Evidence of Radiational Transitions in the Triplet Manifold

of Large Molecules

Haifeng Xu, Philip Johnson

Stony Brook University

Trevor Sears

Brookhaven National Laboratory

6161stst OSU International Symposium on Molecular Spectroscopy OSU International Symposium on Molecular SpectroscopyJune 19-23, 2006, Columbus, OhioJune 19-23, 2006, Columbus, Ohio

OutlineOutline

• Neutral Phenylacetylene and Benzonitrile

• Pump-Probe delayed Ionization

• Long-lived excited states lasting tens of microseconds

• Radiative transitions between triplet states

Pump-Probe ionizationPump-Probe ionization

Excited state dynamics can be studied by time-delayed pump-probe ionization

T1

IP

S1

S0

Pump

Probe

ISC

A. Sur and P. M. JohnsonJ. CHEM. PHYS. 84 1206-1209 (1986)

Decay of PhenylacetyleneDecay of Phenylacetylene

0 1000 2000 3000 4000 5000

ion

sig

nal

(a.

u.)

delay time (ns)

Fast decay Singlet S1 state, ~50ns lifetime

Slow decay anomalously long lifetime

Pump: S1S0 000

Probe: 193nm

Ionization threshold of long-lived Ionization threshold of long-lived phenylacetylene speciesphenylacetylene species

46200 46400 46600 46800 47000

inte

sity

(a.

u.)

probe laser photon energy (cm-1)

30 μs pump-probe delayLong-lived component being from ionization of lowest triplet state T1

~46500cm-1 T1 ~3.05eV (this work)

3.12eV (Phosphorescence1)

3.15eV (solid phase2)

3.04eV (calculation3)

1, Zhuravleva, et al, Opt. Spectroc. 22, 489 (1967). 2, Swiderek, et al, Chem. Phys. Lett. 285, 246 (1998)3, Serrano-Andres,et al, J. Chem. Phys. 119, 4294 (2003)

Why such long lifetime?Why such long lifetime?

• S1-T1 gap ~1.4eV, triplet states formed initially by ISC are expected to be very highly vibrational excited, and should not have such long lifetimes.

• Lifetime of Cold triplet state can be very long.

• From dissociation of the clusters (Hillenbrand et al JCP 95 2237(1991))?

• If cold triplet, how to make up the ~1.4eV S1-T1

energy gap in collision free conditions?

Excitation scan of Excitation scan of PhenylacetylenePhenylacetylene

-2 0 2 4 6 8 10 12

Inte

nsity

(a.

u.)

GHz

20 ns delay

3 μs delay

Simulation

Not from ClustersBut Monomers!

3

Mechanism DiscussionMechanism Discussion

• Long-lived component is from ionization of the cold triplet T1state.

• Rotational resolved structure ensures it is from the monomers, not clusters.

• Only viable mechanism to relax isolated monomers from S1 energy to cold vibrational levels of T1 is by radiative emission.

• Possible electronic triplet states for radiative emission

S0 (1B2) 0

S1(1B2) 4.45eV

T1 (3A1) 3.05eV

T2 (3B2)/ T3 (3A1)

4.26eV*Radiative Transition

ISC

Mechanism Discussion Mechanism Discussion (cont.)(cont.)

* Swiderek, et al, Chem. Phys. Lett. 285, 246 (1998)

BenzonitrileBenzonitrile

0 2 4 6 8 10 12 14 16

/3

(b)

ion

inte

nsi

ty (

a.u

.)

GHz

0 1000 2000 3000 4000

(a)

ion

inte

nsi

ty (

a.u

.)

delay time (ns)

0 500 1000 1500 2000 2500 3000 3500 4000

ConclusionConclusion

• Pump-probe delayed ionization of phenylacetylene and benzonitrile

• Evidence of triplet-triplet emission under supersonic molecular beam condition

• Analysis of emission should give information about radiationless transitions.

Acknowledgment

Prof. Dr. Philip Johnson

Prof. Dr. Trevor Sears

Chemistry Department

Stony Brook University

Supported by U.S. Department of Energy

Experimental arrangement Experimental arrangement for decay measurementsfor decay measurements

Pump Laser(scanned in time)

ProbeLaser

Pulsed Valve

Valve and probe aresynchronized in time

Sample