HOT BAND ANALYSIS AND KINETICS MEASUREMENTS FOR ETHYNYL RADICAL, C - PowerPoint Presentation

Slide1

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 ChemistryDepartment of Energy and Photon SciencesBrookhaven National Laboratory Upton, NY, USA

a. Also Department of Chemistry, Stony Brook University, Stony Brook, New York 11794

International Symposium on Molecular Spectroscopy72st Meeting, June 19-24, 2016Champaign-Urbana, Illinois

TB05Slide2

BackgroundPerić Z

. Phys. D - Atoms, Molecules and Clusters 24, 177-198 (1992)…4s21p45s (X2S+) – Ground state…4s2

1p35

s2 (A2P) – Excited state ~3800cm-1

Potential energies along C-C bond

Vibrational energy levels have mixed electronic character, even for lowest excitation(~

5

%)

Tarroni

and

Carter(2003

)

Potential energies along CC-H bendingSlide3

Last year: RF05-SpectroscopySpectroscopic 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))

Relaxed spectrum

2

P

-

2

S

+

transition

Many unassigned lines

Hot Spectrum at early timeSlide4

50mTorr CF

3C2H precursor in Argon

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.

Total pressure

Last year: RF05 - Kinetics

Continue trend to even lower total pressure with slower relaxation rate

 “Hot” radicals have slower reaction rate than the “cold” radicals.

Molecules were hot initially

Reacting at the same rate when thermalized

C

2

H+CF

3

C

2Hreaction

C2H†+Ar  C

2H+Ar (relaxation)

20000 averagesSlide5

Combination differences and hot bands search

Fortrat Diagram illustrates series of P,Q & R branchesCombination differences from X(0110), 2P state to a 2S

upper state shows series of P, Q, and R branches

Combination differences from X(0200), 2S

state to a 2S

upper

state shows only P & R branches

Q branches

R branches

P branchesSlide6

Hot bands

assignmentSlide7

Last year talk

New band originating from X(0,20,0) Spectroscopic parameters of several C

2H statesSlide8

Experimental set up23 passes inside absorption cellEffective

pathlength of 16 meters1:5 Precursor/Argon at 500mTorr total pressure for spectroscopyRecorded transient absorption to fill the gap between 6780-6990 cm-1

In the laboratory: CF

3C2H +hn

 products

Switch to FM transient absorption

Herriott

absorption cell

InGaAs

Photodiode, Amplifiers and demodulator Slide9

n

2=0Precursor + Argon Improved signal to noise

n

2=1

n

2

=2

Reduced

pathlength

7000 AveragesSlide10

80ms

40ms0ms

S

pectral time evolution

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).

5 bands now assigned,

probing v

2

=0,1,2

40

m

s

80

m

s

40

m

s

80

m

sSlide11

C2H +H

2 productsPeeters et. al J. Chem. Phys.

116

(9), 3700, 2002

X state is highly mixed with A

state

even the lowest vibrational levels, each with different mixing coefficients



Level

dependence

of reaction

rates

?

Rxn

coordinates

Relative Energy (kJ/

mol

)Slide12

H2 affects the relaxation and adds a reactive channel

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 Slide13

Future workAssign 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. Summary

Total 3 bands originating from a X (0,0,0) 2

S+ state to excited vibrational states were observed, consistent with two 2Σ - 2Σ transitions at 6696 and 7088 cm

-1 as well as a 2Π -

2

Σ transition at 7108 cm

-1

.

Two hot bands originating from X(0,1

1

,0)

2

P 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 C

2H, precursor and H2 with argon

for

n2=0, 1, 2Slide14

Acknowledgements

Anh Le

Eisen

Gross

Trevor Sears

Greg Hall

April 2017

Contract No.

DE-SC0012704

Thank you Slide15
Slide16

50mTorr CF

3C2H precursor in Argon

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.

Total pressure

Last year work:

RF05 - Kinetics

Continue trend to even lower total pressure with slower relaxation rate?

 “Hot” radicals have slower reaction rate than the “cold” radicals.

Actual 0.25

Torr

?

Molecules were hot initially

Reacting at the same rate when thermalized

C

2

H+CF

3

C

2Hreaction

C2H†+Ar  C

2

H+Ar (relaxation)