A crossed beam apparatus has been used to measure the emission and ionization cross sections for the prominent spectral features of Na, Ca, Mg, and Fe in collisions with N2 and O2 over the velocity range of 30 to 120 km/s. From the emission and ionization cross sections, the absolute luminous efficiencies in air were determined over the range of meteor velocities. The maximum luminous efficiencies for the brightest features were: greater than 1 percent for the Na D-lines, 0.2 percent for the Ca I(2) singlet, 0.06 percent for the Mg I(2) and Mg I(3) triplets, and 0.4 percent for Fe over the visible spectral range. These luminous efficiencies are valid for free molecular flow conditions for velocities above about 30 km/s and are directly applicable to spectroscopic observations of faint meteors. In contrast to previous work, the luminous efficiency found for stone in the present investigation decreased with velocity above about 50 km/s

Boitnott, C. A.

Savage, H. F.

English

NASA Technical Reports Server

pR_DEWG PAGE BLANK NOT F_
7. Laboratory Determination of the Luminous Efficiency
of Meteor Constituents
HOWARD F. SAVAGE AN2) CHARLES A, BOITNOTT
A rues Research Center, NASA
Moffett Field, California
A crossed beam apparatus has been used to measure the emission and ionization
cross sections for the prominent spectral features of Na, Ca, Mg, and Fe in collisions
with N2 and 02 over the velocity range of 30 to 120 km/s. From the emission and ion-
ization cross sections, the absolute luminous efficiencies in air were determined over the
range of meteor velocities. The maximum luminous efficiencies fl_ the brightest features
were: greater ttmn I percent for the Na D-lines, 0.2 percent for the CaI(2) singlet, 0.06
percent for the MgI(2) and Mg[(3) triplets, and 0._ percent for Fe over the visible
spectral range. These luminous efficiencies are valid for free molecular flow co_ditions
for velocities above about 30 km /s and are directly applicable to spectroscopic observations
of faint meteors. The luminous ea_ciency of a typical stone will be presented and com-
pared witl_ the efficiencies determined by other investigators. In contrast to previous work,
the luminous eJ_ciency found for stone in the present investigation decreases with velocity
above about 50 km/s.
ETEOR LUMINOSITY RESULTS primarily from
the excitation of vaporized meteoric con-
stituents during decelerating collisions occurring
with atmospheric molecules. The naturally oc-
curring process of meteor vapor excitation during
the cascade of energy-transferring collisions can
be simulated in the laboratory by observing the
emission probability under controlled single
collision conditions and integrating over all
collisions to obtain thc total radiating efficiency.
The major aim of our research at Ames Research
Center is to make appropriate laboratory measure-
ments of the light emissions due to collisions
between air molecules and typical meteoric atoms
to allow for a direct calculation of the spectral
luminosity of meteors. The efficiency of converting
kinetic energy to luminous energy is the luminous
efficiency, T, and for a particular spectral feature
the two are related by
83
2dm
5=-½_._v --_
where v is the meteor velocity, din dr is the mass
loss rate of the element and h is the wavelength
of the spectral feature.
The spectral luminous efficiency for free
molecular flow conditions can bc determined from
the relationship given by Sida (1969)
2Ex f So a_ dE
r_ _ tier O'm E
where Eo is the initial kinetic energy, Ex is the
photon energy, E_ is the threshold energy for
exciting the wawlengttl k, a,, is the momentum
transfer cross section, and ax is the cross-section
for the emission of a photon of wavelength k in a
collision between an air molecule and a mercer
atom. Tile m_thematical formulation of r in slip
https://ntrs.nasa.gov/search.jsp?R=19740011330 2020-03-17T14:28:41+00:00Z
84 EVOLUTIONARY AND PHYSICAL
flow is too complex for present analysis and will
not be considered here.
An appreciable fraction of the ablated meteor
atoms is expected to bc ionized during the
deceleration process, The ions formed can produce
either ion lines or, if neutralization occurs, neutral
lines upon subsequent collisions. Therefore, an
accurate determination of rx must take into
account the fraction of ions present during the
deceleration process. The luminous efficiency
equation can be evaluated using an effective
emission cross section given by:
[Ni(E) +_x.(E) 1 No
where axe(E) is the emission cross section for
collisions of ions with air molecules and ax,(E) is
the emission cross section for those collisions in-
volving neutral metal atoms. The ion concen-
tration generated in decelerating the ablated
atoms from initial energy E0 to energy E can be
found from the relation N_(E)/No=I-e -_(_)
where the ionization probability _(E) is deter-
mined from the relation given by Sida (1969)
(E) = 2 f_°
dEff i
o',,, E
where a_ is the ionization cross section. Finally,
the measurable quantities in our experiments,
which are related to ax and z_, are the number of
photons and ions produced in the collision of a
beam of N2 or O5 with a perpendicular beam of
metal atoms. The definitive relationship for these
cross sections is _s=F(1--e-N='_), where (I,x is
the number of photons per second emitted at a
wavelength h when a gas beam of intensity F
passes a distance x through a metal beam of
density N. A similar expression relates a_ to the
number of ions produced.
APPARATUS
The crossed molecular beam apparatus used to
make these measurements shown in figure 1 has
been described in detail in our previous reports
(Boitnott and Savage, 1970; Savage and Boitnott,
1971). Briefly nitrogen or oxygen is ionized in the
source and focused through the analyzing magnet
to select the desired ion. The ion energy is set by
PROPERTIES OF METEOROIDS
FOCUSING,---, _,ON SOURCE
ELECTRODES I V QUARTZ CRYSTAL
_ VAPOR BEAM
"_ _ DETECTOR REFRIGERATED
/ PHOTOMULTIPL IER
....... _ CHARGE EXCHANGE / TUBE
........ ""_ II _ CHAMBER I 1
./2_.... _ i :_._' ,oANALYZING / _'._:_ -= N BEAM
MAGN¥'+- DEFLE/CTOR I_ DETECTOR
' SHUTTER __.OVE N
FIGVRE 1.--Schematic of emission cross-section apparatus.
the potential at the ion source. The resulting N,+
or O_+ beam is then focused into a charge exchange
cell which contains enough N2 or O2 to resonantly
charge exchange about half of the original ion
beam. In such a charge exchange process, there is
no energy exchange or scattering as long as the
densities are low enough so that there are no
secondary collisions. The electrostatic deflection
plates remove the remaining ions from the beam
leaving only N_ or O5 to be injected into the test
chamber. Intersecting at right angles to the N_ or
O2 beam is the metal atom beam which emanates
from a slit in an oven (the flux of metal atoms
being determined by the oven temperature). The
measurements were generally made with metal
atom densities of the order of 10 TM cm -s while the
background density of gas in the test chamber was
about 5 X 10_°em -s. For Na, the flux was measured
with a hot tungsten oxide surface ionizer as
described in our Na paper (Boitnott and Savage,
1970); and the Mg, Ca, and Fe fluxes were
measured with an oscillating quartz crystal.
A portion of the photons generated by the
collisions pass through a bandpass filter and are
counted. The bandpass of these filters is typically
100/_ and is centered on the multiplet of interest.
Now, with the photon count, calibration factors,
metal atom density, and gas beam intensity, the
emission cross section is determined. Then with
the ionization cross section determined from the
ion count, we can work through the equations
previously discussed and determine the luminous
efficiency for the spectral feature being observed.
The conditions of the experiment which can be
controlled are:
LABORATORY DETERMINATION OF TItE LUMINOUS EFFICIENCY OF METEOR CONSTITUENTS 85
(1) The densities of the colliding beams
(2) The energy involved in the collision
(3) The collision partners
(4) The photon wavelength observed.
In regard to the densities of the beams, all of our
measurements have been made under free molecu-
lar flow conditions. As for the energy, unfortu-
nately, our lowest energy is limited to about
150 eV so that the luminous efficiency calculations
must be made with extrapolation of the pertinent
cross sections. Therefore, the efficiency values
below about 30 km/s are much more uncertain
than at the higher velocities.
EMISSION CROSS SECTIONS
The emission cross sections of the most promi-
nent multiplets of Na, Mg, and Ca are shown in
figure 2 over the range of energies investigated.
The wavelengths of the lines of each multiplet
shown in figures 2, 3, and 4 are listed in table 1.
The data shown for Fc are for the total radiation in
the wavelength range between 3500 and 4200 _.
The number of lines in the Fe spectrum is so
numerous and closely spaced that bandpass
filters could only make measurements for collec-
tions of multiplets and could not easily isolate
individual multiplcts. At wavelengths greater
than 4200 _., the blackbody radiation from the
04
E
¢o
i
o
b-
t_
tO
I0 -16
_.)
Z
10-17
0
10-15 _
Na D-LINES
__Fe__500- 4200 A)
-_I (2)
I(2)AND
o o _.F MM_i (5)
A_L_J/ i I .... A_. 1 l-- I
800 1600 2400
BEAM ENERGY, eV
FIGtrRE 2.--Emission cross sections due to N2 collisions
for the prominent multiplets of Na, Mg, anti Ca and
of Fe over the range 3500 to 4200 ,_.
source oven made meaningful measurements
very difficult.
Cross sections for Mg and Ca shown in figure 2
are typical with other multiplets of these elements
10-14 -
04
E
O
ilo-_
o
I-
o
bJ
cO
(,,) 10-16
0
0
Z
o_ 10_17
if)
if)
bJ
10-18
0
-_ N2+-Ca II(,)
__ N2-Ca II(I)
N2-Ca I (4)
1 I I [_ I 1 I
800 1600 2400 5200
BEAM ENERGY, eV
F_GURE 3.--Emission cross sections of Ca due to N_ and
N2 + collisions.
I0-I
>- BECKER AND FRIICHTENICHT (1971)
0
z Fe ( 3400- 5800 _,)
/ _-..-.- Na
,,110 .2 ICHT, ET AL (1968)
(n @ e (CORRECTED)
oz 0-5500_)._1
w 10-5
I- ) "
_J
0
m I(2)AND Mgl(S)
in-4 :, i t l _t_._. J J
IV
0 40 80 120 160
VELOCITY, km/sec
F_O.LTRE4.--Absolute luminous efficiencies for the promi-
nent multiplets of Na, Mg, and Cr_ and of the visible
spectrum of Fe.
86 EVOLUTIONARY AND PHYSICAL PROPERTIES OF METEOROIDS
TABLE 1 .--Wavelengths of Multiplel Lines
Mult iplet Line wavelengths,
Na D-lines 5890, 5896
CaI(2) 4227
CaI(4) 4425, 4435, 4455
CaI[(1) 3934(K), 3938(H)
MgI(2) 5167, 5173, 5183
MgI(3) 3829, 3832, 3838
sho_'ing similar characteristics but with smaller
magnitudes. Collisions involving 0_ instead of N2
generally had lower cross sections, by as much as
a factor of 5, and did not necessarily show the
same variations with energy (Savage and Boitnott,
1971; Boitt)ott and Savage, 1970).
Of considerable interest are the CaII(1) lines,
H and K, s;nce they frequently appear enhanced
in meteor spectra and are thought to indicate slip
flow conditions. The emission cross sections are
shown in figure 3 as a function of energy for both
N2 and N_ + in collisions _ith neutral Ca. In
addition to the large difference in magnitude
between the ion and neutral collision partners, the
variation with energy is completely different. The
high H and I( emission for N_ + collisions with
neutral Ca is due to near resonant charge exchange
including excitation of the Ca ion in a single
collision. Without getting into a complete dis-
cussion of the H and K line enhancement as the
meteor gets lower in the atmosphere, we want
only to show that even in the free molecular flow
regime there is considerable H and K emission as
a result of neutral-neutral collisions. For the
luminous efficiency and a more complete dis-
cussion of the H and K enhancement, see Boitnott
and Savage (1971). The cross sections indicate,
that in the free molecular flow regime, the multi-
plets CaI(4) and CaII(1) should be of nearly
equal intensity. Any enh,mcement of the H and
K lines should tal{e place as the meteor enters the
denser atmosphere where there arc secondary
collisions between neutral calcium and metal or
gas ions, i.e., slip flow conditions.
LUMINOUS EFFICIENCIES
The absolute luminous efficicncies for the most
prominent multiplets of Na, 3[g, and Ca are
shown in figure 4 along with the total luminous
efficiency for the visible spectra of iron. For the
luminous efficiencies of the other multiplcts of
Mg and Ca, see Boitnott and Savage (1971). To
obtain the total for iron, allowances were made
for the visible spectrum above 4200 _ by com-
paring our bandpass data in the 3500 to 4200 *
range with the spectrum obtained by Tagliaferri
and Slattery (1969) and with the natural meteor
data of Harvey (1970). The decrease in the effi-
ciencies of Fe, Mg, and Ca at the higher velocities
is caused by the high relative probability of ioniza-
tion at these velocities. This has the effect of
removing neutral atoms which might be excited
during a later collision in the deceleration process.
Also the emission cross sections for the neutral
atom lines due to collision between metal ions and
air molecules are very low in comparison to those
for neutral-neutral collisions. There is generally
good agreement between our data and the free
molecular flow meteor simulation experiments of
Friichtenieht et al. (1968) and Becker and
Friichtenicht (1971). The data of Friichtenicht
et al. has been increased by 20 percent to account
for the more accurate spectral distribution found
later by Tagliaferri and Slattery (1969).
The luminous efficiencies we have determined
for the multiplets of Na, Mg, and Ca and the
10-12>_"
(D
Z
tO
ta_
it_
tO
[D
_ I0 -13
._I I:_
-1- o
13_
<
0
F-
0
1"
0.
IVERNIANI (1965)
f
AYERS, MCCROSKY,
i//t/ SHAO (1970)
I
----_AVAGE AND BOITNOTTpercent
Fe = 15.4
Mg= 15.1
Ca = 1.8
10-14 _± J I t I • J J
0 40 80 120 160
VELOCITY, km/sec
FIGURE &--The photographic luminous efficiency of a
stone for blue sensitive fihn.
LABORATORYDETERMINATIONOFTHELUMINOUSEFFICIENCYOFMETEORCONSTITUENTS87
total for iron are directly applicable to the deter-
ruination of the relative abundances of these
elements in thos_' free molecular flow photographic
meteors for which good resolution spectra are
available. With an assumption on the relative
abundances of the weak and nonradiating elements
which make up the rest of the meteor, the mass
can be determined.
As an exercise, we have calculated the pho-
tographic luminous efficiency of blue sensitive film
(Ayers, 1965) for a meteor with the assumed
composition of meteoric stone (fig. 5). Variations
in the composition would have only minor effects
on the magnitude of r without changing the general
trends. Other vaporized elements present in the
meteor are assumed to provide negligibly weak
radiation and are neglected. In addition, any con-
tribution by air molecules is not included in the
calculations. Such radiation is toward the red and
is seen in spectra using panchromatic film as are
the Na D-lines.
For comparison, we show the luminous efficiency
deduced by Verniani (1965) from 413 photo-
graphic meteors where he assumed that r is
proportional to the Y", for aii velocities, and
concluded that n = 1. Also shown is the best guess
efficiency of Ayers et aI. (1970) which is pri-
marily based on low velocity, near contimmm
flow, artificial meteor data. The reasons for the
large differences arc not clear to us at this time.
In summary, we have measured in the labora-
tory the fundamental emission and ionization
cross sections needed to determine the spectral
luminous efficiencies of Na, Mg, and Ca and the
total efficiency for iron. We are encouraged by
the good agreement with the luminous efficiency
of iron measured by Friichtenicht et al., and
Becker and Friichtenicht, and feel that the
luminous efficiencies obtained can be applied to
the quantitative analysis of meteor spectra in
free molecular flow conditions.
REFERENCES
AYERS, W. C_., 1965. Luminous efficiency of an artificial meteor at 11.9 kilometers per second,
NASA TN D-2931.
AYErtS, W. G., McCRosKY, R. E., AND _HAO, C. -Y., 1970. Photographic observations of 10
artificial meteors, S,nithson. Astrophys. Obs. Spec. Rept. No. 317.
BECKER, D. G., AND FrtIICrITE__IeHT, J. F._ 197I. Measurement and interpretation of the lumi-
nous efficiencies of iron and copper simulated micrometcors, Aslrophys. J., 166, 699-716.
Boir:_-oz-r, C. A., Ar,-D S._VAC,_:, It. F., 1970. Ligl, t-emission measurements of sodium at simulated
meteor conditions, Astrophys. J., 161, 351-358.
BOIT_OTT, C. A., AND SAVAGE, H. F., 1971. Light-emission measurements of calcium and mag-
ncsiom at simulated meteor conditions. II. Spectral luminous efficieneies, Aslrophgs. J., 167,
349-355.
FRIICHTENICHT, J. F., SLATTERY, .]-. C., AND TAGLIAFERRI, E., 1968. A laboratory measurement
of meteor luminous efficiency, Aslrophys. J., 151, 747-758.
HAnVEY, G. A., 1970. Spectra of faint optical meteors, unpublishcd.
SAVAGE, H. F., AND BoIw_-oa'r, C. A., 1971. Light-emission measurements of calcium and mag-
nesium at simulated meteor conditions. I. Cross-section measurements, Astrophys. J., 167,
341-348.
SIDA, D. W., 1969. The production of ions and electrons by meteorit.ic processes, Monthly Notices
Roy. Aslron. Soe., 155, 37-47.
TAGLIAFERRI, E., AN'q) SLATTERY, J. C., 1969. A spectral measurement of simulated meteors,
Aslrophys. J., 155, 1123-1127.
VERNIAN'I, F., 1965. On the luminous efiiciency of meteors, Smithson. Contrib. Astrophys., 8,
141-172.



Laboratory determination of the luminous efficiency of meteor constituents

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