Single-photon ionization, time-resolved

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.

Transient two-photon ionization experiments on trimer systems were, of course, motivated by a need for time-resolved verification of the pseudorotation motion, which can be considered as a superposition of the asymmetric stretch (Qx) and the bending vibration (Qy) [12]. The triatomic molecule with its three modes is quite different from an isolated oscillating dimer, which vibrates in its single mode until eventually it radiates or predissociates. The interplay of vibrational modes in a trimer system can be considered as the prototype of IVR. 

We turn now to a more detailed description of the photoionization probe step in order to clarify the ideas presented above. Time-resolved photoelectron spectroscopy probes the excited-state dynamics using a time-delayed probe laser pulse that brings about ionization of the excited-state wave packet, usually with a single photon... 

Note that the application of the convolution scheme in the simple form (44) requires that the nonlinear polarization contains only a single interaction with the probe laser field. Apart from the transient transmittance spectrum considered above, this condition is also fulfilled for related detection schemes such as time-resolved fluorescence, ionization, and excited-state absorption. Coherent spectroscopic signals such as the photon-echo, on the other hand, contain two interactions with the probe laser field, thus requiring the calculation of the full three-time response function, followed by a double convolution. 

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