Rotational Spectroscopy of OCS in Superfluid Helium Nanodroplets Paul Raston, Rudolf Lehnig, and Wolfgang Jäger Department of Chemistry, University of Alberta, Edmonton, AB Canada 63 rd International Symposium on Molecular Spectroscopy

MW Spectroscopy of OCS and IR-MW DR Spectroscopy of HCCCN in Superfluid Helium Nanodroplets Paul Raston, Rudolf Lehnig, and Wolfgang Jäger Department of Chemistry, University of Alberta, Edmonton, AB Canada 63 rd International Symposium on Molecular Spectroscopy

Outline J. P. Toennies, A. F. Vilesov, Angew. Chem. Int. Ed. 43, 2622 (2004) MW depletion spectrum of OCS Comparison with MW-IR DR depletion spectrum of OCS IR-MW DR depletion spectrum of HCCCN

diffusion pump 8000 L / s turbo pump 700 L / s turbo pump 700 L / s turbo pump 700 L / s turbo pump 340 L / s Cryostat, 20 K skimmer 500 μm doping cell microwave resonator quadrupole mass-spec nozzle 5 μm Helium Droplet Machine ℓHe-cooled bolometer, 1.4 K

Microwave Resonator Cavity mirrors Helium droplet beam Use of TWT microwave amplifier; output power: 10 W reflectivity R ~ 0.98; total stored power: ~ 50xP in

Previous MW Studies

~6.5 He atoms follow rotation of OCS reducing B to 1/3 of its isolated value OCS Depletion Spectrum *S. Grebenev et al., J. Chem. Phys. 112, 4485 (2000) IR predictions*

J=2-3 Peak Subpeaks spaced 162 MHz apart…

J=2-3 Peak Subpeaks spaced 162 MHz apart are artefacts: Arise from frequency dependant voltage losses in a 2’ cable

True J=2-3 Peak Remove offending cable

Previous MW-IR Double Resonance Study S. Grebenev et al., J. Chem. Phys. 113, 9060 (2000) Laser parked at R 1 (1) peak maximum & microwaves chopped and scanned through 8-18 GHz

Comparison with this Study MW-IR Double Resonance Single Resonance

Comparison with this Study MW-IR DR seems to be probing rotational spectrum within v=1 manifold Rotational lifetime << vibrational lifetime MW-IR Double Resonance Single Resonance

Aculight OPO

i = cm -1 s = cm -1 Aculight OPO

R branch of 1 band; P in =100 mW Test Case – C-H Stretch of HCCCN

C. Callegari et al., J. Chem. Phys. 113, 4636 (2000) Comparison with Previous Study

C. Callegari et al., J. Chem. Phys. 113, 4636 (2000) Previously Reported IR-MW DR Spectrum 3 Microwaves chopped & fixed at J=2-3 peak (9.08 GHz); Laser scanned through R branch IR

What We Get 3 Microwaves chopped & fixed at J=2-3 peak (9.08 GHz); Laser scanned through R branch We observe a blue shift of R branch peaks with MWs on IR

C. Callegari et al., J. Chem. Phys. 113, 4636 (2000) 3 IRComparison Why so different?

In our experiment, droplets are exposed to IR radiation during and after traversing the MW cavity cavity MW IR Different Experiments

In our experiment, droplets are exposed to IR radiation during and after traversing the MW cavity Differences in spectra due to MW pumping of higher energy states that are probed after droplets exit cavity? cavity waveguide MW IR Different Experiments Droplet beam MW

Summary Microwave spectrum of OCS in Helium Droplets measured MW-IR DR seems to be probing rotational spectrum within v=1 manifold IR-MW DR reveals affect of MWs after droplets transverse MW cavity. Work in progress…

Acknowledgements Jäger group Funding: Natural Science and Engineering Research Council of Canada Canada Foundation for Innovation Alberta Science and Research Investments Program

S. Grebenev et al., J. Chem. Phys. 113, 9060 (2000) Previous IR-MW DR study of OCS Positive signal = decrease in depletion

S. Grebenev et al., J. Chem. Phys. 113, 9060 (2000) Experiment Similar to Ours waveguide IR MW IR-MW DR spectrum of OCS has derivative lineshape, similar to that observed for HCCCN in this study

K. K. Lehman, Mol. Phys. 97, 645 (1999)

J=3←2 transition

J=2-3 Peak Subpeaks spaced ~160 MHz apart Real or artifact…

MW Spectrum of HCCCN in Helium Droplets J = 3-4 J = 2-3

MW Spectrum of HCCCN in Helium Droplets

Adjusting for power lost in everything up to the detector, including a 2’ cable after the receiving antenna Microwave Resonator Constant power maintained in resonator by monitoring received power then adjusting the input power accordingly detector

Difference in between adjacent standing waves Dependant Voltage Losses Dependant Voltage Losses Voltage loss greatest when standing wave condition is met along cable L=2’

Subpeaks are Artifacts! Arise from frequency dependant voltage losses in the 2’ cable

“True” J=2-3 Peak Remove offending cable: placed detector in vacuum chamber