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

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?

bright state

CO J=1

dark states

CO J=0 +

droplet phonon

Assume density of states, , and coupling matrix elements. v, are constant. Bixon and Jortner showed eigenvalues, En, given by solution to:

€

Eb − En + πρv 2( )cot πρ En − Eb +α( )( ) = 0

If we assume that we have all values of equally likely due to an inhomogeneous droplet size distribution we can calculate the inhomogeneous lineshape and find:

€

I E( ) =ρv 2

E − Eb( )2

+ πρv 2( )

2

In the “large molecule limit”, v >>1, this is the well known homogeneous lineshape, but it applies even when v << 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 Detection5 µm apertureT=16–35 K

He 100 atm

coldexpansion

clusterformation

dopantpick-up

IR photonabsorption

+He evaporation(103 He/photon)

lase

r

MW multiplephoton absorption

+He evaporation(0.1 He/photon)

dete

ctio

n(b

olom

eter

)

• 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

dopant inside (except alkali)

~ 6500 cm-1

(2 quanta of CH stretch)

9-70 GHz(rotationaltransitions)

NEP:35-45

fW/Hz

flux:1020 atoms/s

~10-4 torr

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

Multiple Zero phonon lines

From Alwkin Slenczka

Poorly Understood.

In at some cases, due tolong lived isomers ofhelium solvation.

May include low frequencylocalized phonons.

Shapes of phonon wings alsonot generally understood!

Lindinger & Vilesov

CPL 406, 386 (2005)

JPCA 105, 6369 (2001)

24200 24400 24600 24800 25000 25200 25400

cm-1

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 NanodropletsTD-DFT (B3LYP/SVP)

S1: 27217 cm-1 f = 0.0003

25000 25200 25400 25600 25800 26000 26200 26400 26600

cm-1

26950 27000 27050 27100 27150 27200

*DT-DFT and CRDS by Tan and Salama JCP 123 014312 (2005)

S2 region Benzo[ghi]perylene: HENDI

CRDS spectrumDT-DFT

S2: 27401 cm-1 f = 0.27

25800 26100 26400 26700 27000 27300 27600

Pick-upPressure

cm-1

Biphenylene in Helium DropletS0 -> S1 (1B3g) (forbidden)(b2u mode 35 induced)T0 ~24550 cm-1

Strong progression in 10

~ -20 cm-1 shift in He.

Jet spectrum (2 color REMPI): Zimmermann, AIP conference proc. 388, 399 (1997)

TD-DFT (B3LYP)6-311+G(d,p) basisS1:26501 cm-1

25000 25200 25400 25600 25800

cm-1

Acenaphthylene S0-S1 (1B2) Spectrum in HeliumNon-fluorescentIn n-pentane: 21,460 cm-1

f = 0.0042TD-DFT: 24,854 cm-1

Long axis polarized

25200 25500 25800 26100 26400 26700 27000 27300

cm-1

Earlier Jet Spectrum: Chen & Dantus, JCP 82, 4771 (1985).

Fluoranthene S0 -> S1(B2) in Helium Droplet

25500 25800 26100 26400 26700 27000 27300 27600

cm-1

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!

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

0 5 10 15

cm-1

Single Vibronic Feature of Coronene in Helium.

Phonon wing too week to beobserved without saturation

-2 0 2 4 6 8 10 12 14

cm-1

Benzo[ghi]perylene in Helium Nanodroplets

0 5 10 15 20 25 30 35

cm-1

Blow up of one vibronic feature of Biphenylene

-5 0 5 10 15 20 25 30cm-1

Blow up of 25,264.3 cm-1

vibronic peak of Acennaphthylene

0 5 10 15 20

cm-1

Fluoranthene: Blow up of single vibronic band

25,595.78 cm-1 (-42 cm-1 shift)

Single ZPL!

0 10 20 30 40

cm-1

0 20 40 60 80 100

cm-1

0 20 40 60 80 100 120 140 160 180 200 cm-1

0-0

Typical line shape Suspected S2 region

Benzo(k)fluoranthene

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

-12 -10 -8 -6 -4 -2 0 2

cm-1

Van der Waals Complexes of Benzo[ghi]perylene

-40 -30 -20 -10 0 10

Ar

-40 -30 -20 -10 0 10cm-1

O2

Van der Waals Complexes of Biphenylene

60 50 40 30 20 10 0 -10

cm-180 60 40 20 0

cm-1

O2Ar

= -65 cm-1 = -44 cm-1

-60 -50 -40 -30 -20 -10 0

cm-1

Multiple sites for 1st pickup ?38-41 cm-12nd pickup

62.3 cm-1

Argon complex with Fluoranthene

(O2 complexes give similar results)

Ar Complex with Benzo(k)fluoranthene

-150 -100 -50 0 50

cm-1

-100 -50 0 50

cm-1

O2 Complex with Benzo(k)fluoranthene

-150 -100 -50 0 50

cm-1

-100 -50 0 50

cm-1