Methanol Photolysis Reaction Dynamics

Methanol photolysis on interstellar grain surfaces is a central aspect of prebiotic astrochemistry. The organic radicals that undergo reaction during the warm-up phase of star-formation arise from the photolysis of methanol on grain surfaces, and it is from these radicals that most prebiotic organic molecules are thought to form [1, 2]. The products of methanol dissociation are extensive, as is shown below in an energy level diagram adapted from ab initio calculations [3, 4]. Remarkably, the branching ratios for methanol photolysis have not yet been adequately measured in either the condensed or the gas phase. The branching ratios assumed for astrochemical modeling studies are essentially educated guesses based on previous laboratory experiments that were ill-equipped to quantitatively determine the relevant values. Laboratory measurement of these branching ratios under a variety of astrophysical conditions is key to understanding any organic chemistry in interstellar environments, particularly prebiotic chemistry.

Quantitative measurement of methanol photolysis branching ratios relies on high-sensitivity THz spectrometer development. THz spectroscopy can be used to monitor the products of methanol photolysis by probing the pure rotational transitions of the resultant radicals. Each of the photolysis fragments has strong rotational lines across the submillimeter and THz ranges. Direct absorption techniques provide the quantitative information needed to accurately determine the photolysis branching ratios. Our group is conducting initial measurements using the direct absorption setup shown below. Here, a low-pressure mercury lamp (shown in the picture on the right) is used to photolyze methanol as it expands into a vacuum chamber in a supersonic expansion. A direct absorption submillimeter spectrometer is then used to monitor the methanol signal depletion and/or the signal increase from the pure rotational signatures of the product channels. While this direct absorption approach works well for monitoring the major product channels, ultimately the higher sensitivity of THz-CRDS may be needed to directly measure the minor product channels.

[1] Garrod, Widicus Weaver, and Herbst (2008), ApJ 682, 283.
[2] Laas, Garrod, Herbst, and Widicus Weaver (2011), ApJ 728, 71.
[3] Chang and Lin (2002) Chem. Phys. Lett., 363, 175.
[4] Chang and Lin (2004) Chem. Phys. Lett., 384, 229.