Abstract
Femtosecond stimulated Raman spectroscopy (FSRS) is an emerging molecular structural dynamics technique for functional materials characterization typically in the visible to near-IR range. To expand its applications we have developed a versatile FSRS setup in the ultraviolet region. We use the combination of a narrowband, ~400 nm Raman pump from a home-built second harmonic bandwidth compressor and a tunable broadband probe pulse from sum-frequency-generation-based cascaded four-wave mixing (SFG-CFWM) laser sidebands in a thin BBO crystal. The ground state Raman spectrum of a laser dye Quinolon 390 in methanol that strongly absorbs at ~355 nm is systematically studied as a standard sample to provide previously unavailable spectroscopic characterization in the vibrational domain. Both the Stokes and anti-Stokes Raman spectra can be collected by selecting different orders of SFG-CFWM sidebands as the probe pulse. The stimulated Raman gain with the 402 nm Raman pump is >21 times larger than that with the 550 nm Raman pump when measured at the 1317 cm−1 peak for the aromatic ring deformation and ring-H rocking mode of the dye molecule, demonstrating that pre-resonance enhancement is effectively achieved in the unique UV-FSRS setup. This added tunability in the versatile and compact optical setup enables FSRS to better capture transient conformational snapshots of photosensitive molecules that absorb in the UV range.
Highlights
The advent of femtosecond lasers has ushered in an exciting era of modern quantum chemistry and molecular spectroscopy [1,2] which has provided previously unavailable or hidden insights about structural dynamics, chemical reactivity, and biological functionality [2,3,4,5,6,7]
To exploit the full potential of ultrafast vibrational spectroscopy to characterize functional materials and biomolecules, we have developed femtosecond stimulated Raman spectroscopy (FSRS) as an emerging structural dynamics technique that has simultaneously high spectral and temporal resolutions [7,8,9,10,11,12,13]
We reported the implementation of cascaded four-wave mixing (CFWM) in a thin transparent medium such as BK7 glass to generate broadband up-converted multicolor array (BUMA) signals [15]
Summary
The advent of femtosecond lasers has ushered in an exciting era of modern quantum chemistry and molecular spectroscopy [1,2] which has provided previously unavailable or hidden insights about structural dynamics, chemical reactivity, and biological functionality [2,3,4,5,6,7]. By incorporating a preceding actinic pump pulse that induces photochemistry or other chemical reactions, vibrational transitions of the molecular system can be tracked in real time via IR absorption or Raman processes with ultrafast IR or visible laser pulses, typically on the femtosecond (fs) to picosecond (ps) timescale which can report on the incipient stage of photoinduced processes. To exploit the full potential of ultrafast vibrational spectroscopy to characterize functional materials and biomolecules, we have developed femtosecond stimulated Raman spectroscopy (FSRS) as an emerging structural dynamics technique that has simultaneously high spectral and temporal resolutions [7,8,9,10,11,12,13]. The conventional FSRS technique consists of an ~800 nm, ps Raman pump pulse from a grating-based spectral filter and a ca
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