Internal Energies of Ion-Sputtered Neutral Tryptophan and Thymine Molecules Determined by Vacuum Ultraviolet Photoionization

Vacuum ultraviolet photoionization coupled to secondary neutral mass spectrometry (VUV-SNMS) of deposited tryptophan and thymine films are performed at the Chemical Dynamics Beam line. The resulting mass spectra show that while the intensity of the VUV-SNMS signal is lower than the corresponding secondary ion mass spectroscopy (SIMS) signal, the mass spectra are significantly simplified in VUV-SNMS. A detailed examination of tryptophan and thymine neutral molecules sputtered by 25 keV Bi-3(+) indicates that the ion-sputtered parent molecules have similar to 2.5 eV of internal energy. While this internal energy shifts the appearance energy of the photofragment ions for both tryptophan and thymine, it does not change the characteristic photoionizaton efficiency (PIE) curves of thymine versus photon energy. Further analysis of the mass spectral signals indicate that approximately 80 neutral thymine molecules and 400 tryptophan molecules are sputtered per incident Bi-3(+) ion. The simplified mass spectra and significant characteristic ion contributions to the VUV-SNMS spectra indicate the potential power of the technique for organic molecule surface analysis.