Direct Measurement of Weak Direct Measurement of Weak Collisions and Collision Collisions and Collision Rates using High-Resolution Rates using High-Resolution Transient IR Absorption Transient IR Absorption Spectroscopy Spectroscopy Daniel K. Havey, Qingnan Liu, Amy S. Daniel K. Havey, Qingnan Liu, Amy S. Mullin Mullin University of Maryland, College Park University of Maryland, College Park The Ohio State University The Ohio State University International Symposium on Molecular International Symposium on Molecular Spectroscopy Spectroscopy June 22 June 22 nd nd , 2007 , 2007
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Direct Measurement of Weak Collisions and Collision Rates using High-Resolution Transient IR Absorption Spectroscopy Daniel K. Havey, Qingnan Liu, Amy.
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Direct Measurement of Weak Direct Measurement of Weak Collisions and Collision Rates Collisions and Collision Rates
using High-Resolution using High-Resolution Transient IR Absorption Transient IR Absorption
SpectroscopySpectroscopy
Daniel K. Havey, Qingnan Liu, Amy S. Daniel K. Havey, Qingnan Liu, Amy S. MullinMullin
University of Maryland, College ParkUniversity of Maryland, College Park
The Ohio State University International The Ohio State University International Symposium on Molecular SpectroscopySymposium on Molecular Spectroscopy
June 22June 22ndnd, 2007, 2007
Collisional Quenching Competes with Collisional Quenching Competes with Chemical ReactionsChemical Reactions
Reaction Coordinate
En
erg
y (a
rb.)
AB* → A + BAB* + M → ABAB + M → AB*
AB*
AB
A + B
Collisional energy transfer is important for understanding chemicalreactions occurring above potential energy wells.
Energy Transfer Probability DistributionsEnergy Transfer Probability Distributions
Strong Collisions – Infrequent / High EWeak Collisions – Frequent / Low E ← Not much is known about these
0
0.2
0.4
0.6
0.8
0 2500 5000 7500 10000
E (cm-1)
P(
E)
/ 1
0-5
(c
m-1
)
Strong CollisionsWeak
Collisions
Donor (Evib)/HOD V → RT Energy Transfer
Probability Distribution Function
Using Spectroscopy to Probe Collisional Using Spectroscopy to Probe Collisional Energy TransferEnergy Transfer
Create highly vibrationally Create highly vibrationally excited azabenzene excited azabenzene molecules with Emolecules with Evibvib ~ 38,000 ~ 38,000 cmcm-1-1
Donor + 266 nm Donor + 266 nm → Donor (Evib)
Allow them to undergo single Allow them to undergo single collisions with HOD molecules collisions with HOD molecules and monitor the transient and monitor the transient population of HOD (000, Jpopulation of HOD (000, JKaKcKaKc))
N
N N N
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) 2458-2460.
Direct Determination of Molecular Direct Determination of Molecular Collision RatesCollision Rates
0 1 2 3 4
Pyrazine
2-Picoline
2,6-Lutidine
kcoll/kLJ
Our measured collision rates are consistently higher than the Lennard-Jonesmodel would predict. The deviation becomes larger upon methylation.
kint (cm3molec-1s-1)
Pyz/HOD = 1.0 x 10-9 Pic/HOD = 1.6 x 10-9
Lut/HOD = 2.2 x 10-9
kLJ (cm3molec-1s-1)
Pyz/HOD = 6.2 x 10-10 Pic/HOD = 6.4 x 10-10
Lut/HOD = 6.9 x 10-10
ConclusionsConclusions
Transient IR absorption spectroscopy is a powerful tool Transient IR absorption spectroscopy is a powerful tool for probing weak collision dynamics.for probing weak collision dynamics.
It has been shown for the first time that strong and It has been shown for the first time that strong and weak collisions can be described by the same single weak collisions can be described by the same single exponential distribution.exponential distribution.
A dramatic effect from isotopic substitution in A dramatic effect from isotopic substitution in collisions of highly vibrationally excited molecules with collisions of highly vibrationally excited molecules with HH22O / HOD has been observed for rotational energy O / HOD has been observed for rotational energy gain.gain.
Lower limits to molecular collision rates can be Lower limits to molecular collision rates can be obtained directly from combined measurements of obtained directly from combined measurements of strong and weak collisions.strong and weak collisions.