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Abstract-The ionisation potentials of a series of alkylketones have been measured by the electronimpact technique using the second derivative method. The ionisation efficiency curves were obtained with a conventional mass spectrometer and the data treated by means of a minicomputer. These results are compared with those given by photoelectron spectroscopy, which has enabled us to test our mass spectrometric method and to distinguish between the adiabatic and the vertical ionisation potentials. It has been observed that the electron-impact technique can lead to the adiabatic ionisation potential in favourable cases.We have determined the appearance potentials of fragmentations +. This kinetic shift becomes larger for an increase in the activation energy and the vibrational degrees of freedom of the molecular ion, as postulated by the quasi-equilibrium theory. An important fraction of the kinetic shift is due to the 'competitive shift,' i.e. the excess energy that must be supplied to the molecular ion for it to yield [B]+ ions in sufficient amounts to be detected by the mass spectrometer. This is confirmed by the calculation of the fragmentation rate constants for the 2-butanone molecular ion. I N T R O D U C T I O NELECTRON-IMPACT mass spectrometry (EIMS), frequently used in the measurement of ionisation potentials (IP), gives questionable results because of the uncertainty in the identification of the processes involved. Ionisation potentials determined by this method are always higher than those given by photoelectron spectroscopy (PES).le3 It is generally assumed that the EIMS technique gives the vertical IP, while the adiabatic IP is measured by photoi~nisation~ or PES, although it has been reported that the adiabatic IP can be determined by electron-impact in some favourable cases.5It has also been reported that the resolution in EIMS rarely makes it possible to separate two electronic states, producing a peak-overlapping and an abnormally high observed IP.6 Even if there is not a second electronic state close to the first ionisation threshold, there is an overlapping of the many peaks corresponding to nonresolved closely spaced vibrational levels of the ion in its ground state. Therefore, the EIMS technique does not provide a means of determining whether the IP measured corresponds to the adiabatic or the vertical IP or to neither.On the contrary, PES is currently the technique which provides the greatest reliability and precision in IP measurement, including those of complex compounds. 7

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