Toward Two-Color Sub-Doppler Saturation Recovery Kinetics in CN (X, v = 0, J) Presented By: Hong Xu 06/22/2015 Chemistry Department Brookhaven National Laboratory Upton, NY 11973, USA.

Motivation Sub-Doppler spectrum has been obtained CN is a benchmark molecule in fundamental processes such as astronomical chemistry and combustion reactions Long chemical life time, ideal to study kinetics processes Collision-induced RET among selected rotational levels of ground state CN radicals has been probed by saturation recovery experiments Lu et al.,The Astrophysics Journal, 1992, 395,

PEM: Photo-Elastic Modulator EOM: Electro-Optic Modulator Experimental set up CW Probe ns Bleach X (v=0) A (v=1) A (v=4) Electronic transition of interest : V-type optical-optical double resonance scheme

Saturation Recovery Vis Bleach Reference Difference Reference UV Photolysis Probe and bleach the same rotational level with different vibronic bands Typical transient FM signal with saturation recovery kinetics Expanded saturation transient, expressed as fractional depletion vs. time for a single polarization  ime  s Intensity /arb. Unit  ime  s Depletion / % CW Probe ns Bleach X (v=0) A (v=1) A (v=4)

Recovery kinetics vs. probe Doppler shift k(  s -1 ) frequencies time Time (ns) Results and discussion

3D FM + time analysis Frequency detuning (in cm -1 ) Time (in ns) Decay kinetics at one detuning Spectrum at one time Global fits to data of this type yield Doppler resolved thermal rate constants for the collision-induced change in rotational state caused by collisions of CN(X,v=0,N) with He or with Ar.

Pressure dependence of the decay rates Pressure dependence of initial recovery rates for selected rotational states in He and Ar. The CH 3 COCN precursor partial pressure was maintained at 10 mTorr. Precursor makes a significant contribution to the RET, increasingly so at low rotational states. The rate vs. He pressure lines are nearly parallel for the three illustrated states, revealing differences almost entirely accounted for by the change in RET due to the precursor. The corresponding lines for Ar show a significant change in slope among the three illustrated rotational states

Rotational state dependence of total inelastic rate constants for CN (X 2 Σ + ) with He/Ar Offset likely due to C 2 N 2 precursor CN-He Offset likely due to precursor CN-Ar The SEP/LIF rates may be overestimated due to an unrecognized contribution by the photolytic precursor, particularly so for the lower rotational states. The calculated total removal rate constants show much better agreement with the present depletion recovery measurements than with the SEP/LIF measurements. The total inelastic rate constants for CN with Ar show stronger dependence on the rotational state. Fei et al., J. Chem. Phys. 1993, 100, Fei et al., Chem. Phys. Lett. 1995, 232,

On going experiment & future work v=1 0, v=2 0 CW Probe ns Bleach X (v=0) A (v=1) A (v=4) (sub-Doppler) CW Bleach ( sub-Doppler) EOM Ampl CW Bleach Probe laser was locked to the bleach laser through a cavity to probe and bleach molecules with the same velocity Detect sub-Doppler profile instead of broad Doppler profile A (v=2) X X X

Hyperfine Saturation Fit Passive stability of bleach laser during scan is critical

Bleach laser was switched on after 12us of Excimer laser; pulse width: 2us Fast increase while bleach laser was switched on, decay to thermalized population after bleach was switched off Future work : Find a range of conditions where the recovery time after the end of the bleach pulse is well resolved, and varies systematically with the pressure and composition of the sample. Investigate sub-Doppler saturation recovery kinetics as a function of rotational state, selected velocity and collision partner to characterize competition between rotational energy transfer and velocity-changing collisions.

Acknowledgements Trevor Sears Gregory Hall Michael Hause, Hua-Gen Yu, Paul Dagdigian Damien Forthomme Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98CH10886 and DE-SC with the U.S. Department of Energy, Office of Science, and supported by its Division of Chemical Sciences, Geosciences, and Biosciences within the Office of Basic Energy Sciences.