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