The large spectral width of ultrashort optical pulses makes it possible to measure the complete time‐resolved absorption spectrum of a sample with a single pulse, offering simultaneously high resolution in both the time and frequency domains. To quantitatively interpret these experiments, we start with the usual perturbative density matrix theory for the third‐order susceptibility of a multilevel system. However, the theory is formulated in terms of four‐time correlation functions which are interpreted as the time‐dependent overlap of bra and ket vibrational wave packets propagating independently on the ground and excited electronic state potential surfaces. This approach captures the critical distinction between electronic population decay and pure dephasing processes, while retaining the intuitive physical picture offered by the time‐dependent wave packet theories of molecular spectroscopy. A useful simplification is achieved by considering the absorption of the probe pulse as the first‐order spectroscopy of the nonstationary state created by the pump pulse. In this case, the dynamic spectrum is obtained through the Fourier transform of the time‐dependent overlap of the initial wave packet propagating on its potential surface and a second wave packet, created by the probe pulse, which evolves simultaneously on the final surface. Calculations for model systems using harmonic surfaces and δ‐function pulses are presented to illustrate the application of this theory and to clarify the unique spectral behavior of the nonstationary states created in femtosecond pump–probe experiments. Finally, we demonstrate the practical application of the theory for anharmonic surfaces and finite pulses by analyzing the dynamic spectroscopy of the excited state torsional isomerization of the bacteriorhodopsin chromophore.
We experimentally demonstrate that atomic orbital parity mix interferences can be temporally controlled on an attosecond time scale. Electron wave packets are formed by ionizing argon gas with a comb of odd and even high-order harmonics, in the presence of a weak infrared field. Consequently, a mix of energy-degenerate even and odd parity states is fed in the continuum by one- and two-photon transitions. These interfere, leading to an asymmetric electron emission along the polarization vector. The direction of the emission can be controlled by varying the time delay between the comb and infrared field pulses. We show that such asymmetric emission provides information on the relative phase of consecutive odd and even order harmonics in the attosecond pulse train.
Theoretical study of photoelectron angular distributions in single-photon ionization of aligned N2 and CO2
the angular distribution of the momenta of XUV ionised electrons is going to bare much relevance to the XUV initiated HHG experiment which is being constucted in the basement at the moment. i would like to take this journal club as an opportunity to involve both theoreticians and experimentalists in a discussion about the differing properties of XUV and tunnel ionised electrons and how we can use these properties to our advantage. i would also like to discuss the possibilities of writing a ral proposal for attempting to measure the ion state resolved angular distribution of the emitted electrons. for experimentalists all that theory has been prompted by J. Phys. Chem. A2008, 112, 9382–9386.