Tunable Infrared Laser Desorption/Ionization Time-of-Flight Mass Spectroscopy of Thin Films Timothy Cheng, Michael Duncan Department of Chemistry, University of Georgia, Athens, GA U.S. Air Force Office of Scientific Research International Symposium on Molecular Spectroscopy June 16, 2008

Previous Work in Tunable IR on Thin Films Infrared spectroscopy of thin films Infrared MALDI (Matrix-Assisted Laser Desorption/Ionization) primarily used on thin films Most research focus on maximizing efficiency, minimizing fragmentation and increasing sensitivity 1 Previous research have shown that the amount of signal is wavelength dependant 2 Mechanism for infrared ionization not fully understood 3 Goal to get a better understanding of IR on thin films and hopefully get a better understanding of ionization mechanism 1 Hillenkamp and Co. Int. J Mass. Spectrom. 13 (2002) Awaza and Co. Int. J. Mass Spectrom. 270 (2008) Murray and Co. J. Mass Spectrom. 39 (2004) 1182

Instrument Schematic The sample is prepared by coating a probe tip with the desired thin film by vapor deposition Sample inserted into a 2-stage Wiley and McLaren time-of-flight mass spectrometer A Laservision OPO/OPA system is used to vary the wavelength of light between cm -1 Pumped by Spectra Physics Pro-230 Nd:YAG at 1064 nm 1 wavenumber linewidth 1-10 mJ/pulse

Mass spectrum at 3880 cm -1 Lots of fragmentation of C 60 K+K+

C 60 Mass Spec at 3930 cm -1 Much less fragmentation of C 60

What we know to help figure out ionization happens Direct ionization of C 60 unlikely because the IP of C 60 is ~7.6 eV while the IR has ~0.5 eV at 4500 cm -1 Delayed Extraction of ions increase resolution Impurities: water, alkali metals present on the sample Very sensitive to impurities Blank probe tip (which has impurities) don’t show any peaks Changing the probe tip material (stainless steel, aluminum, teflon, and copper) doesn’t change the spectrum

Possible Mechanisms for Matrix-free Laser desorption/ionization Surface film of H 2 O absorbing IR light, leading to desorption and ionization of sample Probe itself absorbing laser, then promotes desorption/ionization of sample –Thermionic emission of electrons from probe surface –Secondary ionization by electrons to sample Desorption of sample by passing threshold fluence followed by proton transfer Absorption of salt water leading to photoemission of electrons –Electrons accelerated by plates –Secondary ionization by electrons hitting the plume

Scan of C 60 between 2000 and 4500 cm -1

Combination Bands and Overtones of C 60 Previous research have seen many of the possible combination bands 1 The peaks in the region correspond to combination bands seen previously The peaks around 4100 can correspond to the 2 nd overtone or higher combination bands. The small peaks around the 3300 cm -1 region correspond to impurities Dresselhaus and Co. Phys. Rev. B 48 (1993)1375

Scan of C 60 between 2000 and 4500 cm -1

Potassium Channel for the same scan Potassium can be used as a tracker

Sodium Channel for a C 60 Sample

Potassium Channel on CNT Sample

Conclusions and Future Work Tunable IR laser can be used to probe the vibrational frequencies of thin films Can be used to identify purity or contamination of sample Scan the lower wavenumber region, especially the fundamental C-C stretching vibration around the 1100’s and 1400’s cm -1 Continue working on larger molecules and decrease the amount of undesired impurities in the sample Acknowledgements Michael Duncan Prosser Carnegie Funding from the USAF Office of Scientific Research