VUV photochemistry of small biomolecules

We review our recent results on the vacuum ultraviolet (VUV) photochemistry of small biomolecules. The experimental techniques used, mass spectrometry and photofragment fluorescence spectroscopy, are described. Emphasis is laid on our mass spectrometric results obtained for five nucleic acid bases and five amino acids. Ionisation and appearance energies are determined from photoionisation mass spectrometry, many for the first time. From this, fragmentation pathways following 6–22 eV photoexciation are derived. The adiabatic ionisation energies of the biomolecules studied lie between 8.2 eV (adenine) and 9.6 eV (α-amino-isobutyric acid). We show that the nucleic acid monocations, and chemically related molecular cations, do not fragment even when formed with large internal energies (Eint) ranging from 1.80 to 5.35 eV. In contrast, amino acid monocations are unstable and rapid fragmentation occurs via rupture of the CC(OOH) bond, except for β-alanine, where rupture of the bond between the α-C and β-C is the lowest lying ionic dissociation channel. The VUV photochemistry of the prebiotic species formic acid, acetic acid and methylformate, studied in more detail previously by several techniques, including fluorescence spectroscopy, is also reviewed. Astrophysical implications of our work are discussed in the conclusion.