Efficient calculation of femtosecond time-resolved photoelectron spectra: method and application to the ionization of pyrazine

A computational scheme to calculate the femtosecond time-resolved photoelec tron spectrum of polyatomic molecules is outlined. The method exploits (i) the fact that the calculation of the ionization yield at a particular photo electron energy is formally equivalent to the calculation of the total tran sient absorption into a single excited electronic state, and (ii) a recentl y proposed convolution scheme (S. Hahn and G. Stock, Chem. Phys. Lett., 199 8, 296, 137), which allows for an efficient calculation of the transient ab sorption. Obtaining the complete photoelectron spectrum from a single trans ient-absorption calculation, the approach circumvents the cumbersome discre tization of the electron continuum. To demonstrate its capability, the meth od is applied to a four-mode vibronic-coupling model of pyrazine, which inc ludes the three lowest singlet states (S-0, S-1, S-2) as well as the two lo west cation states (I-0, I-1) of pyrazine. Explicit simulations of femtosec ond ionization experiments are presented for this model and compared to rec ent experiments. It is demonstrated that the time-resolved photoelectron sp ectrum directly monitors the ultrafast S-2 --> S-1 internal conversion proc ess in pyrazine.