No abstract

We propose angle-resolved photoelectron spectroscopy of aerosol particles as an alternative way to determine the electron mean free path of low energy electrons in solid and liquid materials. The mean free path is obtained from fits of simulated photoemission images to experimental ones over a broad range of different aerosol particle sizes. The principal advantage of the aerosol approach is twofold. First, aerosol photoemission studies can be performed for many different materials, including liquids. Second, the size-dependent anisotropy of the photoelectrons can be exploited in addition to size-dependent changes in their kinetic energy. These finite size effects depend in different ways on the mean free path and thus provide more information on the mean free path than corresponding liquid jet, thin film, or bulk data. The present contribution is a proof of principle employing a simple model for the photoemission of electrons and preliminary experimental data for potassium chloride aerosol particles.

The photoionization of thymine has been studied by using vacuum ultraviolet radiationa nd imaging photoelectronp hotoion coincidences pectroscopy after aerosol flash vaporization andb ulk evaporation.T he two evaporation techniquesh ave been evaluated by comparison of the photoelectrons pectra and breakdown diagrams. The adiabatic ionization energies for the first four electronic states were determined to be 8.922 AE 0.008,9 .851 AE 0.008, 10.30 AE 0.02, and 10.82 AE 0.01 eV.V ibrational features have been assigned for the first three electronic states with the help of Franck-Condon factor calculations based on densityf unctional theory and wave function theory vibrational analysis within the harmonic approximation. The breakdown diagram of thymine, as supported by composite methoda bi nitio cal-culations,s uggestst hat the main fragment ions are formed in sequential HNCO-, CO-, and H-loss dissociation steps from the thymine parenti on, with the first step corresponding to ar etro-Diels-Alder reaction. The dissociation rate constants were extractedf rom the photoion time-of-flight distributions and used together with the breakdown curvest oc onstruct as tatistical model to determine 0K appearance energies of 11.15 AE 0.16 and 11.95 AE 0.09 eV for the m/z 83 and 55 fragment ions, respectively.T hese results have allowedu st o revise previously proposed fragmentation mechanismsa nd to proposeamodel for the final, nonstatistical H-losss tep in the breakdown diagram, yielding the m/z 54 fragment ion at an appearance energy of 13.24 eV.

The valence threshold photoionization of oleic acid has been studied using synchrotron VUV radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. An oleic acid aerosol beam was impacted on a copper thermodesorber, heated to 130 °C, to evaporate the particles quantitatively. Upon threshold photoionization, oleic acid produces the intact parent ion first, followed by dehydration at higher energies. Starting at ca. 10 eV, a large number of fragment ions slowly rise suggesting several fragmentation coordinates with quasi-degenerate activation energies. However, water loss is the dominant low-energy dissociation channel, and it is shown to be closely related to the unsaturated carbon chain. In the lowest-barrier process, one of the four allylic hydrogen atoms is transferred to the carboxyl group to form the leaving water molecule and a cyclic ketone fragment ion. A statistical model to analyze the breakdown diagram and measured rate constants yields a 0 K appearance energy of 9.77 eV, which can be compared with the density functional theory result of 9.19 eV. Alternative H-transfer steps yielding a terminal C=O group are ruled out based on energetics and kinetics arguments. Some of the previous photoionization mass spectrometric studies also reported 2 amu and 26 amu loss fragment ions, corresponding to hydrogen and acetylene loss. We could not identify such peaks in the mass spectrum of oleic acid.