Raluca Cireasa

High-order harmonic spectroscopy of the Cooper minimum in argon: Experimental and theoretical study

We study the Cooper minimum in high-order-harmonic generation from argon atoms by using long wavelength laser pulses. We find that the minimum in high-order-harmonic spectra is systematically shifted with respect to total photoionization cross section measurements. We use a semiclassical theoretical approach based on classical trajectory Monte Carlo and quantum electron scattering methods to model the experiment. Our study reveals that the shift between photoionization and high-order-harmonic emission is due to several effects: the directivity of the recombining electrons and emitted polarization, and the shape of the recolliding electron wave packet.

Inhomogeneous high harmonic generation in krypton clusters.

High order harmonic generation from clusters is a controversial topic: conflicting theories exist, with different explanations for similar experimental observations. From an experimental point of view, separating the contributions from monomers and clusters is challenging. By performing a spectrally and spatially resolved study in a controlled mixture of clusters and monomers, we are able to isolate a region of the spectrum where the emission purely originates from clusters. Surprisingly, the emission from clusters is depolarized, which is the signature of statistical inhomogeneous emission from a low-density source. The harmonic response to laser ellipticity shows that this generation is produced by a new recollisional mechanism, which opens the way to future theoretical studies.

High-harmonic transient grating spectroscopy of NO2 electronic relaxation.

We study theoretically and experimentally the electronic relaxation of NO(2) molecules excited by absorption of one ∼400 nm pump photon. Semiclassical simulations based on trajectory surface hopping calculations are performed. They predict fast oscillations of the electronic character around the intersection of the ground and first excited diabatic states. An experiment based on high-order harmonic transient grating spectroscopy reveals dynamics occurring on the same time scale. A systematic study of the detected transient is conducted to investigate the possible influence of the pump intensity, pump wavelength, and rotational temperature of the molecules. The quantitative agreement between measured and predicted dynamics shows that, in NO(2), high harmonic transient grating spectroscopy encodes vibrational dynamics underlying the electronic relaxation.

Time-resolved photoelectron spectroscopy of the CH(3)I B(1)E 6s [2] state.

The predissociation dynamics of the vibrationless level of the 6s (B (2)E) Rydberg state of CH(3)I was studied by femtosecond-resolved velocity map imaging of photoelectrons. By monitoring the decay of the CH(3)I(+) produced by photoionizing the B state, the predissociation lifetime was measured to be 1310 ± 70 fs. Photoelectron spectra were recorded as a function of the excitation scheme (one or two photons to the B state), and as a function of the ionizing wavelength. All of these photoelectron spectra show a simple time dependence that is consistent with the decay time of the CH(3)I(+) ion signal. The photoelectron angular distributions for the ionization of the B state depend on the excitation scheme and the ionizing wavelength, and show a strong dependence on the vibrational modes excited in the resulting CH(3)I(+). At long delays, the photoelectron spectra are characterized by photoionization of the I((2)P(1/2)) fragment formed by predissociation of the B state.

Quantum interference in NO2.

This paper investigates the origin of a quantum interference observed when NO(2) is dissociatively ionized by short pulses of ultraviolet light. We describe time-resolved measurements of NO(+), O(+), and NO(2)(+) ions produced following the interaction of NO(2) with a approximately 70 fs duration pulse centered close to 400 nm and a subsequent time-delayed probe pulse close to 269, 205, or 400 nm. A quantum beat oscillation with a period of 524 fs and a characteristic damping time of 8 ps is observed on all transient ion signals. We investigate the effect of tuning the central wavelength of the excitation pulse over a 12 nm range, and we discuss the potential importance of three possible multiphoton pathways involving one, two, and three pump photons. We conclude that the ionization pathway responsible for the beat signal is most likely due to a process involving the absorption of two pump photons and two probe photons. This presents an interesting problem with respect to the interpretation of the mechanism responsible for the quantum interference signature since the electronic states of NO(2) reached at the two-photon level are all thought to be extremely short-lived and to dissociate on a time scale that is far shorter than the characteristic damping time of the oscillatory signals. We suggest that a possible explanation for the observed dynamics is associated with a minor dissociation channel of the (2)(2)B(2) state of NO(2) through its interaction with the longer lived (2)(2)A(1) state.

Imaging fast relaxation dynamics of NO2

Time-resolved spectroscopy combined with velocity map imaging techniques have been used to investigate the multiphoton dynamics of the NO2 molecule. Two different pump–probe excitation schemes were used to explore different potential energy surfaces (PESs) located in the first dissociation region and in the Rydberg region around 9.2 eV. Integrated and energy-resolved signals of NO2+, NO+ and photoelectrons were recorded as a function of time. When exciting with 403 nm photons, the NO+ signal exhibits an intriguing oscillatory behaviour with a period of 512 fs. The NO+ and photoelectron kinetic energy distributions produced by this pump wavelength were cold, while those produced when employing 269 nm photons as pump were very rich, evidencing the presence of multiple excitation channels. A couple of sharp long-lived photoion–photoelectron peaks represents the most salient feature of the latter. They were assigned to an excitation by two 269 nm photons to a Rydberg state dissociating into NO(A2Σ+)+O(3P). This NO+ peak as well as another one located at 0 eV display very complex time dependencies including the signatures of two dissociation dynamics on timescales of 400 and 600 fs. The different pathways responsible for this temporal behaviour are discussed in view of shedding light onto the underlying multichannel multiphoton dynamics.

Electronic and non-adiabatic dynamics

Andrew J. Orr-Ewing, Jan R. R. Verlet, Tom J. Penfold, Russell S. Minns, Michael P. Minitti, Theis I. Sølling, Oliver Schalk, Markus Kowalewski, Jon P. Marangos, Michael A. Robb, Allan S. Johnson, Hans Jakob Worner, Dmitrii V. Shalashilin, R. J. Dwayne Miller, ̈ Wolfgang Domcke, Kiyoshi Ueda, Peter M. Weber, Raluca Cireasa, Morgane Vacher, Gareth M. Roberts, Piero Decleva, Filippo Bencivenga, Daniel M. Neumark, Oliver Gessner, Albert Stolow, Pankaj Kumar Mishra, Iakov Polyak, Kyoung Koo Baeck, Adam Kirrander, Danielle Dowek, Alvaro ́ Jimenez-Galan, Fernando Martın, Shaul Mukamel, ́ ́ ́ Taro Sekikawa, Maxim F. Gelin, Dave Townsend, Dmitry V. Makhov and Simon P. Neville

Inhomogeneous High Harmonic Generation in Krypton Clusters

High order harmonic generation from clusters is a controversial topic: conflicting theories exist, with different explanations for similar experimental observations. From an experimental point of view, separating the contributions from monomers and clusters is challenging. By performing a spectrally and spatially resolved study in a controlled mixture of clusters and monomers, we are able to isolate a region of the spectrum where the emission purely originates from clusters. Surprisingly, the emission from clusters is depolarized, which is the signature of statistical inhomogeneous emission from a low-density source. The harmonic response to laser ellipticity shows that this generation is produced by a new recollisional mechanism, which opens the way to future theoretical studies.