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

2. 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.

3. 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

4. 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: atoms (45-95 Å diameter)
Droplet temperature: 0.4 K (evaporative cooling)
Sensitivity 3107 He atoms

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

6. 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

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

8. 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)

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

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

11. 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)

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

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

14. 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 =
Note: Different shapes!

15. 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

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

17. Benzo[ghi]perylene in Helium Nanodroplets

18. Blow up of one vibronic feature of Biphenylene

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

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

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

22. 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!)

23. Complexes of PAH’s with Ar and O2

24. Ar Complex with Coronene in Helium

25. Van der Waals Complexes of Benzo[ghi]peryleneAr

26. Van der Waals Complexes of BiphenyleneAr O2
Dn = -44 cm-1
Dn = -65 cm-1

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

28. Ar Complex with Benzo(k)fluoranthene

29. O2 Complex with Benzo(k)fluoranthene