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