Cooling of large and heavy molecules in seeded supersonic beams

Abstract

In this paper we report the application of the techniques of laser fluorescence excitation spectroscopy for the experimental study of the rotational—vibrational cooling of iodine (I2) and of several large molecules, i.e. anthracene (C14H10), tetracene (C18H12), pentacene (C22H14) and ovalene (C32H14), in seeded supersonic beams of rare gases. We have found that the mass of the rare gas expanded at stagnation pressures of p = 20–8300 Torr and through a nozzle of diameter D = 50–200 μ exhibits a marked effect on the degree of rotational—vibrational cooling of the large molecules. The degree of internal cooling increases in the order He < Ne < Ar < Kr < Xe. Cooling of large and heavy molecules in a supersonic expansion of heavy diluents down to a rotational temperature Tr ≈ 5–7 K and vibrational temperature TV < 50 K can be accomplished at moderate values of pD ≈ 2.4–3.0 Torr cm for Ar, pD ≈ 2.0 Torr cm for Kr and pD ≈ 1.4 Torr cm for Xe. Effective internal cooling at moderate values of pD cannot be accomplished in light diluents, i.e., He and Ne, in view of the velocity slip effect. The degree of rotational cooling of large molecules in heavy diluents, such as Ar, Kr and Xe, seems to be as efficient as that of I2 in these media. On the other hand, the degree of vibrational cooling of large molecules in Ar, Kr and Xe is very efficient, in marked contrast to the ineffective vibrational cooling of I2 in heavy diluents under the same circumstances. We have observed that the effective formation of van der Waals molecules between the aromatic molecule and Ar, Kr and Xe is exhibited only after vibrational sequence congestion of the large molecule is eliminated. These observations provide an experimental basis for the use of seeded beams of Ar, Kr and Xe at moderate values of pD for the interrogation of excited-state energetics and dynamics of internally cold, isolated, bare, large molecules. Finally, we demonstrated the possibility of performing laser spectroscopy of large molecules in high-flow seeded supersonic beams expanded through a nozzle for D = 150 μ at p = 10 atm, employing a primitive pumping system based on a mechanical pump without using diffusion pumps.