Rotationally-resolved infrared spectroscopy of
the polycyclic aromatic hydrocarbon pyrene
(C16H10) using a quantum cascade laserbased cavity ringdown spectrometer
Jacob T. Stewart and Brian E. Brumfield,
Department of Chemistry, University of Illinois at UrbanaChampaign
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 challenge
Walk in the park
Trip to the moon
Pyrene C16H10
Coronene C24H12
Ovalene C32H14
400 K
C60
1000 K
Toven increasing with mass to produce necessary
number density
26 atoms
60 atoms
Increasing Qvib
•
•
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
Ne matrix (4 K)
CsI pellet (300 K)
Gas phase (570 K)
Joblin et al., Astron. Astrophys.,
299, 835 (1995).
Getting sample into the gas phase
• Designed an oven to hold•>50
g ofansample
Need
oven that can
• Horizontal orientation allows
liquid sample
operate
up to 700°C
• Can operate up to at least 700°C
for hours
for many
hours
• Needs to be able to
hold large amount of
sample
• Must be able to hold
liquid
Our mid-IR spectrometer
•Fabry-Perot quantum cascade
•Rhomb
and polarizer
act as
an
lasers provided
by Claire
Gmachl
at
optical
isolator
Princeton
•Total
internal
causes
a
•Housed
in areflection
liquid nitrogen
cryostat
phase shift in the light
•Lasers can scan from ~1180-1200
cm-1 (not necessarily continuous)
B. E. Brumfield et al., Rev. Sci. Instrum., 81, 063102 (2010).
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
Each tall peak we observe
is actually a stack of many
transitions
Simulation with B’ decreased by 0.1% relative to B’’
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
Baba et al., J. Chem. Phys., 131,
224318 (2009).
PGOPHER, a Program for Simulating Rotational Structure, C. M. Western, University of
Bristol, http://pgopher.chm.bris.ac.uk
Discrepancy with fluorescence excitation
spectrum
• 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 Our
investigated
– Baba
combination
differences
using our
fit (cm-1)
et al.
Difference % difference
data confirm 300
ourMHz
ground state assignment-4
A’’
0.0337202(12)
0.0339147(45)
-1.95×10
0.6%
B’’
0.0185559(12)
0.0186550(32)
-9.91×10-5
C’’
0.01197271(61)
0.0120406(24)
-6.79×10-5
0.5%
Trot = 20 K
linewidth = 10 MHz
0.6%
Vibrationally excited state
v=0 (cm-1)
0
v=1
Difference
% Difference
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
Observed absorption =
Calculated absorption
𝑄𝑣𝑖𝑏 × 𝐶𝑐𝑙𝑢𝑠𝑡𝑒𝑟
• 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
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
Tvib = 60 – 90 K
S. R. Langhoff, J. Phys. Chem.,
100, 2819 (1996).
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