Investigation of Interferences in Carbon Dioxide through Multidimensional Molecular-Frame High-Harmonic Spectroscopy.

We present molecular-frame high-harmonic spectroscopic measurements of the spectral intensity and group delay of carbon dioxide. Using four different driving wavelengths and a range of intensities at each wavelength for high-harmonic generation, we observe a well-characterized minimum in the harmonic emission that exhibits both a wavelength and intensity dependence. Using the intensity dependence at each driving wavelength, we classify the minimum as due to either a structural two-center interference or dynamic multichannel interference, consistent with previous literature. By additionally measuring the group delay at each driving wavelength and intensity, we find that the sign of the group delay excursion across the interference is an acute probe of the interference mechanism. The experimental results are confirmed by ab initio time-dependent density functional theory calculations of both the spectral intensity and the phase of the harmonic emission.

High-harmonic spectroscopy of transient two-center interference calculated with time-dependent density-functional theory.

We demonstrate high-harmonic spectroscopy in many-electron molecules using time-dependent density-functional theory. We show that a weak attosecond-pulse-train ionization seed that is properly synchronized with the strong driving mid-infrared laser field can produce experimentally relevant high-harmonic generation (HHG) signals, from which we extract both the spectral amplitude and the target-specific phase (group delay). We also show that further processing of the HHG signal can be used to achieve molecular-frame resolution, i.e., to resolve the contributions from rescattering on different sides of an oriented molecule. In this framework, we investigate transient two-center interference in CO2 and OCS, and how subcycle polarization effects shape the oriented/aligned angle-resolved spectra.

Comparison of ultrafast intense-field photodynamics in aniline and nitrobenzene: stability under amino and nitro substitution.

We report on the photoionization and photofragmentation of aniline (C6H5NH2) and nitrobenzene (C6H5NO2) under single-molecule conditions in the focus of 50 fs, 800 nm laser pulses. Ion mass spectra are recorded as a function of intensity ranging from 6 × 1012 to 3 × 1014 W cm-2. Ion yields are measured in the absence of the focal volume effect and without the need for additional deconvolution of data. We observe evidence of resonance-enhanced multiphoton ionization in aniline, in agreement with current literature. Phenyl-based ion fragments, singly-charged parent ions, and dissociative rearrangement processes are observed for both molecules. However, fragmentation in aniline is heavily suppressed in favor of parent ionization while the reverse is true for nitrobenzene, and multiply-charged parent ions are present in aniline and absent in nitrobenzene. We discuss the implications of these and other results as they relate to molecular stability against intense-field ionization and fragmentation, specifically with regards to the opposing behavior of the substituted amino and nitro functional groups.

Ultrafast dynamics. Four-dimensional imaging of carrier interface dynamics in p-n junctions.

The dynamics of charge transfer at interfaces are fundamental to the understanding of many processes, including light conversion to chemical energy. Here, we report imaging of charge carrier excitation, transport, and recombination in a silicon p-n junction, where the interface is well defined on the nanoscale. The recorded images elucidate the spatiotemporal behavior of carrier density after optical excitation. We show that carrier separation in the p-n junction extends far beyond the depletion layer, contrary to the expected results from the widely accepted drift-diffusion model, and that localization of carrier density across the junction takes place for up to tens of nanoseconds, depending on the laser fluence. The observations reveal a ballistic-type motion, and we provide a model that accounts for the spatiotemporal density localization across the junction.

Observation and identification of metastable excited states in ultrafast laser-ionized pyridine.

We report on the fragmentation of ionized pyridine (C(5)H(5)N) molecules by focused 50 fs, 800 nm laser pulses. Such ionization produces several metastable ionic states that fragment within the field-free drift region of a reflectron-type time of flight mass spectrometer, with one particular metastable dissociation being the leading fragmentation process. Because the time of flight is no longer dependent in a simple way on the mass of the ion, the metastable decay is manifested as an unfocused peak on the mass spectrum that appears at a time of flight not corresponding to an integer mass. A previously-developed method is used to identify the precursor and final masses of these ions. The metastable process that creates the most prevalent peak is shown to be C(5)H(5)N(+) → C(4)H(4)(+) + HCN. Simulations confirm this result and place restrictions on the processes for several other observed metastable reactions.

Ultrafast resonance-enhanced multiphoton ionization in the azabenzenes: pyridine, pyridazine, pyrimidine, and pyrazine.

We report on the ultrafast photoionization of pyridine, pyridazine, pyrimidine, and pyrazine. These four molecules represent a systematic series of perturbations into the structure of a benzene ring which explores the substitution of a C-H entity with a nitrogen atom, creating a heterocyclic structure. Data are recorded under intense-field, single-molecule conditions. The pulses (50 fs, 800 nm) are focused into the molecular vapor, and ion mass spectra are recorded for intensities of ~10(13) W/cm(2) to ~10(15) W/cm(2). We measure ion yields in the absence of the focal volume effect without the need for deconvolution of the data. For all targets, stable singly- and doubly-charged parent ions (C(6-n)H(6-n)N(n)(+(+))) are observed with features suggesting resonance-enhanced ionization. From the intensity dependence of the ion yield, we infer that excitation occurs both through (1)ππ* transitions (remnants of the benzene structure) and through (1)nπ* transitions, the latter being a result of Rydberg-like excitations of the lone pair electrons of the nitrogen atoms. Stability against intense-field fragmentation is also discussed.

Ultrafast REMPI in benzene and the monohalobenzenes without the focal volume effect.

We report on the photoionization and photofragmentation of benzene (C(6)H(6)) and of the monohalobenzenes C(6)H(5)-X (X = F, Cl, Br, I) under intense-field, single-molecule conditions. We focus 50-fs, 804-nm pulses from a Ti:sapphire laser source, and record ion mass spectra as a function of intensity in the range ∼10(13) W/cm(2) to ∼10(15) W/cm(2). We count ions that were created in the central, most intense part of the focal area; ions from other regions are rejected. For all targets, stable parent ions (C(6)H(5)X(+)) are observed. Our data is consistent with resonance-enhanced multiphoton ionization (REMPI) involving the neutral (1)ππ* excited state (primarily a phenyl excitation): all of our plots of parent ion yield versus intensity display a kink when this excitation saturates. From the intensity dependence of the ion yield we infer that both the HOMO and the HOMO-1 contribute to ionization in C(6)H(5)F and C(6)H(5)Cl. The proportion of phenyl (C(6)H(5)) fragments in the mass spectra increases in the order X = F, Cl, Br, I. We ascribe these substituent-dependent observations to the different lifetimes of the C(6)H(5)X (1)ππ* states. In X = I the heavy-atom effect leads to ultrafast intersystem crossing to a dissociative (3)nσ* state. This breaks the C-I bond in an early stage of the ultrashort pulse, which explains the abundance of fragments that we find in the iodobenzene mass spectrum. For the lighter X = F, Cl, and Br this dissociation is much slower, which explains the lesser degree of fragmentation observed for these three molecules.

Ultrashort intense-field optical vortices produced with laser-etched mirrors.

We introduce a simple and practical method to create ultrashort intense optical vortices for applications involving high-intensity lasers. Our method utilizes femtosecond laser pulses to laser etch grating lines into laser-quality gold mirrors. These grating lines holographically encode an optical vortex. We derive mathematical equations for each individual grating line to be etched, for any desired (integer) topological charge. We investigate the smoothness of the etched grooves. We show that they are smooth enough to produce optical vortices with an intensity that is only a few percent lower than in the ideal case. We demonstrate that the etched gratings can be used in a folded version of our 2f-2f setup [Opt. Express 19, 7599 (2005)] to compensate angular dispersion. Finally, we show that the etched gratings withstand intensities of up to 10(12) W/cm(2).