Direct Measurement of Weak Collisions and Collision Rates using High-Resolution Transient IR Absorption Spectroscopy Daniel K. Havey, Qingnan Liu, Amy S. Mullin University of Maryland, College Park The Ohio State University International Symposium on Molecular Spectroscopy June 22 nd, 2007

Collisional Quenching Competes with Chemical Reactions AB * → A + B AB * + M → AB AB + M → AB * AB * AB A + B Collisional energy transfer is important for understanding chemical reactions occurring above potential energy wells.

Energy Transfer Probability Distributions Strong Collisions – Infrequent / High  E Weak Collisions – Frequent / Low  E ← Not much is known about these Strong Collisions Weak Collisions Donor (E vib )/HOD V → RT Energy Transfer Probability Distribution Function

Using Spectroscopy to Probe Collisional Energy Transfer Create highly vibrationally excited azabenzene molecules with E vib ~ 38,000 cm -1 Create highly vibrationally excited azabenzene molecules with E vib ~ 38,000 cm -1 Donor nm Donor nm → Donor (E vib ) Allow them to undergo single collisions with HOD molecules and monitor the transient population of HOD (000, J KaKc ) Allow them to undergo single collisions with HOD molecules and monitor the transient population of HOD (000, J KaKc ) Pyrazine 2-Picoline 2,6-Lutidine “Direct Determination of Collision Rates Beyond the Lennard-Jones Model Through State-Resolved Measurements of Strong and Weak Collisions,” D.K. Havey, Q. Liu, Z. Li, M. Elioff, M. Fang, J. Neudel, and A. S. Mullin, Journal of Physical Chemistry A, 111 (2007) Donor (E vib ) + HOD Donor (E vib ) + HOD → Donor (E vib ’) + HOD (000, J KaKc ) Donor Molecules Studied

Kr + laser (647 nm) F-center laser (2.7  m) To: Flowing collision cell Background Detection Reference Gas Detection F-center Transient Infrared Absorption Spectrometer Probe Laser Diagnostics Probe Laser: Tunable Single-mode F-center Laser (2.7  m) Resolution – 10 MHz ( cm -1 ) Power – <1 mW Single-mode / < 20 mW Multi-mode

Reference Gas Detection Galvo Plate Driver Sample Detection Nd:YAG laser 266 nm F-center Transient Infrared Absorption Spectrometer Pump Laser: 4th Harmonic of a Pulsed Nd:YAG Laser (266 nm / 5 ns) Resolution – 30 GHz (1 cm -1 ) Intensity – < 6 MW/cm 2 Background Detection

Using Spectroscopy to Probe Collisional Energy Transfer (E vib ) + HOD → (E vib ’) + HOD (000, J KaKc ) Example: Collect the time-dependent fractional population change of HOD (000, 7 0,7 ) after collisions with 2,6-lutidine Depletion (line center) Appearance (line center ± cm -1 )

Pyrazine/HOD – Translational Energy Gain (E vib ) + HOD = 570 K = 328 K

2-Picoline/HOD and 2,6-Lutidine/HOD – Translational Energy Gain (E vib ) + HOD = 575 K = 376 K = 614 K = 368 K

Pyrazine/HOD – Rotational Energy Gain (E vib ) + HOD Both weak and strong collisions can be described by the same single exponential distribution. D.K. Havey, Q. Liu, Z. Li, M. Elioff, M. Fang, J. Neudel, and A. S. Mullin, Journal of Physical Chemistry A, 111 (2007)

Rotational Energy Gain for Pyrazine/HOD vs. Pyrazine/H 2 O Distinct difference from isotopic substitution is seen for the rotational energy gain. Collisions with water gain 4x more rotational energy than collisions with HOD. M. Fraelich, M.S. Elioff, and A.S. Mullin Journal of Physical Chemistry A 102 (1998)

Lutidine/HOD and Picoline/HOD – Rotational Energy Gain Both Lutidine/HOD and Picoline/HOD have nascent rotational distributions also described by a single T rot. Rotational temperatures for all three systems studied are similar in contrast to H 2 O. Picoline/H 2 O = 590±90 K Lutidine/H 2 O = 490±80 K 394  64 K 426  60 K

Energy Transfer Rates for Collisions with HOD Pyrazine Picoline Lutidine Donor (E vib ) + HOD Donor (E vib ’) + HOD (000, J KaKc ) k app →

Direct Determination of Molecular Collision Rates Our measured collision rates are consistently higher than the Lennard-Jones model would predict. The deviation becomes larger upon methylation. k int (cm 3 molec -1 s -1 ) Pyz/HOD = 1.0 x Pic/HOD = 1.6 x Lut/HOD = 2.2 x k LJ (cm 3 molec -1 s -1 ) Pyz/HOD = 6.2 x Pic/HOD = 6.4 x Lut/HOD = 6.9 x

Conclusions Transient IR absorption spectroscopy is a powerful tool for probing weak collision dynamics. Transient IR absorption spectroscopy is a powerful tool for probing weak collision dynamics. It has been shown for the first time that strong and weak collisions can be described by the same single exponential distribution. It has been shown for the first time that strong and weak collisions can be described by the same single exponential distribution. A dramatic effect from isotopic substitution in collisions of highly vibrationally excited molecules with H 2 O / HOD has been observed for rotational energy gain. A dramatic effect from isotopic substitution in collisions of highly vibrationally excited molecules with H 2 O / HOD has been observed for rotational energy gain. Lower limits to molecular collision rates can be obtained directly from combined measurements of strong and weak collisions. Lower limits to molecular collision rates can be obtained directly from combined measurements of strong and weak collisions.

Acknowledgements Thanks to: The Mullin Group Amy S. Mullin (PI) Qingnan Liu Liwei Yuan Juan Du Shizuka Hsieh Felix Lin Funded by: Department of Energy National Science Foundation Juan Du, Liwei Yuan, Daniel K. Havey, Qingnan Liu