Rotationally-resolved infrared spectroscopy of the polycyclic aromatic hydrocarbon pyrene (C 16 H 10 ) using a quantum cascade laser- based cavity ringdown spectrometer Jacob T. Stewart and Brian E. Brumfield, Department of Chemistry, University of Illinois at Urbana- Champaign Benjamin J. McCall, Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign

Our goal at 8.5 µm Our goal is to observe the 8.5 µm vibrational band of C 60 to aid in astronomical studies We have built a sensitive mid-IR spectrometer and measured the 8 mode of methylene bromide We have attempted to observe C 60, but have not seen any signal yet B. E. Brumfield, J. T. Stewart, B.J. McCall, J. Mol. Spec., 266, 57 (2011).

Seeking an intermediate challenge Trip to the moon Pyrene C 16 H 10 Coronene C 24 H 12 Ovalene C 32 H 14 T oven increasing with mass to produce necessary number density 400 K 1000 K Increasing Q vib 26 atoms 60 atoms C 60 Walk in the park Only pyrene has an IR active mode within QCL frequency coverage Largest molecule to be rotationally resolved using infrared direct absorption spectroscopy

Previous work on this band 1184 cm -1 band previously measured by Joblin et al. Band strength has been measured experimentally Allows us to estimate degree of vibrational cooling Joblin et al., Astron. Astrophys., 299, 835 (1995). Ne matrix (4 K) CsI pellet (300 K) Gas phase (570 K)

Getting sample into the gas phase Designed an oven to hold >50 g of sample Horizontal orientation allows liquid sample Can operate up to at least 700°C for hours Need an oven that can operate up to 700°C for many hours Needs to be able to hold large amount of sample Must be able to hold liquid

Our mid-IR spectrometer B. E. Brumfield et al., Rev. Sci. Instrum., 81, (2010). Rhomb and polarizer act as an optical isolator Total internal reflection causes a phase shift in the light Rhomb and polarizer act as an optical isolator Total internal reflection causes a phase shift in the light Fabry-Perot quantum cascade lasers provided by Claire Gmachl at Princeton Housed in a liquid nitrogen cryostat Lasers can scan from ~ cm -1 (not necessarily continuous) Fabry-Perot quantum cascade lasers provided by Claire Gmachl at Princeton Housed in a liquid nitrogen cryostat Lasers can scan from ~ cm -1 (not necessarily continuous)

The pyrene vibrational mode This mode is a C-H bending mode Pyrene is an asymmetric top (D 2h point group) This is a b-type band (ΔJ = 0,±1; ΔK a =±1; ΔK c =±1)

Overall spectrum PQQR structure of a b-type band with little intensity near the band center Strong P and R-branches indicate a small change in rotational constants in the vibrationally excited state

Changing rotational constants in the excited state Simulation from our assignment of the spectrum Simulation with B’ decreased by 0.1% relative to B’’ Each tall peak we observe is actually a stack of many transitions

Simulating the spectrum We used PGOPHER to fit and simulate the spectrum Ground state rotational constants published by Baba et al. Values obtained from fluorescence excitation spectroscopy PGOPHER, a Program for Simulating Rotational Structure, C. M. Western, University of Bristol, Baba et al., J. Chem. Phys., 131, (2009).

Cannot fit spectrum using Baba et al.’s constants If we allow ground and excited state constants to float during the fitting we obtain a good fit (standard deviation of cm -1 (11 MHz)) Ground state constants from fit are statistically different from Baba et al. This discrepancy between ground state constants is still being investigated – combination differences using our data confirm our ground state assignment Discrepancy with fluorescence excitation spectrum T rot = 20 K linewidth = 10 MHz 300 MHz Our fit (cm -1 )Baba et al.Difference% difference A’’ (12) (45)-1.95× % B’’ (12) (32)-9.91× % C’’ (61) (24)-6.79× %

Vibrationally excited state v=0 (cm -1 )v=1Difference% Difference (32) A (12) (13)6.4× % B (12) (12)5.0× % C (61) (64)1.8× % Rotational constants change very little in the vibrationally excited state B is statistically unchanged between ground and excited states Centrifugal distortion constants were unnecessary to fit the band

Estimating the vibrational temperature Using our assignment, we can calculate the expected spectrum at a vibrational temperature of 0 K Compare expected spectrum to experimental spectrum to estimate T vib Estimate column density from: rate of mass loss from the oven (25 g in ~20 hr) gas velocity in the expansion vertical distribution in the expansion overlap of TEM 00 mode of cavity with expansion

Estimating the vibrational temperature Band strength for pyrene mode is known (10 km/mol) Using this information we can calculate Q vib × C cluster to be ~1.3 Doubling backing pressure did not lead to decrease in absorption – assume C cluster = 1 (no clustering) Use scaled harmonic frequencies to calculate Q vib as a function of temperature S. R. Langhoff, J. Phys. Chem., 100, 2819 (1996). T vib = 60 – 90 K

Conclusions We have measured and assigned rotationally- resolved infrared spectrum of pyrene Largest molecule observed with rotational resolution using infrared absorption Large molecules can be cooled effectively by supersonic expansion

Future Work Try to resolve discrepancy between our work and fluorescence excitation spectroscopy Continue to try and observe C 60 spectrum Develop an external-cavity QCL system to extend frequency coverage Continue on to larger PAHs, such as coronene

Acknowledgments McCall Group Claire Gmachl Richard Saykally Kevin Lehmann