Transcript OHIO STATE.ppt

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 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
2
3
Hillenkamp and Co. Int. J Mass. Spectrom. 13 (2002) 975
Awaza and Co. Int. J. Mass Spectrom. 270 (2008) 134
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
Relative Intensity
Mass spectrum at 3880 cm-1
Lots of
fragmentation
of C60
K+
0
200
400
600
Mass (amu)
800
1000
C60 Mass Spec at 3930 cm-1
+
Relative Intensity
K
C60
+
Much less
fragmentation
of C60
0
200
400
600
Mass (amu)
800
1000
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
+
Relative Intensity
C60
2400
2600
2800
3000
3200
3400
cm
3600
-1
3800
4000
4200
4400
Scan of C60 between 2000 and 4500 cm-1
2886
Relative Intensity
2988
+
C60
3936
4128
2000
2500
3000
3500
cm
-1
4000
4500
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
Scan of C60 between 2000 and 4500 cm-1
2886 Hg x Gu
2988 Hg x Hu
+
C60
3936
4128
2000
2500
3000
3500
cm
-1
4000
4500
Potassium Channel for the same scan
+
K
Relative Intensity
Potassium can be used
as a tracker
2000
2500
3000
3500
cm
-1
4000
4500
Sodium Channel for a C60 Sample
Relative Intensity
2844
Na
+
2986
2500
3000
3500
cm
-1
4000
4500
Potassium Channel on CNT Sample
+
K
4056
CNT, ~5mJ/pulse
pumped overnight
3349
3000
3600
cm
4200
-1
4397
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