Tunable Infrared Laser Desorption/Ionization Time-of-Flight Mass Spectroscopy
of Thin Films
Timothy Cheng, Michael DuncanDepartment of Chemistry, University of Georgia, Athens, GA 30602-2556
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
sensitivity1
• Previous research have shown that the amount of signal is wavelength dependant2
• Mechanism for infrared ionization not fully understood3
• 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) 9752 Awaza and Co. Int. J. Mass Spectrom. 270 (2008) 1343 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 2000-4500 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
0 200 400 600 800 1000
Mass (amu)
Re
lativ
e In
ten
sity
Lots offragmentationof C60
K+
0 200 400 600 800 1000
C60
+
Mass (amu)
K+R
ela
tive
In
ten
sity
C60 Mass Spec at 3930 cm-1
Much lessfragmentationof C60
What we know to help figure out ionization happens
• Direct ionization of C60 unlikely because the IP of C60 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 H2O 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 C60 between 2000 and 4500 cm-1
2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400
cm-1
C+
60
Re
lative
In
ten
sity
Scan of C60 between 2000 and 4500 cm-1
2000 2500 3000 3500 4000 4500
41283936
cm-1
2988
2886
C+
60
Re
lative
In
ten
sity
Combination Bands and Overtones of C60
• Previous research have seen many of the possible combination bands1
• The peaks in the 2800-3000 region correspond to combination bands seen previously
• The peaks around 4100 can correspond to the 2nd 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
2000 2500 3000 3500 4000 4500
Hg x G
u
Hg x H
u
41283936
cm-1
2988
2886
C+
60
Scan of C60 between 2000 and 4500 cm-1
Potassium Channel for the same scan
Potassium can be used as a tracker
2000 2500 3000 3500 4000 4500
cm-1
K+R
ela
tive
Inte
nsi
ty
2500 3000 3500 4000 4500
cm-1
Re
lative
In
ten
sity
Na+2844
2986
Sodium Channel for a C60 Sample
3000 3600 4200
cm-1
K+
4056 4397
3349
CNT, ~5mJ/pulsepumped overnight
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
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