1. Rotational Energy Transfer and Depolarization in Rare Gas + CN (X, v=0) Collisions
Gregory Hall, Damien Forthomme, Trevor Sears
Chemistry Department
Brookhaven National Laboratory
WH June 2014
69th International Symposium on Molecular Spectroscopy
Champaign-Urbana, IL

2. Energy transfer in radical-atom collisions
Why: ET important for thermalization steps in reaction: chemical activation, multi-well reactions, “gateway” mechanisms of non-adiabatic processes; pressure broadening and laser diagnostics
How: Ground state depletion recovery kinetics: high resolution OODR for thermal collision studies of ground state molecules

3. Thermalization from single state

4. Depletion recovery kinetics
Hole kinetics differs from
single-state kinetics only by a sign

5. Initial rate from single state
nj=0 at t=0
“kRET”
For t > 0, decay will slow down due to back-reaction
Initial log slope will be kRET

6. Transient FM Spectroscopy – OODR
CH3COCN nm → CN + CH3CO : Photodissociation
CN(X, v, J) + M → CN (X, v, J’) : Collisions
CN (X) nm → CN (A) v=1 : cw Probe
CN (X) nm → CN (A) v=4 : ns Bleach

7. Population and Polarization
Linearly polarized depletion, partially saturated:
recovery kinetics depend on probe polarization
( ) weighted sum cancels alignment effect to monitor population kinetics
( ) monitors depolarization kinetics
P2(6) line, rotate to abs phase and integrate FM to Absorption, then integrate over full Doppler width for each geometry. Linear scale for Par and Perp, log scale for sum and difference.
DF1-73a data file used. 10% pyruvonitrile in He, total pressure 110 mTorr.

8. X-state hole hyperfine quantum beats
Oscillations in polarization decay are not noise
Coherent depletion of multiple hyperfine levels
If we didn’t already know the ground state hyperfine splittings, we could determine them with about 300 kHz by fitting.
(P2(6) : and instead of 16.2 and MHz)
Interesting to think about the coherent evolution of the “hole” -- the difference between an isotropic thermal sample and those molecules in the lower state that were not excited by a laser pulse!
N″=6
J″=5 ½
J″=6½
F″=6½, 5½, 4½
ΔE=15.91, MHz
J=N+S F=J+I

9. Recovery rate variation with Doppler shift
Rate (μs-1)
2.9
3.2
4.0
3.5 GHz scan in 40 MHz steps
0-20 μs in 5 ns steps
Bleach with broadband ns dye laser after ~10 μs
Fractional depletion vs. time
(5ns/step)
DF1_73A: P2(6) at 10% PV in He 40 MHz/step, 20 microsec full scale, 5 ns/point
2.9e6 s-1 rate at cursor ave (top) middle 3.2e6 s-1 cur e6 s-1 bottom cur 61-62

10. Model for velocity-dependent recovery kinetics
Initial thermal lineshape
Speed-dependent rate
Empirical log curvature
Empirical variation with Doppler shift
Sqrt(1+bv2 ) can exceed a fact to a factor of 2 in the wings of a line, making rate at line center slower than average rate by up to 20%
Initial recovery rate at initial T0
±Corrections for speed-dependent rate and log curvature

11. Total RET removal rates by He & Ar
J-dependent intercepts:
10 mTorr
precursor (CH3COCN)
Trend in He & Ar slopes quite different
Note that precursor CH3COCN has large dipole moment, as does CN(X). For R(0) line, 10 mTorr of pyruvonitrile contributes to the decay as much as 175 mT of Ar or 250 mT of He, whereas for the Q(17) line, the precursor contributes 50% of the decay at Ar pressure ~80 mT or He pressure 60 mTorr. In other words, a 5% mixture of PN/Ar has >40% of the relaxation due to the precursor for R(0) and 30% contribution for Q(17). In He, the 5% mixture has 55% of the R(0) relaxation due to precursor and 23% of the Q(17) relaxation due to precursor.

12. Ar compare with literature & calculations
“normal” trend: decreasing with N
Good agreement with QM rates (Dagdigian, unpublished)
Slower than SEP-LIF experiments, but similar trend with N

13. Compare to previous work: He + CN(X,v=2)
JCP (1994)
Previously reported similar trend for Ar and He
Our rates with He are slower and more independent of N
v=0
v=2

14. Total removal kRET vs N with He
Same calculations in good agreement with our OODR kinetics: weak dependence on N
Slower than previous SEP-LIF expt with v=2, low N
Likely error: uncorrected RET by 3.5% precursor NCCN
v=0
v=2

15. Further analysis, additional measurements
Elastic depolarization insignificant at high J, competitive with RET at low J
Saturation transfer, polarization transfer: bleach J, probe J’
Sub-Doppler saturation recovery:
Sensitive to velocity-changing elastic collisions, competing with RET

16. Conclusions
Ground-state depletion recovery kinetics with continuous transient FM laser probe permits direct single-collision studies of thermal collision dynamics
Speed-dependent collision effects: competition between elastic v-changing and inelastic collisions (pressure broadening applications)
Quantitative agreement with QM scattering calculations – despite qualitative differences with some prior experiments also confirmed by QM scattering!

17. Acknowledgements Damien Forthomme Mike Hause Trevor Sears
Paul Dagdigian
Millard Alexander