AbstractsBiology & Animal Science

We report on the development of a new laboratory tool which is suitable both for generating and quickly cooling short-lived molecules, and also for studying the kinetics and dynamics that take place at the rotational level during the expansion process. By integrating a microplasma device with a supersonic nozzle, temperatures as low as 50 K were achieved for molecules having lifetimes shorter than 40 ns and excitation (internal) energies ??? 11 eV. Additionally, final temperatures ranging from 90 K to 900 K for a set of nested electronic states were observed in the He2 excimer, and a highly non-equilibrium rotational distribution was recorded for the lowest of these nested states. This rotational distribution was analyzed with a kinetic mode and shown to be due primarily to collisional excitation transfer and rotational relaxation. Since collisions are the means by which the supersonic expansion process cools atoms/molecules, this result perhaps demonstrates a fundamental restriction on which molecular states can and cannot be effectively cooled in a supersonic expansion. The rate constant for rotational relaxation within the He2(d3??u+) state was determined to be (9.4 ?? 0.1) ?? 10-13 cm3s-1, while the rate constant for collisional excitation transfer between rotational levels of the He2(e3??g) and He2(d3??u+) states was found to scale as (9.8 ?? 5.9 ?? 10-14 cm3s-1)exp(-(6.4 ?? 10-3)/ ??E*B).