The features in linear absorption spectra can be exquisitely sensitive to the electronic coupling between organic molecules in a molecular aggregate. The spectral signatures of molecular aggregation are the result of electronic coupling, which is determined by the physical arrangement of the molecules. In this work, the absorbance of pseudoisocyanine (PIC) is measured in situ after solution drop casting to reveal a distinct intermediate stage during the aggregation process. A possible composition and structure for the molecular aggregates during this stage is inferred by using a Holstein-like Hamiltonian to calculate an absorption spectrum with spectral features that match those of the measured spectrum. More than one type of aggregate is required to compute a spectrum that agrees with the measured spectrum within this model. In this case, the spectrum can be fit with a trimer and an aggregate with 9 molecules with electronic coupling values of +600 cm<sup>-1</sup> and -600 cm<sup>-1</sup>, respectively. We report a procedure to compute spectra that agree with measured spectra and limits the number of iteratively fit parameters. This strategy will enable the interpretation of in situ absorption data for other conjugated molecules during molecular aggregation and provide insight into the evolving composition of aggregates during the process of film formation.
A novel spectroscopy termed single shot transient absorption (SSTA) is presented that can collect a transient absorption spectrum in 6 ms by using laser pulses with tilted wavefronts to spatially encode the delay between pump and probe pulse arrival times at the sample. The transient absorption technique determines the change in sample transmission that results from sample photoexcitation, and tracks this change as a function of the time delay between the arrival of the pump pulse and the probe pulse. Typically, these time delays are generated using a retroreflecting mirror mounted on a motorized translation stage, with a measurement collected at each translation stage position. Because these measurements must be performed in series, data collection requires a significant amount of time. This limits transient absorption to the measurement of systems that are static for the duration of the experiment. SSTA overcomes this restriction by employing pump and probe pulses which are each focused into a line and tilted with respect to each other to spatially encode time delays within the sample. Here, we describe the SSTA technique and instrumentation, demonstrate the principle of this spectroscopy, and present a method for calibrating the spatially encoded time delay by autocorrelation. This instrument will broaden the scop