Characterization of stimulated Raman scattering of hydrogen and methane gases as a light source for picosecond time-resolved Raman spectroscopy

Abstract

Stimulated Raman scattering (SRS) in compressed hydrogen and methane gas was characterized in terms of pulse energy, temporal width and spectral width in the range of gas pressures 10‐60 atm to use it as a light source for picosecond time-resolved resonance Raman spectroscopy. SRS was pumped by the second harmonic of a Ti : sapphire oscillator‐regenerative amplifier laser system with pulse energy up to 200 mJ, duration2:5 ps and repetition rate 1 kHz. The output spectral region 421‐657 nm was covered by the first and second Stokes SRS components on tuning of the pump wavelength in the range 375‐425 nm. Energy conversion to the first Stokes SRS component was more than 10% with H 2 and more than 20% with CH4. The temporal width of the SRS pulse (1.1‐2.1 ps) was shorter than that of the pump pulse. The spectral band shape was found to be modulated, since the SRS is generated in a transient regime. When more than 100 pulses were averaged, the temporal and spectral profiles of SRS pulses were sufficiently smooth and energy fluctuations were sufficiently small for spectroscopic applications. On the basis of the results obtained, optimized conditions as a Raman shifter were deduced. The Raman shifter served as a light source for two-color pump‐probe time-resolved resonance Raman (TR 3 ) experiments and to demonstrate its capabilities, picosecond TR 3 spectra of nickel tetraphenylporphyrin in toluene solution were measured.