Time-resolved photoelectron spectrum

Figure 7. Time-resolved mass spectrometry. AU-trcms-(2, 4, 6, 8) decatetraene was excited to its 5 2 electronic origin with a femtosecond pulse at A-pump — 287 nm. The excited-state evolution was probed via single-photon ionization using a femtosecond pulse at ApIObe = 235 nm. The time resolution in these experiments was 290 fs (0.3 ps). The parent ion CioH signal rises with the pump laser, but then seems to stay almost constant with time. The modest decay observed can be fit with a single exponential time constant of 1 ps. Note that this result is in apparent disagreement with the same experiment performed at Xprobe — 352 nm, which yields a lifetime of 0.4 ps for the S2 state. The disagreement between these two results can be only reconciled by analyzing the time-resolved photoelectron spectrum.

Calculations of the ion yield in dependence on the pulse delay time At and on the parameters of the laser pulses have been performed in Refs. 86 and 93 for simple one-dimensional models of excited-state vibrational motion and vibronic coupling. It has been found that for the vibronic-coupling examples considered and for suitably chosen pulse parameters, the ion signal as a function of At maps very well the adiabatic electronic population probability. As an example of a molecular system comprising conical intersections. Sec. 5.1 presents a calculation of the time-resolved photoelectron spectrum of pyrazine. 

The first numerically exact simulation of a time-resolved photoelectron spectrum for a multidimensional vibronic-coupling system was reported... 

Applying Eq. (39) to this case, the time-resolved photoelectron spectrum can be written as ... 

Fig. 7. Time-resolved photoelectron spectrum as obtained for the four-mode model of pyrazine, assuming 40 fs pump and probe pulses. Shown are the contributions (a) /22(Bfc, At) and (b) In Ek,A.t), stemming from the ionization of the S2 and Si states of pyrazine, respectively, as well as (c) the total spectrum, defined in Eq. (79).

D. M. Neumark We are currently carrying out somewhat different femtosecond experiments in which time-resolved photoelectron spectroscopy is used to probe the photodissociation dynamics of negative ions. In these experiments, an anion is photodissociated with a femtosecond laser pulse. After a time delay, the dissociating anion is pho-todetached with a second femtosecond pulse and the resulting photoelectron spectrum is measured. The photoelectron spectrum as a function of delay time provides a detailed probe of the anion photodissociation dynamics. First results have recently been obtained for the photodissociation of I2. 

When considering the femtosecond photoionization dynamics of complex systems, a completely exact evaluation of the time and energy resolved photoelectron spectrum is often not really necessary. Approximative schemes which require significantly lower computational effort are valuable in such cases. Within the nonperturbative formalism, Meier et al. have proposed an efficient computational scheme which incorporates the multi-configuration time-dependent Hartree method.An approximate method which is based on a classical-trajectory description of the nuclear dynamics has been elaborated by Hartmann, Heidenreich, Bonacic-Koutecky and coworkers and applied, among other systems,to the time-resolved photoionization spectroscopy of conical intersections in sodium fluoride clusters. 

Figure 5.23 shows the time-resolved photoelectron kinetic energy spectra for a probe pulse polarized parallel to the pump polarization (rr-axis) and for molecules that have been transiently aligned. The spectra for ionization to the triplet state only are shown in Fig. 5.23(a). The evolution of the photoelectron spectrum is plotted in steps of 2 fs. 

Fig. 1.25. Time-resolved pump-probe photoelectron spectra of Au2CO. The topmost spectrum of AU2CO is obtained with solely the probe laser beam (400nm wavelength). The corresponding 400nm probe-only photoelectron spectrum of Au2 is shown in the bottom trace [149]...

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