1. SPECTRA OF C6H6-Rgn (n=1,2) IN THE 3 MIRCON INFRARED BAND SYSTEM OF BENZENE
A. J. BARCLAY, A.R.W. McKELLAR, N. MOAZZEN-AHMADI

2. Outline Experimental set up Spectra Analysis & results Conclusions
Here is the outline of the talk. First I’ll tell you about the experimental set up and improvements in signal to noise using OPO in the rapid scan signal averaging mode. Next, describe the spectra starting with the spectrum of Benzene monomer. Then tell you about our analyses of the spectra of dimers and trimers with rare gases He, Ne, and Ar and finally conclude.

3. Direct IR absorption in a supersonic jet
Spectrometer
Direct IR absorption in a supersonic jet
Pulsed supersonic jet expansion
Slit nozzle (General Valve)
Repetition rate: 0.2 Hz
Backing pressures ~20 atm
10” diffusion pump (Varian VHS-10)
Backing pump (Edwards EM275)
Cooled nozzle (down to -70 C)
Optical parametric oscillator
Toroidal mirrors give >100 traversals of laser through jet
Rapid scan signal averaging with background subtraction in real time
(jet on minus jet off)
Jitter suppression in frequency, improves resolution and S/N for QCL and OPO
Wavelength calibration using reference gas and etalon
Our spectrometer measures direct IR absorption.

4. Signal averaging; Power fluctuations
Five successive scans and averaged signal (Jet OFF)
However, in the case of OPO rapid scan, ¾ of cm-1 per 2 ms, causes power fluctuations which do not average out. Here are 5 successive scans with their average shown on the top trace.

5. Experimental Set Up:
OPO
Argos-SF-10
Module C P S I BD
LN2 cooled InSb
Gas supply
MCT
Reference gas
Fixed etalon
InSb BS M
PZT Driver
YDFL
Function
Generator
DAQ Card
Counter Timers
TTL
“classical” approach (use idler for balanced detection)
To suppress the the power fluctuations we use balance detection by using a portion of the idler signal for real time subtraction of the laser background. We have used this detection technique quite successfully in the past using diode lasers.

6. OPO Power Fluctuations Suppression
Main channel versus Background channel (Classical: Idler and Idler)
.. this is indeed works to some degree for OPO and after some digital filtering, we get a factor of 5 in suppressing the power fluctuations. It turns out we can do better.

7. Experimental Set Up Using OPO Signal
“Twin beam” approach (Signal beam as background)
Experimental Set Up Using OPO Signal
“Twin beam” approach (Signal beam as background)
Gas supply
OPO
Argos-SF-10
Module C P S I BD
LN2 cooled InSb M BS
MCT
YDFL
Reference gas BS
MCT M
Fixed etalon
PZT Driver
InGaAs
If instead of using the idler for suppression of power fluctuations, we now use a portion of the signal channel which is correlated with the idler signal by virtue of down conversion of one photon at 1060 nm to two correlated photons, one in the infrared and another around 1 micron. M
ND filter
1200 nm LP filter
Function
Generator
Counter Timers
DAQ Card
TTL

8. OPO Power Fluctuations Suppression
“Twin beam” approach (Signal beam as background)
We gain a factor of 18 and no need for use of a digital filter.
No need for the use of smoothing (Savitzky-Golay filter)!

9. OPO Power Fluctuations Suppression
A single scan of OCS-He combination band without baseline correction
(1001) ← (0000)
Here is a comparison of the two methods. The etaloning in the twin beams signal is due to a mismatch of the laser polarization and the Brewster window which can be removed.

10. Prior investigations of Rgn-C6H6 complexes
1979: He1;2-C6H6 Electronic spectra (S.M. Beck et al.)
1990: Ar-C6H6, Ar-C6D6 Microwave spectra (T. Brupbacher et al.)
1991: 20,22Ne-C6H6 Microwave spectra (T. Weber et al.)
1992: X-C6H6 (X = Ne, Ar, Kr, Xe, N2 ) Electronic spectra (H.J. Neusser et al.)
1993: Ne-C6H6-H2O Microwave spectra (E. Arunan et al.)
1993: X-C6H6 (X =20Ne, 129Xe, 132Xe) Microwave spectra (T. Brupbacher et al.)
2013: He1;2-C6H6 Electronic spectra (M. Hayashi et al.)
2014: X-C6D6 (X=He, Ne, Ar) Infrared spectra (J. George et al.)
Sample vapor pressure: 15 mTorr
Sample Temperature : 204 K
Path length: 40 m
Weakly-bound rare gas (Rg)–benzene clusters have a rich history because they provide attractive model systems for progressive mircosolvation of an organic molecule cases with rotational resolution.

11. Analysis of the 3  band system of C6H6
J. Pliva and A.S. Pine, J. Mol. Spectrosc. 126, 82 (1987).
Sample vapor pressure: 15 mTorr
Sample temperature: 204 K
Path length: 40 m
# of transitions: 8384
Sample vapor pressure: 15 mTorr
Sample Temperature : 204 K
Path length: 40 m
Analysis of the 3 micron band system of C6H6 was done by Pliva and Pine in This is a very congested band system, as you can imagine, and for this reason they used a low pressure and low temperature sample. The path length was 40 m and over 8000 transitions were included in the analysis.

12. Our rotational temperature: 2.5 K
C6H6 analysis
Four interacting bands: 12(A band), 2+ 13+ 18 (B band), 13+ 16 (C band), 3+ 10+ 18 (D band)
Plus 18 other perturbing states
The usual symmetric top parameters for each state
Five Fermi interactions between them.
WAB = cm-1
WBC = cm-1
WAC = cm-1
WAD = cm-1
WBD = cm-1
130 parameters of which 112 were adjusted.
RMS of the fit: cm-1
Sample vapor pressure: 15 mTorr
Sample Temperature : 204 K
Path length: 40 m
Analysis included lines from 4 interacting bands, +18 other perturbing states, the usual parameters for symmetric top and five Fermi parameters between them. 130 parameters of which 112 were adjusted. Our rotatinal temperature is 2.5 K and therefore we observe a much congested spectrum.
Our rotational temperature: 2.5 K

13. Spectra
Here is the overview of the 3 micron band system at 2.5 K rotational temperature. The nu12 fundamental is centered at cm-1. The other three strong perpendicular bands occur just above the main band as a result of intensity borrowing via anharmonic resonances between the fundamental nu12 and C band, centered at cm-1, and B and D bands, both occurring near 3100 cm-1.
Strongest C-H stretching
fundamental
Centered at cm-1
E1u symmetry
Borrows intensity form 12
via Fermi resonance
Centered at cm-1
Borrow intensity form 12
via Fermi resonances
B centered at cm-1
D centered at cm-1

14. Benzene monomer
Remember a good analysis of the dimers and monomers hinges on having a good model for Benzene monomer. Here is our simulation using the four interacting bands of Benzene.
0.02% benzene in He carrier gas
Backing pressure 19 atm
Trot = 2.5 K

15. Assignment and analysis of the Rgn-C6H6
Small vibrational shifts in all cases.
Slight changes in A/C , B and  for all states from their Benzene monomer values.
Four observed bands: A, B, C and D bands
Fermi parameters were held fixed at their Benzene monomer values, except for WAB .
Sample vapor pressure: 15 mTorr
Sample Temperature : 204 K
Path length: 40 m
Assignment and analysis were done using the following criteria in mind.
WAB = cm Varied
WBC = cm-1 Fixed
WAC = cm-1 Fixed
WAD = cm-1 Fixed
WBD = cm-1 Fixed

16. He-C6H6 The simulation for the He-C6H6 near the band centers.
0.02% benzene in He carrier gas
Backing pressure 19 atm
Trot = 2.5 K

17. He2-C6H6
Her is the simulation in the same regions with the trimer included.
0.02% benzene in He carrier gas
Backing pressure 19 atm
Trot = 2.5 K

18. C6H6, Ne-C6H6, and Ne2-C6H6 0.02% benzene, 0.5% Ne, in He carrier gas
Backing pressure 19 atm
Trot = 2.5 K

19. C6H6, Ar-C6H6, and Ar2-C6H6 0.02% benzene, 0.5% Ar, in He carrier gas
Backing pressure 19 atm
Trot = 2.5 K

20. Results Here is the outline for the rest of the talk.
Small changes in A/C, B and  for all states from their Benzene monomer values
RMS of the fits in all cases were less than cm-1.

21. Conclusions
The 3 micron band system of Rg1,2-C6H6 were observed and analysed following the 3  band system of the benzene monomer.
In all cases the rare gas atoms lie along the C6 symmetry axis of the benzene monomer.
Although we also expect small vibrational shifts for Rgn-C6H6 for n 3, our spectra do not seem contain unassigned features which may be attributed to larger clusters.
Observation of larger clusters may require drastically different jet conditions and/or cooled nozzles.
Sample vapor pressure: 15 mTorr
Sample Temperature : 204 K
Path length: 40 m

22. Many thanks to collaborators and funding agencies
A.R.W. McKellar, National research Council, Canada
A. Barclay
Funding Agencies:
Canada foundation for innovation
Natural Sciences and Engineering Research Council of Canada
University of Calgary
….and thank you.
Mahin Afshari
Mehdi Dehghany
Jalal Norooz Oliaee