We report on a comprehensive transient absorption study with β,β'-linked bis[tetraphenylporphyrinato-zinc(II)] and its corresponding monomer, covering the ultrafast dynamics from femtoseconds up to several microseconds. By exciting these porphyrins either to their first (S(1)) or second (S(2)) electronically excited states and by probing the subsequent dynamics, a multitude of reaction pathways have been identified. In the spectral region associated with the ground-state recovery of the bisporphyrin, transient absorption changes occur within the first few picoseconds, which are ascribable to excitonic interaction both in the S(2) (fs time-domain) and in the S(1) (ps time-domain) state. This is substantiated by complementary experiments with the monomeric porphyrin, in which the S(2) state exhibits a longer lifetime. In contrast to the picosecond dynamics the bisporphyrin and the monomer behave similarly on the nanosecond time-scale, that is nearly all excited molecules eventually reach a long-lived triplet excited state.

The feasibility of sub-Doppler broadband multi-heterodyne spectroscopy with two laser frequency combs is demonstrated with two-photon excitation spectra of the 5S-5D transitions of rubidium vapor.Fourier transform spectroscopy (FTS) has been for nearly fifty years the leading technique in analytical chemistry and molecular spectroscopy, as well as an irreplaceable tool for remote sensing, industrial process control etc. Fourier transform spectrometers record on a single photo-detector, in almost any spectral region, multiplex high-resolution spectra over a broad spectral span. Interferometers with a resolution better than 30 MHz are available, even commercially; however high resolution FTS in the gas phase has been so far limited to the measurements of Doppler-broadened lines, possibly narrowed by cooling the sample e.g. in a supersonic beam or by collisions with a buffer gas.Nonlinear mechanisms for canceling the Doppler effect have been harnessed with tunable lasers or with a single frequency comb. In direct frequency comb Doppler-free two-photon spectroscopy [1,2], all comb lines may contribute and the excitation probability of any given level can be the same as with a continuous-wave (cw) laser of the same average power. Short pulses facilitate nonlinear frequency conversion to access spectral regions where cw lasers are not readily available. However, the spectrum is only retrieved modulo the comb line spacing. Therefore, direct frequency comb spectroscopy with a single frequency comb is only suitable for spectra composed of very few transitions.Here we propose and demonstrate Doppler-free Fourier transform spectroscopy by combining dual-comb spectroscopy and two-photon excitation in a standing-wave field. Dual-comb spectroscopy is a recent technique of Fourier spectroscopy without moving

Multiplex two-photon excitation spectroscopy is demonstrated at Doppler-limited resolution. We describe first Fourier-transform two-photon spectroscopy of an atomic sample with two mode-locked laser oscillators in a dual-comb technique. Each transition is uniquely identified by the modulation imparted by the interfering comb excitations. The temporal modulation of the spontaneous two-photon fluorescence is monitored with a single photodetector, and the spectrum of all excited transitions is revealed by a Fourier transform.

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