The photoionization cross section of atomic nitrogen was measured between the ionization limit and 432 A. The experimental values are well fitted by those from a calculation of HENRY due to the dipole velocity approximation. A Rydberg series converging to the 5S-state of the ion is clearly identified from the ionization measurements and is shown to ionize

Comes, F. J.

Elzer, A.

English

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:NASA TECHNICAL MEMORANDUM NASA TM-77107
A
e.
J	 ^
PHOTOIONIZATION RESEARCH ON ATOMIC
	
RADIATION,	 III.
THE IONIZATIOW CROSS SECTION OF
ATOMIC NITROGEN
F. J. COMES and A. ELZER
Translation of "Photoionizationsuntersuchungen an Atomstramlen. III. Der
Ionisterungsquerschnitt des Atomaren Stickstuffs," Zeitschrift fuer
Naturforschung, Ausgabe A, Vol. 23a, No. 1, January 1968, P.P . 133-136•
(NASA-TM-77107) PBOTOIUNIZATION RESEARCH UN
	 N83-30172
ATOMIC RADIATION. 3: THE IONIZATION CuOSS
SECTION OF ATOMIC NITROGEN (National
Aeronautics and Space Administration) JU p	 Unclas
3C A02/MF A01
	 CS(:L 10H G3/72 28326
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
	
WASHINGTON, DC 20546	 AUGUST 1982
Photoionization Research on Atomic
III. The Ionization Cross Section of Atomic Nitrogen
P. J. Comes, and A. Elzer
Physical Chemistry Institute, Bonn University
Recently, calculations of the photoionization cross section of atomic
oxygen and atomic nitrogen have been published in numerous publications.
Oxygen and nitrogen are unstable in atomic form. Absorption research
on these systems has, for that reason, been primarily conducted on the
"active" form, i.e. a particle mixture, that imparts an electrical charge.
Primarily, these mixtures consist of the particular atoms and not dissociated
molecules, that are partially excited. A measurement of the inionization
cross sections must however be purposefully conducted with pure components,
For this reason, particle beams, that contain the particular atoms in a
defined Atomic state, are required. Such experiments were first conducted
with hydrogen and oxygen atom beams. 1,2. This study will report on the
photoionization of the nitrogen atom in the basic state.
SETTING UP THE APPARATUS
The structural order to measure the photoionization on atomic beams has
already been described elsewhere. l  The measuring equipment con fisted
of a monochromator in Seya-formation (series) and 3 time-of- flight mass
spectrometer. Atomic and photon beams cross vertically in the ioniza-
tion chamber of the mass spectrometer. The formed ions are shot into
the ion tube vertically to both. In contrast to early measurement,, of
hydrogen and oxygen atomic beams, the nitrogen atoms were generated in
1.
(4,
ORIGINAL PAGE 19
`	 OF POOR QUALITY
a condensed discharge. Only weak nitrogen atom concentrations result
from an unconcentrated discharge.
The discharge tube had a length of about 130 cm and was structed in a
U-form. At a pressure of 0.1 to 0.2 Torr, a discharge condensed through
0.2 N r burned in nitrogen between two aluminum electrodes; 10% argon
were added to the nitrogen. Argon was used to measure the temperature
at the portal. (aperture) of the atomic beam generator, where the gases
shot out. 2. Phe noble gas did not measureably affect the discharge.
The discharge sparked twice/second at a tension of 3.5 kV. Nitrogen
atom production with regards to stability and intensity was optimal at
this frequency. Because of the long life span of the ntrogen atom,
the low discharge frequency had little effect on the established measure-
ment speed. The N ion current indicator was stable. The attained
atom concentration was between 25 and 30%. Atom concentrations of
up to 50% could also be attained. To assure stability, the measurements
were conducted with an atom concentration of 25%.
MEASUREMENT RESULTS AND DISCUSSION
The measurement of the N ionization cross section was accomplished
in two stages: 1. Measurement of N ionization as a function of the
beam in wavelength in random units, and 2. Determination of the cross
	
f
K	 section of the N atom for a fixed wavelength A0 in absolute units. There-
fore, Part 1 of the measurement consisted of the N+ production according
to equation 1:
K	 \(4S)+hv->N++e-	 (1)
as a function to determine N+
 production as function of the wavelength
and tied to the photoelectronic yield of a gold cathode. l ' Therewith,
all processes that lead to Nt
 formation through other procedures are to
be rejected. On the one hand, photoionization causes metastable excited
2
(e)
(4)
X CAl
,.
N atoms to move towards N+ formation before the ionization boundary,
and on the other, this ion is formed through dissociative ionization
of N2 . Besides the - 4S basic state, the two excited states 2D and 2S
belong to the s 2 p 3 electron configuration of the N atom. Both states
are metastable *
 Since a transition to the ionic basic state is optically
permissible, both of the ionization boundaries lie above 852 A (1020 and
11.30 A respectively). The contribution of the metastable atoms to N+
ionization can therefore be measured directly before the ionization
boundary (limit) of the 4S atom, by measuring N + formation. Figure 1
shows the N+ ion current for the wavelengths 830 to 923 A. At 852 A,
N+ formation increases rapidly. The proportion of metastable N atoms in the
measured N+ ion current is maximally 0.3%. The formation of nitrogen
ions through dissociative ionization according to equation (2), starts
at a threshhold energy of 24.4 eV (508 A):
Figure I . N+ f.,n current (A) and Photoelectron
current (B) as a function of wavelength
ORIGINAL: PAGE ig
OF POOR QUALITY
3 i{
(2)
Zero-lines (A) and (B)
ORIGINAL PACE 6S
OF POOR QUALITY
An earlier calculation reported the value 511 A. The contribution of
ion formation according to (2) was measureable, and was considered in
the calculation of the effective cross section.
For the second portion of the measurement, the mass spectrometer was
for the time bring replaced by an ionization chamber and the N+ formation
for a fixed wavelength A 0=554 A was determined. The effective cross
section 6(N2 ) for the process
it 	 Ns +e'	 (3)	 (3)
can be recovered from the measured values for the N 2
 pressure, ion
current, and photocell current. Measurements were conducted for 10
different N 2
 pressures. According to (2), the wavelength 554 A did not
suffice for dissociative ionization, and lies in a range of the N2
ionization curve, where little or no structure is to be expected. 3,4.
Calculation: of the N 2
 ionization cross section (N) proceeds according
to equation (4) J _	 (1+) A (N,) . 1 — aQ(1) — o(^?) Ji\;) ° A (N+)	 a	 (4)
(4)
To facilitate this for a particular atom concentration , the ion
current relationship
	 J(N-)/J(N2) and the relationship of the
secondary electron yield A(N 2)/A(N ) must be determined with the mass
spectrometer. After consideration of the probe gas temperature in the
atomic beam through means of an argon measurement, the atom concentration
in the N2
 current with and without charge was determined. The relation-
ship of the secondary electronic yields results from the median impulse
height of the Poisson distribution resultant to both ion types at the
exit portal of the multiplier. To finish determining the atom concentration
it's still necessary to determine the proportion of metastable molecules
in the beam. Fig. 2 shows the N+
 current in the range between 756 and 851 A.
2
Small N2c urrents were measured before the ionization limit at 796 A; their
proportion, compared with the ionization below the limit wavelength, was
4
z ^	 Y
it
ORIGINAL PACE 19
maximally 1.5% of the ion current.	
OF POOR QUALITY
The effective cross section a (N) for the photoionization of the
nitrogen atom in the basic state for the entire wavelength range is
obtainable through the adjusting of relative values to the value
absolute measured for 554 A. Figure 3 shows the measured points and
a connecting median curve M. Additionally, curves A, B, and C are
shown, that show the effective cross section calculated by 11BNRY 5 In
the dipole length and dipole velocity approximations (A and B respec-
tively), as well as the DALGARNO and PARRINSON 6. calculation of the
effective cross section in the dipole velocity approximation (C).
V
U)
4j
ro
.r4
,rj
-P
Fig. 2. N + lon Current (A) and2
Photoelectron Current (B) as a
Function of Wavelength.
Nullinte (5)
N ullinte (A)
I	 I
aso	 goo
>,[:A I
Zero line (B)
Zero line (A)
ORIGINAL PAGE RS
OF POOR QUALITY
HENRY's findings according to the dipole velocity approximation are
mirrored best in the measurements up to a wavelength of 500 A. Under
500 A, the calculated values of this approximation deviate significant-
ly from the measured findings. As is apparent in Fig. 3, the measured
curve shows a much stronger declivity than the calculated ones, whose
slope in the short wave portion of the indicated wavelength range is
about equal. A characteristic deviation of the measured curve shows up
between 600 and 700 A. In absorption and ionization measurements, it
was demonstrated that a Rydberg series exists in this spectral range, that
converges to the 5S state of the ion, and whose termini decay through
autoionization. 7 ' 8. The 5S state is the lowest state of the ion in which
an optically permissible transition of the N atom out of the basic state
is to be expected. The convergence limit discovered in measurements in
active nitrogen 7. correlates well to ERIKSSON's established value for the
excitation energy of this state. 9. The Rydberg series is distinguished
through the transitions 4S0 - 2s2p 3 ( 5 S 0 )np4 P. 10. The selection rules
allow a reciprocal action of these states with the continual states of
the series 2p 2 ( 3 P)ns and 2p 2 ( 3P)nd. The broadening and assymetry of
the absorption lines occasioned by autoicnization is easy to see. The
value Q l for the line form parameter was deduced from this. 8. The
divergence from the theoretical ionization curve caused by the Rydberg
series results from that fact that these transitions were not even taken
into consideration during the calculations. This is to be undertaken in
these calculations.11.
The measured points are widely scattered in the neighborhood of the ioniza-
tion limit. The measurement clearly shows that the stronger divergencies 	 s
lie outside the error limit and can not be explained by statistical scatter- ri
ing,. Transitions into the 1D and 1S ionic states, that likewise result
	 }
from the 2s 2 2p 2 configuration and whose entrance potentials lie at 755 and
6
ik-1	 F y
o^U, t
^3 N
^ u
^l
to o
N
o -
U
OH
'	 ORIGINA PAGE ES
OF BOOR QUALITY'
j
667 A, are spin prohibitted and should not occur within the absorption 	
WI
spectrum.
Since the ionization curve was only measured for discrete wavelengths
and not with a continual spectrum, statements about potential new termini
in the ionization continuum of the nitrogen atom are extremely problematical.!:
However, it seems possible to repeat the measurements reported here
against the background of a continuum.
We thank the German Research Society for its generous support of the study.
Light Energy (eV)
—►
 LichtenercieleV)
15	 20	 25	 30
3,	 1I	 1'	 SI	 3i 3I
l 5
I
j	 I
00	 I	 ^^ ^I^
0	 0—°0—
	 O	 A
o
C
900
	 B00	 700	 600
	 540	 400
Weltenidnge(A) •
Wavelength
Figure 3. Effective
Cross Section for the
Photoionization of th
N-atom.
M: Measured Curve
A&B: HENRYII.
Dipole Length
Dipole Velocity
C: DALGARNO/PARKINSON6.
Dipole Velocity
. 
f	 ♦ C
. 	 W
LITERATURE
1. F.J. Comes, F. Speier and A. Elzer, Z. Naturforsch., 23a,
114 [1968].
2. F.J. Comes, F. 5peier and A. Elzer, Z. Naturforsch., 23a,
125 [1968].
3. F.J. Comes and W. Lessmann, Z. Naturforsch., 19a 65 [1964).
4. R.E. Huffman, Y. Tanaka, and J.C. Larrabee, J. Chem. Phys.
39, 910 [1963].
5. R.J. Henry, J. Chem Phys. 45, 4357 [1966].
6. A. Dalgarno and D. Parkinsori, J. Atmospheric Terrest. Phys.
18, 335 (19601.
7. P.K. Carroll, R.E. Huffman, J.C. Larrabee and Y. 'anaka,
Atrophys. J. 146, 553 [1966].
8. F.J. Comes and A. Elzer, Phys. Letters 25 A. 334 [1967].
9. K.B.S. Eriksson, Ark. Fys. 13, 303 [1958].
10. Odd parity is indicated by ° on the termini symbols.
11. R.J.W. Henry, private communication.
8
z^


Photoionization research on atomic radiation. 3: The ionization cross section of atomic nitrogen

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