Astrochemistry

Low temperature Kinetics of stellar winds, interstellar medium, and planetary atmospheres

The low temperature production/loss rate coefficient behavior of gas species is crucial for modeling environments such as stellar winds, interstellar media, and planetary atmospheres, and yet the majority of the most important low temperature rate coefficients in these environments have not been determined.  This is due in part to the difficulty of theoretically calculating low temperature behavior of rate coefficients due to the need for extremely accurate Potential Energy Surfaces (PES), PESs which are capable of describing the long-range interactions that often dictate low temperature reaction rate behavior.  Lack of low temperature rate coefficient data is also due in part to the difficulty of measuring low temperature rate coefficients due to the fast rate of condensation of reagents at these temperatures.  The fast rate of condensation at low temperatures often makes conventional gas phase rate coefficient measurement techniques ineffective.  However, by utilizing a pulsed de Laval nozzle apparatus with a PLP-LIF technique, the Heard group is able to quickly generate and measure low temperature gas reactions before the reagents have time to condense.

Stellar Winds:

The majority (~85%) of Complex Organic Molecules (COMs) in the InterStellar Medium (ISM) are generated in the stellar winds of Asymptotic Giant Branch (AGB) stars.  COMs of increasing complexity are generated in AGB stellar winds in part by low temperature gas phase reactions with atoms and small molecule species as the stellar wind carries the COMs radiatively away from the AGB stars.  So far, ~70 molecules have been identified in AGB stellar winds, some of which are prebiotic molecules, and there is great uncertainty in many of the unmeasured low temperature reaction rate coefficients that are currently used to model the production of COMs in AGB stellar winds.  The Heard group is currently working with the Decin group in Leuven, Belgium in order to identify unmeasured low temperature rate coefficients that are predicted via sensitivity analysis to have the largest impact on the production of COMs in AGB stellar winds and to measure these rate coefficients.

InterStellar Media (ISM):

The ISM is composed of the regions of space between stars within a galaxy.  The unique conditions of the ISM (densities less than 106 molecules cm-3, temperatures as low as 10 K, and ionizing radiation often present) provide a challenging environment to recreate in the lab.  With the de Laval nozzle apparatus the Heard group has helped to provide low temperature kinetic data that can be fit and extrapolated to the conditions of the ISM with much greater certainty than estimated reaction rate coefficients that were previously used.

One type of system studied in the Heard group is reactions between the OH radical and hydrocarbon species.  This type of system often occurs on potential energy surfaces that exhibit a pre-reaction complex. These reactions often exhibit rate coefficients that have a negative temperature dependence that, if not included in kinetic models, can cause the rate coefficients to be off by orders of magnitude.  Therefore, continued experimental measurements at low temperatures are required in order to investigate this further.

Planetary Atmospheres:

The kinetics of reactions involving Volatile Organic Compounds (VOCs) is important because VOCs are an important class of pollutants in our atmosphere, VOCs are involved in combustion processes, and VOCs have been observed in atmospheres of other planets. The dominant removal of many of these species is through reaction with the OH radical and the Heard group has studied the reaction of several oxygenated VOCs with the OH radical at low temperatures.

Additionally, the low temperature reaction of 1CH2 with a variety of species relevant to Titan’s atmosphere has been studied in the Heard group.  It was found that there is an inverse temperature dependence for the loss rate coefficient of 1CH2 with many co-reagents and that the branching ratio between collisional deactivation of 1CH2 and reactive loss of 1CH2 increases with decreasing temperature.