70 th International Symposium on Molecular Spectroscopy Coblentz Award Lecture 6/24/15 Gary E. Douberly Laser Spectroscopy of Radicals, Carbenes and Ions in Superfluid Helium Droplets Department of Chemistry, University of Georgia Athens, Georgia, USA

Program Scope and Goals Mid-IR spectroscopy of molecular radicals relevant to combustion and atmospheric chemistry Provide a starting point for high-resolution gas-phase studies Challenge emerging electronic structure methods Rational in situ synthesis of reaction intermediates Formation and spectroscopy of entrance and exit channel open- shell molecular complexes

“pick-up cells” ~10 11 molecules·cm -3 Laser spectroscopy Stark/Zeeman spectroscopy Mass Spectrometry

Cooling timescale < 10 ns, pick-up timescale ~10  s He atoms can dissipate ~6 eV (~140 kcal/mol) T=0.4 K “pick-up cells” ~10 11 molecules·cm -3 He droplets are optically transparent below ~20 eV. Spectroscopic study of the outcome of cold collisions between sequentially picked-up reactants (both reactive and non-reactive collisions)

Kinetic Trapping of Metastable Clusters Rapid cooling of the condensing molecular system Long range forces can steer “reactants” into local minima G.E. Douberly and R.E. Miller J. Chem. Phys, 2005, 122,

He Droplet Source Pick-up cells Pyrolysis of Organic Precursors Molecular Radical and Carbene Production F.P. Lossing, Canadian J. Chem., 49, 357 (1971) “pick-up cells” ~10 11 molecules·cm -3 H 2 O cooled Cu electrodes Quartz Pyrolysis Tube

cw-“Chen” Nozzle

He Droplet Source Pick-up cells Molecular Radical and Carbene Production “pick-up cells” ~10 11 molecules·cm -3 Methyl (CH 3 ) J. Phys. Chem. A (2013), 117, Vinyl (C 2 H 3 ) J. Chem. Phys. (2013), 138, Ethyl (C 2 H 5 ) J. Chem. Phys. (2013), 138, Propargyl (C 3 H 3 ) and (C 3 H 3 OO) J. Phys. Chem. A (2013), 117, Allyl (C 3 H 5 ) and (C 3 H 3 OO) J. Chem. Phys. (2013), 139, Hydroxyl (OH) J. Phys. Chem. A (2013), 117, Hydridotrioxygen (HOOO) J.C.P. (2012), 137, ; J.P.C. Lett (2013), 4, OH  (C 2 H 2 ) J.C.P. (2015), 142, ; J.M.S. (2015) accepted. OH  (CH 3 OH) J. Phys. Chem. A, (2015) accepted. Hydroxymethylene (HCOH) J. Chem. Phys. (2014), 140, Dihydroxymethylene (HOCOH) J. Chem. Phys. (2015), 142,

(He) n + (He) 3 + Helium Droplet Detection via Mass Spectrometry e - (70 eV) 2e He 2 + (He) n + (n ≥ 2)

(He) n + (H 2 O) + (He) 3 + Helium Droplet Detection via Mass Spectrometry e - (70 eV) 2e H2OH2O H2O+H2O+ IP difference (He – molecule) ≈ eV

Helium Droplet Detection via Mass Spectrometry (1-butyne-4-nitrite) IP difference (He – molecule) ≈ eV He + + { } +* (C 3 H 3 ) + + NO + CH 2 O 39 (C 3 H 3 + )

Neat droplet beam + + Add O 2 to downstream PUC 39 (C 3 H 3 + ) 30 (H 2 CO +, NO + ) (C 3 H 2 + ) Propargyl Radical via 1-butyne-4-nitrite Pyrolysis

39 38 m/z=39 depletion m/z=38 depletion 300 K 1000 K h Infrared cw-OPO

m/z=38 detection

 = -1.4 cm -1 B Gas = 2730 MHz B He = 1020 MHz T rot = 0.37 K

Grebenev, Toennies, Vilesov, Science 279, 2084 (1998) Narrow Linewidths ~200 MHz Inhomogeneous Broadening Mechanisms Dominate Increasing number of 4 He atoms The Microscopic Andronikashvili Experiment OCS in a pure 3 He droplet OCS in a pure 4 He droplet

Grebenev, Toennies, Vilesov, Science 279, 2084 (1998) The Microscopic Andronikashvili Experiment Conclusion: The appearance of rotational fine structure is a microscopic manifestation of 4 He superfluidity.

OCS

Exactly what we expect for an (a 1 ) band of a C 3v symmetric top B gas / B He =2.2 Group theory and the gas-phase effective Hamiltonian approach still work!

2 B 2 2 B 1

E laser E Stark E laser E Stark or  M = 0  M = ±1 Droplet Beam cw-OPO (idler  3  m) OH C 2 H 2 Detect laser- induced depletion of ionization cross- section

Accepted last week

E laser E Stark  M = ±1 Infrared Stark Spectroscopy

E laser E Zeeman E laser E Zeeman or  M = 0  M = ±1 OH C 2 H 2 Detect laser- induced depletion of ionization cross- section 1 Tesla Rare Earth Permanent Magnets (iron caps) Joe Brice still has all 10 fingers!

Infrared Zeeman Spectroscopy E laser E Zeeman E laser E Zeeman  M = 0  M = ±1

Wavenumbers (cm -1 ) Infrared Zeeman Spectroscopy Tesla OH  CO 2  3/2

VPT2 f S. Davis, D. Uy, and D. J. Nesbitt, J. Chem. Phys. 112, 1823 (2000) h P. M. Johnson and T. J. Sears, J. Chem. Phys. 111, 9222 (1999)

resonance polyad 100 ps 5 ps

Infrared Spectra of the n- and i-propyl radicals Christopher Moradi

Schriener et.al. Angew. Chem. Int. Edit. (2008), 47, 7071.

b a b a (b2)(b2) (a1)(a1) (a)(a)

Stark Spectroscopy of trans,cis-dihydroxycarbene b a E laser E Stark  M = 0

Stark Spectroscopy of trans,trans-dihydroxycarbene b a E laser E Stark  M = 0

Bimolecular Reactions in Helium Droplets? Spectroscopic Detection of C 3 H 3 OO

CCSD(T)/ANO spin density calculations: 65% C(1), 35% C(3) E.B. Jochnowitz…J.F. Stanton, G.B. Ellison JPC A 109, 3812, (2005). Resonance stabilization Energy ~11 kcal/mol High concentrations in flames; self reaction first step to soot formation 65% 35%

Sequential pick-up of C 3 H 3 and O 2 ~2 kcal/mol~5 kcal/mol QCISD(T) calculations Barrier heights need to be reduced to reproduce experimental rate constants

Sequential pick-up of C 3 H 3 and O 2 ~2 kcal/mol~5 kcal/mol QCISD(T) calculations + O 2 → ∆H  19 kcal/mol ~1500 He atoms

Depends strongly on O 2 C3H3C3H3 C 3 H 3 OO m/z=27 u

Mass Spec (Laser OFF – ON) (C 2 H CO 2 )

P.S. Thomas, N.D. Kline, T.A. Miller, J. Phys. Chem. A 114, (2010) acetylenic-trans isomer ( 2 A′′) Solid Argon: 3326 cm -1 CCSD(T) VPT2: 3332 cm -1

Barrier Heights? “…given the high spin-contamination at these entrance saddlepoints, it would be worthwhile, in future calculations, to perform a multi-reference based analysis…” D. K. Hahn, S. J. Klippenstein, and J. A. Miller

J. Phys. Chem. A (2013), 117,

Pyrolytic decomposition of organic precursors combined with mass spectrometry allows for the spectroscopic study of a broad range of molecular radicals and carbenes within helium droplets. Low temperature and isotropic environment allows for a detailed study of the vibrational complexity associated with hydrocarbon radicals relevant to combustion. Small radical and carbene systems often exhibit rotational fine structure in the IR spectra, and these can be probed with Stark and Zeeman spectroscopy. Bimolecular reactions can be carried out within the helium droplet, and the products or intermediates associated with these reactions are carried downstream. (so far… Methyl + O 2 / Ethyl + O 2 / Allyl + O 2 / Propargyl + O 2 ) Summary * * * * IONS in Helium Droplets!! Gert von Helden WG01

Acknowledgments Post-Docs: Paul Raston (JMU), Christopher Leavitt (UGA), Bernadette Broderick (UGA) Graduate Students: Alexander Morrison, Tao Liang, Christopher Moradi, Joseph Brice Collaborators: Mark Marshall, John Stanton, Henry F. Schaefer, Wesley Allen, Jay Agarwal, Stephen Klippenstein Support: University of Georgia AFRL Eglin Air Force Base ACS-Petroleum Research Fund U.S. National Science Foundation (CAREER) U.S. Department of Energy, Office of Science (BES-GPCP)