The character of the long-lived state formed from S1 of Phenylacetylene

Philip M. Johnson and Trevor J. SearsBrookhaven National Laboratory and Stony Brook

UniversityColumbus Talk TK01, Tuesday June 18th, 2013

Support: Department of Energy Basic Energy Sciences

1

Introduction to the photophysics of substituted benzenes

Simplified state diagram of anisole

3.7

5.0

T1

S1,

8.0 D0

S0

S1-T1 gap = 1.3 eV (10500 cm-1)

ISC, IC

E(eV)

Ion

Pump

Isolated small or medium-sized aromatic.

Photoexcitation can lead to a variety of processes:Fluorescence,Inter-System Crossing,Internal Conversion… Less likely: Phosphorescence,Chemistry (bond breaking)

2

X

Deactivation via collisions not possible in a beam

Probing the collisionless photophysics

3.7

5.0

T1

S1,

8.0 D0

S0

S1-T1 gap = 1.3 eV (10500 cm-1)

ISC, IC

E(eV)

Ionization

Pump

Pump-Probe Ionization Spectroscopy

Beam-cooled sample.

Resonantly pump to S1(v) with nsec pulsed dye laser.

Wait, then probe the system using a second (dye) laser tuned to an energy at or above the IP. Excite along the beam for longer time delays.

Time-of-flight mass spectrometric detection of the ion, as parent or fragments.

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Typical Results

From: J. Phys. Chem. 1988, 92, 183, Colson Group

S1

ISC to T1*

ISC: to excited S0 :loss of signal

DELAY TIME (ns)

0 60 120

Instrument-limited rise

0 2 4microsecs

Benzene v6’=1

From: J. Phys. Chem. 1983, 87, 2232 Johnson Group

Biexponential decay due to singlet and triplet*. Energy in T* is important for determining its lifetime.

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ISC to T1*

S1

ISC: to excited S0 :loss of signal

Model

0 2 4microsecs

Benzene v6’=1

From: J. Phys. Chem. 1983, 87, 2232 Johnson Group

Singlet level mixed with few levels {|k>} from a triplet state, which are themselves more weakly coupled to a set of {|l>}. |s> may also be weakly coupled to (another) set of {\l>}. Solve T-D Schroedinger equation or coupled rate equations. 5

ISC to T1*

S1

ISC: to excited S0 :loss of signal

Kommandeur, Adv. Chem. Phys. 70, 133 (1988).

Another view

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Photoelectron spectra as a function of the delay time. Watch the triplet spectrum gain intensity as the singlet decays.

This confirms the longer-lived triplet is formed from the initially excited singlet in benzene.

Triplet S1

Photoelectron energy/eV

0.0 1.0 2.0

By tuning to higher S1 states in the excitation step, one can watch the T1 decay rate increase.

Benzene delayed PES

Phenylacetylene

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S1-S0 spectrum has a band origin at 35877 cm-1 and, in a jet-cooled sample, is simple looking. Vibrational assignments were made by comparison to related molecules and by calculations of vibrational frequencies and intensities.

Rotationally resolved spectrum of the origin band derived from high resolution LIF (Pratt group).

7

Fast decay (~75ns) of S1 state due to fluorescence

Slow decay (>100ms) of ???

From: J. Phys. Chem. A 2008, 112, 1195DELAY TIME (ns)

Phenylacetylene photophysics

Pump-Probe delay ionization experiment. S1 decay looks normal, but we find a long-lived species that survives for as long as we can measure. Limited by the length of the beam apparatus!

Checked the spectrum of the long-lived species is the same as that of S1, and it is. Spectra here are at 30 ns (top), 3μs middle and calculated (bottom).

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Photoelectron SpectraTime-dependent PES of benzene, as before.

Same experiment on PA. The long-lived species appears instantaneously in this experiment, and does not decay.

Long lifetime species

S1

Photoelectron energy/eV0.0 1.0 2.0

Triplet S1

0.0 1.0 2.0Photoelectron energy/eV

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S1 multi-exponential decay

Examining the S1 photoelectron spectrum at longer delays, one sees residual S1 signal. Typical of intermediate sized molecules, coupling of bright state to manifold of T-states.

See the same signature in fluorescence spectrum.

However, the long-lived state is formed during the laser pulse and does not decay, i.e. not coupled to other states at all?

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Summary of the observations

By comparison with related molecules, the state energies in PA would argue for a Triplet lifetime in the 10-100s of ns.

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Following excitation of the S1 state of PA in a cold beam, we see decay of the singlet state with a multi-exponential lifetime matching the known fluorescence + ISC decay and a separate, much longer-lived component.

Benzonitrile (C6H5CN) shows similar behaviour.

Production of the long-lived state occurs synchronously with S1, and it has a distinct PES.

Mass spectrometry experiments

Possibilities: PA has some alternative route to a low internal energy Tn level. Or:Isomerization?

Compare ion fragmentation patterns in mass spectra of S1 and the long-lived state.

Any isomer will have a different ion fragmentation pattern. Reflectron TOF used has mass resolution of >500. 12

Mass spectraThe fragmentation patterns are essentially identical. Differences are due to fragmentation of metastable ions at different points along the flight path.

13

This means the long-lived state is PA in some excited electronic state, probably with low internal excitation.

b

c

d

Summary

The long-lived state observed following excitation to S1 of PA (and, by extension benzonitrile) is not an isomer, but an excited (triplet) state with low vibrational excitation.

The state is formed during the laser pulse, and does not couple to underlying continua.

New class of radiationless transition behavior? Raman-type process?

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Probe laser delay (ns)

Sig

nal (

arb)

?

Acknowledgements

Phil Johnson, Gary Lopez, Jason Hofstein, Chih-Hsuan Chang, Haifeng Xu, Greg Hall

BNL, Department of Energy, for financial support.

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State Symm. Energy-au Δ eV Δ cm-1 λ nm Δ cm-1Zero pt. corr.-au Δ eV Δ cm-1 λ nm Expt. eV

From S0 S1-Tn From S0 with ZPC

S0 A1 -308.509241335 0.109349

S1 B2 -308.332190987 4.818 38858 257.3 0.103649 4.663 37607 265.9 4.448a

T1 A' -308.393654958 3.145 25368 394.2 13490 0.103693 2.991 24127 414.5 3.154b

T2 A1 -308.357574529 4.127 33287 300.4 5571 4.255b

T3 B2 -308.354502990 4.211 33961 294.5 4897T4 B2 -308.341191807 4.573 36883 271.1 1975

Calculated state energies

Gaussian-09 TD-DFT aug-cc-pVTZ basis

T1 state has significant quinoidal structure and trans-configuration about the C-C triple bond

Related Work

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• “Fully” rotationally-resolved spectra

Pratt group recorded LIF in a beam using a c.w. dye laser. This is much higher resolution than we get with the pulse-amplified c.w. system. Line widths commensurate with fluorescence lifetime.

There are ‘extra’ lines in the spectrum, that may be due to triplet perturbing levels. Or something else? With our pulsed laser linewidths, they would be coherently excited in our experiment.

Also, the Meijer group has recently recorded ultracold spectra of BN-and seen extra lines and shifts too.