Total energy distribution measurements of field emitted electrons detect molecular infrared spectra of adsorbed molecules. Tunneling electron gives up energy to excite various modes of adsorbed molecule. These electrons, when energy-analyzed, show up on the collector and exhibit the spectra of various modes excited by tunneling electrons

Swanson, L. W.

English

NASA Technical Reports Server

June 1969	 Brief 69-10172 
.
	
NASA TECH BRIEF C' 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Detection of Molecular Infrared Spectra 
is
The effect of adsorbed molecules on the total energy 
distribution (TED) of field emitted electrons has been 
investigated. The expectation was that tunneling 
electrons would interact electronically with bonding 
electrons of the adsorbate so as to perturb the TED 
in such a way as to give information regarding the 
binding forces between the adsorbed atom and the 
surface. These theoretical expectations have been 
realized to some degree in practice and thus have 
provided insight as to bonding modes of certain 
adsorbate-substrate systems. On the basis of work by 
others using internal solid state field emission devices, 
another possible aspect of this application was de-
duced: namely, the detection of molecular infrared 
spectra of adsorbed molecules from TED measure-
ments of field emitted electrons. Basically, the idea is 
as follows: 
A tunneling electron can give up a quantum of 
energy h p to excite various vibrational, rotational, 
and possibly electron modes of the adsorbed molecule. 
These electrons, when energy-analyzed by a retarding 
potential analyzer, will show up on the collector at a 
tip-to-collector bias potential Vb = -O - h v, where O 
is the collector work function. The derivative of the 
collector current dl c/dV b versus Vb will show a peak 
at Vb = °c v and thus exhibit the spectra of the 
various modes excited by the tunneling electrons. 
In recent experiments these results were confirmed 
with various organic compounds on a tungsten surface. 
These findings provide a new and significant tool for 
obtaining spectra of adsorbed molecules. The range 
of equivalent wavelength covered has been shown to 
extend over the near and far infrared spectrum (1.0
to 30 microns). The sensitivity is such that a monolayer 
of material is sufficient, and the resolution is theoreti-
cally limited by the thermal electrons above the bar-
rier, which can be reduced to less than 10 mV by 
cooling the emitter to 4°K. Molecules can be examined 
after heating the tip to various temperatures in order 
to study the decomposition modes for catalytic-type 
investigations. 
The ease of sample introduction (evaporation and 
condensation onto the tip) and the large spectral 
range of this device provides decided advantages over 
the internal field emission method described by J. 
Lambe and R. Jaklevic, Phys. Rev. 165, 821 (1968). 
This tool has significant interest for the surface 
chemist in that it provides a way to obtain surface 
infrared spectra of adsorbed molecules and decompo-
sition products. Because of this spectral range, it has 
prospects as a general analytical spectrometer to the 
organic and inorganic spectroscopist. 
Note: 
Documentation is available from: 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP69- 10172 
Patent status: 
No patent action is contemplated by NASA. 
Source: Lynwood W. Swanson of 
Field Emission Corporation 
under contract to 
Headquarters, NASA 
(HQN- 10377) 
Category 02 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
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Detection of molecular infrared spectra

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