1. 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

2. 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, (1992)

3. Last year: RF05-Spectroscopy
Spectroscopic parameters for 3 states at 6696, 7088 and 7109 cm-1 were determined (Recorded spectra at & 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

4. 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.

5. 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

6. Hot bands assignment

7. 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)

8. 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 cm-1

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

10. Spectral time evolution
80ms
40ms
0ms
5 bands now assigned, probing v2=0,1,2
40ms ms
40ms ms
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, (11) 2016).

11. 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

12. 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

13. 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 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 cm-1
Search for higher vibrational levels of hotbands for Kinetics measurement
Master equation model to understand kinetics measurements.

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

16. 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.