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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
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