Sum-Frequency-Generation-Based Laser Sidebands for Tunable Femtosecond Raman Spectroscopy in the Ultraviolet

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. OPEN ACCESSAppl. Sci. 2015, 5 49

Monitoring Photochemical Reaction Pathways of Tungsten Hexacarbonyl in Solution from Femtoseconds to Minutes

“…This early stage endeavor to capture the desirable transient Raman features was likely affected by the electronic potential energy landscape of the photoproduct species which absorbs in the bluer region to our stimulated Raman pulses. We plan to tune the Raman pump further to the blue and collect the anti-Stokes signal, , which should increase the resonance enhancement factor and expose stronger Raman peaks along the reaction coordinate from the reactant, intermediate(s), to photoproduct(s). The anharmonic couplings between the CO stretch and low-frequency motions ,, could be assessed by analyzing the time-resolved vibrational peak evolution at multiple spectral locations, without complication from the conventional transient absorption that typically shows overlapping electronic bands. , As the technical advance is underway, we expect to provide a much fuller picture of the UV photolysis of tungsten hexacarbonyl in solution in the near future.…”

“…Our ultrafast laser system consists of a mode-locked Ti:sapphire oscillator (Mantis-5) and an ultrafast regenerative amplifier (Legend Elite-USP-1K-HE, Coherent, Inc.), ,, which provides the fundamental pulse (FP) centered at 800 nm with ∼35 fs pulse duration, 4 W average power at 1 kHz repetition rate. In our fs transient absorption (TA) setup, part of the FP converts to 400 nm through second harmonic generation (SHG) in a 1 mm-thick type-I β-barium borate (BBO) crystal.…”

“…For the ground state femtosecond stimulated Raman spectroscopy (GS-FSRS), a home-built tunable FSRS apparatus has been described in detail. ,, Briefly, a small portion of FP is focused on a 2 mm-thick sapphire plate (with higher spectral stability so no motor is needed) to generate SCWL which goes through a prism pair to compress to ∼30 fs pulse duration and used as Raman probe (R pr ). The pulse duration of R pr is measured using the optical Kerr effect (OKE) signal between R pr and another fs pulse at 400 nm generated via SHG of the FP. , The fwhm of the OKE signal is ∼40 fs in a 1 mm-thick fused silica glass slide, and assuming the two incident pulse durations are comparable with similar Gaussian spectral profiles, the retrieved pulse duration is ∼30 fs.…”

Femtosecond stimulated Raman line shapes: Dependence on resonance conditions of pump and probe pulses

“…The anti-Stokes stimulated Raman can be attributed to an IRS term where probe acts as the excitatory pulse. Unlike spontaneous Raman, the anti-Stokes stimulated Ra-man signal is not necessarily weaker than its Stokes counterpart in spite of the inversely mirrored vibrational modes [39,40,48,55,56]. This is because: (i) R pr initiates the Raman process from the most populated vibrational ground state (v=0) in IRS, (ii) the resonance of both Raman pump and probe pulses are important for the signal strength.…”

Excited State Structural Evolution of a GFP Single-Site Mutant Tracked by Tunable Femtosecond-Stimulated Raman Spectroscopy

“…In the tunable FSRS setup with 539 nm (results in Figure 5 ) and 504 nm (results in Figure 6 ) Raman pump, the detailed experimental procedures were previously reported [ 11 , 73 , 74 , 75 ]. In brief, the generation of narrowband Raman pump in the visible range (from ca.…”