Session C7: Strong Field and Multiphoton Induced Dynamics

Quantum Phase Measurements in Two Color Above Threshold Ionization

We have observed angle- and energy-dependent phase shifts between spectral peaks in above-threshold ionization of argon. We utilize a weak second field at half the frequency of the strong drive laser to form interfering pathways between neighboring peaks and measure the oscillations in the angular distribution as a function of relative optical delay, similar to RABBITT-type experiments. Our results show direct evidence of the Coulomb potential distortions on the quantum phases of strong field ionized electrons. There has been recent interest in the phase shifts and time delays in single-photon ionization of atoms by attosecond pulses. Our work extends these types of measurements for the first time into the strong field regime.   

Experimental evidence of light induced conical intersection in photodissociation of diatomic molecules

We show experimental evidence for the existence of light induced conical intersections (LICI's) in diatomic molecules. Using a strong laser field, ${\rm H}^+_2$ is photodissociated, and interference in the dissociated part of the angular distribution is observed for vibrational states that overlap the LICI. At the point of the LICI, non-Born-Oppenheimer dynamics dominate transitions between the ground ($1{\rm s}\sigma_{\rm g}$) and excited ($2{\rm p}\sigma_{\rm u}$) electronic states, and as a result strong rovibrational coupling takes place. Multiple quantum paths around the LICI contribute to the final dissociative state of the wavefunction, which produce interference in both the vibrational and rotational parts of the wavefunction. Numerically solving the time-dependent Schr\"odinger equation, we highlight how the observed interference arises in the presence of the LICI. Tuning laser frequency and polarization, we can change the LICI's position in phase space, and the simulation sheds insight into how to manipulate non-Born-Oppenheimer dynamics in diatomic molecules.   

Multielectron effects in strong field ionization of molecules

Multielectron effects are studied for strong field ionization of di- and polyatomic molecules at their equilibrium geometries, using time dependent density functional theory. Strong field ionization of molecules have been previously often analyzed using ``single active electron'' (SAE) approximation based theories such as for example Intense Field Many Body $S$-matrix Theory and typically the contributions from inner valence orbitals and multielectron effects were concluded to be of less importance. For several di- and polyatomic molecules we show that ionization rate from inner valence orbitals can increase dramatically due to a novel resonant coupling which influences the molecular dynamics. We discuss the dependence of the results on the orientation of the molecules and laser parameters. Moreover we show how such a mechanism can lead to localization of electron depending on the symmetry of the orbitals involved. Finally, we propose how the novel mechanism can be observed experimentally and show how the multi-electron effects can help explain several experimental results which have shown disagreement with SAE approximation based theories.   

Mapping Rotational Wavepacket Dynamics with Chirped Probe Pulses

We develop an analytical model description of the strong-field pump-probe polarization spectroscopy of rotational transients in molecular gases in a situation when the probe pulse is considerably chirped: the frequency modulation over the pulse duration is comparable with the carrier frequency. In this scenario, a femtosecond pump laser pulse prepares a rotational wavepacket in a gas-phase sample at room temperature. The rotational revivals of the wavepacket are then mapped onto a chirped broadband probe pulse derived from a laser filament. The slow-varying envelope approximation being inapplicable, an alternative approach is proposed which is capable of incorporating the substantial chirp and the related temporal dispersion of refractive indices. Analytical expressions are obtained for the probe signal modulation over the interaction region and for the resulting heterodyned transient birefringence spectra. Dependencies of the outputs on the probe pulse parameters reveal the trade-offs and the ways to optimize the temporal-spectral imaging. The results are in good agreement with the experiments on snapshot imaging of rotational revival patterns in nitrogen gas.   

Time-resolved measurements of two-pulse enhanced ionization

Recent measurements have found greatly enhanced fluorescence in atmospheric density gases when two temporally separated ultrashort pulses are used [1]. We explore two-pulse ionization in xenon using supercontinuum spectral interferometry. This technique allows a time resolved measurement of absolute electron densities for inter-pulse delays from 200 fs to 50 ps, from the ionization threshold of neutral xenon to a mostly Xe$^+$ plasma. Use of a thin gas target allows careful characterization and control of the pulse intensities and neutral gas densities. We find that the ionization rate from the second pulse depends strongly on the time delay between the two pulses and the ionization fraction due to the first pulse. Notably, it is possible with the two-pulse configuration to ionize with the second pulse at intensities below the single pulse ionization threshold of neutral Xe. \\[4pt] [1] L. Shi, W. Li, Y. Wang, X. Lu, L. Ding, and H. Zeng, Phys. Rev. Lett. 107, 095004 (2011)   

Studies of inner-orbital ionization though velocity map imaging and Fourier transform spectroscopy

Since the first observation of inner-orbital ionization (IOI) of molecules by strong laser fields [PRL 67, 1230 (1991)], the influence of the inner-orbitals on ionization and high-harmonic generations has attracted much attention. We discuss a new technique to study IOI which is sensitive to both the geometry and phase of the orbitals. By combining velocity map imaging and Fourier transform spectroscopy, we can directly measure the populations in non-dissociating states of the molecular ion and their spatial distribution. We specifically report on the X $^2\Pi_{g,3/2}$ and A $^2\Pi_{u,3/2}$ states of I$_2^+$, which represent ionization of the HOMO ($\pi_g$) and HOMO-1 ($\pi_u$) orbitals of I$_2$, respectively. Once we have access to the populations of these states, we can study their dependence on polarization and coherent redistribution via a third coupling pulse.   

Survival of atoms in strong microwave fields

Recent experimental work on the ionization of atoms by intense laser and microwave fields has shown that bound atoms in highly excited states remain after the intense radiation pulse, even though the orbital period of the detected atoms exceeds the duration of the laser or microwave pulse. In both cases the fields are orders of magnitude larger than required for static field ionization of the highly excited atoms. Here we report a large population (10-25{\%}) left bound in the states with n \textgreater 350, when atoms are exposed to strong 16.9-GHz microwave fields in the presence of a well-controlled static field of 6 mV/cm. A production of such extremely high lying states is observed for a wide range of initial Rydberg states, as low as n $=$ 21, for Li and Na, and is, in fact, a general feature of microwave ionization. As well as the survival of the highly excited states in quasi stable orbits, threshold ionization fields also appear to depend strongly on the static field during the experiment. We observe the 1/3n$^5$ dependence only if the static fields are non-zero, and larger fields are required to ionize 50{\%} of atoms if the static field is canceled out.   

Multiphoton and above-threshold ionization in the XUV energy range

Recent experiments at the free-electron laser in Hamburg FLASH have succeeded in measuring direct multiphoton ionization and above-threshold ionization (ATI) in the XUV. We present a theoretical description of the phenomena seen in argon and xenon in an ab initio approach using the time-dependent configuration interaction singles method. Upon solving the time-dependent Schr\"odinger equation for the N-electron problem the photoelectron spectrum is calculated. Multiphoton absorption cross sections are derived and employed to quantify the ATI process for different photon energies.