A femtosecond pump–probe spectrometer for dynamics in transmissive polymer films

An experimental setup and data collection strategy for femtosecond transient absorption spectroscopy on thin ($$<$$<1 $$\upmu \hbox {m}$$μm) solid polymer film samples is described. The experiment allows for parallel detection of the changes in optical density $$\Delta {\mathrm{OD}}$$ΔOD via broadband supercontinuum probing in the VIS/UV range and single-color detection at an independently selected wavelength from the deep UV to the IR with a sensitivity of $$\Delta {\mathrm{OD}} \approx 10^{-3}$$ΔOD≈10-3 per laser shot (r.m.s. standard deviation) and a time resolution below 40 fs. A fast and reproducible bi-directional translation of a two-dimensional film sample of $$20 \times 20\,\hbox {mm}^2$$20×20mm2 in size is used to measure fresh sample spots at each detection interval. Signal readout at a 1 kHz rate enables single-shot analysis and automated signal discrimination, as well as detailed statistics on sample homogeneity, signal evolution with increasing number of pump pulses, and reproducibility. The technique was employed to study the photoisomerization of Disperse Red 1 in films of polymethylmethacrylate after $$\pi \pi ^*$$ππ∗ photoexcitation at $$\lambda =473$$λ=473 nm. The results revealed excited-state dynamics characterized by time constants of $$\tau _1 = 0.06$$τ1=0.06 and $$\tau _2 = 1.0\,\hbox {ps}$$τ2=1.0ps, almost identical as in solution, but evidently enhanced vibrational excitation and slower vibrational cooling (time constant $$\tau _3 = 14\,\hbox {ps}$$τ3=14ps) after return to the electronic ground state due to the constraining polymer environment.