Monoenergetic, internal energy selected polyatomic ions are produced in a new two-laser pump-pump experiment. Low energetic parent ions produced with the first laser are further excited with the tunable second laser to a specified internal energy after extraction from the ion source. The metastable decay of energy-selected ions is monitored in a reflectron time-of-flight mass spectrometer, and as a first example, dissociation rates have been measured for two decay channels of C6H6'. The result clearly demonstrates directly that C-loss as well as H-loss channels are competing and originate from the same electronic state of the benzene cation. IntroductionDuring the past three decades the unimolecular decay of polyatomic ions in a mass spectrometer has been studied in great detail.' The results, e.g. breakdown curves, metastables, and kinetic energy release, have been suggestive of a statistical behavior in the unimolecular decay. A critical test of the assumption made in statistical theories would, however, be only really optimal if the ions are prepared in a defined internal energy level rather than with a broad energy distribution as e.g. after electron impact ionization or vacuum-UV excitation. Even though the energy of the vacuum-UV photons is well-defined, this type of excitation still produces a broad range of final ion energies associated with differing relative kinetic energies of the electron.An alternative method to produce molecular ions is multiphoton and in particular two-photon ionization via a resonant intermediate state of the neutral molecule.2 In this way at low light intensities soft ionization is possible3 whereas at high light intensities strong fragmentation is ob~erved.~" In the multiphoton excitation process the original parent ions are produced with relatively little internal energy as a consequence of the "ladder-switching" p r~c e s s .~ Then, during the laser pulse the parent ions begin to absorb further photons until they finally dissociate. When the two-or threephoton energy is tuned close to the ionization threshold, the parent ions are preferentially produced in their vibrationless ground state. This wavelength, however, fixes the internal energy of the ions such that further photon absorption is specified, and hence one cannot vary the internal energy as would be desirable for any study of unimolecular decay behavior. On the other hand, if the photon energy is increased, ions are produced in higher vibrational states and the final energy of the excited ions, even though variable, is no longer defined, an unknown quantity going into kinetic energy of the electron. Recent one-laser experiments yielding metastable decay rat& for aniline6 and chlorobenzene' cations inherently display this uncertainty.In this work it will be shown that in a two-laser pump-pump experiment polyatomic ions can be produced with differing energy content but now internal energy selected. Dissociation rates of two decay channels of energy-selected benzene cations at different

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