Helium Nanodroplet Isolation Spectroscopy of NO 2 and van der Waals Complexes Robert Fehnel Kevin Lehmann Department of Chemistry University of Virginia

Why study Nitrogen Oxides? Reactions between “NO x ” and related compounds are important in atmospheric and combustion chemistries. – Include NO, NO 2, NO 3, N 2 O 3, N 2 O 4, N 2 O 5, HNO, HNO 2, HNO 3. (with multiple isomers). – Reactions between these and with O n (n = 1-3) often have low or no barrier. Several are free radicals (NO, NO 2, NO 3 ). – Association reactions should occur on multiple spin surfaces but higher spin compounds or complexes are not yet known. -- May trap these in Helium. – Potential to use Spin orientation in Magnetic field to control chemistry -- Requires fast spin relaxation – Potential to do optical detected ESR studies in helium droplets. -- Extend to molecules what Callegari & Ernst have done for alkali atoms on droplets.

Previous Work Done by Conjusteau Studied the photodissociation of NO 2 in He nanodroplets – Used 1-photon process at 398nm – Did not find an LIF signal below threshold showing that the relaxation time is less than 90ns as to suppress the signal – Did not see dissociation and came up with 4 reasons for that Vibrational relaxation in ground state is faster than dissociation It recombined in three different ways either exactly as it was, van der Waals complex, and or a complex is made mixing the original doublet state with a new quartet state Conjustea, A. Thesis. (2002)

No Dissociation

Previous Work Done by Wittig A mass spec depletion signal for 17,700 – 18,300 cm -1 was observed In this region the intramolecular dynamics are believed to be chaotic Found that the spectra could be fitted well by shifting the R 0 line positions by 7 cm -1, adding 7 cm -1 widths to all lines and adjusting intensities – The increased line widths are due to rapid vibrational relaxation in the He E. Polyakova et al. Chem. Phys. Lett. 375 (2003) 253.

Nozzle Diameter = 10 μm Skimmer = 400 μm Nozzle T ≥ 16 K Backing Pressure ≤ 60 Bar L He Skimmer Nozzle Closed Circuit Refrigerators He Chopper Pickup cell Gas Cracker Multipass Cell 1.5K bolometer N.E.P. ~ 2x W/Hz 1/ cm >5000 L/s 2500 L/s IR OPO 2560 – 3125 cm -1 H Magnets with movement in and out Machine Schematic Bolometer noise ~ beam noise ~ of chopped beam signal(1 Hz BW)

Acculight Argos OPO SPI Wavemeter To Spectrometer OPO Power meter 150 MHz etalon 7.5 GHz etalon Approximately 1.75 W of power measured entering the machine. Produces over 2 W of CW over the tunable range of 3.2 – 3.9 μm. Continuous scans of 45 GHz. Also produces W of 1.5 μm light.

NO 2 O 2 spin states ○ ○ Dipole = Dipole = O 2 bond length = ÅO 2 bond length = Å DoubletQuartet

NO 2 spectrum in He ~5000 NO 2 v 1 + v 3 R(0).

NO 2 spectrum Showing Baseline NO 2 v 1 + v 3 R(0) -> NO 2 + N 2 NO 2 + O 2 P(2) ? R(2)? Based upon tentative P(2), R(2) assignments, B(NO 2 ) = 0.24 cm -1 (60% gas phase) Compared to: B(CO 2 ) = cm -1 (40% of gas phase) B(N 2 O) = cm -1 (17% of gas phase)

NO 2 Fit of v 1 + v 3 R(0) line for R(0) = cm cm -1 FWHM Gas Phase: (J = 1.5) (J = 0.5)  = cm -1 We lack a predictive theory for the shape of ro-vibrational lines In helium.

NO 2 R(0) Showing Wings & Lorentzian Fit These “steps” are reproducible. Similar step seen on red wing of CO 2 R(0) transition.

NO 2 + H 2 O spectrum NO 2 R(0) NO 2 + H 2 O Bleeding in H 2 O vapor through cracker And optimizing signal on Q branch Q R P

NO 2 + H 2 O Subtracting NO 2 R(0) Q R P R branch modestly stronger than P branch suggest this is a K = 0 -> 1 sub-branch NO 2 v 1 + v 3 R(0)

NO 2 + H 2 O Subtracting NO 2 R(0) Appears to be B-type dominated A,B hybrid band. Suggests Hydrogen bonding to lone pair on Nitrogen. B eff = 0.02 cm -1 K = 0 -> 1 K = 0 -> cm -1 FWHM cm cm -1 FWHM cm cm -1 FWHM cm cm -1 FWHM cm -1

NO 2 + H 2 O Q Branch Splitting Fit Splitting of Q branch likely due to tunneling between different Hydrogen atoms H-bonding. Areas of the two peaks ~ 3:1

NO 2 +O 2 spectrum NO 2 v 1 + v 3 R(0) NO 2 + O 2 NO 2 + H 2 O

NO 2 +O 2 Fit for K = 0 → 1 Strongly B-type band, Suggesting C 2v symmetry. B ~ 0.02 cm -1 Moving ~1T magnet up to second pickup region had no effect on intensity!

NO 2 +N 2 spectrum K = 1 → 0 K = 0 → 1 K = 1 → 2

NO 2 +N 2 Fit Line spacing of Approximately cm -1 in each transition. B-type transition (B+C)/2 = cm -1 A = 0.17 cm -1 K = 1 branches => Not T shaped!

Things that have not worked (yet?) We could not find the symmetric N 2 O 4 dimer band (2973 cm -1 in gas phase). – Did see a weak, broad transition that likely is trans- asymmetric dimer. We could not find any evidence of NO 2 formed by sequential pickup of NO and then O 2 passed through the cracker (which is expected to produce ~80% dissociation of O 2 ). Could not detect HNO ( 2681 cm -1 in gas phase) formed by pickup of H atoms (from cracker) after NO pickup.

Near Term plans Make N 16 O 18 O to firmly determine (B+C)/2 value. Look for NO 3 formed from either NO + O 2 or from NO 2 + O. – NO 3 has strong electronic band at 662 nm. Look for IR spectrum of N 2 O 3 formed from NO + NO 2. Study microwave spectrum (Brooks Pate) and do ab initio calculations for NO 2 complexes. Use Mass Spec to characterize products from various inputs to the cracker.

Acknowledgements Dr. Ozgur Birer who help construct the HENDI machine at UVa. Funding: National Science Foundation, UVa