MRI: Development of a 3-D Imaging for Vibrationally Resolved Cross Section Measurements

Pacific Union College, Angwin CA

Investigators

Abstract

The goal of this project is to develop a versatile device that probes quantum properties of small diatomic molecular ions (particles composed of two atoms that have lost one or more electrons). The singly charged hydrogen molecular ion (two protons, bound together by a single electron) is a simple but fundamental example of such an ion. Diatomic molecular ions are important because they are abundantly found in space, and might also briefly exist in other settings, including living organisms. The mechanism binding the two atoms is very similar to a coiled spring, allowing the atoms to move one with respect to the other without going astray. According to the basics of Quantum Mechanics, a diatomic molecular ion is continually vibrating in a range of discrete quantum states at different temperatures, which are defined by integers that state the degree of vibration on a given scale for which the bigger the integer, the stronger the pulsation. The proposed device probes these discrete states and is comparable to a thermometer reading a low temperature for a cold object and high temperature for hot one. It will also provide data that will help to understand how the vibrational states affect their environment during interaction with other particles. Not well understood, these slow interactions, technically called low energy interactions, are known to play important roles in the interstellar medium, in the cold part of nuclear fusion plasmas, and in the processes of DNA strand breaks, etc. Thus, this project will promote the progress of science and may have implications to a broad spectrum of areas of importance to society; at the same time, it will give participating undergraduate students the opportunity to take part in the development of a sophisticated 3-D imaging device at their own institution. The charge transfer in collisions between hydrogen and diatomic molecular ions touches a variety of disciplines spreading from physical science to life science. First of all, it is of foremost importance in fundamental physics because it involves the smallest atom. It is also one of the dominant reactions in environments such as the cold divertor plasma regions of a fusion tokamak or in interstellar clouds where the main constituents are neutral H, the positive hydrogen ion, and H-molecules. Moreover, understanding of this simplest fundamental system is a key for mastering more complex systems which exist in, e.g., biophysics where radical attacks on biomolecules such as DNA potentially involve charge transfer at very low energy. However, it is often almost impossible to compare laboratory measured cross sections to existing theories and calculations because the vibrational state distribution of the molecules is not known. The proposed 3-D imaging device will ultimately improve previously measured absolute cross section measurements by making them vibrationally resolved, enabling a more detailed comparison between theoretical and experimental results. In this 3-D imaging technique, the molecular ion undergoes a resonant dissociative charge exchange with an alkali atom and releases its vibrational energy in the form of kinetic energy of the two fragments. The detection of the positions of the daughter particles and their flight time differences made possible with the detectors allows the reconstruction of the molecular ion's initial vibrational energy via simple dynamics. This detection technique is equivalent to taking a time resolved snapshot picture of the molecular ion fragments (thus the name 3-D imaging). The whole 3-D imaging apparatus is envisioned to be a portable device which could be easily transported to and used at a variety of research facilities.