1. Near-infrared spectroscopy of ethynyl radical, C2H
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
71st Meeting, June 19-24, 2016
Champaign-Urbana, Illinois
RF05
a. Also Department of Chemistry, Stony Brook University, Stony Brook, New York 11794

2. Motivation
Ethynyl plays an important role in both processes forming Carbon nanotubes and soot formation
Also a main photodissociation product of the acetylene which is ejected (via strong stellar winds) into the outer envelope of carbon-rich evolved stars.
Astrophysic people are the first people seeing it in the gas phase
C2H in interstellar medium
Tucker et. al. Astrophys. J. 193 L115–L119 (1974) Wootten et. al. Astrophys. J –854 (1980).
Nyman, Astron. Astrophys. J –327 (1984).
Combes et. al. Astrophys. J. 147 L25–L26 (1985)
Saleck et. al. Can. J. Phys –754 (1994)
Keady and K. H. Hinkle, Astrophys. J –546 (1988)
Tenenbaum et. al. Astrophys. J. 720 L102–L107 (2010)

3. 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)

4. Previous work A(0,0,0) A(0,0,1) A(0,0,2) Trapped in Solid Ne
Gas-phase experimental
C-C stretching (1846cm-1): Hirota et. al. J. Chem. Phys. 87, 73 (1987)
C-H stretching(3200cm-1): Curl and co-worker
Origin of A state is divided: , , , and 4107cm-1 each has 20-30% of A state character (Curl & Nesbitt)
UV bands (Yen-Chu Hsu)
Theory: Tarroni and Carter: JCP 119, (2003) &
Molecular Physics 102, 21–22, 2167–2179 (2004)
D. Forney, M.E. Jacox, W.E. Thompson J. Mol. Spec.170(1), (1995)
A(0,0,0)
A(0,0,1)
A(0,0,2)
Trapped in Solid Ne
(not rotationally resolved)

5. Experimental set up Excimer Laser 193nm 2 meters Herriot cell
In the laboratory: CF3C2H +hn  products
C2H2 +hn  products
in a glow discharge of a mixture of acetylene and helium.
Laser block
Excimer
Laser
193nm
Sacher
Diode laser
Oscilloscope
2 meters Herriot cell
Detector
35 passes inside absorption cell
Effective pathlength of 25 meters
1:1 Precursor/Argon at 1Torr total pressure for spectroscopy
Recorded both time and frequency

6. Results Typical scan Time l l ~0.6 cm-1 full scale
Total 3 bands were measured and assigned:
two 2S+-2S+ transitions at 6696 and 7088 cm-1
2P-2S+ transition at cm-1
Time
~200ms full scale

7. Results - Spectroscopy
2S+-2S+ transition at 6696 cm-1
At 3.6 ms
At 0 ms
2P-2S+ transition at cm-1
At 3.6 ms
At 0 ms

8. Analysis 2S+ state: 2P state
0ms
3.6ms
Energy levels were model using simple equations:
Fine structure splitting of the 2S+-2S+ band at 7088cm-1 is due to the difference in the spin-rotation splittings between the ground and the excited states
2P state
Energy levels were modeled including: the spin-orbit interaction (A), rotational constant (B), centrifugal distortion correction (D), the spin-rotation (g), and the l-doubling (p + 2q) in the Hund's case (a) basis.
Converts to Hund's case (b) at higher rotational levels >13

9. Simulation
2S+-2S+ transition
2P-2S+ transition
At 3.6 ms

10. Spectroscopic parameters in wavenumber (cm-1) of observed C2H bands
Parameters and Perturbations
Spectroscopic parameters in wavenumber (cm-1) of observed C2H bands
Next Slide
b. From Tarroni and Carter (2004) paper
The 7088 cm-1 2S+ state is regular at low N, but suffers a large perturbation at high N
This perturbation affects both + and - parity levels equally
Plot of obsed-calced of upper level term values illustrating the N-dependent perturbations in the X(0,20,3) A(0,3,0)0k level (S symmetry) at 7088 cm-1
Can not make any conclusion at high N, how about low N.

11. Are the 2S+ at 7088 cm-1 and 2P at 7110 cm-1 states perturbing each other?
Parity-seperated plots of the energy levels of the 2S+ at 7088cm-1 and the 2P state at 7110 cm-1
Perturbing levels need to have same Parity and J
J=8.5 (+ parity) – no interactions
J=6.5 (- parity) of 2S+ is 0.5cm-1 higher in energy than 2P
Observed-calced (cm-1)
The simple answer is “NO”
Tarroni and Carter (Mol. Phys, 2004) implies 2D or higher angular momentum states.
N(N+1)

12. Kinetics Total pressure 50mTorr CF3C2H precursor in Argon
If all molecules were made in the ground state, rapid rise at t=0 and single exponential reactive loss independent of Argon pressure.
Total pressure
50mTorr CF3C2H precursor in Argon
X(000) level initially
(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.

13. Kinetics Molecules was 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.

14. Conclusion Spectroscopy Kinetics
Spectroscopic parameters for 3 bands at 6696, 7088 and 7109 cm-1 are determined.
Origins and intensity ratio between three bands are in good agreement with Tarroni and Carter (Molecular Physics (2004))
PES complex, X state is highly mixed with A state.
Many unassigned transitions originate from higher vibrational levels of X state.
Kinetics
Non-Thermal reaction rate  relaxation rate vs reaction rate.
Level dependence of reaction rates

15. Acknowledgements Thank you Anh Le Hong Xu Sylvestre Twagirayezu
Trevor Sears
Greg Hall
Thank you
May 2016
Contract No. DE-SC

16. Kinetics C2H can be made both with C2H2 and CF3C2H precursor
But very different reactivity
a. Nodelay, 1 ms time gate
b. 3ms delayed

17. Photolysis products
C2H2+hnC2H + H
CF3C2H+hnC2H+CF3
Internal energy distribution of C2H from C2H2 photodissociation at 193 nm
Very little X2S+(000) level  Most radicals in the ground level get there by collisions.
No global measurement of internal energy distribution of C2H from CF3C2H.
CF3 should carry a larger fraction of the energy than H
Expected to be “cooler” from CF3C2H than from C2H2
Energy would be favor CF3 than C2H in CF3C2H. Compare with C2H2 C2H gets most of the energy.

18. Kinetics C2H2+C2H products * Rate of relaxation vs rate of reaction
Ground state
C2H2+C2H products k1 *
Slope of decay rate vs C2H2 ~3x slower than C2H+C2H2 published thermal rate constant
Increment amounts of precursor
Fixed amount of Argon 250mTorr
Rate of relaxation vs rate of reaction
What are the differences?
Most published work used ~100x more inert collision partners (He or Ar). But should not effect the decay rate since He and Ar are inert.
150mTorr
Just looking at ground state kinetics, you see rise time gets faster, decay gets faster with increasing acetylene. If you just plot decay rate vs p(C2H2) you get an apparent rate constant about 3x too slow compared to lit values.
Also note that the amount of c2h increases x3 as we increase c2h2, but signal amplitude does not increase. Make more, but smaller fraction makes it down to v=0 before removed by reaction.

19. Kinetics 50mTorr of C2H2 precursor in Argon Total pressure
Pressure dependent growth is similar to CF3C2H precursor
Observed decay rate remains pressure dependent even up to 10 Torr
Evidence of continuing vibrational relaxation.
“Hot” radicals also less reactive vs “cold” radicals