CYCLOPROPYLACETYLENE STUDIED IN COLD FREE JET EXPANSION, ROOM TEMPERATURE

GAS, AND DILUTE SOLUTION: TIER MODEL IVR

PAM L. CRUM, GORDON G. BROWN, KEVIN O. DOUGLASS, BRIAN C. DIAN JAMES E. JOHNS, PRADEEP

M. NAIR, HYUN S. YOO, AND BROOKS H. PATE

Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904

CC

C

C

C

H

H

H

H

H

H

Spectroscopic Techniques for Range of Molecular Environments

Frequency Domain

Isolated molecule

MW-IR Double

Resonance

Time Domain

Isolated molecule

FS Transient Absorption

Spectroscopy

Solvated molecule

FS Transient Absorption

Spectroscopy

Vibrational Dynamics of Isolated and Solvated Molecules

Intramolecular Vibrational Energy Redistribution (IVR) and Solvent-Induced Vibrational Energy Relaxation (VER)

IVR– Total amount of vibrational energy constant– Threshold density of states ~10 states/cm-1

– Only pathway for energy flow in isolated molecule– Rate is molecule dependent

VER– Energy removed from molecule (solute)– Rate is solvent dependent

E kIVR

kVER

v=1

v=0

Acetylenic C-H Stretch Fundamental Measurements:kTOT = kIVR + kVER

Acetylenic C-H Stretch Fundamental Measurements:kTOT = kIVR + kVER

Yoo, H.S. et al. J. Phys. Chem A 2004 108(8) 1348-1364Yoo, H.S. et al. J. Phys. Chem A 2004 108(8) 1365-1379Yoo, H.S. et al. J. Phys. Chem A 2004 108(8) 1380-1387

Vibrational Dynamics in Solution

[≡C―H]

[≡C―H]

Intensity (V)

Fre

quen

cy (

cm-1

)

Nor

mal

spe

cies

13C

_413C

_213C

_1

3310

3320

3330

3340

13C

_5

2

5

14

2

Cyclopropylacetylene (CPA)

Exceptionally fast CCl4 relaxation time (sub ps)

What made this molecule different from the other Terminal Acetylenes studied?

2 TOOLS TO GET AT THE DYNAMICSMW-IR DR : FS TAS : t

Pulsed-IR FTMW Detector

• Advantages– Scan rate = 18 cm-1/hr– No hot bands– Sensitive

• Limitations– Linewidth ~ 0.03 cm-1 – Can’t tell where energy goes

OPO/OPA4 mJ/pulse

of 3 m light0.03 cm-1 bandwidth

Pulsed IRNd:YAG

Laser

CPA Ground State Depletion

Frequency (cm-1)3320 3340 3360

Inte

nsity

(V

)

0.2

0.4

0.6

FTMW monitor 13103.80 MHz

101

202

CC

C

C

C

H

H

H

H

H

H

Slow Scans of CPA

Frequency (cm-1)3336 3337 3338

C.

3332 3333 3334

B.

3327 3328 3329 3330

Inte

nsity (

V)

A.

3316 3317 3318 3319

D. R(1)

P(2)

P(1)

R(2)

101

202

FTMW Monitor13103.80 MHz

0 50 100 150

Sur

viva

l Pro

babi

lity

0.0

0.2

0.4

0.6

0.8

1.0 CPA Survival Probability

= 70 ps

time (ps)

0 5 10 15

Sur

viva

l Pro

babi

lity

0.0

0.2

0.4

0.6

0.8

1.0CPA Survival Probability

= 1.5 ps

= 7 ps

Timescales for IVR in CPA

Frequency DomainFTMW-IR

1 ~ 1 ps

2 = 7 ps

3 = 70 ps

(Single Rotational Level Analysis)

Tier Model of IVRState Specific Dynamics of Ultracold CPA 

BrightState

FirstTier

Bath States

1 1 ps 2 = 7 ps

SecondTier

3 = 70 ps

Direct information on the identity of the interacting states is unavailable from IR spectrum

Two ColorTransient Absorption Spectroscopy

TOPAS 1

PUMP

TOPAS 2

PROBE

Variable Delay Stage

CaF2 lens

Sample

/2 plate

Chopper

Probe Reference

Pump Reference

Transmitted Intensity

InSb

InSb InSb

v=2

v=0

v=1

X-H – 100cm-1

X-H

νpump1 ~ 3320cm-1

νprobe ~ 3220cm-1

~ 10μJ~ 12μJ

v=0

v=1 X-H – 40cm-1

X-H

νpump1 ~ 3320cm-1

νprobe ~ 3280cm-1

~ 10μJ~ 10μJ

300K Gas Phase Dynamics by Femtosecond Transient Absorption Spectroscopy

Time (ps)

0 20 40 60 80 100

Nor

mal

ized

Abs

orpt

ion

Cha

nge

(OD

)

0.0

0.2

0.4

0.6

0.8

1.0CPA (g)

v = 2-1 probe1 = 0.6 ps

2 = 6.0 ps

3 = 38 ps

-2 0 2 4 6 8 10 12 14

0.0

0.2

0.4

0.6

0.8

1.0CPA (g)

v = 2-1 probe

1 = 0.6 ps

2 = 6.0 ps

3 = 38 ps

Initial Decay, Recurrences, and Oscillations: Data Agreement

Time (ps)

0 10 20 30 40 50

Nor

ma

lize

d A

bso

rptio

n C

ha

nge

(O

D)

0.0

0.2

0.4

0.6

0.8

1.0 CPA (g)v = 2-1 probe

Survival Probability

0 10 20 30 40 50

-0.05

0.00

0.05

0.10 Residuals

Gas Phase Measurement of Population Relaxation into CH Bend Overtone Levels

CPA (g)PS Measurement

Time (ps)

0 50 100 150 200 250 300

0

2

4

6

8

rise ~ 1 ps (laser limited)fall1 ~ 7 psfall2 ~ 40 ps

Normal Vibrational Mode Shifts due to 13C

Isotope Shift (cm-1)

-60 -50 -40 -30 -20 -10 0 10

Nor

mal

Mod

e F

requ

ency

of

Nor

mal

Spe

cies

(cm

-1)

0

500

1000

1500

2000

2500

3000

3500

C5C4C3C2 and C1

Inte

nsity

(V

)

Frequency (cm-1)

Normal species

13C_4

13C_2

13C_1

3310 3320 3330 3340

13C_5

2

5

1

4

2

C5C4C1C2

GSD IR Spectra of CPA:

Normal Species and 13C

(Natural Abundance)

Inte

nsi

ty (

V)

Frequency (cm-1)

Normal species

13C_4

13C_2

13C_1

3310 3320 3330 3340

13C_5

2

5

1

4

2

13C Isotopomers:Effect on Dynamics

time (ps)

0 5 10 15 20

Inte

nsity

0.0

0.2

0.4

0.6

0.8

1.0NS13C_213C_5

Inte

nsity

(V

)

Frequency (cm-1)

Normal species

13C_4

13C_2

13C_1

3310 3320 3330 3340

13C_5

2

5

1

4

2

Vibrational Dynamics of CPA in 300K Gas  

Acetylenic CH StretchBrightState

Acetylenic CH Bend Overtone

Excitation (1200 cm-1)

Third Tier

1 ~ 1 ps 2 = 7 ps

SecondTier

3 = 40 ps

Comparison of 300K Gas and Dilute Solution Dynamics (0.05 M CCl4)

Initial rates are the samein gas and solution butsolution relaxation ismore efficient

Solution relaxation is not single exponential and “breaks off” at about 1/5 of the full amplitude

Vibrational Dynamics of CPA in Dilute Solution  

Acetylenic CH StretchBrightState

Acetylenic CH Bend Overtone

Excitation (1200 cm-1)

Third Tier

1 ~ 1 ps 2 = 7 ps

SecondTier

3 = 40 ps

Solution Phase Bleach Recovery and 300K Gas Dynamics

CPAGas v = 2-1 ProbeCCl4 v = 1-0 Probe

Time (ps)0 50 100 150 200

Nor

mal

ized

Abs

orpt

ion

Cha

nge

(OD

)

0.0

0.4

0.8

1.2

0 10 20 30

0.0

0.4

0.8

1.2

CCl4: 1 = 0.6 ps 2 = 7 ps 3 = 41 ps

Gas: 1 = 0.6 ps 2 = 6 ps 3 = 38 ps

Summary of CPA Dynamics

Free jet expansion (FTMW):

1 ~ 1 ps

2 = 7 ps

3 = 70 ps

300K Gas phase fs data:

1 ~ 0.6 ps

2 = 7 ps

3 = 38 ps

Dilute Solution fs data:

1 ~ 0.6 ps

2 = 7 ps

3 = 40 ps

Thermal rate enhancement in the final IVR time scale is typicallyobserved (coupled modes are thermally populated).

Conclusions

• Dynamics in all cases dominated by intramolecular processes

• CPA IVR dynamics follow Tier Model

• Strong coupling to first tier– States with two quanta in C-H bend

Acknowledgements

Pate Lab Group Members

Funding:• NSF Chemistry• SELIM Program• University of Virginia• John D. and Catherine T. Macarthur Foundation• The Jeffress Trust

Quantum DecoherenceModel Calculation: Coupled Degenerate States

H = Hmol(Q1,Q2) + Hbath({qi}) + Hmol-bath

Hmol = H1(Q1) + H2(Q2) + W12 : E1 = E2

Hmol-bath kQn2qi : Only pure dephasing terms included

The dynamics are modeled using random matrix methods:

W12E1 + Ebath

H =W12

E2 + Ebath

The dephasing interactions for the two coupled intramolecular states are assumed to be uncorrelated.

N = 24 N = 24 N1 = 9 N2 = 40

Extending the Quantum Decoherence Model to Other Common IVR Models

I. IVR in a Sparse II. IVR Dominated III. Tier ModelIntramolecular Bath by a Single, Strong

Resonance

Solvent Dephasing Effects for Tier Model IVR

Solvent Dephasing Effects for Tier Model IVR

GSD Slow Scans of CPA

frequency (cm-1)

3335 3336 3337 33383331 3332 3333 3334

3327 3328 3329 3330

Inte

nsity

(V

)

3316 3317 3318

P(1)R(1)

P(2) R(2)

Inte

nsity

(V

)

0.2

0.3

0.4

0.5

0.6

Frequency (cm-1)

3320 3330 3340 3350

0.01

0.02

0.03

0.04

12C species

13C species

GSD of 13C Species

CC

C

13C

C

H

H

H

H

H

H

Dynamics from frequency-domain spectrum:

All Bands

Bands 6-13

Bands 9-13

Frequency (cm-1)

3320 3330 3340

Calc.1.5ps6 ps

time (ps)

0 5 10 15

Free jet expansion timescales:

1 ~ 1.5 ps

2 ~ 6 ps

Comparing ultracold free jet expansion to gas-phase room-temperature

Gas-Phase Ultrafast Time-Domain

Isolated molecule

Room-Temp Dynamics of all

thermally populated levels (vibrational hot bands and rotational)

fs transient absorption spectroscopy

Free Jet Expansion Frequency-Domain

Isolated molecule

Ultracold Dynamics of a

single rotational level (State-Resolved)

MW-IR Double-Resonance

Frequency (cm-1)3300 3320 3340 3360

Inte

nsi

ty

0.2

0.4

0.6

0.8

1.0

frequency spectrumfs pulse (norm)

~ 300 fs pulse

~ 100 cm-1 bandwidth

0 50 100 150

Sur

viva

l Pro

babi

lity

0.0

0.2

0.4

0.6

0.8

1.0 CPA Survival Probability

= 70 ps

time (ps)

0 5 10 15

Sur

viva

l Pro

babi

lity

0.0

0.2

0.4

0.6

0.8

1.0CPA Survival Probability

= 1.5 ps

= 7 ps

0 50 100 150

Sur

viva

l Pro

babi

lity

0.0

0.2

0.4

0.6

0.8

1.0 CPA Survival Probability

= 70 ps

time (ps)

0 5 10 15

Sur

viva

l Pro

babi

lity

0.0

0.2

0.4

0.6

0.8

1.0CPA Survival Probability

= 1.5 ps

= 7 ps

Timescales for IVR in CPA

Frequency DomainFTMW-IR

1 = 1.5 ps

2 = 7 ps

3 = 70 ps

Time DomainGas Phase

1 = 0.1 (0.02) ps

2 = 7 (0.9) ps

3 = 50 (15) ps

Time (ps)

0 5 10 15 20

No

rma

lized

Ab

sorp

tion

Ch

ang

e (

OD

)

0.0

0.2

0.4

0.6

0.8

1.0 CPA (g)v = 2-1 probe

1 = 0.6 ps

2 = 7.0 ps

3 = 42 ps

0 50 100 150

Sur

viva

l Pro

babi

lity

0.0

0.2

0.4

0.6

0.8

1.0 CPA Survival Probability

= 70 ps

time (ps)

0 5 10 15

Sur

viva

l Pro

babi

lity

0.0

0.2

0.4

0.6

0.8

1.0CPA Survival Probability

= 1.5 ps

= 7 ps

Timescales for IVR in CPA

Frequency DomainFTMW-IR

1 = 1.5 ps

2 = 7 ps

3 = 70 ps

Time DomainGas Phase

1 = 0.6 ps

2 = 7 ps

3 = 42 ps

Time (ps)

0 5 10 15 20

No

rma

lized

Ab

sorp

tion

Ch

ang

e (

OD

)

0.0

0.2

0.4

0.6

0.8

1.0 CPA (g)v = 2-1 probe

1 = 0.6 ps

2 = 7.0 ps

3 = 42 ps

Time DomainSolution Phase

1 = 0.6 ps

2 = 7 ps

3 = 60 ps

CPACCl4 v = 1-0 Probe

Time (ps)

0 50 100 150 200

No

rma

lize

dA

bso

rptio

n C

ha

ng

e (O

D)

0.0

0.4

0.8

1.2

0 10 20 30

0.0

0.4

0.8

1.2

Comparison of Solution Rate and Initial Gas-Phase IVR Rate

kTOT = kIVR + kVER

Dynamics of the Acetylenic C-H stretch for

Gas-Phase CPA

Time (ps)

0 5 10 15 20

No

rma

lized

Ab

sorp

tion

Ch

ang

e (

OD

)

0.0

0.2

0.4

0.6

0.8

1.0 CPA (g)v = 2-1 probe

1 = 0.6 ps

2 = 7.0 ps

3 = 42 ps

CPA (g)PS Measurementv=1 - v=0 Probe

0

4

8

12

16

CPA (g)PS Measurementv=2 - v=1 Probe

Abs

orba

nce

Cha

nge

(mO

D)

0

2

4

6

8

CPA (g)PS Measurement-40 cm-1 Probe

Time (ps)

0 50 100 150 200 250 300

0

2

4

6

8

Femtosecond Resolution Picosecond Resolution

Dynamics of the Acetylenic C-H stretch for Solution-Phase CPA

0.00

0.01

0.02

0.03 CPA (CCl4)

CPA (Gas)

Time (ps)

0 4 8 12 16 20

Abs

orpt

ion

Cha

nge

(OD

)

0.00

0.01

0.02

0.03 CPA (CCl4)

1 = 0.65 ps

CPA

Gas v = 2-1 Probe

CCl4 v = 1-0 Probe

Time (ps)

0 50 100 150 200

No

rmal

ized

Abs

orp

tion

Ch

ang

e (O

D)

0.0

0.4

0.8

1.2

0 10 20 30

0.0

0.4

0.8

1.2

13C Isotopomers:Effect on Dynamics

time (ps)

0 5 10 15 20

Inte

nsity

0.0

0.2

0.4

0.6

0.8

1.0NS13C_413C_5

Data Agreement

time (ps)

0 10 20 30 40 50

Inte

nsi

ty

0.0

0.2

0.4

0.6

0.8

1.0 Average of all speciesGas femto

time (ps)

0 5 10 15 20

Inte

nsity

0.0

0.2

0.4

0.6

0.8

1.0NS13C_113C_213C_413C_5

Average Survival Probability(all species)

FS Gas Dynamics

13C Isotopomers:Normal Vibrational Mode Shift &

Effect on Dynamics

The survival probability calculation for the normal species of CPA is compared to the same calculation for each of the 13C isotopomers.

time (ps)

0 5 10 15 20

Inte

nsity

0.0

0.2

0.4

0.6

0.8

1.0NS13C_113C_213C_413C_5

Isotope Shift (cm-1)

-60 -50 -40 -30 -20 -10 0 10

Nor

mal

Mod

e F

requ

ency

of

Nor

mal

Spe

cies

(cm

-1)

0

500

1000

1500

2000

2500

3000

3500

C5C4C3C2 and C1

Solution Phase Measurement of Population Relaxation into CH Bend

Overtone Levels

Time (ps)

-5 0 5 10 15 20 25 30

Abs

orpt

ion

Cha

nge

(OD

)

0.00

0.05

0.10

0.15

CPA (CCl4)

PS Measurementsv = 2-1 Probe

-40 cm-1 Probe

rise = 1.2 ps (laser limited)fall1 = 6.4 psfall2 = 45 ps

Transient Absorption Spectroscopy for

Terminal Acetylenes

v=2

v=0

v=1

0

0–108cm-1

1 2

Excited StateAbsorption

SEP

GSD

0– 40cm-1

vCHs = 0,

vCHb= 2

+ 2100 cm-1 in other modes

0

vCHs = 0,

vCHb= 0

+ 3300 cm-1 in other modes

1

1

1

2

2

Excited State Absorption

Bend Overtone Absorption

Bleach Recovery