VUV photoelectron imaging of biological nanoparticles: ionization energy determination of nanophase glycine and phenylalanine-glycine-glycine.

The ionization energies of biological nanoparticles are determined using the velocity map photoelectron imaging technique. A beam of nanoparticles produced by aerosol methods is photoionized with tunable vacuum ultraviolet (VUV) synchrotron radiation. The resulting photoelectrons are detected and their angular and energy distributions are measured, yielding an angle-resolved photoelectron spectrum. The ionization energies of the nanoparticles are derived from plots of the photoelectron spectrum versus incident photon energy. The ionization energies of nanophase glycine and phenylalanine-glycine-glycine are 7.6 +/- 0.2 eV, and 7.5 +/- 0.2 eV, respectively. X-Ray powder diffraction studies on the glycine nanoparticles indicate that they are crystalline in nature. The reduced ionization energy when compared to gas phase results suggests that the polarization energy in the solid is significant. The difference in the ionization energy between the nano and gas phase reflects this polarization energy and is derived to be 1.7 +/- 0.2 eV and 1.6 +/- 0.2 eV for glycine and phenylalanine-glycine-glycine, respectively. Using these results the molecular polarizability of glycine is estimated to be 4.7 +/- 0.3 A3 (31.9 +/- 1.9 au).