Time-Resolved Spectroscopic Study of Diatomic Molecular Sodium

Miami University, Oxford OH

Investigators

Abstract

This project will measure the "excited state lifetime" of a molecule, which is the amount of time a molecule remains in a high energy state after absorbing light. The characteristic timescale of this lifetime is determined by the laws of nature, and is one of the fundamental properties of the molecule. It is typically a few tens of nanoseconds (billionths of a second). The lifetime of sodium molecules is measured in this project using very precisely pulsed lasers in a technique called "time-resolved spectroscopy." When the lasers are pulsed on, a high-speed electronic stopwatch is switched on to record the amount of time that lapses between the absorption and the emission of the photon. The data obtained has applications in fields such as astrophysics (determining what kind of molecules are in space and how they formed), plasma physics (determining the temperatures of very hot materials), and laser physics (helping to predict what materials could be used to make better lasers). This project focuses on experimental studies of radiative lifetime measurements (for a variety of rotational quantum numbers) of the excited ion-pair state of sodium molecules from direct fluorescence decay using a time-resolved spectroscopic technique. Sodium molecules can exhibit exotic behavior since their structural and radiative properties vary strongly as a function of internuclear distance. This is due to the effect of the ion pair character, which causes the formation of double well in the molecular potential energy curves. Recently, the radiative lifetimes of some of the ion-pair states of sodium molecules have been calculated and mapped out. This provides benchmark values against which the reliability of recent theoretical approaches can be tested in the laboratory. Measurements of excited state lifetimes play an important role in understanding the interplay between molecular potential energies, molecular electronic structure, and related quantities such as transition probabilities, oscillator strengths, and line intensities. The experimental approach is to create sodium molecules in a heatpipe oven and excite them by a two-step laser double resonance excitation to reach the ion-pair state. Molecular fluorescence detection, atomic fluorescence detection from dissociating molecules (a process which happens on different time scale than molecular radiative timescale), and polarization of atomic fluorescence from predissociating molecule will be measured. If a dissociated molecule is detected, polarization measurement will provide detail information about the nonadiabatic dynamics of the predissociation process. Thus, the time-resolved spectroscopic technique provides a tool for studying the intramolecular vibrational energy distribution in molecules and the dynamics of atomic motion in a molecule.