1. COLLISIONALLY-MEDIATED SINGLET-TRIPLET CROSSING IN ã1A1 CH2 REVISITED: (010) COUPLING
ANH T. LE, GREGORY HALL, TREVOR SEARSa
Department of Chemistry
Brookhaven National Laboratory
Upton, NY, USA
International Symposium on Molecular Spectroscopy
69th Meeting, June 16-20, 2014
Champaign-Urbana, Illinois
a. Department of Chemistry, Stony Brook University, Stony Brook, New York 11794

2. Previous work CH2 bending potential a state X state |S> |T>
CIISC occurs only through the few pairs of levels which
are mixed, and thus have partial singlet and partial triplet character. These pairs then
act as “gateways” for the relaxation between spin-rotation-vibration levels associated with
different electronic states.

3. Types of experiment to study mixed state pairs :
Doppler limited spectroscopy to identify the perturbed lines from level shifts
Saturation dip spectroscopy to study the ortho hyperfine structure:
No hyperfine splitting unless both both I and S are non-zero
Kinetics
At 1MHz resolution the hyperfiine component the ortho level in the ã1A1 state remain the same while the ortho level in the X3B1 state split due to the coupling between the unpaired electron spin (S) with 1H nuclear spin (I).
Bley and Temps, J. Chem. Phys., 98, 1058 (1993)
kISC = S 2fi krot b2 (1-b2)

4. Perturbed lines from level shifts?
Kobayashi et. al [J. Chem. Phys. (2006)] rotational analysis of ã(010).
b1B1(030)0-ã1A1 (010)1 ˜
717
515T
515S
111
212
313
414
515
616
515 the only one that has triplet partner identified
Only 5 of 54 lowest levels >.1 cm-1 deviation from rigid rotor fit

5. How about kinetics of ã1A1(010)?
Single exponential decay on ã1A1(010) after the initial thermalization.
Slope=0.115 ms-1Torr-1
Slope= ms-1Torr-1
Total rate afte
Plotting the observed decay rate vs helium pressure measure the intersystem crossing rate constant from the slope get nearly identical slope between v=1& v=0. In each case the para rate is 30% faster than the ortho.
ã1A1(000) kinetic
ã1A1(010) kinetic
Kisc rate is similar for ã1A1(000) & ã1A1(010)
Hard to reconcile with the small number of identified perturbed levels in ã1A1(010)

6. Experiment setup
CH2CO+308nm CH2+CO
EOM Φ FM
Receiver
1.5m

7. Time
wRF=192MHz l
Unmixed state
no hyperfine splitting
EOM

8. ã1A1(010) Sub-Doppler measurement on 616 line
1 broadened hyperfine feature or 3 unresolved features?
313
Linewidth(FWHM): 2.4 MHz
Out of those lines only are in the region that are in the Msquare laser. 616 is the only possibility to have any splitting
Linewidth 2.4MHz
Linewidth 6MHz
But no good reason for a 2.4-6MHz change in linewidth

9. ã1A1(010) Sub-Doppler measurement
Linewidth(FWHM): 2.4 MHz
313 was selected as the control line: unmixed, and similar intensity as the 616
Splitting: 2MHz
Splitting: 3MHz
Splitting: 1MHz
Estimate of mixing coefficient 2%<b2<5%  its triplet partner is too dark to be observed (consistent with observation)

10. Kinetic signatures of mixed states
Time l
616 & 523 have similar decay at longer time however, the decay curve appears to be different during the first few hundreds ns.
523 was selected to be a control line because it is unmixed and has similar rotational energy with 616
Ratio of the FM signal looks constant at the longer time
Slope at first 400ns show that 616 has about <5% triplet mixing

11. How about the para lines?
Sub-Doppler measurements can’t be done since it’s para level
515(T) appears very differently with a much slower growth at early time
Since the partner of the 515 was identified  Fraction of total mixed state signal in S orT .
515 Mixing coefficient b2 is about 10% of the triplet mixing

12. Summary
New spectrometer was constructed with improvement of saturation dip spectral linewidth
Sub-Doppler and kinetics measurement on the ã(010) 616 line suggests a very small mixing coefficient between singlet and triplet states
Kinetics measurement of the ã(010) 515 S&T lines show the mixing coefficient, b2 is about 10%.
The question remains the same: ã(010) & ã(000) have similar kisc rates, yet search for gateway state finds inadequate strength and abundance of mixed states

13. Acknowledgements Dr. Gregory Hall Prof. Trevor Sears
Group members at BNL
U.S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of
Chemical Sciences, Geosciences and Biosciences
Contract No. DE-AC02-98CH10886

14. V=0
18e

15. 313 line
28w

16. 18h

17. 616 1 line gamma: 3.
28s

18. 616 3 lines fitted fixed gamma:1.44 residue: 0.000586123