Transcript OSU_2008.ppt

RYDBERG ELECTRONS
STEALTHY SPIES OF
MOLECULAR
STRUCTURE
International Symposium on Molecular
Spectroscopy
17 June 2008
Michael P. Minitti
•
•
Rydberg
States
an ion or molecule
State of
where an
excited electron has a high principal
quantum number
Hydrogenic in nature, with a binding
energy given as:
EB 
RRyd
n   
2
Rydberg I
Experimental Setup
Ti:Sapphire
BBO
upconversion
Regenerative Amplifier
YLF Pump
4ω
2ω
• 5 kHz rep. rate
• 209 nm pump / 418 nm
e- MCPs
probe
• ~230 fs 4ω pulse width
CPU
Timing Electronics
Molecular beam
Ion MCPs
Structural Dispersion in
Flexible Molecules
• RFS spectra of molecules w/ various internal rotation DOFs show
multiple structures are populated
• Well
resolved even in the presence of large vibrational
temperatures
1,4-Dimethyl-piperazine
(DMPZ)
N,N-Dimethyl-2-butanamine
(DM2BA)
N,N-Dimethyl-isopropanamine
(DMIPA)
N,N-Dimethyl-1-butanamine
(DM1BA)
M.P. Minitti, J.D. Cardoza and P.M. Weber, JPCA, 110, 10212
(2006)
N,N-Dimethyl-3-hexanamine
(DM3HA)
SD I
DM3HA
Near
time
zero
Intensity (arbitrary units)
DM1BA
30 ps
delay
DM2BA
DMIPA
DMPZ
2.0
2.5
3.0
Electron Binding Energy (eV)
2.0
2.5
3.0
Electron Binding Energy (eV)
SD II
Vibrational Temperatures
M.P. Minitti, J.D. Cardoza and P.M. Weber, JPCA, 110, 10212
SD III
History says...
due
to
vibrational
Intensity
1. Linewidth
congestion
Spectral line shape
{ν}
{ν’}
BE
“Electronic transitions consist of a series of bands,
each band corresponding to a transition between a
given pair of vibrational levels.”
-Ira N. Levine, Physical Chemistry
History says...
Spectral line shape
2. Linewidth due to lifetime of the
state
Long lifetimes = sharp lines
Short lifetimes = broad lines
How is it then that we see sharp lines in the
presence of large vibrational energies in addition
to very fast intermediate lifetimes?
Time-Dependent Structural
Dispersion
0 ps
2.5
2.6
2.7
2.8
2.9
Binding Energy (eV)
70 ps
2.5
3.0
150 ps
2.6
2.7
2.8
2.9
Binding Energy (eV)
3.0
2.5
2.6
2.7
2.8
2.9
Binding Energy (eV)
3.0
Time-dependent structural dispersion observed
in 3s Rydberg peak of DM2BA
M.P. Minitti and P.M. Weber, Phys. Rev. Lett, 98, 253004
TD I
Fractional Population
Fractional Population
1.0
N,N-Dimethyl-2-butanamine
0.9
0.8
1.0
N,N-Dimethyl-3-hexanamine
0.7
0.8
0.6
0.6
0.5
0.4
0.2
0.4
0.0
0 50 100 150 200
0.3
Delay Time (ps)
0.2
0.1
0.0
-25 0 25 50 75 100 125 150 175 200
Delay Time (ps)
Experimentally
determined fractional
populations
(area
under the curves)
Two dominant conformeric
forms, A and B, in
equilibrium via opposing
first order reactions
At  Ae  xe  e
kA  kB t

M.P. Minitti and P.M. Weber, Phys. Rev. Lett, 98, 253004
(2007)
TD II
DFT Calculation
Observed t = 0 fractional
ground state population:
0.67/0.33
Calculated t = 0 fractional
ground state population:
0.65/0.35
(using RT distributions)
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Observed t = ∞ fractional
excited state population:
0.78/0.22
Calculated t = ∞ fractional
excited state population:
0.70/0.30
(using previously estimated*
vibrational temperature of
950 K)
M.P. Minitti and P.M. Weber, Phys. Rev. Lett., 98, 253004
* M.P. Minitti, J.D. Cardoza and P.M. Weber, J. Phys. Chem. A., 110,
10212 (2006)
TD III
Structural Dispersion
Spectra are insensitive towards vibrational
excitation and provide a purely electronic
spectrum dependent on the coordinates of all
electrons and nuclei and therefore the
molecular structure
What other spectral features can our Rydberg
electron spies tell us about the molecular
structure?
NEXT MISSION: N,N,N’,N’ - TMEDA
fs-resolved TMEDA
PES
3s linewidth as a function of
pump-probe delay
3s Rydberg Peak - Full Width Half Maximum
(cm-1)
80 0
70 0
60 0
5 ps
50 0
40 ps
40 0
200 ps
30 0
0
20
40
60
80
1 0 0 12 0 1 40 1 6 0 1 8 0 2 0 0
P u m p -P ro b e D e lay (ps )
Linewidth comparisons
to similar tertiary amines
900
N,N-Dimethyl-1butanamine
800
700
FWHM (cm-1)
600
500
N,N,N’,N’-TMEDA
400
300
200
1,4 -Dimethyl-piperazine
100
0
0
20
40
60
80
100 120 140 160
Pump-Probe Delay (ps)
180
200
220
What’s the cause?
TMEDA condenses in a
minimum
hν
q
configuration
coordinate
The molecule contains vibrational energies that are
significant to the barriers in its energy landscape
As vibrational energy dissipates, the molecule condenses
in a minimum on its surface
TMEDA Mass
Spectra
Dimer
Parent
+ He
Paren
t
Presence of Noble gas clusters and
multimers
Closing
Remarks
• Rydberg Fingerprint Spectroscopy has
been proven to be sensitive to an array
of molecular properties
• Coupled with mass spectroscopy, RFS
has multiplexing advantages
• Chemically relevant systems can be
investigated
Acknowledgement
s
• Prof. Peter Weber
• Dr. Job Cardoza
• Fedor Rudakov
• Joe Bush
• Sanghamitra Deb
• Brad Taylor
• Jie Bao
• Brian Bayes
$$$
DOE - Basic Sciences