Transcript Femtosecond Laser Spectroscopy of C 60 M. Boyle, Max Born Institute

Max-Born-Institut
Femtosecond Laser Spectroscopy
of C60
M. Boyle, Max Born Institute
Outline
1.) Rydberg Structure of C60
•Excitation and ionization processes
2.) Ionization and Fragmentation Dynamics
•Energy redistribution times
2
Max-Born-Institut
Experimental System : C60
800nm~1.55eV
Energy /eV
8
7
Sn
6
Tn
5
Dipole allowed:
S0S2
4
3
•Complex model system
•High symmetry, thus still treatable
IP=7.58eV
1T
1u
2 1T
1g
1
1A
g
0
S2
S1
T2
T1
S0
3
Max-Born-Institut
Experimental Method : Time of Flight
Wiley-McLaren
Reflectron TOF
Electron TOF
e-
Ion+
y
Double µ-Metal Shielding
C60-Oven
x
z
4
Max-Born-Institut
What is a Rydberg State?
A Rydberg state of a molecule describes the situation
when an electron is far from the core, thus becoming
hydrogenic in nature.
IP
Take the Bohr model of the Hydrogen atom:
e-
 13.6
BE 
n2
+
Experimental Spectrum
-1.5
-3.4
3
2
n=1
-13.6
Binding Energy (assuming 1 photon)
BE=hn-KE
BE
KE
hn
0
5
Max-Born-Institut
Experimental Variable Parameters
•Intensity - 1011-1013 W/cm2
•Wavelength- 800nm, 400nm, 660nm
•Bandwidth Limited Pulse Duration
•Chirp
•Polarization
6
Max-Born-Institut
Intensity Dependence of Photoelectron Spectra
l = 800 nm, t = 1.5 ps
10
10
10
10
10
10
electron yield / log units
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
6
90
5
4
3.6x1012 W/cm2
60
3
2
30
1
electron yield *104 / arb. units
10
0
30
0
5
4
20
3
2
10
1
0
3
0
4
2
3
2
1
1
0
0
4
1
3
2.2x1012 W/cm2
1.5x1012 W/cm2
1.1x1012 W/cm2
2
0.5
1
0
0
5
10
15
20
0
KE / eV
0
0,5
1
1,5
2
7
Max-Born-Institut
Comparison between 400nm and 800nm
***assuming 1-photon ionization
Electron intensity / arb. units
800nm
1.5 ps
BE=hn-KE
400 nm
2.1 ps
IP
x5
0,0
0,5
1,0
1,5
2,0
Binding Energy / eV
2,5
3,0
8
Max-Born-Institut
Wavelength Dependence of Binding Energy
***assuming 1-photon ionization
Normalized Signal
1,0
800nm , 1.5 ps
660nm, 120 fs
400nm, 2.1 ps
0,8
0,6
0,4
IP
0,2
0,0
-0,15
-0,2
0
1
2
3
Binding Energy [eV]
9
Max-Born-Institut
Bandwidth Limited Pulse Duration
DE ~10meV
Dt ~180 fs
DE*Dt=0.441
Rydberg spectra seen for pulse
durations as short as 30 fs
Indicates very fast population
process
DE~20meV
Dt ~100 fs
DEmeas = DElaser  DEdecay ~ DElaser
DE ~ 85meV
Dt ~ 30fs
0,0
0,5
1,0
1,5
2,0
DEradiative decay << DElaser
Lifetimes of Rydberg states are
400 fs or longer
Energy[eV]
10
Max-Born-Institut
Photoelectron Spectra for two chirps
negative chirp (blue leads red)
positive chirp (red leads blue)
Excitation and emission
occur in same laser pulse
IP
Shift=energy bandwidth of laser
0,6
0,8
1,0
1,2
Kinetic Photoelectron Energy [eV]
1,4
11
Max-Born-Institut
Effect of different polarization
Electrons emitted
directly have an
angular dependence
with respect to
polarization axis
TOF
Counts [arb. Units]
e-
e- TOF
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
Kinetic Energy [eV]
12
Max-Born-Institut
Experimental Summary
•Intensity - R.S. emerges from background in low intensity pulses
•Wavelength - indicate a direct excitation process
(no intermediate state)
•Pulse duration - indicates a very fast process
•Chirp - indicates electron emitted within one pulse duration
•Polarization - indicates the electrons are emitted directly
13
Max-Born-Institut
Modeling of Rydberg Series in C60
•Solved the Schrödinger equation for the bound energies of C60
assuming
a simple one electron system
using a jellium model
r [a.u.]
-12
-8
-4
0
-0.5
-1.0
-1.5
4
8
12
Resultant BE values were in
agreement with literature
values of
Puska and Nieminen,
Phys. Rev. A 47, 1181
(1993)
14
Max-Born-Institut
Calculated Rydberg Series
16
14
ns
npnd nfng nh ni
12
nj
n
10
n corresponds to the
number of modes in the
resultant wave function
(typical jellium notation)
8
6
4
2
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
-1/2
[Binding Energy / eV]
15
Max-Born-Institut
Results from Calculation and fitting
Standard Deviation
for selected series
0.02
for other series
>0.03
14
Solid
points from Calculation
12
Open points from fitting of exp.
10
8
n
l=1
l=5
l=3
l=7
l=9
6
4
l=5
2
l=3 l=5
l=7
0
0
1
2
3
(Binding Energy / eV)
4
5
-1/2
16
Max-Born-Institut
Excitation Method
Still under investigation
•Directly?
•energy bandwidth of laser too narrow
•AC Stark Shift (~60meV) to small
•Through super excited state (similar to Schick, et al
JCP A 3735 (2001) for Phenol)?
•three wavelengths indicate no step-wise process
•coupling of vibrational and electronic energy???
17
Max-Born-Institut
Excitation Method
Our current hypothesis:
Oven heated C60 contains
vibrational energy:
at 500 ° C C60
has 5.8 eV of vibrational energy
Energy /eV
IP=7.58eV 8
7
6
5
4
Shifting of Density of States?
3
2
1
Doesn’t explain sharpness of lines
0
1.4eV
18
Max-Born-Institut
Excitation Method
Energy /eV
e-
8
IP=7.58eV
7
6
5
4
3
Under the framework of the
Inverse Born Oppenheimer
Approximation*:
2
1
Electrons move slow in relation to the
Picture when electron
nuclear motion, thus, each
is far from the core
vibrational level has its
own set of electronic levels.
To test this hypothesis, we will use
a cold source that will provide a C60
molecular beam at ~80K
*Thanks go to Prof. R.D. Levine for his insight
19
Max-Born-Institut
Excitation of Rydberg Series in C60
•Resolved Rydberg Structure has been observed
with Laser Photoelectron Spectroscopy of C60.
•Electrons populate the Rydberg states with a
four photon process, and are then single photon
emitted during the same pulse.
• a simple two particle model shows excellent
agreement to experimental spectra.
Excitation method is not yet fully understood.
20
Max-Born-Institut
Next Experimental Steps
1.) C70, NC59, ...
2.) cold C60 source (vibration-less source)
3.) Two color pump probe
4.) angular distribution electrons
21
Max-Born-Institut
Comparison to C70
1.4
C60, 1500 fs, 1*10
1.2
13
W/cm
2
13
2
Normalized Counts
C70, 1300 fs, 2*10 W/cm
1.0
0.8
0.6
0.4
+0,15
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Kinetic Energy [eV]
22
Max-Born-Institut
Time Resolved Measurements of C60
Motivation:
Mechanisms and time scales of energy redistribution are of considerable
interest for understanding and perhaps controlling molecules.
(i.e. control over the fragmentation pattern)
23
Max-Born-Institut
Cluster Energy redistribution
e-
Direct electron emission
and fragmentation
Time scales??
Coupling to vibrational
degrees of freedom
hn
e- statistical electron
emission
delayed
fragmentation Equilibration among vibrational
degrees of freedom
e“thermionic”
emission
(delayed ionization)
Metastable
fragmentation
24
Max-Born-Institut
Time Resolved Measurements of C60
Previous pulse duration dependence measurements in the gas phase
indicate three different ionization regimes for C60
(Campbell, et al. PRL 84 (2000) 2128):
Time
scale
t<100fs
Photo Electron Spectra
characteristics
ATI peaks
Photo Ion Spectra
characteristics
multiply charged C60
t<700fs
Hot electrons with high KE
multiply charged C60 and
small fragments
t>700fs
(up to 5ps)
Cooler electrons
‘typical’ bimodal distribution
delayed ionization tail
25
Max-Born-Institut
Cluster Energy redistribution
e-
< 100fs (ATI)
non-existent for C60
Coupling to vibrational ~ 240 fs modelled
300-500fs measured
degrees of freedom
hn
e-
statistical electron
emission (Hot Electrons)
ps time scale(<5ps)
delayed
fragmentation Equilibration among vibrational
Metastable
degrees of freedom
fragmentation
e
“thermionic”
26
emission
nsms time scale
(delayed ionization)
Max-Born-Institut
Time Resolved Measurements of C60
One Color pump-probe measurements at 800nm
30 fs or alternatively 100 fs pulses
unequal pulse intensity (3:1 ratio)
co-linear Michelson arrangement (fringes)
Weaker pulse leads stronger
Pump-probe allows for an
controlled input of energy
Stronger pulse leads weaker
+ time
- time
t=0
27
Max-Born-Institut
Fragmentation and Ionization Dynamics
Normalized to single pulse
3.0
+
 C60-2n
2.5
 C60-2n
2.0
 C60-2n
t<500fs
electronic
to
select
vibrational
modes
Fragmentation
++
+++
1.5
1.0
1.5
1.0
+
C60
++
C60
0.5
Ionization
+++
C60
0.0
-10
0
10
Time Delay [ps]
20
30
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Max-Born-Institut
Fragmentation and Ionization Dynamics
500fs
Normalized to single pulse
3.0
+
C60-2n
++
2.5
C60-2n
2.0
C60-2n
+++
1.5
1.0
1.5
1.0
+
C60
++
C60
0.5
+++
C60
0.0
-2
0
Time Delay [ps]
2
29
Max-Born-Institut
Mass Spectra
2000
+
++
C60
1000
C60
Long Positive Delay
+++
C60
Counts
0
1000
Short Positive Delay
0
2000
Negative Delay
1000
0
5000
6000
7000
8000
9000
TOF
30
Max-Born-Institut
Mass Spectra
1.0
++
C60
Normalized Signal
0.5
Long Positive Delay
+
C60
+++
C60
0.0
1.0
0.5
Short Positive Delay
0.0
1.0
Negative Delay
0.5
0.0
5000
6000
7000
8000
9000
TOF
31
Max-Born-Institut
Total Signal
Normalized to Single Pulse
1.5
Strong competition
of fragmentation
and ionization
1.4
1.3
1.2
1.1
1.0
0.9
-10
0
10
20
30
Time Delay [ps]
32
Max-Born-Institut
Direct Fragmentation-Double Charged
4000
++
C48
++
C50
++
C52
++
C54
3000
++
C56
Counts
++
C58
2000
1000
-20
-10
0
10
20
30
Time Delay [ps]
33
Max-Born-Institut
Direct Fragmentation - Single Charged
+
C48
1600
+
C50
+
C52
+
C54
Counts
1200
+
C56
+
C58
800
400
0
-20
-10
0
10
Time Delay [ps]
20
30
34
Max-Born-Institut
Direct Fragmentation - Triple Charged
1200
+++
C48
+++
C50
+++
C52
+++
900
C54
+++
C56
Counts
+++
C58
600
300
-20
-10
0
10
Time Delay [ps]
20
30
35
Max-Born-Institut
Ratio between metastable and direct Fragmentation
++
C48
0,9
Ratio (meta/direct)
++
C50
0,8
++
C52
++
C54
0,7
++
C56
++
C58
0,6
0,5
Indication of
internal energy
0,4
related to the energy
absorbed
0,3
0,2
-20
-10
0
10
Time Delay [ps]
20
30
36
Max-Born-Institut
Hot vs. Cold C60
Normalized to single pulse
3
Hot C60 Source 20mJ/8mJ
Cold C60 Source 19mJ/7mJ
2
1
0
-10
0
10
20
30
Time Delay [ps]
37
Max-Born-Institut
Cold C60 Source
Electron
TOF
Liquid N2 Walls
He Buffer Gas
Aggregation
chamber
Ion
RETOF
38
Max-Born-Institut
Hot vs. Cold C60
Normalized to single pulse
4
C48
3
C52
2
++
HOT
++
C56
C58
++
++
1
C56
3
++
C52
++
2
C58
C48
1
-10
COLD
0
++
++
10
Time Delay [ps]
20
30
39
Max-Born-Institut
Conclusions
•Pump-probe measurements indicate time scales for energy relaxation
Time
scale
t < 500 fs
Comment
Energy relaxation from electronic to SPECIFIC vibrational modes
t < 15ps
Energy remains in these modes, which lead to fragmentation
t < 20ps
Non-exponential equilibration of energy
t > 20ps
Vibrational equilibration
•difference between vibrationally hot and cold C60 sources
easier coupling of vibrational modes when hot
40
Max-Born-Institut
Thanks
Max Born Institute
•Dr. C.P.Schulz
•Prof. I.V.Hertel
Göteborg University and
Chalmers University of Technology
•M.Hedén - cold source
•Prof. E.E.B. Campbell
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Max-Born-Institut