Cryogenic continuous flow Raman

Raman scattering is produced when photon is absorbed and immediately re-emitted. Raman effect is responsible for the color of blue sky on a sunny day.

Raman effect and Raman spectroscopy are based on an inelastic scattering of light by molecules and atoms. The blue color of clear sky, deep water or ice - all are due to Raman phenomenon. It is an instantaneous process (unlike fluorescence, for example) where a molecule absorbs and immediately emits (scatters) a photon. Energy of a small fraction of emitted photons is slightly different from the energy of initial photon - by the change in energy of internal motion of molecule itself. Because every molecule has a finite and distinct set of vibrations it produces a unquiet scattering pattern - a Raman spectrum. This phenomenon is used to measure internal vibrations of molecules and thus interpret their structures, especially with the development of lasers as light sources.

Raman spectroscopy is very sensitive and rich in structural information. However, it has few limitations, such as low fraction of Raman photons and natural threshold for power damage from excitation light on another: typical Raman measurement requires much longer accumulation than absorption spectra. This is especially significant in dynamic measurements. Transient species that form and quickly disappear are usually measured by either trapping them at very low temperature (freeze-quench) or by using a so-called continuous flow technique. In continuous flow technique samples are mixed and flown through the beam during entire accumulation.

We are developing a cryogenic continuous flow transient Raman approach, which combines features of freeze-quench and traditional continuous flow methods. Reactants are mixed and measured at temperatures down to -40^oC in aqueous samples and -70^oC in organic solvents. On one hand low temperature slows down overall reaction and prevents thermal damage dramatically decreasing sample consumption. On the other hand geometry and optical properties of liquid sample improves collection of scattered light while sample flow prevents photodecomposition.

Structures of transient reactive species at temperatures of -40^oC and below are studied using this continuous flow time-resolved resonance Raman setup.

Cryogenic temperatures present significant challenges in sample delivery and that we successfully resolved. Escalating viscosities of solutions lead to samples pressures of up to 40 atm while hindering effective mixing of reagents. Our current setup can deliver a sub-second mixing time with dead volume under 0.01 ml. Work on developing this approach is continuing.

Vibrational spectroscopic techniques are extremely sensitive and rich in structural information, but lack simplicity and speed of electronic (optical) absorption. Electronic absorption, on the other hands, is fast, accessible, very sensitive to kinetics of the reaction, but lacks direct structural information. We often complement vibrational studies with parallel or simultaneous in situ optical studies. This approach provides a real time probe and allows us to carry direct cross correlation between structural and kinetic data of several spectroscopic methods greatly enhancing overall sensitivity. Furthermore, it allows to develop structurally meaningful optical probes that can be used more independently.