Time-Resolved Holography with Photoelectrons

To describe the microscopic properties of matter, quantum mechanics uses wave functions, whose structure and time dependence is governed by the Schrödinger equation. In atoms the charge distributions described by the wave function are rarely observed. The hydrogen atom is unique, since it only has one electron and, in a dc electric field, the Stark Hamiltonian is exactly separable in terms of parabolic coordinates (η, ξ, φ). As a result, the microscopic wave function along the ξ coordinate that exists in the vicinity of the atom, and the projection of the continuum wave function measured at a macroscopic distance, share the same nodal structure. In this Letter, we report photoionization microscopy experiments where this nodal structure is directly observed. The experiments provide a validation of theoretical predictions that have been made over the last three decades.

Midinfrared strong-field laser ionization offers the promise of measuring holograms of atoms and molecules, which contain both spatial and temporal information of the ion and the photoelectron with subfemtosecond temporal and angstrom spatial resolution. We report on the scaling of photoelectron holographic interference patterns with the laser pulse duration, wavelength, and intensity. High-resolution holograms for the ionization of metastable xenon atoms by 7-16  μm light from the FELICE free electron laser are presented and compared to semiclassical calculations that provide analytical insight.

Highly parallel, active pixel detectors enable novel detection capabilities for large biomolecules in time-of-flight (TOF) based mass spectrometry imaging (MSI). In this work, a 512 × 512 pixel, bare Timepix assembly combined with chevron microchannel plates (MCP) captures time-resolved images of several m/z species in a single measurement. Mass-resolved ion images from Timepix measurements of peptide and protein standards demonstrate the capability to return both mass-spectral and localization information of biologically relevant analytes from matrix-assisted laser desorption ionization (MALDI) on a commercial ion microscope. The use of a MCP-Timepix assembly delivers an increased dynamic range of several orders of magnitude. The Timepix returns defined mass spectra already at subsaturation MCP gains, which prolongs the MCP lifetime and allows the gain to be optimized for image quality. The Timepix peak resolution is only limited by the resolution of the in-pixel measurement clock. Oligomers of the protein ubiquitin were measured up to 78 kDa.

In mass spectrometry imaging, spatial resolution is pushed to its limits with the use of ion microscope mass spectrometric imaging systems. An ion microscope magnifies and then projects the original spatial distribution of ions from a sample surface onto a position-sensitive detector, while retaining time-of-flight mass separation capabilities. Here, a new type of position-sensitive detector based on a chevron microchannel plate stack in combination with a 512 ϫ 512 complementary metal-oxide-semiconductor based pixel detector is coupled to an ion microscope. Spatial resolving power better than 6 m is demonstrated by secondary ion mass spectrometry and 8 -10 m spatial resolving power is achieved with laser desorption ionization. A detailed evaluation of key performance criteria such as spatial resolution, acquisition speed, and data handling is presented. (J Am Soc Mass Spectrom 2010, 21, 2023-2030 © 2010 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry M ass spectrometry imaging (MSI) [1-3] measurements allow the visualization of the spatial structure and identification of the molecular masses from complex surfaces. High spatial resolution is accomplished with ion-microscope mass spectrometers, where ions are extracted from the sample surface and projected onto a position-sensitive detector. A spatial resolution better than 4 m has been reported using UV/IR laser surface probes in matrix assisted laser desorption ionization (MALDI) [4 -8]. A pulsed primary ion beam as a surface probe can achieve higher spatial resolving powers (1 m) [9,10]. The spatial resolution can be further improved by using a more focused primary ion/laser desorption ionization surface probe. However, fragmentation of the surface molecules and long measurement times are undesired side effects of decreasing the surface probe area. For instance, at a 2 ϫ 2 m pixel size (4 m lateral resolution) and a sample size of 1 ϫ 1 mm, a typical measurement comprises 250,000 measurement points and can last several hours. An alternate approach to increase the spatial resolution is the use of microscope mode MSI. In the microscope mode, surface molecules are desorbed and ionized over a large sample area, typically 200 -300 m in diameter. An ion microscope employs ion optics to project the ionized surface compounds onto a position-sensitive detector while magnifying the image and retaining the spatial information defined by the sample surface. With a field of view of 200 ϫ 200 m and a sample size of 1 ϫ 1 mm, a microscope mode imaging experiment involves 25 measurement points and retains the 4 m lateral resolution given the corresponding ion optical magnification factor. The ion optical magnification allows high-resolution images to be obtained independent of the ionization source. Microscope mode MSI enables fast, highresolution large area imaging provided that an adequate, i.e., fast and position-sensitive, detector is used to record high quality molecular images [4].Position-sensitive detectors most commonly used for microsc...