Electronic spectra of Polyaromatic hydrocarbons in helium Ozgur Birer, Paolo Moreschini, Giacinto Scoles & Kevin Lehmann

This talk does not fit well with the theme SPECTRAL SIGNATURES OF MOLECULAR DYNAMICS I had hoped to get further than we have with the analysis. Let me start with a result I recently found that I believe does fit.

Do Lorentzian Lines Imply Lifetime Broadening? Assume density of states, r, and coupling matrix elements. v, are constant. Bixon and Jortner showed eigenvalues, En, given by solution to: dark states bright state a If we assume that we have all values of a equally likely due to an inhomogeneous droplet size distribution we can calculate the inhomogeneous lineshape and find: CO J=0 + droplet phonon CO J=1 In the “large molecule limit”, rv >>1, this is the well known homogeneous lineshape, but it applies even when rv << 1 where one eigenstate is almost always dominated by the bright state and there is no decay into the bath. This Lorentzian lineshape is essentially purely inhomogeneous due to variation in the shift of the bright state due to long range perturbations by phonons! I believe this situation could be common in spectroscopy of nanosystems

Droplet Production and Detection ~ 6500 cm-1 (2 quanta of CH stretch) 9-70 GHz (rotational transitions) NEP: 35-45 fW/Hz 5 µm aperture T=16–35 K flux: 1020 atoms/s dopant inside (except alkali) ~10-4 torr He 100 atm laser cold expansion cluster formation dopant pick-up IR photon absorption + He evaporation (103 He/photon) MW multiple photon absorption + He evaporation (0.1 He/photon) (bolometer) detection Droplet sizes: 1000-10000 atoms (45-95 Å diameter) Droplet temperature: 0.4 K (evaporative cooling) Sensitivity (S/N=1@1Hz): 3107 He atoms

from Toennies and Vilesov, Angew. Chem. 43, 2622 (2004)

Multiple Zero phonon lines Poorly Understood. In at some cases, due to long lived isomers of helium solvation. May include low frequency localized phonons. Shapes of phonon wings also not generally understood! From Alwkin Slenczka

CPL 406, 386 (2005) Lindinger & Vilesov JPCA 105, 6369 (2001)

Coronene S0 -> S1(B2u) in Helium droplets Like Benzene, this is a forbidden transition made allowed by e2g modes. Many weaker lines were previously assigned to non-e2g vibrational states, these are stronger in our spectrum. Triplets? Previous jet spectrum: Bermudez & Chan, JPC 90, 5029 (1986)

Benzo[ghi]perylene in Helium Nanodroplets TD-DFT (B3LYP/SVP) S1: 27217 cm-1 f = 0.0003

S2 region Benzo[ghi]perylene: HENDI DT-DFT S2: 27401 cm-1 f = 0.27 CRDS spectrum *DT-DFT and CRDS by Tan and Salama JCP 123 014312 (2005)

Biphenylene in Helium Droplet S0 -> S1 (1B3g) (forbidden) (b2u mode n35 induced) T0 ~24550 cm-1 Strong progression in n10 ~ -20 cm-1 shift in He. TD-DFT (B3LYP) 6-311+G(d,p) basis S1:26501 cm-1 Jet spectrum (2 color REMPI): Zimmermann, AIP conference proc. 388, 399 (1997)

Acenaphthylene S0-S1 (1B2) Spectrum in Helium Non-fluorescent In n-pentane: 21,460 cm-1 f = 0.0042 TD-DFT: 24,854 cm-1 Long axis polarized

Fluoranthene S0 -> S1(B2) in Helium Droplet Earlier Jet Spectrum: Chen & Dantus, JCP 82, 4771 (1985).

Benzo(k)fluoranthene in Helium Nanodroplets First vibronically resolved spectrum TD-DFT: (B3LYP) S1(1A1): 25891 cm-1 f = 0.22 S2(1B2): 28131 cm-1 f = 0.0007 Note: Different shapes!

In these cases, rotational structure below our resolution Let’s look at blow ups of single vibronic bands to reveal spectroscopic structure induced by helium solvation In these cases, rotational structure below our resolution

Single Vibronic Feature of Coronene in Helium. Phonon wing too week to be observed without saturation

Benzo[ghi]perylene in Helium Nanodroplets

Blow up of one vibronic feature of Biphenylene

Blow up of 25,264.3 cm-1 vibronic peak of Acennaphthylene

Fluoranthene: Blow up of single vibronic band 25,595.78 cm-1 (-42 cm-1 shift) Single ZPL!

Benzo(k)fluoranthene 0-0 Typical line shape Suspected S2 region

Conclusions... Most PAH’s have complex helium induced structure visible without saturation Trends of structure nonsystematic In order to attack this theoretically, we need to know the change in He-molecule potentials upon electronic excitation With potentials, helium time dependent density theory provides attractive approach to calculation of real time correlation functions (another talk!)

Complexes of PAH’s with Ar and O2

Ar Complex with Coronene in Helium

Van der Waals Complexes of Benzo[ghi]perylene Ar

Van der Waals Complexes of Biphenylene Ar O2 Dn = -44 cm-1 Dn = -65 cm-1

Argon complex with Fluoranthene (O2 complexes give similar results) Multiple sites for 1st pickup ? 38-41 cm-1 2nd pickup 62.3 cm-1

Ar Complex with Benzo(k)fluoranthene

O2 Complex with Benzo(k)fluoranthene