UV pump vibrational sum frequency generation probe spectroscopy with fluorescence suppression using a spatial filter

Time-resolved vibrational sum-frequency generation (TR-VSFG) spectroscopy probes photoinduced structural dynamics at interfaces, but UV-pump TR-VSFG experiments are often hampered by fluorescence from chromophores, which can overwhelm the weak SFG signal. Here, we demonstrate a spatial filter as a simple, effective method to suppress this fluorescence background. Because fluorescence is emitted incoherently across a broad range of angles while the coherent SFG signal propagates in a well-defined spatial mode, the spatial filter selectively rejects the former while preserving the latter. Using the UV-photoexcitation of phenol at the air-water interface as a test case, we show that the spatial filter suppresses the fluorescence background by 86.4% while reducing spectral noise by up to an order of magnitude, all with little attenuation of the resonant vibrational signal itself. This approach should be broadly applicable to UV-pump TR-VSFG studies of photoinduced dynamics at aqueous and other interfaces.