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 Chemistry University of Leicester Talk R th International Symposium on Molecular Spectroscopy Columbus, Ohio June 22, 2012

Helium droplet properties A helium droplet is a large cluster of helium atoms (10 3 – 10 6 ) 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 a quadrupole mass spectrometer UV or Infrared radiation

IR depletion spectroscopy Vibrational depletion spectrum of water clusters in the O-H stretching region obtained from the helium dimer channel T = 15 K P 0 = 20 bar = 5000 (H 2 O) n (H 2 O) 2 (H 2 O) 3 (H 2 O) 4 (H 2 O) 5 (H 2 O) 6 [caged]

Electronic depletion spectroscopy Aim of experiment To record the electronic spectrum of toluene using depletion spectroscopy Investigating the S 1 - S 0 electronic transition hv I.II.III.

Continuous source Closed-cycle cryostat Source chamber Cryo-trap chamber Pickup cell Reflectron time-of-flight mass spectrometer Experimental setup Quadrupole mass spectrometer

TOF spectrum of toluene T = 11K P 0 = 20 bar = 11000

Depletion signal in different mass channels BackgroundC 7 H 7 + channelC 5 H 5 + channel

Selected vibrational features (S 1 -S 0 ) Enhancement Depletion %

Intersystem crossing Depletion spectroscopy is less efficient for detecting electronic transitions as electronic excitation relaxes much more slowly. S1S1 S0S0 T1T1 Intersystem crossing Internal conversion Non-radiative decay Phosphorescence Fluorescence Absorbance  ISC   0.64  LIF   0.30  Fluorescence: ~ ns  Intersystem crossing: ~10  s  T 1 lifetime in helium droplets ~ ms  S 1  T 1 = 8,400 cm -1 vib. energy

Excitation of toluene to the S 1 state followed by ISC creates metastable toluene in the T 1 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 Δk 1 / k p or Δk 2 / k p, depending on the fragment mass channel monitored. Suggested mechanism C7H8C7H8 hvhv C7H8*C7H8* (C 7 H 8 + )*+ 2e - + e - C5H5+C5H5+ C7H7+C7H7+ k1k1 k2k2 C7H8+C7H8+ C7H8*C7H8* S0S0 S1S1 T1T1 ISC (Parent ion) kpkp

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