In situ measurement of three-dimensional ion densities in focused femtosecond pulses

Effect of the Impact Angle on the Kinetic Energy and Angular Distributions of β-Carotene Sputtered by 15 keV Ar2000 Projectiles

Molecular dynamics (MD) computer simulations are used to model ejection of particles from β-carotene samples bombarded by 15 keV Ar₂₀₀₀. The effect of the incidence angle on the angular and kinetic energy distributions is investigated. It has been found that both of these distributions are sensitive to the variation of the incidence angle, particularly near the normal incidence. For impacts along the surface normal, material ejection is azimuthally symmetric, and a significant emission occurs along the surface normal. The kinetic energy distribution of intact molecules has a maximum near 1 eV and terminates below approximately 2 eV. An increase of the incidence angle breaks the azimuthal symmetry. Most of the intact molecules become ejected in the forward direction. The maximum in the polar angle distribution shifts toward large off-normal angles. In addition, the most probable kinetic energy of ejected molecules is significantly increased. The mechanisms of molecular emission responsible for the observed changes are delineated. The implications of the observed ejection characteristics for the utilization of large gas cluster projectiles in secondary neutral mass spectrometry are discussed.

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.

Optical device for the precision control of the electric field in the focus of a beam

We present a common-path optical device consisting of four optical components that produce a laser field consisting of two sub-beams with one radially polarized and the other linearly polarized. In the focus, the radially polarized sub-beam produces longitudinal polarization while the linearly polarized sub-beam produces polarization perpendicular to the propagation direction. By rotating the optical components, the orientation of the resulting electric field in the focus can be continuously varied in any direction. Estimates of the angular resolution of the device are given within the paraxial approximation.

Time slicing in 3D momentum imaging of the hydrogen molecular ion photo-fragmentation

Photo-fragmentation of the hydrogen molecular ion was investigated with 800 nm, 50 fs laser pulses by employing a time slicing 3D imaging technique that enables the simultaneous measurement of all three momentum components which are linearly related with the pixel position and slicing time. This is done for each individual product particle arriving at the detector. This mode of detection allows us to directly measure the three-dimensional fragment momentum vector distribution without having to rely on mathematical reconstruction methods, which additionally require the investigated system to be cylindrically symmetric. We experimentally reconstruct the laser-induced photo-fragmentation of the hydrogen molecular ion. In previous experiments, neutral molecules were used as a target, but in this work, performed with molecular ions, the initial vibrational level populations are well-defined after electron bombardment, which facilitates the interpretation. We show that the employed time-slicing technique allows us to register the fragment momentum distribution that reflects the initial molecular states with greater detail, revealing features that were concealed in the full time-integrated distribution on the detector.

Laboratory transferability of optimally shaped laser pulses for quantum control

Optimal control experiments can readily identify effective shaped laser pulses, or "photonic reagents," that achieve a wide variety of objectives. An important additional practical desire is for photonic reagent prescriptions to produce good, if not optimal, objective yields when transferred to a different system or laboratory. Building on general experience in chemistry, the hope is that transferred photonic reagent prescriptions may remain functional even though all features of a shaped pulse profile at the sample typically cannot be reproduced exactly. As a specific example, we assess the potential for transferring optimal photonic reagents for the objective of optimizing a ratio of photoproduct ions from a family of halomethanes through three related experiments. First, applying the same set of photonic reagents with systematically varying second- and third-order chirp on both laser systems generated similar shapes of the associated control landscape (i.e., relation between the objective yield and the variables describing the photonic reagents). Second, optimal photonic reagents obtained from the first laser system were found to still produce near optimal yields on the second laser system. Third, transferring a collection of photonic reagents optimized on the first laser system to the second laser system reproduced systematic trends in photoproduct yields upon interaction with the homologous chemical family. These three transfers of photonic reagents are demonstrated to be successful upon paying reasonable attention to overall laser system characteristics. The ability to transfer photonic reagents from one laser system to another is analogous to well-established utilitarian operating procedures with traditional chemical reagents. The practical implications of the present results for experimental quantum control are discussed.

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.

Transfer-matrix-based method for an analytical description of velocity-map-imaging spectrometers

We propose a simple and general analytical model describing the operation of a velocity-map-imaging spectrometer. We show that such a spectrometer, possibly equipped with a magnifying lens, can be efficiently modeled by combining analytical expressions for the axial potential distributions along with a transfer matrix method. The model leads transparently to the prediction of the instrument's operating conditions as well as to its resolution. A photoelectron velocity-map-imaging spectrometer with a magnifying lens, built and operated along the lines suggested by the model has been successfully employed for recording images at threshold photoionization of atomic lithium. The model's reliability is demonstrated by the fairly good agreement between experimental results and calculations. Finally, the limitations of the analytical method along with possible generalizations, extensions, and potential applications are also discussed. The model may serve as a guide for users interested in building and operating such spectrometers as well as a tutorial tool.

Single-snapshot and intensity-resolved two-photon fluorescence measurements

We present a single-snapshot (SSS) method for obtaining intensity-resolved two-photon fluorescence (TPF). This simple method uses a digital camera to image the TPF spot on a liquid dye jet. By making a comparison between the local laser and TPF intensities, TPF probabilities are reconstructed. We compare our intensity-resolved TPF results with those obtained by the more common intensity scanning (IS) and z-scan methods. The dependence of the TPF probability on intensity obtained by the SSS method for coumarin-30 exhibits a clear maximum around I approximately 4 x 10(12) W/cm(2) and a postsaturation decrease, while no such effects were found in the data obtained by the other methods. Additionally, theoretical models are presented to extract the overall probability from within the volume integral. To our knowledge, we present the first reported measurements of such intensity-resolved TPF.