Time-dependent delayed electron spectra: a direct measurement of total decay rate as a function of internal energy

We present a general method to measure internal energy dependant decay rate in the case of systems emitting thermal electrons. Our approach is based on the measurement of the time-dependent kinetic energy spectra of delayed electrons using time-resolved velocity map imaging spectrometry. We illustrate this method in the case of C60 molecules. Indeed electron spectra for C60 have been studied in great details in the past few years, allowing a complete analysis of the observed features. Moreover, C60 offers the opportunity to study competing decay mechanisms demonstrating that the technique may have broader applicability to other molecules. Using a model that includes all contributing decay channels relevant in our time delay range (namely delayed ionization and dissociation by C2 emission) we are able to derive quantitative information on the decay channels of the molecule. In the situation considered here, the time-dependent electron temperature extracted from the kinetic energy spectra is used to determine more precisely the rate constant for the dominant process, namely neutral C2 dissociation channel. In other words, the measurement of the cooling of an ensemble of C60 molecules as a function of time delay after heating provides a direct and quantitative access to its decay dynamics. This method may be used to map out the total rate for complex decay mechanisms. PACS. 36.40.-c Atomic and molecular clusters – 33.60.+q Photoelectron spectra