Probing gaseous photothermal effects via vibrational dispersion spectroscopy

Photothermal spectroscopy in gases enables sensitive molecular detection by inferring absorption from laser-induced refractive-index changes following rovibrational excitation. In gas mixtures, however, the photothermal response varies with composition because it is governed by collisional energy transfer and coupled thermodynamic and transport processes. These species-dependent dynamics are difficult to disentangle when the refractive index is probed only at an off-resonant wavelength. In this work, we employ background-free vibrational dispersion spectroscopy to characterize photothermal responses in gases. The method probes the refractive index at vibrationally resonant wavelengths to retrieve both steady-state density and differential density changes associated with individual molecular species in gas mixtures. This capability provides a versatile platform for studying the thermodynamics and transport underlying photothermal phenomena, with applications in combustion diagnostics and chemical-synthesis monitoring.