. These are typically proportional to the excited-state population, an example in 2D electronic spectroscopy (2DES) is fluorescence-detected 2DES (F-2DES), and in 2D infrared (2DIR) tag-loss 2DIR (TL-2DIR). Very recently, a new type of photo-thermal 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 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 photo-thermal 2DES (PT-2DES). PT-2DES closely resembles conventional 2DES, while enjoying the advantages of action detection.