www.le.ac.uk

Hot molecules in helium nanodroplets:a new route to optical spectra

Benjamin Shepperson, Adrian Boatwright, Cheng Feng, Daniel Spence, Shengfu Yang, Andrew M. Ellis

Department of ChemistryUniversity of Leicester

Talk R10267th International Symposium on Molecular Spectroscopy

Columbus, OhioJune 22, 2012

Helium droplet properties

•A helium droplet is a large cluster of helium atoms (103 – 106)

•Remains a liquid even at temperatures close to absolute zero

•Exhibits superfluid properties(Helium II) - An immeasurable viscosity - Ultra high thermal conductivity - Ultra-high cooling rate

• Evaporation of each helium atom from the surface removes 5 cm-1 of energy from the droplet until a steady state temperature of 0.37 K is reached.

Low temperature nozzle

Skimmer Pick-up cells Electron ionization in mass spectrometer

P 10 – 50 bar

6 T 25 K

Formation and doping of helium droplets

Depletion spectroscopy

Low temperature nozzle

Skimmer Pick-up cell Ionization in aquadrupole mass

spectrometer

UV or Infrared radiation

0 5 10 15

Time/ms

3100 3200 3300 3400 3500 3600 3700 3800 3900

0.00

0.05

0.10

0.15

0.20 %

Dep

letio

n

Wavenumber/cm-1

IR depletion spectroscopy

Vibrational depletion spectrum of water clusters in the O-H stretching region obtained from the helium dimer channel

T = 15 KP0 = 20 bar

<N> = 5000

(H2O)n

(H2O)2

(H2O)2

(H2O)3

(H2O)4

(H2O)5

(H2O)6 [caged]

Electronic depletion spectroscopy

Aim of experiment • To record the electronic spectrum of toluene using depletion spectroscopy

• Investigating the S1- S0 electronic transition

hv

I. II. III.

Continuous source

Closed-cyclecryostat

Source chamber

Cryo-trap chamberPickup

cell

Reflectron time-of-flightmass spectrometer

Experimental setup

Quadrupolemass spectrometer

TOF spectrum of toluene

T = 11KP0 = 20 bar

<N> = 11000

Depletion signal in different mass channels

Background C7H7 + channel C5H5

+ channel

0 10 20 30

Time/ms

-5 0 5 10 15 20 25 30 35

Time/ms

-5 0 5 10 15 20 25 30 35

Time/ms

Selected vibrational features (S1-S0)

Enhancement

Depletion

%

Intersystem crossing•Depletion spectroscopy is less efficient for detecting electronic transitions as electronic excitation relaxes much more slowly.

S1

S0

T1

Intersystem crossing

Internalconversion

Non-radiative decay

Phosphorescence

Fluorescence

Absorbance

ISC 0.64

LIF 0.30

Fluorescence: ~ ns

Intersystem crossing: ~10 s

T1 lifetime in helium droplets ~ ms

S1 T1 = 8,400 cm-1 vib. energy

Excitation of toluene to the S1 state followed by ISC creates metastable toluene in the T1

state, which is sufficiently long-lived to be ionized in the mass spectrometer. This

changes the relative ionization cross sections of some ionization channels.

We essentially measure the relative change of the branching ratios Δk1 / kp or Δk2/ kp ,

depending on the fragment mass channel monitored.

Suggested mechanism

C7H8 hv

C7H8* (C7H8

+)*+ 2e-+ e-

C5H5+

C7H7+

k1

k2

C7H8+C7H8

*

S0 S1 T1

ISC (Parent ion)

kp

1.A new method for observing spectroscopic transitions of molecules in helium nanodroplets has been found.

2. Spectral absorption is registered by changes in the production rate of specific fragment ions ejected from the droplets, rather than a change in helium droplet size. Light absorption must deliver the molecule into a sufficiently long-lived state to ensure that the molecule stays in this state long enough to reach the mass spectrometer (ms timescale) – otherwise the technique will not work.

3.Depending on the particular ion channel chosen for data collection, optical spectra can appear as either ion- depletion or enhancement signals.

4.As opposed to conventional depletion spectroscopy, larger droplet sizes are accessible with the limit being the ability of the molecular ions to form and

escape from the helium droplets following electron impact ionization

Conclusions

Acknowledgements

Adrian Boatwright

Cheng Feng

Daniel Spence

Shengfu Yang

Andrew M. Ellis

Workshop technicians

EPSRC & Royal Society for funding