Transcript ultrafast-v2.ppt

Flow of Vibrational Energy in Polyatomic Molecules:
Using Acetylenic Anharmonic Couplings to Follow
Vibrational Dynamics
Steven T. Shipman and Brooks H. Pate
Department of Chemistry
University of Virginia
Tier Model of IVR
Bright
State
First
Tier
Second
Tier
Bath States
Vibrational Energy Relaxation (VER) of
Polyatomic Molecules in Solution
Solute Vibrational Levels
C-H Stretch
IR Pump
Solvent Accommodates Energy
Difference at Each Step of the
Relaxation “Cascade”
Watching Localized Energy Transport
Many IVR studies follow the departure of energy from an
initially prepared state rather than the arrival of energy
elsewhere in the molecule.
Exceptions: Time-Resolved IR-Raman (Dlott)
IR pump - UV probe (Crim, Abel)
A few difficulties: most low-frequency modes are delocalized
across the entire molecule and are hard to directly monitor.
Terminal acetylenes have several nice features:
CC-H bend is at low frequency (625 cm-1) but localized
Stretch-bend coupling larger than linewidths in RT sol’n phase
CC-H stretch is very intense
Using Anharmonic Couplings to Follow Dynamics
Following late-stage relaxation dynamics is hindered
because directly probing low-frequency modes is difficult.
So let’s indirectly probe them…
E(v1,v2,v3,…) = ∑ wi (vi + 1/2) + ∑ ∑ xij (vi + 1/2) (vj + 1/2) + …
i
i
j≥i
Anharmonic interaction between acetylenic C–H stretch (3330 cm-1)
and acetylenic C–H bend (600 cm-1) is -20 cm-1.
The transient absorption signal at 3330, 3310, and 3290 cm-1 allows
us to extract the dynamics of the low-frequency bending mode.
Two Color Transient Absorption Spectroscopy
Probe
Reference
InSb
Variable Delay Stage
Chopper
Polarizer
InSb
CaF2 lens
Transmitted Intensity
Monochromator
Pump
Reference
InSb
Sample
λ/2 plate
OPA 1
OPA 2
PROBE
PUMP
1 kHz repetition rate
Independently tunable OPAs
Tuning range ~1–6 µm, ~5–10 µJ/pulse
Gas and liquid samples
Vibrational Transitions of Terminal Acetylenes
v=1
≡C–H
v=1
=C–H
v=1
bend
3290 cm-1
3310 cm-1
2950 cm-1
3100 cm-1
v=1
–C–H
Ground State
Pump
Probe
v=2
bend
3270 cm-1
v=0
bend
What States are Being Prepared?
mbey 202-101 (13567.966 MHz)
1.5 ps pulse (20 cm-1 FWHM)
Ultrafast pulses are broad!
High-resolution data is necessary to know what states are prepared.
Molecular Beam Data – Butyne
GSD-DP Measurements
202 – 101 (17259.905 MHz)
10.8 cm-1 / hr, 20 averages
Butyne – C–H Stretch Identification
Scaled Harm Anharm Cubic Rediag Mode Description Intensity
3007.1
2982.9
2984.2
asym –CH3
26.8
3000.3
2979.3
2983.1
asym –CH3
30.9
2943.3
2911.2
2912.6
asym –CH2
6.7
2929.8
2969.1
2986.6
sym –CH3
32.1
2914.5
2920.0
2939.5
sym –CH2
20.0
Butyne in CCl4 – The 2941 cm-1 Band
3290 cm-1
16.9 / 29.7 ps
cm-1
2827
4.9 ps
v=2
v=1
v=1
v=0
v=0
v=1
Molecular Beam Data – Methylbutenyne
GSD-DP Measurements
202 – 101 (13567.966 MHz)
10.8 cm-1 / hr, 20 averages
Methylbutenyne in CCl4 – The 3101 cm-1 Band
3290 cm-1
vbend = 1
18.4 / 31.0 ps
3270 cm-1
vbend = 2
15.3 / 32.1 ps
Transient Absorption (mOD)
Methylbutenyne in CCl4 – The 2925 cm-1 Band
4
2
0
-2
4
2
0
-2
4
2
0
-2
-50
3310 cm-1
vbend = 0
10.7 / 34.6 ps
3290 cm-1
vbend = 1
16.1 / 32.0 ps
3270 cm-1
vbend = 2
9.9 / 21.0 ps
0
50 100 150 200
Delay Time (ps)
Methylbutenyne Results
Pump Probe Bend Fast
2925 2925
0
3.7 (0.5)
Slow
N/A
2925
3310
3290
3270
0
1
2
10.7 (1.1)
16.1 (1.3)
9.9 (2.2)
34.6 (5.4)
32.0 (1.9)
21.0 (5.1)
3101
3290
3270
1
2
18.4 (3.4)
15.3 (5.9)
31.0 (4.6)
32.1 (8.2)
Pump 2925 cm-1
Pump 3101 cm-1
Butyne and Methylbutenyne – Relaxation Tiers
t = 30 ps
t = 5 ps
t = 15 ps
t = 0 ps
Bright
State
First
Tier
–C–H stretches
–C–H bends
Second
Tier
≡C–H bends
Bath States
Solvent modes
Summary
The strong stretch-bend coupling of the acetylenic C–H stretch
can be exploited to probe late stages of the vibrational relaxation
process.
In methylbutenyne, a comparison of the dynamics of states with
vbend = 1 and vbend = 2 indicates that the methylic and ethylenic
stretches explore qualitatively different relaxation pathways.
Gas phase measurements need to be made for comparison, but
the solvent contribution to the relaxation is no faster than 30 ps.
Acknowledgements
Current and former Pate Lab members
NSF - Chemistry
Weak Butyne Bands Near 2981
GSD-DP Measurements
202 – 101 (17259.905 MHz)
10.8 cm-1 / hr, 20 averages
Methylbutenyne
3290
3310 (x –1.05)
Butyne one-color measurement
1.4 (0.4) ps recovery
pump 2941
probe 2941
One-color measurements hindered by stimulated emission.
Vibrational Dynamics in Solution
E
kIVR
v=1
kVER
v=0
Acetylenic C-H Stretch Fundamental Measurements:
kTOT = kIVR + kVER
Yoo, H.S. et al. J. Phys. Chem A 2004 108(8) 1348-1364
Yoo, H.S. et al. J. Phys. Chem A 2004 108(8) 1365-1379
Yoo, H.S. et al. J. Phys. Chem A 2004 108(8) 1380-1387
Properties of Acetylenic Compounds
3500
Frequency (cm) -1
3000
H
CHstretch and (CCstretch + 2CHbend)
C
C
2500
CCstretch
R
2000
1500
v=2 CHbend
Acetylenic CH stretch is a
local mode oscillator with
motion dominated by the
terminal H-atom.
1000
CHbend
500
-CH2F
-CH2CH3 -C(CH3)=CH2
-CH2Br
-CHFCH3
-CH(CH3)2
-CH2Cl
R group structure
-C(CH3)3
-CH3
-Si(CH3)3
This atom extends
approximately 3A from
the R group.
Bright States vs. Eigenstates
W
C-H
Bright State
Dark States
Molecular
Eigenstates
Pulsed-IR FTMW Detector
Pulsed IR
Nd:YAG
Laser
OPO/OPA
0.02 cm-1 bandwidth
10 Hz repetition rate
Tuning range ~1 – 4 µm, ~5 – 10 mJ/pulse
Molecular Beam Samples – No Hot Bands!
The Molecules Under Study
1-butyne
tert-butyl acetylene
methylbutenyne
trimethyl silylacetylene
Preliminary Data – TBA and TMSA in CCl4
2973 / 2973 cm-1
15.4 ps
2964 / 2964 cm-1
8.4 ps
Long lifetime of bright state implies that couplings are not large.
Butyne – Coupling to Other C–H Bends
2982 cm-1
13.4 ps
5 cm-1 resolution makes identification of probed band difficult…
… But the bleach indicates a frequency shift of at least that amount.
From Xij, most likely couplings are to bending modes near 1470 cm-1.
(Couplings on the order of 20 cm-1 to all –CH3-based stretches.)
Butyne Results
Pump Probe Bend
Fast
Slow
2941 2941
0
1.4 (0.4)
N/A
2827
0
4.9 (0.7)
N/A
2982
0
13.4 (0.7)
N/A
2941 3290
1
16.9 (2.2) 29.7 (2.9)
3290 time constants are in agreement with methylbutenyne results.
Same general relaxation pathways? Need to check at 3270…
Difficult to say if 2982 and 3290 time constants are in fact identical.