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

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.

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.

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.

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.

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.

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

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)!

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.

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.

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 1987. 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.

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 =23.023 cm-1 WBC = 9.048 cm-1 WAC =-1.111 cm-1 WAD =-1.064 cm-1 WBD =-1.064 cm-1 130 parameters of which 112 were adjusted. RMS of the fit: 0.0015 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

Spectra Here is the overview of the 3 micron band system at 2.5 K rotational temperature. The nu12 fundamental is centered at 3047.908 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 3079.6 cm-1, and B and D bands, both occurring near 3100 cm-1. Strongest C-H stretching fundamental Centered at 3047.9 cm-1 E1u symmetry Borrows intensity form 12 via Fermi resonance Centered at 3079.6 cm-1 Borrow intensity form 12 via Fermi resonances B centered at 3101.9 cm-1 D centered at 3100.4 cm-1

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

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 =23.023 cm-1 Varied WBC = 9.048 cm-1 Fixed WAC =-1.111 cm-1 Fixed WAD =-1.064 cm-1 Fixed WBD =-1.064 cm-1 Fixed

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

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

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

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

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 0.001 cm-1.

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

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