The character of the long-lived state formed from S 1 of Phenylacetylene Philip M. Johnson and Trevor J. Sears Brookhaven National Laboratory and Stony Brook University Columbus Talk TK01, Tuesday June 18 th, 2013 Support: Department of Energy Basic Energy Sciences 1

Introduction to the photophysics of substituted benzenes Simplified state diagram of anisole T1T1 S 1, 8.0 D0D0 S0S0 S 1 -T 1 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 T1T1 S 1, 8.0 D0D0 S0S0 S 1 -T 1 gap = 1.3 eV (10500 cm -1 ) ISC, IC E(eV ) Ionization Pump Pump-Probe Ionization Spectroscopy Beam-cooled sample. Resonantly pump to S 1 (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. 3

Typical Results From: J. Phys. Chem. 1988, 92, 183, Colson Group S1S1 ISC to T 1 * ISC: to excited S 0 :loss of signal DELAY TIME (ns) Instrument-limited rise microsecs Benzene v 6 ’ =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. 4 ISC to T 1 * S1S1 ISC: to excited S 0 :loss of signal

Model microsecs Benzene v 6 ’ =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 T 1 * S1S1 ISC: to excited S 0 :loss of signal Kommandeur, Adv. Chem. Phys. 70, 133 (1988).

Another view 6 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 S1S1 Photoelectron energy/eV By tuning to higher S 1 states in the excitation step, one can watch the T 1 decay rate increase. Benzene delayed PES

Phenylacetylene 7 S 1 -S 0 spectrum has a band origin at 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 S 1 state due to fluorescence Slow decay (>100  s) of ??? From: J. Phys. Chem. A 2008, 112, 1195 DELAY TIME (ns) Phenylacetylene photophysics Pump-Probe delay ionization experiment. S 1 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 S 1, and it is. Spectra here are at 30 ns (top), 3μs middle and calculated (bottom). 8

Photoelectron Spectra Time-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 S1S1 Photoelectron energy/eV Triplet S1S Photoelectron energy/eV 9

S 1 multi-exponential decay Examining the S 1 photoelectron spectrum at longer delays, one sees residual S 1 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? 10

Summary of the observations By comparison with related molecules, the state energies in PA would argue for a Triplet lifetime in the s of ns. 11 Following excitation of the S 1 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 (C 6 H 5 CN) shows similar behaviour. Production of the long-lived state occurs synchronously with S 1, and it has a distinct PES.

Mass spectrometry experiments Possibilities: PA has some alternative route to a low internal energy T n level. Or: Isomerization? Compare ion fragmentation patterns in mass spectra of S 1 and the long-lived state. Any isomer will have a different ion fragmentation pattern. Reflectron TOF used has mass resolution of >

Mass spectra The 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 S 1 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? 14 Probe laser delay (ns) Signal (arb) ?

Acknowledgements Phil Johnson, Gary Lopez, Jason Hofstein, Chih-Hsuan Chang, Haifeng Xu, Greg Hall BNL, Department of Energy, for financial support. 15

16 StateSymm.Energy-auΔ eVΔ cm -1 λ nm Δ cm -1 Zero pt. corr.-auΔ eVΔ cm -1 λ nm Expt. eV From S 0 S 1 -T n From S 0 with ZPC S0S0 A1A S1S1 B2B a T1T1 A' b T2T2 A1A b T3T3 B2B T4T4 B2B Calculated state energies Gaussian-09 TD-DFT aug-cc-pVTZ basis T 1 state has significant quinoidal structure and trans-configuration about the C-C triple bond

Related Work 17 “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.