rydberg electrons international symposium on molecular spectroscopy 17 june 2008 michael p. minitti...
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RYDBERG ELECTRONS
International Symposium on Molecular Spectroscopy
17 June 2008Michael P. MinittiBrown University
STEALTHY SPIES OF MOLECULAR STRUCTURE
• State of an ion or molecule where an excited electron has a high principal quantum number
• Hydrogenic in nature, with a binding energy given as:
Rydberg States
Rydberg I
€
EB
=
RRyd
n − δ( )
2
Experimental Setup
YLF Pump
Ti:Sapphire
Regenerative Amplifier
CPU Timing ElectronicsIon MCPs
e- MCPs
4ω 2ω
BBO upconversion
Molecular beam
• 5 kHz rep. rate
• 209 nm pump / 418 nm probe
• ~230 fs 4ω pulse width
Structural Dispersion in Flexible Molecules
SD I
• RFS spectra of molecules w/ various internal rotation DOFs show multiple structures are populated
• Well resolved even in the presence of large vibrational temperatures
M.P. Minitti, J.D. Cardoza and P.M. Weber, JPCA, 110, 10212 (2006)
1,4-Dimethyl-piperazine(DMPZ)
N,N-Dimethyl-isopropanamine(DMIPA)
N,N-Dimethyl-2-butanamine(DM2BA)
N,N-Dimethyl-1-butanamine(DM1BA)
N,N-Dimethyl-3-hexanamine(DM3HA)
Near time zero
30 ps delay
Inte
nsit
y (a
rbit
rary
uni
ts)
DMPZ
DMIPA
DM2BA
DM3HA
DM1BA
2.0 2.5 3.0
Electron Binding Energy (eV)
2.0 2.5 3.0
Electron Binding Energy (eV)
Inte
nsit
y (a
rbit
rary
uni
ts)
SD II
Vibrational Temperatures
M.P. Minitti, J.D. Cardoza and P.M. Weber, JPCA, 110, 10212 (2006)
SD III
History says...Spectral line shape
“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
1. Linewidth due to vibrational congestion
{ν’}{ν}
Inte
nsi
ty
BE
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?
2.5 2.6 2.7 2.8 2.9 3.0Binding Energy (eV)
Time-Dependent Structural Dispersion
TD I
2.5 2.6 2.7 2.8 2.9 3.0Binding Energy (eV)
2.5 2.6 2.7 2.8 2.9 3.0Binding Energy (eV)
Time-dependent structural dispersion observed in 3s Rydberg peak of DM2BA
M.P. Minitti and P.M. Weber, Phys. Rev. Lett, 98, 253004 (2007)
0 ps
150 ps70 ps
TD II
-25 0 25 50 75 100 125 150 175 2000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Delay Time (ps)
0 50 1001502000.00.20.40.60.81.0
Delay Time (ps)
N,N-Dimethyl-3-hexanamine
N,N-Dimethyl-2-butanamine
Fo l di n g /u n f o l d in g k i n e tic p ar a m e te r s f or i n ve s ti g a te d hy d r o c a rbo n c ha i ns
Initial population
(A e + x e )
Final population
(A e )
k A /k B
Obser v ed
k A +k B
k A
( )
k B
( )
D 2M B A
(950 )K 0.67 0.78 0.28 6.8 10·
10
s- 1
1.5 10·
10
s- 1
(66 )ps
5.3 10·10
s- 1
(19 )ps
D M 3H A
(810 )K 0.85 0.64 0.56 6.7 10·
10
s- 1
2.4 10·
10
s- 1
(41 )ps
4.3 10·10
s- 1
(23 )ps
€
A t( ) = Ae
+ xe⋅ e
− kA
+ kB( ) ⋅ t
Experimentally determined fractional populations (area under the curves)Two dominant conformeric forms, A and B, in equilibrium via opposing first order reactions
M.P. Minitti and P.M. Weber, Phys. Rev. Lett, 98, 253004 (2007)
DFT Calculation
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
M.P. Minitti and P.M. Weber, Phys. Rev. Lett., 98, 253004 (2007)
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)
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, J.D. Cardoza and P.M. Weber, J. Phys. Chem. A., 110, 10212 (2006) TD III
Structural DispersionSpectra 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
5 ps
40 ps
200 ps
0 20 40 60 80 100 120 1 40 160 180 200
30 0
40 0
50 0
60 0
70 0
80 0
Pum p-Probe Delay (ps)
3s
Ryd
be
rg P
ea
k -
Fu
ll W
idth
Ha
lf M
axi
mu
m
(cm-
1)
3s linewidth as a function of pump-probe
delay
Linewidth comparisons to similar tertiary
amines
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0
0
1 0 0
2 0 0
3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
9 0 0
Pump-Probe Delay (ps)
N,N-Dimethyl-1-butanamine
N,N,N’,N’-TMEDA
1,4 -Dimethyl-piperazine
What’s the cause?TMEDA condenses in a
minimum
q
hν
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
configuration
coordinate
Dimer
Parent
Presence of Noble gas clusters and multimers
TMEDA Mass Spectra
Parent+ He
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
Acknowledgements
• Prof. Peter Weber• Dr. Job Cardoza• Fedor Rudakov• Joe Bush• Sanghamitra Deb• Brad Taylor• Jie Bao• Brian Bayes
$$$DOE - Basic Sciences