LOW-ENERGY-ELECTRON TOTAL SCATTERING CROSS-SECTION FOR THE PRODUCTIONOF CO WITHIN CONDENSED METHANOL

We propose a method based on high-resolution electron-energy-loss spec troscopy to measure in situ the neutral fragmentation products arising from the impact of low energy electrons on thin solid films at low te mperature. We show more particularly that the detection of electronic states from a dissociation product is a good alternative when the corr esponding vibrational levels are obscured by those of the deposited fi lm. In the case of thin methanol film condensed at 18 K, we find that low energy electrons can dissociate the CH3OH molecules into CO fragme nts that remain within the film. The production of CO fragments, clear ly identified from its lowest electronic state a (3) Pi, is studied as a function of the electron dose, electron energy, and film thickness. The energy dependence of the CO production rate, which is also calibr ated in terms of an electron total scattering cross section sigma(p), is characterized by an energy threshold at 8eV, a shoulder at about 11 .5 eV, a broad maximum centered around 14 eV, and a rise above 19 eV. A value of sigma(p) approximate to 4.2 x 10(-18) cm(2) is Obtained at 14 eV. The shoulder and the broad maximum are specifically attributed to the ...(6a')(1)(3sa')(2),(2)A', ...(1a '')1(3sa')2(,2)A '', and ... (5a')(1)(3sa')(2),(2)A' core excited electron resonances, which decay into their parent repulsive states. The rise above 19 eV is correlated to the lowest dissociative photoionization processes known to produce neutral CO in the gas phase. (C) 1997 American Institute of Physics.