High-resolution gas-phase infrared spectroscopy of buckminsterfullerene (C60) was attempted near 8.5
Ever since its discovery in 1985 [
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 [
Despite great interest in a high-resolution spectrum of C60, a rotationally resolved, gas-phase absorption spectrum of C60 has not yet been observed. There are several obstacles which must be overcome to record such a spectrum. First, it is difficult to generate a gas-phase sample of C60. C60 has negligible vapor pressure at room temperature and must be heated to temperatures in excess of 875 K to reach a vapor pressure on the order of 10–100 mTorr [
Plot of C60 vibrational partition function (on a logarithmic scale) versus temperature. The vibrational frequencies used in the calculation were obtained from [
We have developed an experiment which attempts to address these obstacles. We have built a high-temperature oven source which has been used to generate C60 vapor. To attempt to relax the vibrational degrees of freedom, the hot C60 vapor has been cooled using a supersonic expansion. The supersonic jet is then probed using continuous wave cavity ring-down spectroscopy (cw-CRDS) [
Previous work has shown effective vibrational cooling of large molecules, such as polycyclic aromatic hydrocarbons (PAHs), in supersonic expansions [
Our high-resolution mid-IR spectrometer has been described in detail previously [
We have constructed a high-temperature oven which we used to produce gas-phase C60. The oven is described in detail in our previous work on pyrene [
Knowledge of where to scan in frequency space to detect the first signal from the 8.5
We have simulated the 8.5
Estimating the expected
To estimate the number density, we use the rate of mass loss from the oven per unit time, combined with the velocity of molecules in the expansion. We account for the fact that not all of the molecules in the expansion will overlap with the
We can now use (
Plot of calculated
Our attempts to observe C60 are summarized in Table
Summary of attempted absorption spectroscopy of C60. The measured mass loss rate for each attempt was used to calculate
Attempt |
|
|
NEA (ppm)a | Estimated |
---|---|---|---|---|
1 | 955 | 150 | 1.0 | 130 |
2 | 955 | 500 | 0.9 | 140 |
3 | 965 | 1900 | 0.6 | 74 |
bThese values represent the largest
Representative absorption spectrum of the Ar/C60 expansion from 1184 to 1186 cm−1. This plot is from attempt 3 listed in Table
As can be seen in Table
The observed reduction in the mass loss of C60 from the source is consistent with prior reports in the literature. In a previous study on the thermal decomposition of C60, Sundar et al. observed that C60 held at 975 K for an extended period of time (24 h) decomposed to amorphous carbon [
Because of the high temperatures involved in producing gas-phase C60, we were concerned that the alignment of our optical cavity with our oven might be affected by thermal expansion or other heating effects. To eliminate this possibility, we observed absorption spectra of D2O in expansions from our oven at similar temperatures as our C60 searches. There are many lines of the bending mode as well as a hot band within the spectral coverage of our spectrometer, which also allowed us to monitor the rotational and vibrational temperature of D2O molecules in the high temperature supersonic expansion.
We introduced a small amount of D2O into an Ar slit expansion and observed low-lying states which were populated in the expansion. We observed the 111
Measuring several transitions of D2O in the expansion from the hot oven permitted calculation of both the rotational and vibrational temperatures for D2O. Table
List of observed D2O transitions. Rotational levels are denoted in the usual way
Vibrational band | Transition | Frequency (cm−1) |
---|---|---|
|
|
1199.793 |
|
1194.038 | |
|
1193.255 | |
|
1198.536 | |
| ||
|
|
1181.311 |
|
1199.154 | |
|
1192.497 | |
|
1199.690 | |
|
1198.379 |
Boltzmann plot of D2O rotational levels observed in the supersonic jet at 875 K. The error bars indicate
From the material presented in Section
Previous studies have shown that large molecules (specifically PAHs) can be effectively cooled in supersonic expansions [
Vibrational cooling of large molecules seeded in a supersonic expansion of a monatomic carrier gas proceeds by transferring vibrational energy from the large molecule to translational energy in the expansion by collisions with the carrier gas (V-T transfer). Because of this, the number of collisions that a molecule experiences in the supersonic expansion will have a significant effect on the amount of vibrational cooling that is possible in the expansion. We have estimated the number of two-body hard sphere collisions which occur in our attempted C60 spectroscopy and previous work on PAHs by our group [
Estimated number of two body hard sphere collisions experienced by large molecules seeded in an argon supersonic expansion. The first three entries in the table are from the attempted C60 spectroscopy reported in this work. The pyrene entry is from our previous work observing pyrene in the same experimental setup which we used to attempt C60 spectroscopy [
Molecule |
|
|
Nozzle type | Number of collisions |
---|---|---|---|---|
C60 (att. 1) | 955 | 150 | Slit | 320 |
C60 (att. 2) | 955 | 500 | Slit | 1100 |
C60 (att. 3) | 965 | 1900 | Slit | 4000 |
Pyrene | 430 | 150 | Slit | 560 |
Anthracene | 410 | 270 | Pinhole | 600 |
Tetracene | 485 | 175 | Pinhole | 360 |
Pentacene | 550 | 250 | Pinhole | 500 |
Ovalene | 630 | 300 | Pinhole | 620 |
The first step in estimating the number of collisions in a supersonic expansion is to describe the temperature, density, and velocity of molecules in the expansion. These properties are given by the following equations for the centerline of the expansion, which are reproduced from [
The next step in calculating the number of collisions is to calculate the mean free path of the large molecules in the expansion. To do so, we use a hard sphere model, which gives the usual equation:
Hard sphere radii used to compute the mean free path of molecules in the supersonic expansion.
Species | Radius |
---|---|
Ar | 1.88 |
C60 | 5.0 |
Pyrene | 4.0 |
Anthracene | 3.8 |
Tetracene | 4.2 |
Pentacene | 4.6 |
Ovalene | 5.1 |
We can then calculate the average frequency of two-body collisions of the large molecule with Ar atoms in the expansion by dividing the average speed of the molecules by the mean free path. We assume that the translational temperature of the large molecules is thermalized with the translational temperature of the Ar atoms and calculate the average speed according to
From Table
Theoretical harmonic vibrational frequencies for all of the PAHs listed in Table
Vibrational partition function, average vibrational energy (above zero-point energy) in cm−1, frequency of the lowest energy vibrational mode, and energy to collision ratio for the molecules listed in Table
Molecule |
|
|
|
|
---|---|---|---|---|
C60 (att. 1) |
|
56000 | 267 | 174 |
C60 (att. 2) |
|
56000 | 267 | 50 |
C60 (att. 3) |
|
57000 | 267 | 14 |
Pyrene |
|
4000 | 99 | 7 |
Anthracene |
|
3200 | 91 | 5 |
Tetracene |
|
6400 | 56 | 18 |
Pentacene |
|
10000 | 38 | 21 |
Ovalene |
|
18000 | 61 | 29 |
D2O | 1.20 | 260 | 1178 | 0.8 |
Looking at these values, it is somewhat surprising that C60 was not efficiently cooled using a supersonic expansion for attempts 2 and 3. Considering the previous work with PAHs, it appears that there should be enough collisions to carry away the vibrational energy for these attempts. One possible reason why C60 was not efficiently cooled is that the lowest energy vibrational mode for C60 is at 267 cm−1, which is significantly higher than the lowest modes for the PAHs (see Table
Another possibility is that the extremely large size of C60 is causing a significant velocity slip effect to occur because of the large difference in mass between the C60 molecules and the Ar carrier gas [
Finally, it is worth noting that other molecules have been observed to have anomalously low cooling efficiency in supersonic expansions. In particular, Sulkes observed that benzene displays a lack of collision-induced vibrational relaxation in He and Ar expansions, despite the fact that substituted benzene derivatives showed excellent cooling under similar conditions [
It would be preferable to produce C60 vapor at a much lower temperature than is possible in our oven. Doing so would significantly decrease the vibrational partition function and vibrational energy of the molecules prior to the supersonic expansion. For example, if C60 vapor could be produced at a temperature similar to our pyrene work (430 K), then the vibrational partition function would be only
We are aware of three possible methods for producing C60 vapor at lower temperatures: supercritical fluid expansion, laser desorption, and collisional cooling in the gas-phase. In the supercritical fluid expansion method the C60 sample would be dissolved in a supercritical fluid. It has been demonstrated that C60 can be dissolved in supercritical toluene, or to a lesser extent in supercritical carbon dioxide containing toluene as a co-solvent [
A laser desorption/supersonic expansion source has been successfully employed for resonant two-photon ionization measurements of C60 [
The final method to discuss for producing cold gas-phase C60 relies upon the modified use of a gas-phase aggregation cluster source [
Although we expect all of the proposed methods to be superior to our current oven in terms of vibrational temperature, we also expect them to produce significantly lower number densities of gas-phase C60. Based on reports of supercritical fluid extractions of C60 [
Obtaining a high-resolution gas-phase spectrum of C60 represents a significant challenge for molecular spectroscopy. We have attempted to obtain such a spectrum using a highly sensitive cw-CRDS spectrometer but were unable to detect any absorption signal from C60. We have shown that an absorption signal should be expected with our current experimental setup if we are able to vibrationally cool C60. Our lack of signal is most likely due to the fact that we must heat our sample to >950 K to obtain sufficient vapor pressure for spectroscopy, which leads to an incredibly large vibrational partition function because of the large size of C60. We have compared these results with previous work that has shown efficient vibrational cooling of large polycyclic aromatic hydrocarbons. To overcome this problem it will be necessary to produce gas-phase C60 at much lower temperatures so that the vibrational degrees of freedom can be cooled. While there may exist alternative methods that are capable of generating a cold gas-phase sample, these methods will result in low number densities of C60. The low C60 number density will require the use of more sensitive and complex absorption spectroscopy techniques than cw-CRDS to enable collection of a rotationally resolved spectrum.
The authors have no direct financial relations with the commercial identities named in this work.
The authors would like to thank the Gmachl group at Princeton for providing the quantum cascade lasers which have made this work possible. The authors also thank Gregory S. Girolami for useful discussions about decomposition of the C60 sample. Their efforts to observe the spectrum of C60 have been supported by the NASA Laboratory Astrophysics Program (APRA NNG05GE59G), a Camille and Henry Dreyfus New Faculty award, a Packard Foundation Fellowship, and the University of Illinois. Jacob T. Stewart has been supported by a Robert C. and Carolyn J. Springborn Fellowship from the University of Illinois.