Abstract

UV resonance Raman excitation using low-duty-cycle pulsed lasers such as the Nd:YAG can result in photophysical processes that interfere with Raman spectral studies of ground-state species due to the high incident laser energy fluxes. Depletion of the ground state occurs due to optical absorption and due to the population of intermediate levels which have lifetimes comparable to or longer than the excitation pulse width. In addition, formation of photochemical intermediates can occur. For example, excitation in resonance with the tyrosinate <i>L</i>_<i>a</i> electronic transition (∼240 nm) results in formation of tyrosyl radicals which deplete the concentration of ground-state tyrosinate molecules; as a result, decreased resonance Raman intensities are observed for vibrational modes of ground-state tyrosinate. For pyrene, excitation in resonance with the <i>S</i>₄ electronic transition results in population of the long-lived <i>S</i>₁ state via rapid internal conversion. This long-lived state bottlenecks relaxation back to the ground state, thus causing saturation of the ground-state pyrene Raman intensities. Given similar incident <i>average</i> laser powers and focusing conditions, higher-duty-cycle lasers result in decreased saturation. A comparison between a 20-Hz Nd:YAG and a 200-Hz excimer laser-based UV Raman excitation source demonstrates superiority of the excimer in avoiding both Raman saturation and interferences from photochemical transients. For the identical energy flux per pulse, the accompanying tenfold increase in <i>average</i> energy flux for the excimer, over the YAG, results in a dramatic improvement in the spectral signal-to-noise ratios. We report the first measurement of the absolute resonance Raman cross section of pyrene within the <i>S</i>₄ transition. The Raman cross section of 48 barns/str measured for the 1632-cm⁻¹ vibration with 240-nm excitation is the largest observed to date.

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