Ultrafast vibrational dynamics: An impulsively stimulated resonance Raman perspective

Abstract

Accessing excited-state vibrations is crucial to elucidate physical and chemical properties of photoreactive molecules and solids. Time-resolved resonance Raman spectroscopy provides unique vibrational fingerprints of different electronic levels, but its spontaneous version often suffers from strong fluorescence backgrounds and has its spectral/temporal resolutions fundamentally limited by the Fourier principle, hindering spectral assignments and the investigation of ultrafast dynamics. Here, we discuss how these issues can be overcome using Impulsive Stimulated Raman spectroscopy, a pump-probe technique that tracks coherent vibrational motion on different electronic states in real time. Using the highly fluorescent cyanine dye IR-775 as an experimental benchmark, we show that broadband probe detection isolates Raman spectra across different resonances, enabling unambiguous excited-state assignment. The time-domain scheme effectively measures fluorescence-free spectra, revealing a low-frequency Raman mode at 130 cm−1 in the electronic ground and first excited states, which plays a critical role in vibronic coupling with higher-lying electronic levels. Finally, we highlight the technique's versatility in light of recent experiments and related perspectives. These include two-color schemes with pulses spanning from X-rays to the infrared, the use of pulse chirp as a powerful control knob to unveil specific spectroscopic signatures, and strategies to amplify the scattering cross section.