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![Page 1: Rotationally-resolved infrared spectroscopy of the polycyclic aromatic hydrocarbon pyrene (C 16 H 10 ) using a quantum cascade laser- based cavity ringdown.](https://reader035.fdocuments.us/reader035/viewer/2022062515/56649cc15503460f949894aa/html5/thumbnails/1.jpg)
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
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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).
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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
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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)
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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
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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)
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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)
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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
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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
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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).
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• 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%
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Vibrationally excited state
v=0 (cm-1) v=1 Difference % Difference
0 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
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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 absorption
𝑄𝑣𝑖𝑏×𝐶𝑐𝑙𝑢𝑠𝑡𝑒𝑟
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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
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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
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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
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Acknowledgments
• McCall Group• Claire Gmachl• Richard Saykally• Kevin Lehmann