|Example 2D spectrum of a crosspeak due to energy transfer. The crosspeak arises an initial excitation of mode A and so appears along frequency A on the pump axis. Energy then transfer from mode A to mode B before the arrival of the probe pulse, so it appears at frequency B along the probe axis. In a pump-probe spectrum, the crosspeak would overlap with and be obscured by the diagonal peaks from mode B.|
Vibrational Polaritons: Throwing Away a Standard Assumption
Spectroscopists often study systems where they assume that they are observing the vibrations or electronic excitations of a target molecule. Light excites the system but only weakly interacts with it, giving spectroscopic information about the molecular response.
But in a recent paper in Science, the Xiong group used ultrafast spectroscopy and pulse shaping to study a system where one can’t tell where the “light” ends and the “molecule” begins. By placing molecules in an optical microcavity whose length is tuned to match the frequency of light, they have created a system where photons and molecular vibrations mix to form a hybrid state called a vibrational polariton.
Mixing Light and Matter: Why Does It Matter?
Moving from molecular vibrations to vibrational polaritons completely changes how vibrational energy flows through the system. Normally, energy trapped in molecular vibrations moves through the molecule,and is deposited in the solvent as heat. Transfer of vibrational energy between molecules (population transfer) is rare.
The polaritons created by the Xiong group, however, showed highly efficient energy transfer between different molecules, much faster than transfer to the solvent.
This type of efficient intermolecular vibrational energy transfer is essential for pushing forward several frontiers of chemistry: Bose–Einstein condensation of vibrational polaritons (“lasing without inversion”), remote energy transfer (in semiconductors and photovoltaics), intra-cavity chemistry, and others.
Where Does Ultrafast Pulse Shaping Fit In?
The Xiong group used coherent two-dimensional infrared spectroscopy (2D IR) to directly observe the ultrafast vibrational energy transfer, and to distinguish it from competing effects like direct coupling of the high and low energy polariton modes.
The presence of crosspeaks in the 2D IR spectrum of the polariton showed vibrational energy transfer between the two systems, as energy that started in the delocalized high energy mode flowed downhill to the delocalized low energy mode.
Crosspeaks are broadly useful for studying many different chemical systems. They show connections between vibrations, which are related to molecular structure. They have been widely used in studying the biophysics of proteins and nucleic acids, as well as interaction between small molecules. Crosspeaks also improve spectral resolution and enhance detection of weaker vibrations through coupling to stronger modes. Crosspeaks have been used in studying proteins, ultrafast chemical exchange of small molecules, equilibrium dynamics in thermally populated vibrations, vibrational energy transfers, Fermi resonances, and delocalized vibrational excitons. The ability to detect crosspeaks is one of the advantages of 2D spectroscopy over traditional pump-probe measurements.
IR pulse shaping allowed the Xiong group to convert an existing pump-probe spectrometer to a 2D IR spectrometer, without which these experiments would have been impossible. Specifically, the pulse shaper generated two IR pump pulses from a single pump pulse and scanned the delay between them - the core approach to a 2D experiment. Using an IR pulse shaper also allowed the use of phase cycling and rotating frame techniques to the pulses, both of which improve signal-to-noise. You can read more about all of these ideas in the links below.
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