%0 Journal Article
%T Inefficient Vibrational Cooling of C60 in a Supersonic Expansion
%A Jacob T. Stewart
%A Brian E. Brumfield
%A Bradley M. Gibson
%A Benjamin J. McCall
%J ISRN Physical Chemistry
%D 2013
%R 10.1155/2013/675138
%X High-resolution gas-phase infrared spectroscopy of buckminsterfullerene (C60) was attempted near 8.5 ¦Ìm using cavity ring-down spectroscopy. Solid C60 was heated in a high-temperature (~950£¿K) oven and cooled using an argon supersonic expansion generated from a 12.7£¿mm ¡Á 150 ¦Ìm slit. The expected ratio is ~140 for vibrationally cold C60, but no absorption signal has been observed, presumably due to a lack of vibrational cooling of C60 in the expansion. Measurements of D2O at 875£¿K are presented as a test of instrument alignment at high temperature and show that efficient rotational cooling of D2O occurs in the hot oven ( £¿=£¿20£¿K in the expansion), though vibrational cooling does not occur. The attempted C60 spectroscopy is compared to previous work which showed efficient vibrational cooling of polycyclic aromatic hydrocarbons (PAHs). Possible alternative experiments for observing a cold, gas-phase spectrum of C60 are also considered. 1. Introduction Ever since its discovery in 1985 [1], buckminsterfullerene (C60) has been the subject of an enormous body of research (the publication of its discovery has been cited more than 8100 times at the time of writing according to Web of Science). One particular area of interest is the astrochemistry of C60. C60 was long believed to be present in the interstellar medium (ISM) and has recently been detected via emission spectroscopy in the mid-infrared (mid-IR) [2¨C7]. Though C60 has been detected in the ISM, the mechanism by which it is formed is still under debate [8]. Obtaining an astronomical absorption spectrum of C60 would yield additional information about the abundance and temperature of C60 in the ISM, which could provide useful information in determining the formation mechanism. Unfortunately, astronomical searches for absorption from cold gas-phase C60 are hampered by the lack of a high-resolution gas-phase laboratory spectrum of any of the four infrared active vibrational bands of C60. A rotationally resolved spectrum of C60 would also be of great fundamental interest. The acquisition of such a spectrum would be a significant milestone in the field of molecular spectroscopy, as C60 would be the largest and most symmetric molecule to be observed with rotational resolution. In addition, due to boson exchange symmetry restrictions on the overall symmetry of the molecular wave function there are many rotational levels in the ground and vibrationally excited state that are rigorously forbidden to exist [9, 10]. Such missing levels will manifest as ¡°gaps¡± in the normal progression of rovibrational lines.
%U http://www.hindawi.com/journals/isrn.physical.chemistry/2013/675138/