Time-resolved Raman spectroscopy with a tunable ultraviolet kilohertz nanosecond laser

Abstract

Time-resolved resonance Raman (TR3) spectra are obtained with a pair of Q-switched Nd : YLF-pumped Ti : sapphire lasers, generating tunable (810–920 nm) ∼20 ns pulses at a 1 kHz repetition rate. Frequency doubling in lithium borate (LBO) provides blue (405–460 nm) pump and probe pulses, while UV probe pulses, tunable from 205 to 230 nm, can be generated by doubling the second harmonic in β-barium borate (BBO). Pump and probe pulse timing are controlled electronically. A timing sequence is implemented in which exposure of the multichannel detector alternates between positive and negative time delays between pump and probe pulses, so that accumulated difference spectra are free of artifacts from spectrograph drift or gradual decomposition of the sample. The system was tested on the carbonmonoxy hemoglobin (HbCO) photocycle, for which UV TR3 spectra have previously been reported. HbCO was pumped at 419 nm, at the maxima of the strong Soret absorption band, and saturation of the photoresponse (maximum deligation) was established by measuring the intensity ratio of the HbCO and deoxyHb ν4 porphyrin RR bands, generated with 425 nm probe pulses. UV TR3 difference spectra were obtained at time intervals from 0.06 to 20 µs using 229 nm probe pulses. They are in good agreement with those recorded previously with a pair of 300 Hz excimer-dye lasers. The time required to achieve a comparable signal-to-noise ratio was eight times shorter with the 1 kHz Nd : YLF-Ti : S lasers. Copyright © 1999 John Wiley & Sons, Ltd.