ANH T. LE, GREGORY HALL, TREVOR SEARSa Division of Chemistry HOT BAND ANALYSIS AND KINETICS MEASUREMENTS FOR ETHYNYL RADICAL, C2H, IN THE 1.49 mm REGION ANH T. LE, GREGORY HALL, TREVOR SEARSa Division of Chemistry Department of Energy and Photon Sciences Brookhaven National Laboratory Upton, NY, USA International Symposium on Molecular Spectroscopy 72st Meeting, June 19-24, 2016 Champaign-Urbana, Illinois TB05 a. Also Department of Chemistry, Stony Brook University, Stony Brook, New York 11794

Background …4s21p45s (X2S+) – Ground state …4s21p35s2 (A2P) – Excited state ~3800cm-1 Tarroni and Carter(2003) Potential energies along CC-H bending Potential energies along C-C bond Any small distortion form equilibrium can cause a change in the electronic order  vibronic effect are very large. It was pointed out by Tarroni & Carter that even lo Vibrational energy levels have mixed electronic character, even for lowest excitation(~5%) Perić Z. Phys. D - Atoms, Molecules and Clusters 24, 177-198 (1992)

Last year: RF05-Spectroscopy Spectroscopic parameters for 3 states at 6696, 7088 and 7109 cm-1 were determined (Recorded spectra at 6990-7130 & 6640-6780 cm-1). Origins and intensity ratio between three bands are in good agreement with Tarroni and Carter (Molecular Physics (2004)) 2P-2S+ transition Relaxed spectrum Hot Spectrum at early time Many unassigned lines

Last year: RF05 - Kinetics Molecules were hot initially Total pressure C2H†+Ar  C2H+Ar (relaxation) C2H+CF3C2Hreaction Signal grows slower at lower argon pressure, indicating it takes longer for molecules to relax to the ground level X(000), while they are reacting at the same rate. 50mTorr CF3C2H precursor in Argon Reacting at the same rate when thermalized (slow vib relaxation) conditions. In order for the signals to be larger at long time that when vib relaxation is faster, the excited C2H must be less reactive than thermal C2H. 20000 averages Continue trend to even lower total pressure with slower relaxation rate  “Hot” radicals have slower reaction rate than the “cold” radicals.

Combination differences and hot bands search Combination differences from X(0110), 2P state to a 2S upper state shows series of P, Q, and R branches Q branches R branches P branches Fortrat Diagram illustrates series of P,Q & R branches Combination differences from X(0200), 2S state to a 2S upper state shows only P & R branches

Hot bands assignment

New band originating from X(0,20,0) Spectroscopic parameters of several C2H states Last year talk New band originating from X(0,20,0)

Experimental set up 23 passes inside absorption cell In the laboratory: CF3C2H +hn  products Switch to FM transient absorption Herriott absorption cell InGaAs Photodiode, Amplifiers and demodulator 23 passes inside absorption cell Effective pathlength of 16 meters 1:5 Precursor/Argon at 500mTorr total pressure for spectroscopy Recorded transient absorption to fill the gap between 6780-6990 cm-1

Precursor + Argon n2=0 n2=1 Improved signal to noise Reduced pathlength 7000 Averages

Spectral time evolution 80ms 40ms 0ms 5 bands now assigned, probing v2=0,1,2 40ms 80ms 40ms 80ms We had two undergraduate research project students working on the early time data attempting to identify combination differences for bands originating in known vibrationally excited levels. First results published now: The near infrared spectrum of ethynyl radical, A. T. Le, G. E. Hall and T. J. Sears, J. Chem. Phys. 145, 074306(11) 2016).

C2H +H2 products Peeters et. al J. Chem. Phys. 116(9), 3700, 2002 Relative Energy (kJ/mol) X state is highly mixed with A state even the lowest vibrational levels, each with different mixing coefficients Level dependence of reaction rates? Barrierless A state reaction C2H+H2 system: simpliest interesting rxn beside precursor is rxn with H2 which it occurs on vinyl potential energies surfaces and we have good theoretical support on. Rxn coordinates

H2 affects the relaxation and adds a reactive channel Now add H2 as secondary reactant. Complicated again, H2 both reacts and relaxes the C2H. But we have a very good data set now. H2 both reacts and affects relaxation and we can probe this interplay as a function of C2H energy… Have measurements on n2=0,1&2 … higher vibrational level will be useful

Summary Total 3 bands originating from a X (0,0,0) 2S+ state to excited vibrational states were observed, consistent with two 2Σ - 2Σ transitions at 6696 and 7088 cm-1 as well as a 2Π - 2Σ transition at 7108 cm-1. Two hot bands originating from X(0,11,0) 2P and X(0,20,0) 2S+ states were identified in the spectral region 6635-6740 cm-1. Kinetics measurements of C2H, precursor with argon and C2H, precursor and H2 with argon for n2=0, 1, 2 Future work Assign the newly recorded spectra 6780-6990 cm-1 Search for higher vibrational levels of hotbands for Kinetics measurement Master equation model to understand kinetics measurements.

Acknowledgements Thank you Anh Le Eisen Gross Trevor Sears Greg Hall April 2017 Thank you Contract No. DE-SC0012704

Last year work: RF05 - Kinetics Molecules were hot initially Total pressure C2H†+Ar  C2H+Ar (relaxation) C2H+CF3C2Hreaction Actual 0.25 Torr Signal grows slower at lower argon pressure, indicating it takes longer for molecules to relax to the ground level X(000), while they are reacting at the same rate. 50mTorr CF3C2H precursor in Argon Reacting at the same rate when thermalized (slow vib relaxation) conditions. In order for the signals to be larger at long time that when vib relaxation is faster, the excited C2H must be less reactive than thermal C2H. ? Continue trend to even lower total pressure with slower relaxation rate?  “Hot” radicals have slower reaction rate than the “cold” radicals.