Rotationally-resolved infrared spectroscopy of the polycyclic aromatic hydrocarbon pyrene (C16H10) 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 C60 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 C60, 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 challengeTrip to the moon

Pyrene C16H10

Coronene C24H12Ovalene C32H14

Toven increasing with mass to produce necessary

number density

400 K 1000 K

Increasing Qvib

26 atoms 60 atoms

C60

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, 063102 (2010).

•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 ~1180-1200 cm-1 (not necessarily continuous)

The pyrene vibrational mode• This mode is a C-H bending mode

• Pyrene is an asymmetric top (D2h point group)

• This is a b-type band (ΔJ = 0,±1; ΔKa=±1; ΔKc=±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, http://pgopher.chm.bris.ac.uk

Baba et al., J. Chem. Phys., 131, 224318 (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 0.00036 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

Trot = 20 Klinewidth = 10 MHz

300 MHzOur fit (cm-1) Baba et al. Difference % difference

A’’ 0.0337202(12) 0.0339147(45) -1.95×10-4 0.6%

B’’ 0.0185559(12) 0.0186550(32) -9.91×10-5 0.5%

C’’ 0.01197271(61) 0.0120406(24) -6.79×10-5 0.6%

Vibrationally excited state

v=0 (cm-1) v=1 Difference % Difference0 1184.035561(32)

A 0.0337202(12) 0.0337138(13) 6.4×10-6 0.019%

B 0.0185559(12) 0.0185554(12) 5.0×10-7 0.002%

C 0.01197271(61) 0.01197111(64) 1.8×10-6 0.013%

• 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 Tvib

• 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 TEM00 mode of cavity with expansion

Observed   absorption=Calculated   absorption𝑄𝑣𝑖𝑏×𝐶𝑐𝑙𝑢𝑠𝑡𝑒𝑟

Estimating the vibrational temperature

• Band strength for pyrene mode is known (10 km/mol)• Using this information we can calculate Qvib × Ccluster to be

~1.3• Doubling backing pressure did not lead to decrease in

absorption – assume Ccluster = 1 (no clustering)• Use scaled harmonic frequencies to calculate Qvib as a

function of temperature

S. R. Langhoff, J. Phys. Chem., 100, 2819 (1996).

Tvib = 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 C60 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