Frequency and Time Domain Studies of Toluene

Adrian M. Gardner, Alistair M. Green, Julia A. Davies, Katharine L. Reid and Timothy G. Wright

Introduction: Vibrational Modes

• Often, vibrations are considered in the harmonic approximation.

• Within this approximation normal vibrational modes are considered to be independent.

• However, true vibrational eigenstates are a linear combination of the harmonic levels.

Introduction: Previous Investigations• Many vibrational levels in the S1 electronic state have been

assigned by Hickman et al.1

• This was achieved by recording dispersed fluorescence spectra via the observed S1 vibrational levels.

• Knowledge of the vibrational levels of the toluene cation is limited.

• Meek et al.2 employed time of flight photoelectron spectroscopy.

• Later Whiteside et al.3 used TOF-PES and velocity map imaging to investigate the toluene cation.

• Lu et al.4 utilized Zero Electron Kinetic Energy Spectroscopy , while Gunzer and Grotemeyer5 used the closely related Mass Analysed Threshold Ionization, MATI, technique.1. C. G. Hickman, J. R. Gascooke and W. D. Lawrance, J. Chem. Phys., 104, 4887 (1995)

2. J T. Meek, S. R Long and J. P. Reilly, J. Phys. Chem., 86, 2809 (1982)3. P. T. Whiteside, A. K. King and K. L. Reid, J. Chem. Phys., 123, 204316 (2005)4. K. T. Lu, G. C. Eiden and J. C. Weisshaar, J. Phys. Chem., 96, 9742 (1992)5. F. Gunzer and J. Grotemeyer, Phys. Chem. Chem. Phys., 4, 5966 (2002)

Present Investigations• Photoelectron spectra have been recorded via several

vibrational levels in the S1 electronic state.

• Nanosecond Zero Electron Kinetic Energy (ZEKE) photoelectron spectroscopy results in spectra with a resolution of ≈ 8 cm-1.

• This allows detailed information of the vibration levels of the toluene cation to be obtained.

• Picosecond Slow Electron Velocity map Imaging (SEVI) results in energy resolution ≈ 30 cm-1 being achievable, but offers time resolution of ≈ 1 ps.

• This provides a method of probing the dynamics of the S1 electronic state.

Experimental: Techniques

Investigating electronically excited states

(1+1) Resonance Enhanced Multi-Photon Ionization, REMPI, Spectroscopy

Experimental: Techniques

Investigating cationic ground states

ZEKE-PFI Photoelectron Spectroscopy

Experimental: Techniques

Investigating cationic ground states

ZEKE-PFI Photoelectron Spectroscopy

Experimental: TechniquesInvestigating cationic ground states: Time resolved photoelectron spectroscopy

• If the bandwidth of the laser light overlaps two (or more) coupled vibrational modes which are each a linear combination of zero order bright and dark states.

• Initially the wavepacket will resemble the bright state.• The wavepacket will evolve in time to resemble the dark state.• Hence the resulting photoelectron spectrum will evolve in time.

Results: (1+1) ns-REMPI Spectrum

Results: PES via a Fermi Resonance

• Three modes, all of a1 symmetry are present within a 10 cm-1 energy

range.• Each eigenstate has been deduced to be a linear combination

of16a2, 6a1 and 10b116b1 zero order states.66. J. A. Davies, A. M Green and K. L. Reid, PCCP, 12, 9872 (2010)

Results: PES via a Fermi Resonance

6. J. A. Davies, A. M Green and K. L. Reid, PCCP, 12, 9872 (2010)

• Three modes, all of a1 symmetry are present within a 10 cm-1 energy

range.• Each eigenstate has been deduced to be a linear combination

of16a2, 6a1 and 10b116b1 zero order states.6

Results: Higher Frequency Vibrational Levels

• No ZEKE signal could be observed via 6.

• The broad features to higher energy in the ZEKE spectra, along with the raised baseline are indicative of IVR.

Conclusions

• The cationic ground state and S1 excited electronic state of toluene have been investigated in both the frequency and time domains.

• Photoelectron spectra have been recorded via many intermediate vibrational levels.

• Extensive IVR has been observed via relatively low frequency S1 vibrational levels.

• We have shown the complementary information which may be gained thorough employing both nanosecond and picosecond investigations into electronically excited states and cationic ground states of aromatic molecules.

Frequency and Time Domain Studies of Toluene

Alistair M. Green, Julia A. Davies, Katharine L. Reid and Timothy G. Wright

Results: PES via S1 00

• The ion origin peak has the largest intensity in both spectra, confirming the Δv = 0 propensity rule holds for toluene.

• All bands (aside from the very weak 16a1 feature) are assigned to modes of a1 symmetry, confirming that the approximation of toluene to the C2v point group is a reasonable assumption.

Experimental: Techniques

Investigating cationic ground states: Slow Electron Velocity map Imaging (SEVI)

• Probe wavelengths which are only slightly greater than a cationic vibrational level are employed.

• Resulting photoelectrons will have a lower kinetic energy, and therefore a lower spread of kinetic energy.

Experimental: Techniques

Investigating cationic ground states: Slow Electron Velocity map Imaging (SEVI)

• Many different probe wavelengths are used.• SEVI spectra are spliced together resulting in a complete spectrum with a higher

resolution than if a conventional two colour VMI scheme was employed.

Experimental: Nanosecond Apparatus

Apparatus in the Wright Laboratory, University of Nottingham.

Experimental: Picosecond Apparatus

Apparatus in the Reid Laboratory, University of Nottingham.

• A bespoke picosecond laser system is used to generate two independently tuneable UV beams.

• A delay stage is used to vary the time between the pump and the probe pulses entering the vacuum chamber from -10 – 510 ps.

• Bandwidth of the generated light is ≈ 13 cm-1.

• Velocity map imaging is employed as the detection method.• In order to increase resolution Slow Electron Velocity-map Imaging, SEVI is

used.