Laser Raman Spectrometer

• Subject Area: Analytical Chemistry
• Term: Funded in 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 562966163

The requested instrumentation includes a high-resolution Laser Raman spectrometer with a tunable continuous wave laser emitting from 450 nm to 1300 nm (with only small energy gaps), detectors and polarization accessories. The spectrometer will be equipped with a sample stage to allow measurements at variable temperature (-195 to +600 °C). With this instrument, the applicants will characterize the (photo)physical properties of metal complexes, molecules and nanoparticles in fluid and frozen solutions, solids and films by Raman spectroscopy. As far-infrared spectra below ca. 350 cm-1 are challenging to obtain, Raman spectroscopy will deliver important information on low-frequency bands such as metal-ligand vibrations thus complementing IR spectroscopy in this energy range and providing information on the metal-ligand bonding situation. Systems with (near-)degenerate electronic ground states, such as pseudo-octahedral d2 and low-spin-d5 transition metal and lanthanide complexes, show significant splitting of the ground states that will be quantified via electronic Raman (eR) transitions. Resonance Raman (rR) spectroscopy is a useful technique for assigning the character of electronic transitions as vibrational modes of chromophores that resonate at the Raman excitation wavelength are selectively enhanced. This will give unprecedented insights into the characters of individual Franck-Condon (FC) states of photoactive systems. In the visible spectral region, the FC states often possess charge transfer (CT) character (MLCT, LMCT, ILCT, LL’CT, IVCT). Transition metal complexes with d3 and d2 electron configuration can additionally possess spin-flip (SF) states, often located in the near-infrared (NIR) spectral region. Both types of excited states will be characterized by rR spectroscopy. rR spectra can furthermore display resonance-enhanced overtones or combination bands, potentially yielding even more information on the excited states such as the horizontal displacement of potential energy minima of the ground and excited states. Raman spectra can display fluorescence artifacts as a competitive pathway to the Raman scattering when excitation occurs into an electronically excited state. Spin-allowed fluorescence is often undesired as it is more intense than the Raman scattering. However, SF or CT NIR-emissive systems can show spin-forbidden phosphorescence after resonance excitation of a suitable state with a Raman laser in addition to the Raman bands providing valuable detailed insight. Sample integrity, phase transitions and spin-crossover transitions of respective systems at variable temperature will be probed by Raman/rRaman/eRaman spectroscopy using the requested instrument.

• DFG Programme: Major Research Instrumentation
• Major Instrumentation: Laser-Raman-Spektrometer
• Instrumentation Group: 1840 Raman-Spektrometer
• Applicant Institution: Johannes Gutenberg-Universität Mainz
• Leader: Professorin Dr. Katja Heinze