With a three-dimensional (3D) momentum imaging technique, we investigated the laser desorption ionization dynamics initiated by both chirped picosecond and femtosecond pulses. 3D momentum images of desorbed ions from 2,5-dihydroxybenzoic acid (DHB), a common laser desorption matrix, were obtained for the first time. A striking difference was observed between the processes initiated by femtosecond and picosecond pulses. The lack of initial momentum in ions produced by femtosecond pulses suggests a suppression of plume formation, which can be exploited to increase the sensitivity of matrixassisted laser desorption ionization.

The velocity map imaging (VMI) technique was first introduced by Eppink and Parker in 1997, as an improvement to the original ion imaging method by Houston and Chandler in 1987. The method has gained huge popularity over the past two decades and has become a standard tool for measuring high-resolution translational energy and angular distributions of ions and electrons. VMI has evolved gradually from 2D momentum measurements to 3D measurements with various implementations and configurations. The most recent advancement has brought unprecedented 3D performance to the technique in terms of resolutions (both spatial and temporal), multi-hit capability as well as acquisition speed while maintaining many attractive attributes afforded by conventional VMI such as being simple, cost-effective, visually appealing and versatile. In this tutorial we will discuss many technical aspects of the recent advancement and its application in probing correlated chemical dynamics.

We report a new implementation of three-dimensional (3D) momentum imaging for electrons, employing a two-dimensional (2D) imaging detector and a silicon photomultiplier tube (siPMT). To achieve the necessary time resolution for 3D electron imaging, a poly(p-phenylene)-dye-based fast scintillator (Exalite 404) was used in the imaging detector instead of conventional phosphors. The system demonstrated an electron time-of-flight resolution comparable with that of electrical MCP pick-off (tens of picoseconds), while achieving an unprecedented dead time reduction (∼0.48 ns) when detecting two electrons.

We report a new implementation of a recently developed 3D momentum imaging technique [Lee et al. J. Chem. Phys. 141, 221101 (2014)]. The previously employed high-speed digitizer in the setup is replaced by a portable USB3 oscilloscope. A new triggering scheme was developed to suppress trigger jitters and to synchronize the signals from a camera and the oscilloscope. The performance of the setup was characterized in the study of laser desorption/ionization of 2,5-dihydroxybenzoic acid on a velocity map imaging apparatus. A ∼60 picosecond time resolution in measuring time-of-flight is achieved with a count rate of ∼1 kHz, which is comparable to the system using high-speed digitizers. The new setup affords great portability and wider accessibility to the high-performing 3D momentum imaging technique.

To achieve an efficient 3-D imaging detection of electrons/ions in coincidence, a conventional 2D imaging detector (MCP/phosphor screen) and a fast frame camera are used in the 3D velocity map imaging (VMI) technique [1, 2] . However, it is still difficult to obtain two separate TOF events for two electrons using a conventional MCP detector coupled with a photomultiplier tube (PMT). This is because the phosphor screen is usually made of P47 phosphor which has longer decay time and thus not good to achieve high temporal resolution. Furthermore, due to the very short time separation interval between two electrons, it is imperative to use different phosphor/scintillator for improved 3D electron momentum imaging. Herein, we demonstrate that a scintillator screen coated with poly-para-phenylene laser dye (Exalite 404) can be used to achieve a greatly improved TOF resolution, which is sufficienct for 3D electron imaging.. A silicon photomultiplier tube (si-PMT) is also adopted to suppress the ringing in electric signals, typically associated with MCP pick-off.. The shorter emission lifetime of the poly-paraphenylene dye compared to the conventional P47 phosphor helps achieve an unprecedented dead time ( 0.48 ns). This has greatly enhanced the multi-hit capability of the 3D VMI technique in detecting two or more electrons in coincidence.