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.

Donor (EDonor (Evibvib) + HOD ) + HOD → Donor (Evib’) + HOD (000, JKaKc)

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 InfraredAbsorption Spectrometer

Probe Laser Diagnostics

Probe Laser:Tunable Single-mode F-center Laser (2.7 m)Resolution – 10 MHz (0.0003 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 InfraredAbsorption Spectrometer

Pump Laser:4th Harmonic of a Pulsed Nd:YAG Laser (266 nm / 5 ns)Resolution – 30 GHz (1 cm-1)Intensity – < 6 MW/cm2

Background Detection

Using Spectroscopy to Probe Collisional Using Spectroscopy to Probe Collisional Energy TransferEnergy Transfer

N

(Evib) + HOD →

N

(Evib’) + HOD (000, JKaKc)

Example: Collect the time-dependent fractional population change of HOD (000, 70,7) after collisions with 2,6-lutidine

-0.01

-0.0075

-0.005

-0.0025

0

0.0025

-2.5 0.0 2.5 5.0

Time (s)

I/

Io

-0.0025

0

0.0025

0.005

-2.5 0.0 2.5 5.0

Time (s)

I/

Io

Depletion(line center)

Appearance(line center ± 0.0125 cm-1)

Pyrazine/HOD – Translational Energy GainPyrazine/HOD – Translational Energy Gain

0

300

600

900

0 500 1000 1500

Erot (cm-1)

Ttr

ans,

lab (

K)

N

N

(Evib) + HOD <Tapp,lab> = 570 K<Tdep,lab> = 328 K

2-Picoline/HOD and 2,6-Lutidine/HOD – 2-Picoline/HOD and 2,6-Lutidine/HOD – Translational Energy GainTranslational Energy Gain

0

300

600

900

0 500 1000 1500

Erot (cm-1)

Ttr

ans,

lab (

K)

0

300

600

900

0 500 1000 1500

Erot (cm-1)

Ttr

ans,

lab (

K)

(Evib) + HOD

(Evib) + HOD

<Tapp,lab> = 575 K<Tdep,lab> = 376 K

<Tapp,lab> = 614 K<Tdep,lab> = 368 K

N

N

Pyrazine/HOD – Rotational Energy GainPyrazine/HOD – Rotational Energy Gain

N

N

(Evib) + HOD

Both weak and strong collisions canbe described by the same singleexponential 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) 2458-2460.

Rotational Energy Gain for Pyrazine/HOD Rotational Energy Gain for Pyrazine/HOD vs. Pyrazine/Hvs. Pyrazine/H22OO

Distinct difference from isotopicsubstitution is seen for the rotational energy gain.

Collisions with water gain 4x morerotational energy than collisionswith HOD.

M. Fraelich, M.S. Elioff, and A.S. MullinJournal of Physical Chemistry A 102 (1998) 9761-9771.

Lutidine/HOD and Picoline/HOD – Lutidine/HOD and Picoline/HOD – Rotational Energy GainRotational Energy Gain

Both Lutidine/HOD andPicoline/HOD have nascentrotational distributionsalso described by a single Trot.

Rotational temperaturesfor all three systems studiedare similar in contrast to H2O.

Picoline/H2O = 590±90 KLutidine/H2O = 490±80 K

19

20

21

22

23

24

25

0 500 1000 1500

Erot (cm-1)

ln(p

op

ula

tio

n/g

)

N

39464 K

42660 K

N

Energy Transfer Rates for Collisions with Energy Transfer Rates for Collisions with HODHOD

0

1

2

3

4

5

6

0 500 1000 1500 2000 2500

Erot (cm-1)

kJ

ap

p /

10

-11 (

cm

3m

ole

cu

le-1

s-1

)

N

N

N

N

Pyrazine

Picoline

Lutidine

Donor (Evib) + HOD Donor (Evib’) + HOD (000, JKaKc) kapp

→

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.

AcknowledgementsAcknowledgements

Thanks to:

The Mullin Group

Amy S. Mullin (PI)Qingnan Liu

Liwei YuanJuan DuShizuka HsiehFelix Lin

Funded by:

Department of EnergyNational Science Foundation

Juan Du, Liwei Yuan, Daniel K. Havey, Qingnan Liu