Can free electron lasers answer critical questions in ultraviolet photobiology?

DNA and other biological macromolecules are sensitive to damage by ultraviolet light with wavelengths less than about 320 nm. Fortunately, such wavelengths are efficiently absorbed by ozone in the stratosphere. DNA can also be: damaged by longer wavelength ultraviolet and (perhaps) visible light, albeit with far lower efficiencies. Indeed, the cross section for damage induction in DNA by longer wavelengths is so low that it has been difficult to measure easily in controlled experiments using monochromatic laboratory sources. However, the flux of solar photons reaching the biosphere is far higher than at the shorter wavelengths. Thus the damage induced by wavelengths > 320 nm may constitute a significant fraction of the total UV burden in a particular organism or ecosystem. Understanding the nature and extent of this long wavelength induced damage is crucial in several important societal problems including: Evaluating the biological impact of depletion of stratospheric ozone. Determining if sunscreen preparations offer the same protection against long term effects such as cutaneous malignant melanoma as they do against prompt effects such as erythema (sunburn). Determining the safest spectrum for use in indoor tanning. The light sources required for such studies must provide high intensities of nearly monochromatic light that can be tuned to any wavelength in the required spectral range. This paper will evaluate the potential of ultraviolet free electron lasers, and particularly the soon to be available W-FEL at the Thomas Jefferson National Accelerator Facility for such experiments.