Research Methods

The Bragg lab utilizes several ultrafast spectroscopic techniques to examine the photophysical dynamics and structure of transient states within molecules and materials of interest. These methods permit examination of events occurring on timescales as fast as 10s of femtoseconds (10-14 s) to microseconds (10-6 s) following excitation). These methods include:

  • Transient absorption spectroscopy (TA). TA interrogates the response of a molecule or material to excitation with a light pulse (the “pump” pulse) according to changes in the transmission of a second pulsed light beam (the “probe” pulse), the intensity of which can be collected and corrected for emission and pump light scatter from the sample.  Changes in probe transmission reflect changes in the absorption or scattering characteristics of a sample associated with transient states.  The lifetimes of transient states can be determined by measuring the probe intensity at specific times after sample excitation.

  • Femtosecond stimulated Raman spectroscopy (FSRS). FSRS is a coherent Raman technique that is used to measure vibrational spectra, which are related to molecular structure.  In this technique vibrational features are observed as a signal gain on a white light continuum induced by excitation with a narrowband Raman excitation pulse (in a time-resolved variant, this pulse pair is preceded by a “pump” pulse that photoactivates the sample as in TA).

  • Pump repump probe spectroscopies (PRP).  PRP is a variation of TA in which a second  pulse is applied (between the pump and probe) to perturb a population of transient states or species at some time after their initial preparation with the pump or photoexcitation pulse.  The second pulse is tuned energetically to align with a particular absorption (a ‘re-pump’ pulse) or stimulated emission (‘dump’) band.  Changes in the absorption relative to the absence of this second pulse are monitored over time similar to a TA measurement. This technique can be used to interrogate mixtures of conformers or other sub-populations transient species (a technique called transient hole burning), separate the presence of competing photochemical dynamics that appear in TA data, or characterize the photophysical and photochemical properties of metastable chemical species.

  • Broadband polarization anisotropy (BPA). Polarized excitation of an isotropic (randomly aligned) sample results in asymmetry or anisotropy the alignment of excited chromophores; the realignment of these molecules (i.e. orientational diffusion) has different time dependence than the energetic relaxation of excited transient states.  In standard TA measurements, orientational diffusion can be eliminated by probing changes in transmission at a specific relative polarization.  In contrast, BPA monitors the transmission of two probe pulses with different polarizations to isolate information about orientational alignment. Measurements of this type can be used to determine relative directions between transition dipoles, and can be used to monitor the extent and timescales of energy migration or transfer in a material or track molecular rotation in solution with time.