Femtosecond wave-packet propagation in spin-orbit-coupled electronic states of 39,39K2 and 39,41K2

Nuclear quantum dynamics on the ground electronic state of neutral silver dimer 107Ag109Ag probed by femtosecond NeNePo spectroscopy

The nuclear quantum dynamics on the ground electronic state of the neutral silver dimer 107Ag109Ag are studied by femtosecond (fs) pump-probe spectroscopy using the 'negative ion - to neutral - to positive ion' (NeNePo) excitation scheme. A vibrational wave packet is prepared on the X1Σ+g state of Ag2via photodetachment of mass-selected, cryogenically cooled Ag2- using a first ultrafast pump laser pulse. The temporal evolution of the wave packet is then probed by an ultrafast probe pulse via resonant multiphoton ionization to Ag2+. Frequency analysis of the fs-NeNePo spectra obtained for a single isotopologue and pump-probe delay times up to 60 ps yields the harmonic (ωe = 192.2 cm-1), quadratic anharmonic (ωexe = 0.637 cm-1) and cubic anharmonic (ωeye = 3 × 10-4 cm-1) constants for the X1Σ+g state of neutral Ag2. The fs-NeNePo spectra obtained at different pump wavelengths provide insight into the excitation mechanism. At a pump wavelength of 510 nm instead of 1010 nm, resonant excitation of a short-lived electronically excited state of the anion followed by autodetachment results in population of higher-energy vibrational levels of the neutral ground state. In contrast, at 1140 nm dynamics with a slightly shorter beating period and different relative phase are observed. The present study demonstrates that isotopologue-specific fs-NeNePo spectroscopy provides accurate vibrational constants of mass-selected neutral clusters in their electronic ground state in the terahertz spectral region, which remains difficult to obtain directly in the frequency domain with any other type of spectroscopy of comparable sensitivity.

Probing Rydberg-Rydberg interactions in N2 by ultrafast EUV-NIR photoelectron spectroscopy

The ultrafast dynamics of molecular nitrogen (N₂) just below the ionization threshold has been investigated by time-resolved photoelectron spectroscopy using a single harmonic centered at hν = 15.38 eV. The evolution of the Rydberg wavepacket launched by the ultrashort EUV pulse is probed by a time-delayed femtosecond NIR laser pulse. The observed photoelectron spectra show two series of vibrational peaks to the ground X²Σg+ state and the first excited A²Πu state of N2+. Among these, two photoelectron peaks with the vibrational quantum numbers v_X+ = 4 and v_A+ = 1 exhibit clear anti-phase oscillation with a period of 300 fs, showing that two Rydberg states converging to the X²Σg+ and A²Πu ionic states interact with each other, thus causing periodic switching in the population of the ion core states.

Parametrically polarization shaped pulses guided via a hollow core photonic crystal fiber for coherent control

We present ultrafast polarization pulse shaping through a micro structured hollow core photonic crystal fiber. The pulses are shaped in pulse sequences in which the energy, distance, phases, and chirps as well as the state of polarization of each individual sub-pulse can be independently controlled. The application of these pulses for coherent control is demonstrated for feedback loop optimization of the multi-photon ionization of potassium dimers. In a second experiment, this process is investigated by shaper-assisted pump-probe spectroscopy which is likewise performed with pulses that are transmitted through the fiber. Both techniques reveal the excitation pathway including the dynamics in the participating electronic states and expose the relevance of the polarization. These methods will be valuable for endoscopic applications.

Slow periodic oscillations in time domain dynamics of NO2

The authors investigated the time domain nonadiabatic dynamics of NO2 on the coupled X 2A1 and A 2B2 electronic states by launching wave packets on the excited electronic state and focused on the evolution at long times (t>200 fs), which has received little attention up to now. The authors showed that the initial fast spreading of the wave packets is followed at all energies by slow periodic intramolecular vibronic energy redistribution (IVER) with periods in the range of 0.3 to several tens of picoseconds. These energy transfers lead to oscillations with the same periods in the population of each electronic state. Propagation of wave packets indicates that IVER frequencies also dominate the fluctuations of the squared modulus of the autocorrelation function |A(t)|2 at energies not too high above the bottom of the conical intersection, but this is no longer the case at higher energies. For example, for initial wave packets prepared by almost vertical excitation of the vibrational ground state of the ground electronic surface, the oscillations of |A(t)|2 essentially reflect the detuning from 1:2 resonance between the frequency of the bend and that of the symmetric stretch in the excited electronic state. These theoretical results were used to discuss the possible origin of the low-frequency oscillations which were recently observed in time domain experimental spectra of NO2.

Learning from the acquired optimized pulse shapes about the isotope selective ionization of potassium dimers

Selective optimization of the 39,39K2 and 39,41K2 isotopomers in a three-photon ionization process is presented by applying evolution strategies on shaped fs pulses in a feedback loop. The optimizations at different center wavelengths show considerably large enhancements of one isotope compared to the other and reversed. We compare the acquired optimized pulse shapes for combined phase and amplitude with pure amplitude modulation. Particularly from their spectra we are able to extract information about the optimally chosen differing ionization paths via the involved vibrational states. Furthermore, a comparison of the temporal shape of the optimized pulse forms for combined phase and amplitude with pure phase optimization is given. The presented pulse form analysis demonstrates the potential of restricted optimization to gain insight into the underlying dynamical processes. Our approach reveals how the optimization algorithm precisely addresses the vibrational wave functions both spectrally and temporally.

Isotope selective ionization by optimal control using shaped femtosecond laser pulses

We report on selective optimization of different isotopes in an ionization process by means of spectrally broad shaped fs-laser pulses. This is demonstrated for (39,39)K2 and (39,41)K2 by applying evolution strategies in a feedback loop, whereby a surprisingly high enhancement of one isotope versus the other and vice versa is achieved (total factor approximately 140). Information about the dynamics on the involved vibrational states is extracted from the optimal pulse shapes, which provides a new spectroscopical approach of yielding distinct frequency pattern on fs-time scales. The method should, in principle, be feasible for all molecules.

SEMICLASSICAL DESCRIPTION OF MOLECULAR DYNAMICS BASED ON INITIAL-VALUE REPRESENTATION METHODS

Recent progress in the development of semiclassical methods to describe quantum effects in molecular dynamics is reviewed. Focusing on rigorous semiclassical methods that are based on the initial-value representation of the semiclassical propagator, we discuss several promising schemes that have been developed in the past few years to extend the applicability of semiclassical approaches to complex molecular systems. In particular, integral-filtering techniques and forward-backward methods are surveyed. Furthermore, recently proposed approaches that allow the semiclassical description of nonadiabatic molecular dynamics are discussed. The potential and efficiency of these methods is illustrated by selected applications.