In matrix isolation spectroscopy, individual molecules of a chemical compound are trapped and isolated from one another in a solid, inert matrix at low temperature while their spectrum is measured. Under these conditions, the molecules cannot interact with each other and interact only weakly with the surrounding inert matrix, thereby simulating the gas phase. What's more, a sample thus prepared can be preserved as long as the matrix is maintained. Thus, the matrix isolation technique is particularly well-suited to the study of highly reactive chemical species such as ions and free radicals which are difficult to generate and maintain in appreciable abundance in the gas phase. Reactive species such as these are typically generated either in the gas phase prior to deposition in the matrix, or after deposition by in-situ photolysis of an appropriate matrix-isolated precursor.
The matrix-isolation equipment currently in use in the Astrochemistry Lab is depicted in the photographs below. It consists of a cryogenically-cooled cold finger mounted in a stainless steel, oil diffusion-pumped high vacuum system. This cryostat is mounted on the vacuum system in such a way that it can be rotated through a full 180o without breaking vacuum.
The Equipment.
A small (~25 mm) window (cesium iodide for infrared work; sapphire for UV/visible work) is suspended at the tip of the crystat within the vacuum chamber and can be cooled to temperatures as low as 10 K (4 K with liquid helium cryocooling). Vacuum windows on the chamber permit spectroscopic measurements of samples prepared within. Additional ports permit the admission of the inert gaseous matrix material (usually argon for IR work; neon for UV/visible work) and vacuum ultraviolet light for sample photolysis.
The steps in a typical experiment are illustrated in the series of schematic diagrams at right. First, the sample window is cooled to 10 K (4 K for neon matrices) and positioned to face the beam axis of the spectrometer where a background spectrum of the bare substrate is recorded.
The Experiment.
Afterwards, the window is rotated to face the sample deposition ports. PAH vapor is generated by sublimation of a solid PAH sample which is placed in a pyrex test tube and attached to the system through a standard stainless steel Cajon Ultratorr vacuum fitting. The inert gas flows from a reservoir bulb, through a length of copper tubing, and into the system through an adjacent port. The two vapor streams coalesce and freeze at the surface of the cold window. Use of a great overabundance of inert gas ensures that in the resulting solid solution, the sample molecules are effectively isolated in the inert matrix. Once a suitable amount of sample has been deposited, the substrate is returned to the first position and its spectrum is recorded and ratioed to the background spectrum.
Finally, for spectroscopic studies of species which are generated by in situ ultraviolet photolysis (e.g. ionized PAHs), the substrate can subsequently be rotated to face a third port upon which is mounted a flowing hydrogen, microwave discharge lamp. Photolysis of the sample with the 120 nm (10.1 eV) Ly-a output of this lamp for as little as 2 minutes is usually sufficient to produce a measurable population of ions, and an equilibrium ion population is normally achieved within 10 - 20 minutes of photolysis. Afterwards, the photolyzed sample is returned to the scan position and its spectrum is recorded. In that spectrum, the intensity of the bands of the precursor neutral species will be diminished, and a series of new bands will appear. Comparison of the spectra obtained before and after photolysis allow identification of the ion spectral features which appear upon photolysis. Percentage ionizations in the range from 5% to 20% are typically realized with this technique.
