Pump-Probe Photoionization &

Mass Spectroscopy of Pentamethylcyclopentadiene

Fedor RudakovPeter Weber

Molecular Spectroscopy

June 21, 2007

CONCLUSIONS:

The 21A1 state is reached with a time constant of 55 fs...

.... Decays with a 84 fs time constant

h + +

Kuthirummal, Rudakov, Evans, and Weber, J. Chem. Phys. 125, 133307 (2006).

Recent Work on 1,3-cyclohexadiene

267nm

Pentamethyl Cyclopentadiene

e-

400nm

1B2

2A1

1A1

RyRy

Experimental setup

CPUTiming Electronics

Regenerative Amplification Laser System

Ion MCPs

e- MCPs

3ω 2ω

BBO upconversion

Molecular beamTime Delay

Stage

Pentamethyl Cyclopentadiene Time-Resolved

Time-delayed experiments: Time-delayed experiments: • Pump: 267 nm pulsePump: 267 nm pulse• Probe: 1 or 2 photons of 400 nmProbe: 1 or 2 photons of 400 nm

Signals: Signals: • Mass spectraMass spectra• Photoelectron spectraPhotoelectron spectra

2A1

267nm

e-

400nm

1B2

TimeDelay

1A1

400nm

Time-resolved mass spectrum

As the wave packet evolves on the As the wave packet evolves on the electronic surfaces, energy is deposited electronic surfaces, energy is deposited in vibrational coordinates. in vibrational coordinates.

Once ionized, this energy may Once ionized, this energy may suffice to fragment the molecule on suffice to fragment the molecule on the way to the mass spec detector. the way to the mass spec detector.

The early decay of the parent mass The early decay of the parent mass reflects the loss of electronic energy as reflects the loss of electronic energy as the wave packet moves out of the 1Bthe wave packet moves out of the 1B22 state state (165 fs).(165 fs).

The fragments show an initial rise (165 The fragments show an initial rise (165 fs) when the molecule crosses from 1Bfs) when the molecule crosses from 1B22 to to 2A2A11. The decay of the fragment peak shows . The decay of the fragment peak shows the curve crossing through the lower the curve crossing through the lower conical intersection (101 fs). conical intersection (101 fs).

All Fragments CombinedAll Fragments Combined

Parent Parent MassMass

x10x10

Xe-Instrument Xe-Instrument function function FWHM=163(7)FWHM=163(7)

Time-Resolved Mass Spectrum

As the wave packet evolves on the As the wave packet evolves on the electronic surfaces, energy is deposited electronic surfaces, energy is deposited in vibrational coordinates. in vibrational coordinates.

Once ionized, this energy may Once ionized, this energy may suffice to fragment the molecule on suffice to fragment the molecule on the way to the mass spec detector. the way to the mass spec detector.

The early decay of the parent mass The early decay of the parent mass reflects the loss of electronic energy as reflects the loss of electronic energy as the wave packet moves out of the 1Bthe wave packet moves out of the 1B22 state state (165 fs).(165 fs).

The fragments show an initial rise (165 The fragments show an initial rise (165 fs) when the molecule crosses from 1Bfs) when the molecule crosses from 1B22 to to 2A2A11. The decay of the 2A. The decay of the 2A11 state is seen by state is seen by the decay of the fragment signal (101 the decay of the fragment signal (101 fs). fs).

All Fragments CombinedAll Fragments Combined

Parent Parent MassMass

267nme-

1B2

2A1

TimeDelay

1A1

400nm

400nm

Time-resolved photoelectron spectra267nm

e-

400nm

1B2

0 fs

200fs

2 ps

1A1

2A1

Mass and Photoelectron spectra

All Fragments All Fragments CombinedCombined

Parent Parent MassMass

2A1

State

1B2 State

Summary: Curve Crossing Dynamics in Pentamethyl Cyclopentadiene

We watched the transition from the 1BWe watched the transition from the 1B22 state state

to the 2Ato the 2A11 state … state …

… … before transition to the ground state 1Abefore transition to the ground state 1A11

We observed molecule in We observed molecule in 11BB22 , the , the 22AA11 and the and the 11AA1 1 statestate

by photoelectron and mass spectroscopyby photoelectron and mass spectroscopy

2A1

We learned:We learned:

Energetics: Energetics:

The ionization energy is at The ionization energy is at 7.30 eV7.30 eV

Dynamics: Dynamics:

It takes 165 fs to leave the It takes 165 fs to leave the 11BB22 state state

The life time of the The life time of the 22AA11 state is 101 fs state is 101 fsNo structural changes take place on the No structural changes take place on the ground Stateground State

267nm

165 fs165 fs

101 fs

101 fs

1B2

FastFast

1A1

Acknowledgements

Job Cardoza Mike Minitti

Narayanan KuthirummalJoe BushXinli Yang

Funding: DOE