Rotationally-resolved infrared spectroscopy of the polycycl - PowerPoint Presentation

Slide1

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-ChampaignBenjamin J. McCall, Departments of Chemistry and Astronomy, University of Illinois at Urbana-ChampaignSlide2

Our goal at 8.5 µmOur goal is to observe the 8.5 µm vibrational band of C60 to aid in astronomical studiesWe have built a sensitive mid-IR spectrometer and measured the 8 mode of methylene bromideWe 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).Slide3

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 spectroscopySlide4

Previous work on this band1184 cm-1 band previously measured by Joblin et al.Band strength has been measured experimentallyAllows 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)Slide5

Getting sample into the gas phase

Designed an oven to hold >50 g of sampleHorizontal orientation allows liquid sampleCan 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 liquidSlide6

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)Slide7

The pyrene vibrational modeThis mode is a C-H bending modePyrene is an asymmetric top (D2h point group)This is a b-type band (ΔJ = 0,±1; ΔKa

=±1; ΔKc=±1)Slide8

Overall spectrumPQQR structure of a b-type band with little intensity near the band centerStrong P and R-branches indicate a small change in rotational constants in the

vibrationally excited stateSlide9

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 transitionsSlide10

Simulating the spectrumWe used PGOPHER to fit and simulate the spectrumGround 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).Slide11

Cannot fit spectrum using Baba et al.’s constantsIf 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 assignmentDiscrepancy with fluorescence excitation spectrum

T

rot

= 20 K

linewidth

= 10 MHz

300 MHz

Our 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)-9.91×10-50.5%C’’0.01197271(61)0.0120406(24)-6.79×10-50.6%Slide12

Vibrationally excited statev=0 (cm-1)

v=1Difference% Difference0

1184.035561(32)

A

0.0337202(12)

0.0337138(13)

6.4×10-60.019%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 statesCentrifugal distortion constants were unnecessary to fit the bandSlide13

Estimating the vibrational temperatureUsing our assignment, we can calculate the expected spectrum at a vibrational temperature of 0 KCompare expected spectrum to experimental spectrum to estimate TvibEstimate column density from:rate of mass loss from the oven (25 g in ~20 hr)

gas velocity in the expansionvertical distribution in the expansionoverlap of TEM00 mode of cavity with expansion

 Slide14

Estimating the vibrational temperatureBand 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 KSlide15

ConclusionsWe have measured and assigned rotationally-resolved infrared spectrum of pyreneLargest molecule observed with rotational resolution using infrared absorptionLarge molecules can be cooled effectively by supersonic expansionSlide16

Future WorkTry to resolve discrepancy between our work and fluorescence excitation spectroscopyContinue to try and observe C60 spectrumDevelop an external-cavity QCL system to extend frequency coverageContinue on to larger PAHs, such as coroneneSlide17

Acknowledgments

McCall Group

Claire

Gmachl

Richard

Saykally

Kevin Lehmann