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 10 18 June 2014 69th International Symposium on Molecular Spectroscopy Champaign-Urbana, IL

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

Thermalization from single state

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

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

Transient FM Spectroscopy – OODR CH3COCN + 193 nm → CN + CH3CO : Photodissociation CN(X, v, J) + M → CN (X, v, J’) : Collisions CN (X) + 900 nm → CN (A) v=1 : cw Probe CN (X) + 625 nm → CN (A) v=4 : ns Bleach

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.

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) : 15.91 and 20.43 instead of 16.2 and 20.17 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, 20.43 MHz J=N+S F=J+I

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 43-44 ave (top) middle 3.2e6 s-1 cur 50-51 4.0e6 s-1 bottom cur 61-62

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

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.

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

Compare to previous work: He + CN(X,v=2) JCP 100 1190 (1994) Previously reported similar trend for Ar and He Our rates with He are slower and more independent of N v=0 v=2

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

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

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!

Acknowledgements Damien Forthomme Mike Hause Trevor Sears Paul Dagdigian Millard Alexander